US20180368899A1

US20180368899A1 - Preshearing method and apparatus for the control of the rheology and the injectability of aqueous cement suspensions for bone repair and regeneration

Info

Publication number: US20180368899A1
Application number: US15/966,185
Authority: US
Inventors: Erdem Şahin; Dilhan M. Kalyon
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-04-30
Filing date: 2018-04-30
Publication date: 2018-12-27
Also published as: US12053218B2; US20210353345A1

Abstract

The invention provides a system for the preshearing based control of the flow and deformation behavior, i.e., the setting kinetics, and the time dependent shear viscosity, elasticity of aqueous cementitious suspensions that can be used for bone repair and regeneration. The dynamic cement microstructure is tailored to the demands of the surgical tasks (faster/slower setting) or additive manufacturing tasks (lower/higher viscosity) by application of various preshearing conditions. The relationships between the preshearing and pressurization conditions and the setting kinetics and the time dependent changes in elasticity and viscosity are complex and characterization of viscoelastic properties using advanced rheological characterization techniques including small-amplitude oscillatory and steady torsional rheometry is needed a priori to enable such tailoring. The preshearing system is intended to give control on the injectability and setting time of any calcium phosphate cement formulation to the surgeon during an orthopedic surgery where a batch of bone cement is processed. Other possible utilizations of the system include controlling the setting kinetics, shear viscosity and facilitating the resultant flow stability of cementitious ceramic suspensions processed in direct ink writing assemblies for additive manufacturing of cement constructs, in injection systems for restoration and fracking.

Description

BACKGROUND OF THE INVENTION

Technical field

The present invention relates to ceramic cements used in bone repair and regeneration, and more particularly to methods and devices for mixing, pressurization, and preshearing based in situ treatment for the control of the flowability, setting kinetics, and injectability/extrudability during manual or robotic deposition of such cementitious suspensions.

Prior art

Cementitious materials are the materials of choice in design and manufacturing of structural, monolithic or injectable components due to their abundancy in nature, and by virtue of their flowability before setting. The latter is exploited most commonly in biomedical applications for bone repair and regeneration using calcium phosphate based cements (driven by the exceptional osteoconductivity of calcium phosphates) as injectable biomaterials (bone cement pastes) especially following cancerous bone removal and for minimally invasive surgeries. The minimally invasive clinical applications of bone cement pastes include spinal fusion, vertebroplasty, khyphoplasty, cranioplasty and periodontal surgery. Such inorganic bone cement pastes typically exhibit relatively low shear viscosity and elastic modulus and gain elasticity and shear viscosity with time. The rates of growths of the elasticity and viscosity of calcium phosphate based cements are higher than those of conventional cements as a result of the rapid dissolution and crystallization of various calcium phosphate phases in water. For this reason they are especially suitable for additive manufacturing of geometrically complex constructs where the deposited paste is made to gain elasticity for immediate shape retention. However additive manufacturing of relatively slow setting cementitious suspensions have been realized by pressurization through hoses of relatively large diameters (>10 mm) due to the propensity of such dynamically evolving suspensions for flow instabilities and related clogging issues during pressure-driven flows through narrow capillaries with diameters in the micronmeter scale.
During surgical applications the precise placement of the bone cement paste by the surgeon is very important. Various means are available for the placement of the cement paste into the repair site. Generally a syringe with a hypodermic needle with a diameter around 1 mm can be used. During the injection of the cement paste a pressure drop of the ceramic paste is developed as the paste flows out of the syringe and the needle and as it is forced into the treatment site. This pressure drop represents the bottle neck to injection and is overcome by the surgeon applying a sufficient pressure on the ram of the syringe that holds the cement to overcome the pressure drop. The applicability and the injectability of the cement suspension are governed by the shear viscosity and the elasticity of the ceramic paste (functions of temperature, time, solid content and shear rate/stress). Once the ceramic suspension attains certain upper thresholds of viscosity and elasticity the injection of the cement paste to the treatment site via the pressure applied on the ram by the surgeon is no longer possible. The rapid increase of the shear viscosity of the cement paste (transition from flowable suspension to a gel and then to a rigid solid) is associated with the cement reaching its setting time. Thus, the setting time restricts the duration of time that the cement remains viable for injection during surgery. Currently in clinical practice, the setting time and flowability of the cementitious suspension are not adjustable parameters for a given formulation. The surgeon needs to select and use a suitable cement formulation (a commercial product which typically comes in a syringe or is mixed in the operating room) to obtain the targeted setting time and flowability.
Synthesis of ingredients for bone cements is intensively researched since there are unlimited combinations of possible constituents. The most popular methods for generation of new types of calcium phosphate based bone cements include the addition of chemical groups that have affinity to calcium ions such as citric acid or citrate salts to prolong the setting period, addition of salts to the cement setting liquid to increase the ionic strength and decrease the supersaturation of precursor ions, addition of phosphate containing salts such as sodium phosphate to increase the supersaturation, addition of acids to increase the solubility, and addition of chemicals like gentamycin that electrostatically stabilize calcium phosphate crystals by increasing their surface charge. The major problem that is faced is that every time there is a formulation change that involves the use of ingredients that are not approved by US Food and Drug Administration, FDA, for in vivo usage, new and very costly FDA approvals may be necessary for implementation of these ingredients and formulations. Furthermore, the problem associated with the development of many choices for the surgeon is that the cement needs to be tailored via changes in composition to the specific application at hand. The surgeon currently has no recourse but to switch formulations depending on the requirements of the specific surgery since there are typically no adjustable parameters available during the injection of the bone cements to allow their tailoring for specific applications at the surgery site. Furthermore, multiple changes in flowability and elasticity may be necessary during the course of surgery to accommodate the complications that arise during surgery.
The mixing, conveying and delivery of the bone cement are time consuming in practice when done manually and separately. Bone cement mixing and delivery devices have been invented either as separate automated parts or as all-in-one facilities to improve the efficiency of preparation and delivery of the material. Previous inventions that introduce novelty into the area are as follows. US Patent # 2008/0065088 A1 provides a mixing device for bone cements that is comprised of multiple chambers and pistons. U.S. Pat. No. 8,409,211 B2 introduces a bone cement delivery device with a tubular inner wall and a tubular outer wall, helical thread and vacuum and a pressure sensor. US Patent # 2008/0154229 A1 discloses a cement mixing and delivery device that uses a helical element and vibration to mix and deliver. These US patents and applications lack method and control on the flow behavior, setting time and kinetics of the ceramic bone cements. Other bone cement delivery devices, such as a system for injecting a low viscosity fluid into a bone cement reservoir [U.S. Pat. No. 8,870,888 B2], a delivery device that uses a screw actuator to push the plunger [U.S. Pat. No. 6,712,794 B2], a delivery device that has an electronic actuator controller [U.S. Pat. No. 8,403,888 B2], a device for transferring bone cement into a syringe for delivery [U.S. Pat. No. 8,408,250 B2], and a device which has a vibration element attached to the delivery needle [U.S. Pat. No. 8,834,481 B2, U.S. Pat. No. 7,901,407 B2] are available.
Overall, none of these methods and devices can control the rheology (flow and deformation behavior), the setting kinetics and the injectability/extrudability window of the bone cement paste to accommodate the specific demands of a particular task that is being undertaken. On the other hand US Patent # 2009/0112365 A1 discloses a method for determination and mechanical modification of the setting kinetics of polymerisable cements used in orthopedic applications. Various environmental factors including temperature, humidity, sound wave velocity, etc. within and around the polymer based cements have been correlated with this method to cement setting kinetics and the level of polymerization.
Subsequent torsional or oscillatory mixing under non-specific stress conditions have been shown to facilitate the attainment of the level of maximum polymerization. In another disclosure on modification of rheology of fluids, U.S. Pat. No. 6,293,754 B1 explains direction of acoustic energy at frequencies between 1 kHz and 10 MHz to pressurized fluids for controlling viscosity by shear-thinning. The invention proposes a novel method of exploiting the shear-thinning mechanism and does not specifically apply to cementitious suspensions. In yet another method of agitation to alter the microstructure of cementitious suspensions, U.S. Pat. No. 4,202,413 discloses a method to improve the effectiveness of swelling clay particles that extend the inorganic cement suspension, by similar vigorous mixing. This method is only for the enhancement of the dispersion, swelling and extender function of inorganic, non-setting clay particles by application of simple blending action of a rotating blade. Conversely, preshearing method and apparatus disclosed in the present invention utilize a distinctly different mechanism than shear-thinning and specifically apply to inorganic, non-polymerisable, aqueous cements such as calcium phosphate bone cements by employing torsional shear stresses at specific modes, rates and amplitudes that are determined upon accurate rheological characterization a priori in order to shorten and lengthen the setting times and to increase or decrease the resultant development of cement viscosity. Clearly, there is a need for a method and apparatus to realize the ability to in situ tailoring of the injectability, the shear viscosity, and elasticity of such ceramic paste materials, during surgery. In addition, extrusion through a micronmeter-diameter nozzle during robotic deposition of cementitious suspension necessitates such precise control of the evolving microstructure and pressurization conditions to achieve flow stability. The present disclosure provides a method and an apparatus that can address the aforementioned requirements for the effective utilization of inorganic cementitious suspensions for such applications

