GB2470034A

GB2470034A - Reinforced polymer composite

Info

Publication number: GB2470034A
Application number: GB0907766A
Authority: GB
Inventors: Jaya Luxshmi Nemchand; Anthony Walter Anson
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-05-05
Filing date: 2009-05-05
Publication date: 2010-11-10
Also published as: GB0907766D0

Abstract

A process of forming a reinforced polymer composite comprises providing a polymer in a liquid state, adding a reinforcing material to the liquid polymer, and allowing the polymer to solidify in order to form a solid structure. The reinforcing material may be randomly distributed and orientated throughout the polymer. The polymer is preferably selected from epoxy resins, polyurethanes, or silicone elastomers. The reinforcing material may be in the form of particulates or fibres and may be a polymeric, ceramic, or glassy material. Alternatively, metal wires or particles of hydroxyl calcium phosphate may be used as the reinforcing material. The composite is used to form an artificial bone structure which is intended for use in testing by mimicking the mechanical properties of real bone.

Description

A Biological Mimic Test Composite

Background

Bone fracture is common in human beings and other vertebrates. There are four main causes of fractures: fracture caused by injury, by fragility of the bone, fatigue or stress fractures, due to repetitive cyclic loading and pathological fractures such as bone weakening by disease such as osteoporosis Fracture repair is the process of rejoining and realigning broken bone fragments. The essential requirement to effect successful repair of a fracture is reduction and stabilisation. These requirements are met by ensuring good contact between the fractured parts and by maintaining the spatial relationship between broken for stabilisation.

Different methods are employed to ensure effective reduction and stabilisation; this will depend upon the nature of the fracture. The simplest form is by splinting or setting a cast to affect immobilisation of bones. Surgical intervention may be required in certain fracture types necessitating the use of instrumentation within the bone or fixed externally on the bone surface. Each treatment method should lead to a correctly aligned and stabilised bone that will then permit healing processes to begin.

Treatment method depends on the nature and position of the fracture, the gravity of the injury and the condition of the patient.

Fracture healing occurs in two way; primary healing or secondary healing. Primary healing involves direct attempt by the bone on one side of the cortex (outer surfaces of the bone) to fuse with the bone on the other side of the cortex to restore mechanical stability.

Secondary healing involves five stages of healing; formation of haematoma, inflammation, soft callus formation, callus mineralization and callus remodelling.

Haematoma formation occurs at the fracture site soon after the fracture has occurred, this is followed by inflammation. Fibro vascular tissue then replaces the haematoma helping to stabilise the fracture area. Cell proliferation is seen within the periosteum leading to soft callus formation, where dead bone cells are resorbed and immature woven bone is laid down. During callus mineralization, the woven bone is replaced by S. " lamellar-type bone and the fracture is united. Further callus remodelling occurs, restoring the medullary cavity and the bone returns to a more normal shape and function. * *.

: There is a wide variety of established orthopaedic implants in use for fracture fixation.