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing background, a method and an apparatus for the tailoring of the flow behavior (shear viscosity, elasticity), setting time and the injectability/extrudability of cementitious ceramic suspensions prior to or during the course of delivery by an orthopedic surgeon or a robotic additive manufacturing system are disclosed. The present disclosure specifically relates to the modification of flow properties of a ceramic cement suspension by preshearing (i.e., via subjecting the cementitious suspension to steady or oscillatory torsional deformations at critical values of the shear rate, shear strain and frequencies), pressurization (i.e., delivery of flowable cement through a capillary by a screw-driven mechanism) and an apparatus for both preshearing and pressurization. The present disclosure shows that the shear viscosity, elasticity and the setting kinetics of various relevant calcium phosphate cement formulations are dependent on such preshearing of the cement suspensions. Thus preshearing and simultaneous or subsequent pressurization by a handheld or robotically controlled apparatus would allow specific tailoring of the flow properties and setting kinetics, and hence the control of the injectability/extrudability of the ceramic paste on one hand and the physical properties of the set cement on the other. The surgeon is given the freedom to choose the appropriate preshearing mode and preshearing parameters similar to considering different bone cement formulations that may be suitable for an application in terms of ease of injection, the setting time and ultimate mechanical properties. The invention also facilitates extrusion of cementitious suspensions for related applications including construction, structural restoration and additive manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is made to the following detailed description of an embodiment considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a flow chart showing the standard procedural steps of rheological characterization of cements for predetermination of their key viscoelastic properties;

FIG. 2 is a flow chart showing standard operational procedure for utilization of the various modes of the preshearing method;

FIG. 3A is a schematic diagram of a rheometer based testing device and steady torsional method of preshearing,

FIG. 3B is a schematic diagram of a rheometer based testing device and oscillatory torsional method of preshearing;

FIG. 4A is a schematic diagram of a preshearing apparatus constructed in accordance with an embodiment of the present invention, the preshearing apparatus being configured for batch processing and injection to a treatment site,

FIG. 4B is a schematic diagram of the preshearing apparatus in FIG. 4A configured to pump bone cement into a syringe;

FIG. 5A is a schematic diagram of a preshearing apparatus constructed in accordance with another embodiment of the present invention, the preshearing apparatus with two independent screws being configured for semi-batch processing of cements;

FIG. 5B is a schematic diagram of the proximal end of the preshearing apparatus in FIG. 5A;

FIG. 6A is a schematic diagram of the axial section of a preshearing apparatus constructed in accordance with another embodiment of the present invention, the preshearing apparatus being equipped with a thermal jacket for the control of temperature within the apparatus,

FIG. 6B is a schematic diagram of the proximal end of the preshearing apparatus in FIG. 6A;

FIG. 7 is a chart showing the time-dependent setting behavior of a ceramic sample held under quiescent conditions, as represented by its elastic storage modulus, G′, and its magnitude of complex viscosity, η * wherein each point refers to a specific elapsed time under quiescent conditions (without shearing) following mixing;

FIG. 8 is a flow chart showing the partitioning between the data measurement and application of the effects of preshearing to conduct measurement of the time-dependent rheological behavior and setting kinetics without creating strain history;

FIGS. 9A and 9B are a pair of charts showing the time-dependent development of the storage modulus (FIG. 9A) and the magnitude of complex viscosity (FIG. 9B) for brushite cement samples that are not presheared (kept under quiescent conditions) and presheared at various frequencies at a strain amplitude of 0.04;

FIG. 10A and 10B are a pair of charts showing the time-dependent development of the storage modulus (FIG. 10A) and the magnitude of complex viscosity (FIG. 10B) for cement samples that are presheared at various strain amplitudes at a frequency of 1 rps;

FIGS. 11A and 11B are a pair of charts showing the time-dependent development of the magnitude of complex viscosity when the preshearing involves steady torsional flow (constant shear rate) and oscillatory shearing at various frequencies (FIG. 11A) and strain amplitudes (FIG. 11B);

FIGS. 12A and 12B are a pair of charts showing the variation of the storage modulus, G′, of bone cement as a function of time (FIG. 12A) and frequency (FIG. 12B) at a constant strain amplitude of 1;

FIG. 13 is a chart showing the time-dependent development of the storage modulus, G′, of the bone cement as a function time at various strain amplitudes, γ⁰and a constant frequency of 0.1 rps;

FIGS. 14A, 14B and 14C are a set of charts showing the cyclic changes in the frequency, i.e., from 0.1 rps to 100 rps, applied for 4, 7, and 15 cycles respectively; and

FIG. 15 is a chart showing the time-dependent development of the storage modulus, G′,of bone cement following cyclic changes in frequency during preshearing at various step sizes (strain amplitude is held constant at 1).

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The following disclosure is presented to provide an illustration of the general principles of the present invention and is not meant to limit, in any way, the inventive concepts contained herein. Moreover, the particular features described in this section can be used in combination with the other described features in each of the multitude of possible permutations and combinations contained herein.
All terms defined herein should be afforded their broadest possible interpretation, including any implied meanings as dictated by a reading of the specification as well as any words that a person having skill in the art and/or a dictionary, treatise, or similar authority would assign thereto.
Further, it should be noted that, as recited herein, the singular forms ‘a,’ “an,” and “the” include the plural referents unless otherwise stated. Additionally, the terms “comprises” and “comprising” when used herein specify that certain features are present in that embodiment, however, this phrase should not be interpreted to preclude the presence or additional of additional steps, operations, features, components, and/or groups thereof.
The present disclosure generally relates to cementitious ceramic formulations, the rheological behavior and setting kinetics of which are very sensitive to preshearing. By definition, cementitious ceramics are single phase or a combination of ceramic powders that react with an aqueous setting solution. The mechanisms of the reactions may vary starting from hydration to dissolution, nucleation and crystallization. Initially flowable fluid mixture eventually gels, sets and hardens with time to a rigid mass with mechanical integrity. The invention is based on our recent observation that preshearing a cementitious calcium phosphate suspension in various shearing modes and under critical shearing conditions prior to injection can significantly increase or decrease the setting time, and thus can replace or complement the chemical setting retardants or enhancers that are utilized in the synthesis of clinically relevant calcium phosphate cements.
The present disclosure contemplates calcium phosphate and other types of inorganic cements that set at times close to or within the typical bone cement processing time of 10 to 45 minutes. Generally calcium phosphate cements with or without chemical retardants at the powder/liquid ratios between 1.0 and 4.5 and starting calcium phosphate particles with mean sizes that are generally less than 10 micrometers set within that period. Setting of water consuming cements like brushite forming calcium phosphate cements can generally be effectively retarded or delayed by preshearing whereas setting relatively slow setting cements like hydroxyapatite forming cements can generally be promoted by various mechanisms. Typical brushite forming cements set relatively rapidly in a few minutes and this period even shortens further when the water content decreases. For this reason setting retarder chemicals are necessary and become integral parts of most of the commercially available brushite forming calcium phosphate cements used in bone repair. The preshearing method and the apparatus offer the possibility to reduce/eliminate setting retarder or promoter chemicals from the cement formulation and their adverse effects on the biological and mechanical properties of the set biomaterial by increasing or decreasing the setting time mechanically.
The present disclosure discusses subjecting the cementitious ceramic formulations to time-dependent, strain amplitude-dependent or frequency-dependent shearing in a rheometer to determine the strain and strain rate conditions under which the setting of the ceramic paste is significantly altered. The rheometer is used to preshear the cement paste as well as to characterize the time dependent developments of its shear viscosity, elasticity, and injeadbility in a quiescent approach by removing the effect of strain history from measurements.
The present disclosure discloses a method comprising of subjecting the cementitious ceramic paste to predetermined preshearing conditions of shearing modes, shearing rates, and shearing strains in situ using a preshearing apparatus to control the time-dependency of the shear viscosity, and elasticity, hence the injectability of the bone cement using the data base generated by the use of the rheometer. The time it takes for cement paste to set can be increased or decreased by selecting the appropriate shearing mode (steady torsional flow versus oscillatory shear) or by selecting the appropriate values of the frequency or the strain amplitude in oscillatory shearing or shear rate in steady torsional flow.
The present disclosure also introduces a novel preshearing apparatus for preshearing-based control of the rheology, setting times and injectability/extrudability of inorganic cementitious suspensions.

Preshearing Method for Control of Rheological Behavior and Setting Time (Injectability)

A preshearing method in accordance with an embodiment of the present invention has the following steps:
(1) subjecting of the cementitious ceramic formulation to steady or time-dependent preshearing in a small-amplitude oscillatory rheometer under systemically varied conditions of shear rate, shear strain amplitude and frequency;
(2) characterization of the time-dependent development of shear viscosity, elasticity and setting kinetics following preshearing and determining the strain and strain rate conditions under which the setting of the ceramic paste is significantly altered; steps #1 and #2 being undertaken to determine a priori the relationship between preshearing conditions and provide guidance to the surgeon on the evolution of the time dependent shear viscosity and elasticity (and hence the injectability window) of the ceramic suspension on preshearing conditions.
(3) preshearing of the bone cement by the surgical staff in the operating room or by a robotic operator using a preshearing apparatus under the conditions that are necessary to achieve the desired injectability window of the bone cement on the basis of the particular requirements of the task on hand.
Characterization of the time-dependent development of shear viscosity, elasticity and setting kinetics following preshearing in the small amplitude oscillatory rheometer involves measurement of the mechanical response of the cement suspension to a predetermined torsional strain or stress state, as a function of time. Various strain modes, rates, amplitudes and frequencies are known to elicit different rheological behaviors in calcium phosphate cements. Conducting these measurements at relatively low strain amplitudes (below critical linear viscoelastic strain amplitude) and low frequencies in an oscillatory torsional rheometer ensures that the cement setting kinetics are probed at a condition closest to the quiescent equilibrium state when the setting characteristics of a specific cement formulation is clearly exhibited. Hence determination of the linear viscoelastic strain limit (LVSL) for a cement formulation is necessary prior to these measurements. The linear viscoelastic strain amplitude limit of a cement suspension increases with growing crystals as the cement sets and as temperature increases, and with viscous loss of mechanical wave energy as frequency decreases. Its dependence on frequency is weak so that a wide range of frequencies can be applied to cement suspensions without exceeding the LVSL.
The setting kinetics of a cement suspension is determined from the variation of the storage modulus (the primary measure of elasticity) with time during oscillatory preshearing. Cement set at quiescent conditions represents the baseline reference in preshearing method with which various preshearing modes and conditions are related. Conventional testing in oscillatory rheometer is unable to precisely determine the setting kinetics of cements because of the effect of applied strain history on the setting kinetics. The only way to remove this effect is to partition the testing run into preshearing and measuring steps (or waiting and measuring steps for quiescent samples). This way the rheometer is used both to preshear the cement paste and to precisely characterize the time dependent developments of the shear viscosity, the elasticity and hence the injectability of the bone cement. During testing of quiescently setting cement this is accomplished by keeping the cement suspension at rest for various periods of time prior to the measurement step as demonstrated in the examples.
Testing the cement formulation at quiescent conditions and by preshearing in oscillatory rheometer at various frequencies enables the determination of five important data for any cement system (the LVSL, the dough time, the initial setting time, the final setting time, and the setting promoting frequency limit (SPFL)) as shown in FIG. 1 and in the example given later. The LVSL, 1, of a cement can be conveniently measured by subjecting the cement suspension sample to oscillatory torsion at a constant frequency and systematically increasing the strain amplitude from a low value (<1%) to macroscopically observable values, and determining the strain amplitude at which the dynamic properties start to decrease indicating the transition to nonlinear viscoelastic behavior. The dough time, 2 is the first of the setting times observed in rheological characterization during setting of a cement and it represents the end of injectability period when the suspension starts to thicken macroscopically. It is generally observed when the ratio of the loss modulus to the storage modulus (G″/G′=tan delta) decrease below 0.5. The initial setting time, 3 is the second kinetic data of importance as it indicates that cement has gained elasticity and further working under pressure may damage the solid structure. It is observed when the second time derivative of the time dependent storage modulus function reaches maximum value. The final setting time, 4, the last of the kinetic data that marks the attainment of a rigid structure that can withstand pressure is observed when the storage modulus reaches a constant maximum value. SPFL, 5, is the analogue of LVSL to determine the critical frequency similar to the critical strain amplitude. It is the critical frequency that produces the same setting kinetics in comparison to the quiescently set cement at the constant strain amplitude used to measure quiescent cement properties. SPFL is determined iteratively and frequencies exceeding it promotes setting at strain amplitudes less than LVSL.
The alterations in the kinetic properties 2,3,4 resulting from the preshearing process are related to the baseline reference as promoting or delaying of the quiescent setting kinetics. These various preshearing effects are compiled and utilized as a guide for preshearing to tailor the setting kinetics by adjustment of the preshearing parameters. The preshearing method can be used to increase or to decrease the time it takes for the cement paste to set by selecting the appropriate shearing mode (steady torsional flow versus oscillatory shear) or by selecting the appropriate values of the frequency or the strain amplitude in oscillatory shearing or shear rate in steady torsional flow. The appropriate shearing mode and the parameters of shearing are obtained from rheological characterization and applied according to the kinetic requirements of the task as described schematically in FIG. 2. For example the setting of the cement formulation can be promoted to earlier times by increasing frequency at a constant strain amplitude, for the necessary injectability window of the surgical task at hand, based on the rheological data inputs on effects of the preshearing conditions. Similarly its shear viscosity can be decreased by the same preshearing effect to facilitate pressurization for the targeted volumetric delivery rate of a robotic deposition device. For both manual and automatic preshearing operations, the extent of application is critical for the effective modification of the setting kinetics. A pair of indicators complement the preshearing process by providing the operator guidance on the extent of application to safeguard the intended microstructure of the presheared material. The “setting effectively delayed”, 6, indicates that setting kinetics has slowed down and is continuing to slow down; and the “end preshearing”, 7, indicates that preshearing period has exceeded the target delivery time. Manual or automatic delivery should start between these indicated time periods for optimum injectability/extrudability.
We have recently reported that oscillatory torsional preshearing enhances the setting kinetics and decreases the setting times of calcium phosphate cements provided that the applied strain amplitude is below the LVSL and the frequency is higher than the SPFL. [E. Şahin, D. M. Kalyon, “The rheological behavior of a fast-setting calcium phosphate bone cement and its dependence on deformation conditions”, Journal of the Mechanical Behavior of Biomedical Materials, Volume 72, 2017, Pages 252-260]. Furthermore, each unique cement system exhibits specific limits of these parameters that should be determined a priori according to the generally observed preshearing effects of oscillatory torsion given in Table 1,. The strain amplitude is the primary parameter for the preshearing method whereas the frequency of the applied oscillatory torsional strain is a fine tuning parameter. Injection or extrusion time periods and viscosities of bone cements can be tailored by application of the findings from preshearing experiments.

TABLE 1

Effect of various combinations of oscillatory torsional strain amplitudes
and frequencies on calcium phosphate cement setting kinetics

Applied oscillation strain	Frequency higher	Frequency lower
amplitude and frequency	than the SPFL	than the SPFL

Strain amplitude lower	Positive effect	Negative effect
than the LVSL	(Lower setting time)	(Higher setting time)
Strain amplitude higher	Negative effect	Negative effect
than the LVSL	(Higher setting time)	(Higher setting time)

In theory, the mechanical energy applied to a cementitious suspension by oscillatory torsion is proportional to the strain amplitude. The viscous drag force on the particle is critical for the integrity of the inter-particle network that forms during setting of a cementitious suspension. At high applied energy levels particles and inter-particle crystal bonds disrupt and cement setting is retarded. This critical energy level for any suspension is manifested by the deviation from linear viscoelastic behavior and accurately monitored as a function of time by small amplitude oscillatory rheometry.
FIG. 3A and 3B give the schematics of the steady and oscillatory torsional modes of application of drag shear forces to cementitious suspensions, i.e. preshearing by torsion of the suspension between circular plates, 8. The strain rate {dot over (γ)}, 9, applied during steady torsional preshearing (strain monotonously increasing with time t, i.e., the integral of the strain rate that is imposed, γ={dot over (γ)}*t, 10), and the strain amplitude γ°, 11, applied to the cement suspension during oscillatory torsional preshearing (strain cyclically applied in the form of a sinus wave as γ=γ° sin ωt, 12) mainly determine the extent of the retardation of cement setting. Shear stresses τ, 13 develop in the cement in response to the applied strains γ, 10 and 12, according to the Hooke's law and depending on the shear modulus G, if the deformation occurs linear viscoelastically (τ=G*{dot over (γ)}*t or τ=G*γ° sin(ωt+δ)). There exists a phase difference δ, 14 between the applied oscillatory strain and the resultant stress due to the viscoelastic character of the material. Hence the oscillatory torsion mode of a rotational rheometer (FIG. 3B) is an effective tool to discern information about the viscous and elastic character of a fluid according to the phase difference δ, by virtue of the cyclic nature of the deformation and the adjustable frequency ω, 15, i.e., the time frame of a cycle of deformation.
The crystal network forming during setting is prone to structural damage when the applied oscillatory or torsional strains exceed the linear viscoelastic strain limit as crystals cannot attenuate the applied deformation energy elastically and break. This limit increases with the cement setting extent since bigger, well developed crystals are stronger. Hence the effectiveness of applied strain amplitudes or shear strain rates depend on the setting kinetics of a particular cement formulation which can be determined by the empirical methods mentioned above. Furthermore, different preshearing modes have the capability to delay or retard the setting of calcium phosphate cements. Oscillatory torsional strains exceeding the linear viscoelastic strain limit cause a temporary reduction in cement setting extent as setting recovery occurs subsequently. Alternatively steady torsional strains applied to a calcium phosphate cement suspension cause continuous damage to the forming crystal network due to the monotonous increase in strain at a rate higher than the crystal growth rate.
Still referring to FIG. 3B, frequency ω, 15, of the applied oscillation energy determines the rate of energy transfer and affects time dependent processes in cement setting including dissolution, crystallization, and ion transportation. Higher frequencies cause an improvement in diffusion of calcium and phosphate ions similar to the effect of increasing temperature of the cement suspension. The effectiveness of this mechanism depends on the variation of the LVSL with frequency as well as the intrinsic chemistry of the cement suspension such as the concentration of ions and the extents of the dissolution, nucleation and crystallization of species in a specific cement formulation at a particular setting time. As these properties are very hard to model mathematically for any cement suspension system containing multiple phases and chemical species, an empirical correlation between the chemistry of the cement formulation and the effect of preshearing parameters needs to be sought using a rheometer. Hence preshearing method is basically an implementation of time-dependent microstructural control on cementitious suspensions based on the utilization of advanced rheological techniques as a means to accurately map the dynamic cement setting kinetics.
Preshearing Apparatus to be Used as a Handheld Device or Integrated into a Robotic System
The preshearing device can generate and apply targeted strain rates and strains as a function of time for preshearing-based control of the rheology and setting time of injectable cementitious ceramic suspensions. The characterization steps provide the surgeon or robotic operator with a wealth of information as to how to control the injectability and workability of the bone cement via changes in the operating parameters of the preshearing device which is also capable of on-site mixing of the ingredients of the ceramic paste formulation and on-site pressurization and the delivery of the ceramic paste to the treatment site or the translating printing stage.
FIGS. 4A and 4B show schematics of a preshearing apparatus in accordance with two embodiments of the present invention. Accordingly, the preshearing based control of the viscosity, elasticity and the setting time of injectable cementitious ceramic suspensions can be realized using a rotating and oscillating screw mechanism that is placed inside a barrel. Such a mechanism involving a cyclic extrusion system can be used to first mix the ingredients of the cement formulation if such mixing is necessary, and following mixing, for the preshearing of the cement paste at frequencies, strains, and strain rates that are identified a priori to be effective in delaying/promoting the setting of the cement and the decreasing/increasing of the elasticity and the shear viscosity of the bone cement. This preshearing apparatus is not a conventional single screw or twin screw extruder. The screw(s) (only one screw if the device is based on single screw extrusion, schematic of which is seen in FIG. 4A., 16, or two screws if the device is based on a twin screw extruder) of the preshearing apparatus do not rotate in the same direction but can oscillate between clockwise (“CW”), and counterclockwise (“CCW”) directions at the desired frequencies to generate the 445 identified oscillatory strains and steady strain rates as a function of time. There are comprehensive mathematical models and simulation packages which would allow the translation of the desired frequencies and strains obtained via the use of the rheometer to the operating parameters of the preshearing apparatus [M. Malik, D. M. Kalyon, and J. C. Golba Jr., “Simulation of co-rotating twin screw extrusion process subject to pressure-dependent wall slip at barrel and screw surfaces: 3D FEM Analysis for combinations of forward- and reverse-conveying screw elements”, International Polymer Processing, 29, 1, 51-62 (2014); D. Kalyon and M. Malik, “An integrated approach for numerical analysis of coupled flow and heat transfer in co-rotating twin screw extruders”, International Polymer Processing, 22, 293-302 (2007)] and these capabilities can be used for each formulation to determine what the operating parameters of the preshearing device should be (screw speeds, CW or CCW rotation based oscillations) to generate the strain histories that are suggested by the rheometry tests.
Still referring to FIGS. 4A and 4B, the preshearing apparatus comprising screw(s) 16 and barrel(s) 17, has inlet ports 18 to allow the feeding of various ingredients of the bone cement formulation if mixing is desired and the facility to rotate the screw/s in sequential or cyclic manner. Furthermore, the preshearing device has an exit gate 19 that is attached preferentially by a screw mechanism to a die 20 that can fully or partially seal the exit off by a manually or automatically closing a plug 21. If mixing of the ingredients of the formulation is desired, the ingredients of the formulation are fed into the extrusion hardware through ports 18 with the apparatus fixed on a table top and with attachments to solid and liquid feeders. The screw(s) is/are steadily rotated by an electronically controlled gear box 22 that is connected to the screw shaft 23 and powered by the power pack 24 in the forwarding direction to allow the introduction of the ingredients into the mixing volume of the preshearing apparatus that is operated in the batch configuration. Inlet ports 18 are closed manually or automatically upon completion of the feeding of the ingredients. The screw(s) is/are continued to be rotated by the gear box 22 for mixing and conveying of the cement. The exit gate 19 is initially sealed by closing the plug 21 so that the resulting circulation and back-mixing of the ingredients in the mixer gives rise to a well-mixed bone cement paste.
After the completion of the interspersing of the ingredients (mixing) additional rotation and oscillation of the screw(s) is/are carried out. This stage is defined as the “preshearing prior to delivery/injection” stage. During preshearing the shear rate and the shear strain history are tailored to affect the setting kinetics and the time-dependent development of the viscosity and the elasticity of the ceramic paste. For example, the increase of the rotational speed of the screw increases the shear rates that the cement paste is exposed to. Thus, the duration of the preshearing and the shear rate and strain history that are applied during preshearing become parameters that the operating room personnel can adjust according to the requirements of the specific surgery. This can conveniently be done by manually entering the parameters to the electrically controlled gear box through a control unit shown in FIG. 4B. The apparatus can be powered by a single button 28 on the control unit. The modes and parameters of operation are read on the digital screen 29, and can be controlled by using a set of adjustment buttons 30. The control unit also contains two indicators 31 and 32 that assist the operator by visually indicating when the setting is effectively delayed 6, and when preshearing can safely be ended 7 which are determined according to the algorithm explained in FIG. 2.
Applied shear rate, time and strain history would alter the setting time and shear viscosity and the elasticity (i.e., the injectability, workability and the setting time) of the ceramic paste. Therefore, with a single formulation a wide range of setting times, shear viscosity and elasticity behavior (i.e., a range of injectabilities and working times) become possible. Another novelty of the apparatus is its capability to generate sequential cyclic oscillatory shearing during which the frequency of the deformation can be altered from one value to another in a cyclic manner. As shown in the examples, the most effective mode of preshearing involves the application of multiple frequencies during oscillatory shearing.
In the final step, the gate 19 of the preshearing device that connects the barrel 17 and the die 21 acting as the nozzle is opened. Manual timing is the default in clinical practice due to the manual nature of the operations and automatic timing is the default for robotic deposition. In the embodiment shown in FIG. 4A, the apparatus is equipped with a hypodermic needle 22 to perform direct injection of the presheared cement. In this case, the unit is detached from its feeder connections that are not shown in the figure, with sealing of the inlet ports and used as a pumping device. Cementitious suspension of appropriate viscosity and elasticity is discharged during the final step via the steady rotation of the screw(s) acting as a pressurizing pump at a predetermined rate that is suitable delivery rate. Discharged bone cement may also be transferred to a syringe 26 (see FIG. 4B) using different adapters as the die 27.
In another embodiment of the present invention, the preshearing apparatus is a semi-batch mixing and extruder device consisting of two or more successive screws that are driven by concentric shafts with independent rotation capability. As schematically described in FIGS. 5A and 5B, the concentric shafts 33 and 34, and the screws 35 and 36 are powered by different gears 37 and 38 that are controlled by separate electronic controller units that are not shown schematically. The screws are in contact with each other to enable synchronous operation at the same mode (oscillation or steady rotation). The utility of the present embodiment is due to partitioning of the pressurization and preshearing functions into successive screws so that cement mixing, conveying, preshearing and delivery can be done at various positions along the length of the barrel of the apparatus, at various time intervals. For example the screw 35 positioned closest to the inlet ports 18 at the proximal end, majorly functions to mix and convey the cement suspension by steady torsion while the successive screw(s) 36 have the additional functions of preshearing by oscillatory torsion and delivery by steady torsion. This partitioning enables the proximal screw(s) 35 also store the cement suspension as a reservoir when such idle waiting is beneficial for the development of cement viscosity according to the requirements of the task at hand.
In another embodiment that is schematically represented in FIG. 6A, the preshearing apparatus is equipped with a thermal jacket 41 that encloses the screw(s) 16 and the barrel(s) 17 made of a metal of high thermal conductivity, in order to control the temperature of the cement paste during processing. I he thermal jacket consists of helically wound metal tubes 42 of high thermal conductivity (i.e., aluminum or copper) around the barrel and facilitates circulation of a fluid of high heat capacity (i.e., water) by the help of a pump in a fluid bath that is not shown schematically. The temperature of the fluid bath can be conventionally controlled by electrical heating or cooling elements. Circulating fluid enters the jacket through the inlet tube 43, leaves the jacket through the outlet tube 44. As schematically described from the back view of this embodiment in FIG. 6B, the gear connected to the screw shaft 23, and the electronically controlled gear box, the power pack, the gate, the valve, the adapter dies that are not shown in the figure, have no contact points with the thermal jacket so that heat transfer is confined to the periphery of the preshearing barrel.
In another embodiment of the present invention, continuous preshearing and delivery of cementitious suspensions are enabled by coupling at least two batch preshearing apparati (FIG. 4A and 6A) or at least two semi-batch preshearing apparati (FIG. 5A). Thus prehearing at the screw closest to the die opening at the distal end in one apparatus, 36 in FIG. 5A, occurs at the same time period as the pressurization at the same screw in the apparatus operating in parallel. The alternating and reverse order of operation of the screws and also the inlet ports and outlet dies in parallel apparati can produce a continuous cement stream by quickly switching the apparati once the delivery from the active apparatus ends as the die opening closes. The timing for this coordinated operation mode is automatic since both preshearing and pressurization periods are fixed for a cement system according to the characterization of its setting kinetics and the kinetic requirements of delivery. The preshearing is applied through a period that is set according to the aforementioned procedure given in FIG. 2. The pressurization period is dependent on the shear rate of the steady torsion, which is determined according to the intended speed of delivery. The optimum value of shear rate in terms of setting kinetics may also be determined by comparison of several shear rates with the baseline reference during rheological characterization. In this continuous operation mode that is suitable for additive manufacturing by robotic control, the automatic control of the inlet ports and outlet die opening can be facilitated by making use of pressure transducers as indicators that normally measure minimum and maximum pressures when the screw is empty or full respectively. Pressure at the wall of the barrel at the end of the screws may be measured by electronic transducers, 39 in FIG. 5A. Alternatively the torque applied to the screws can be measured using pressure transducers in the electronically controlled gear box. The critical preshearing parameters that should be exceeded or avoided for the promotion, delaying or retardation of setting of various clinically relevant calcium phosphate cement formulations were determined as well as the setting times at various preshearing conditions and discussed in the following examples where a rheometer was utilized as both characterization and preshearing apparatus.

EXAMPLE 1

The formulations and the preshearing method have been tested using a small amplitude oscillatory rheometer, i.e., a parallel plate based shearing device with one plate (disk) stationary and the second either rotating in one direction continuously (CW or CCW) or oscillating between CW and CCW directions. The shearing device has the ability to impose a constant or cyclic shear rate and to measure the torque and the normal force as a function of time, temperature and rate of shear. The rheometer can thus characterize the elasticity and the shear viscosity of the cement paste as a function of the previous shearing history. The diameter of the two disks can be varied between 8 to 50 mm. The calcium phosphate cement formulations of this invention are placed in between the two plates at a typical gap of separation of 0.5 to 4 mm. In the steady torsional mode the shearing device can typically generate shear rates which are in the 0.01 to 50 1/s (the shear rate is defined as the linear velocity of the disk over the gap between the two discs). At the oscillating mode the typical sinusoidal shear strain and shear stress waves are obtained. The typical frequency range is 0.01 to 1000 rad/s (rps).
Brushite forming calcium phosphate cement formulation was mixed with powder/liquid ratio of 1.0, 1 wt % brushite seed and citric acid concentration of 0.5 M using a sonicator for 60 seconds according to the stoichiometry of the following setting reaction and transferred to the shearing device.
β−Ca₃(PO₄)₂+Ca(H₂PO₄)₂.H₂O+7H₂O→4CaHPO₄.2H₂O
The characteristic starting particle sizes were in the 2 μm range. The temperature of the sample holder chamber was set to 25° C. The preshearing device was operated at two modes: steady flow (steady torsional) and oscillatory flow (oscillatory torsional) to preshear the bone cement at different shearing modes and with different frequency and amplitude for oscillatory shearing and at different shear rates for steady torsional flow based shearing and for different durations. Furthermore, the dynamic moduli (storage modulus, G′, which represents the amount of energy stored as elastic energy during one cycle of deformation “the elasticity” and loss modulus, G″, which represents the energy dissipated as heat during one cycle of deformation, the magnitude of complex viscosity η* (the value of which approaches the shear viscosity as the shear rate and frequency approach zero), tangent(δ)=G″/G′ and normal stress were measured as functions of time by oscillatory torsional flow at various frequencies of 0.1 rad/s, 1 rad/s, 10 rad/s, 100 rad/s and at various strain amplitudes.
The development of the storage modulus, G′ and the magnitude of complex viscosity, η* of the bone cement sample that is kept under quiescent conditions with time are shown in FIG. 7. The elasticity (as represented by the storage modulus, G′ 46) and the viscosity (as represented by the magnitude of complex viscosity 47) increase in parallel as a function of time during which the sample is held under quiescent conditions. Each point refers to a specific elapsed time under quiescent conditions (without shearing) following mixing and it belongs to the average of measurements from different cement samples. Starting each measurement with a freshly prepared cement sample is necessary to eliminate the strain history. The curve fits of the data points represent the setting curves 46 and 47. The cement is typically injectable until the initial setting time (injection period 48) when the storage modulus (setting rate) starts to abruptly increase. It is measured as the time when the second derivative of storage modulus reached maximum value and the final setting time is conveniently determined from the plateau region of the storage modulus vs time curve. Any manipulation of the cement needs to be avoided during this setting period 49 that spans the majority of the duration after mixing of the cement with water.
These data obtained for quiescent conditions indicate that this specific formulation would have relatively low elasticity and shear viscosity for about 600 s following mixing. The bone cement starts to harden at a relatively high rate after 600 s and becomes completely solid-like in 1200 s, at which time the bone cement would not flow at all. Thus, upon mixing this specific formulation needs to be delivered into the treatment site by the surgeon within 10-20 minutes, with the maximum time cut-off depending on what the surgeon needs in terms of viscosity and elasticity of the bone cement. It should be noted that this is a water based formulation and would require some thickening prior to injection otherwise there will be significant demixing (segregation) effects during injection through a needle (see references on segregation of suspensions with relatively low viscosity binders [Yaras et al., “Flow Instabilities in Capillary Flow of Concentrated Suspensions,” Rheologica Acta, 33, 48-59 (1994); Yilmazer et al., “Mat Formation and Unstable Flows of Highly Filled Suspensions in Capillaries and Continuous Processors,” Polymer Composites, 10 (4), 242-248 (1989)]
The schematics of how the preshearing effect is documented are shown in FIG. 8. In one set of experiments the bone cement was held under quiescent conditions (no preshearing) for various durations of time and then its dynamic properties, i.e., storage modulus, G′, which represents “the elasticity” and loss modulus, G″, which represents the energy dissipated as heat during one cycle of deformation, and the magnitude of complex viscosity η* were characterized as a function of time at a constant frequency of 1 rps and at a constant strain amplitude of 0.04. In a second series of experiments the bone cement was presheared at various strain amplitudes, γ° and frequencies, ω for various durations and then subjected to the same oscillatory shear parameters of frequency of 1 rps and strain amplitude of 0.04 and time dependent development of the elasticity and viscosity were documented. Thus, following preshearing or quiescent conditions for the same durations of time, the time dependent elasticity and viscosity behavior of the bone cement was documented by collecting the dynamic properties as a function of time at 1 rps and strain amplitude of 0.04.
The comparisons of the time dependencies of the elasticity, i.e., the storage modulus, and viscosity, i.e., the magnitude of complex viscosity, of brushite cement that were not presheared or were presheared at various frequencies at the same strain amplitude of 0.04 are presented in FIGS. 9A and 9B. It is worthwhile to emphasize again that these results are free from strain history and the associated shear thinning effects due to partitioning of the preshearing and measurement steps. Delaying of the setting reaction 50 is observed for a low frequency of 0.1 rps. At this low strain amplitude the strains applied by oscillatory torsion are thought to affect the viscous liquid phase rather than the solid phase, slightly disrupting the crystal microstructure that develops with time. On the other hand, the setting rate of brushite cement increased gradually with increasing oscillation frequency at the same strain amplitude. Finally, at the frequency of 100 rps 51, the setting time decreased considerably in comparison to quiescent conditions 52. The increase in the setting rate with higher frequencies of oscillation is thought to result from enhanced dissolution and the consequent rise in the supersaturation of the crystallizing brushite phase by the agitation of the viscous liquid phase. This mechanism seems to dominate over the slight disruption of crystallization, reaching its maximum effect at the maximum frequency of 100 rps 51. At this preshearing condition the final setting time is shifted to about 800 seconds, indicating a decrease in setting time of about 400 seconds compared to cement sample that is kept under quiescent conditions 52. Therefore, the kinetics of the setting reaction can be altered in both directions via the adjustment of the frequency at the low strain amplitude of 0.04.
Thus, the typical results provided in FIGS. 9A and 9B indicate that cement formulations would be sensitive to preshearing at critical frequencies at a constant strain amplitude below the linear viscoelastic strain limit. Above a critical frequency the setting time would be shortened and below a critical frequency the setting time would be increased. The critical values of the frequencies can be determined a priori by carrying out such experiments in a rheometer device functioning simultaneously as a preshearing device and as an elasticity and viscosity testing apparatus. Thus, assuming that similar drag shear oscillation frequencies and strain amplitudes are generated in a preshearing device in a surgery setting the surgeon would have the options of increasing or decreasing the setting rate of the cement paste by selecting the appropriate preshearing conditions.
The effects of changes in the strain amplitude (this is representative of the angular displacement of the disk of the rheometer over the gap) used during preshearing on the time-dependent development of the elasticity and viscosity of the paste are shown in FIGS. 10A and 10B (at a constant frequency of 1 rps). In comparison to lower strain amplitudes a strain amplitude of 1, 53 caused significant delay in the setting of the cement paste. The initial and final setting times more than doubled with a slight increase in strain amplitude at a relatively low frequency of 1 rps. It is clear from these results that the parameters of oscillatory preshearing, i.e., frequency and strain amplitude both have significant roles in the time dependent development of elasticity and viscosity of cement pastes. The latter has a particularly strong effect on the development of elasticity as represented by the storage modulus above the linear viscoelastic strain limit for a specific oscillation frequency. The delaying of cement setting induced at this condition has a wide range of extents from a few seconds to hours depending on the frequency of the applied disruptive force on the intergrowing crystal structure that is driven by the dissolution and crystallization thermodynamics.
The comparisons of the time-dependent changes in the viscosity of the cement paste following preshearing that is based on oscillatory shear versus steady torsional flow are shown in FIGS. 11A and 11B. Both the viscosity and the elasticity (not shown) grow significantly slower with steady torsional flow 54, 55 in comparison to oscillatory shear. In oscillatory shear the strain is cyclic (sinusoidal) whereas in steady torsional flow the strain that is imposed is equal to the strain rate multiplied with the time. The application of steady torsional flow even at relatively low shear rates may eventually disrupt the network structure of the paste and increase the setting time significantly, giving rise to a significant increase in the Injectabillity time window of the cement paste. Mild strain rates 55 typically retard cement setting such that the average yield strength of the cement structure is exceeded relatively earlier and multiple times. This retardation is caused by the continuous disruption of cement network structure the growth rate of which competes with the shear strain ratè. Depending on the magnitude of steady shear strain rate, the disrupted cement microstructure may recover completely as the growing crystals are able to accommodate the applied strains or go through a series of recovery-disruption cycles that lead to a weakly set cement.

EXAMPLE 2

The cement precursors in Example #1 without seed particles were mixed at a powder/liquid ratio of 1.0 and citric acid concentration of 0.1 M. Here the effects of preshearing are followed via systematic changes in the frequency as well as the strain amplitude. FIGS. 12A and 12B show that as the frequency of the preshearing is increased at a constant strain magnitude of 1, the setting of the cement paste is delayed significantly with increasing frequency, consistent with the findings associated with Example #1. Despite the fact that applied strain amplitude of 1 was greater the linear viscoelastic strain limit for this cement formulation, preshearing at a frequency of 0.1 rps and strain amplitude of 1, 56 resulted in setting kinetics similar to the quiescently set cement (not shown). Further increasing the frequency at this relatively high strain amplitude to 1 rps, 57 and 10 rps, 58 slowed down the setting proportionally.
FIG. 13 shows the effect of the strain amplitude on the storage modulus, G′, of the same cement formulation at the same oscillation frequency of 1 rps. The strain amplitude of 0.1 is lower than the linear viscolelastic strain limit for this formulation and results in similar setting kinetics 59 with the quiescently set cement (not shown). On the other hand a high strain amplitude of 100 applied at the same frequency 60 significantly delayed the development of the crystal network. Clearly the strain amplitude provides another controlling variable with increasing strain amplitude delaying further the setting reaction and increasing the injectability/extrudability of the cement paste.

EXAMPLE 3

The application of interrupted cyclic deformation of the ceramic paste (cyclic increase and then decrease of the frequency) versus continuously oscillated (at constant frequency and strain amplitude) was tested using the cement precursors of Example #1 with a powder/liquid ratio of 0.8 and citric acid concentration of 0.4. The effects of stepwise cyclic change in applied frequency on bone cement dynamic rheological properties were investigated by varying the frequency from 0.1 rps to 1 rps, from 0.1 rps to 10 rps, or from 0.1 to 100 rps as well as increasing the number of times the frequency is altered during the course of total deformation period. The frequency was altered between 4 and 15 number of times. The schemes for cyclic changes in frequency between 0.1 rps to 100 rps in conjunction with three different number of cycles over the total deformation period are shown in FIG. 14A, FIG. 14B, and FIG. 14C. The application times for each step change 61, 62, 63 varied from 250 to 170 to 100 seconds respectively with a concomitant increase in the number of cycles applied for a given total duration.
FIG. 15 shows the storage modulus, G′, (indicative of the elasticity of the paste) upon the application of five different preshearing modes. In two of the runs the frequency has been kept constant at 0.1 rps, 64 and 100 rps, 65 during preshearing. On the other hand, the frequency has been cyclically shifted between 0.1 and 100 rps with different periods over the course of the total duration of the experiment under the other three preshearing conditions. When the period of the application is rather long, for example, when each frequency is applied for a duration of 250 s (this is indicated as 4 shearing cycles) the cyclic alteration of the frequency 66 does not lead to any retardation in the setting time over the application of a constant frequency during the deformation. On the contrary, the setting kinetics was promoted (decrease of the setting time) at a small number of 4 cycles possibly due to agitation of the liquid phase during frequency shifts. However, when the period of the application of each frequency is reduced to 100 s (this is indicated as 14 shearing cycles), i.e., the frequency is shifted from 0.1 to 100 rps repetitively for 100 s durations at each frequency, the dynamic properties of the paste 67 decrease considerably in comparison to samples subjected to oscillatory shear at constant frequencies of 0.1 and 100 rps. Thus, these results suggest that various other cyclic modes than continuous shearing or continuous oscillation can even be more effective in prolonging setting time and increasing the injectable time window of the cementitious ceramic pastes.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. Certain features that are described in this specification In the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in combination in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. All such variations and modifications are intended to be included within the scope of the invention.

Claims

We claim:

1. A process for controlling the setting rate, deformation behavior and injectability/extrudability of cementitious ceramic suspensions consisting of a binder liquid phase and mixtures of ceramic particles, the process comprising the steps of

Characterization of the setting kinetics and deformation behavior (rheology) of a cementitious ceramic suspension under quiescent and different shearing conditions “preshearing” using a rotational rheometer which is able to generate time dependent shear viscosity at various shear stresses and linear viscoelastic material functions (dynamic properties) as a function of frequency and strain amplitude;

Compiling the setting kinetics and deformation behavior during and after preshearing into a data base to provide guidance on the relationship between both the preshearing and pressurization conditions and the injectabillty/extrudability window of the suspension during the surgical or additive manufacturing procedure;

Definition by the surgeon or the operator what the injectability period, viscosity or the extrusion flow rate of the cementitious ceramic suspension should be during manual or robotic deposition

Preshearing and pressurizing the cementitious ceramic suspension in a preshearing apparatus using the guidelines provided by the preshearing database to control its setting kinetics, deformation behavior and injectability/extrudability during manual or robotic deposition

2. The process of claim 1, where the cementitious ceramic suspension is an inorganic bone cement used in bone repair and regeneration applications

3. The process of claim 1, where the surgical procedure includes spinal fusion, vertebroplasty, khyphoplasty, cranioplasty, periodontal and endodontal surgeries

4. The process of claim 1, where the binder liquid phase of the cementitious ceramic suspension is pure water or an aqueous solution of water and water soluble chemicals including citric acid, sodium chloride salt, trisodium citrate salt, sodium hydrogen phosphate, orthophosphoric acid

5. The process of claim 1, where the particles of the cementitious suspension are inorganic ceramics including phases of calcium aluminate, calcium sulphate, calcium silicate, calcium aluminum silicate, zinc phosphate, magnesium phosphate, magnesium oxychloride, magnesium oxysulphate or calcium carbonate ceramics

6. The process of claim 1 where the rheological data in the preshearing database is compiled by monitoring the variations in the viscosity and elasticity with time, temperature, and preshearing history, thus pinpointing the characteristic data of the linear viscoelastic strain limit, cement dough time, cement initial setting time, cement final setting time and the setting promoting frequency limit under quiescent and preshearing conditions using a rotational rheometer with the ability to generate steady shearing at various shear rates or oscillatory shearing at various frequencies and strain amplitudes.

7. The process of claim 2, where the particles of the inorganic bone cement are calcium phosphate ceramics

8. The process of claim 7, where the particles of the calcium phosphate ceramics are brushite forming cement precursors

9. The process of claim 7, where the particles of the calcium phosphate ceramics are hydroxyapatite forming cement precursors

10. The process of claim 7, where the calcium phosphate cements have starting calcium phosphate particles with mean sizes that are generally less than 10 micrometers.

11. The process of claim 7, where the calcium phosphate cements have brushite or hydroxyapatite seed particles that are less than 5% in concentration.

12. The process of claim 6, where the viscosity is described by shear viscosity measured as a function of time, temperature and shear rate or the magnitude of complex viscosity measured as a function of time, temperature, strain amplitude and frequency

13. The process of claim 6, where the elasticity is described by the storage modulus measured as a function of time, temperature, strain amplitude and frequency.

14. An apparatus to carry out preshearing and pressurization as a compounding extruder characterized in that the suspension can be given the targeted preshearing history and delivered by it under manual or robotic control, and comprising a barrel containing inlet ports and outlet opening; at least one screw inside said barrel, a shaft connected to said screw, a gear driving said shaft and screw; an electronic controller and power pack moving said gear.

15. The apparatus of claim 14, where the geometry of the apparati and the operating conditions during preshearing and pressurization are determined either empirically or via mathematical modeling and simulation of the flow and heat transfer occurring in the batch or semi-batch apparatus as a function of time.

16. The apparatus of claim 14, where the extruder is a batch single screw extruder with a screw that is capable of rotating in both clockwise and counterclockwise direction and alternating between rotation in the clockwise and counterclockwise direction in a cyclic manner.

17. The apparatus of claim 16, where the extruder is a batch single screw extruder with a die, the opening of which can be controlled to provide either no flow out of the extruder or flow out of the extruder at the desired flow rate.

18. The apparatus of claim 14, where the extruder is a batch twin screw extruder with two screws that are rotating in either clockwise of counterclockwise direction or oscillating screws which alternate between rotations in the clockwise and counterclockwise direction in a cyclic manner.

19. The apparatus of claim 18, where the extruder is a batch twin screw extruder with a die, the opening of which can be controlled to provide either no flow out of the extruder or flow out of the extruder at the desired flow rate.

20. The apparatus of claim 14, where the extruder is a modified semi-batch single screw extruder with at least two successive, independent screws that are driven by concentric hollow shafts that are powered by different electronically controlled gears.

21. The apparatus of claim 20, where the extruder is a modified semi-batch single screw extruder with a die, the opening of which can be controlled to provide either no flow out of the extruder or flow out of the extruder at the desired flow rate.

22. The apparatus of claim 14, where continuous preshearing and pressurization is realized by coupling at least two batch or two semi-batch mixers in parallel so that the inlet ports and the corresponding screws of each apparatus operate in reverse and alternating order such that the coupled apparati receive, mix, convey and deliver the cement suspension at different periods that are synchronized according to the duration of the preshearing determined according to the guidelines of the database and the requirements of the particular task on hand.

23. The apparatus of claim 17, where the batch single screw extruder can provide a time dependent preshearing history to the cement suspension by oscillatory or steady torsion following the guidelines of the data base and the requirements of the particular task on hand.

24. The apparatus of claim 21, where each independent screw is capable of rotating in both clockwise and counterclockwise direction and alternating between rotation in the clockwise and counterclockwise direction in a cyclic manner.

25. The apparatus of claim 21, where each independent screw can independently provide a time dependent preshearing history to the cement suspension by oscillatory or steady torsion following the guidelines of the database and the requirements of the particular task on hand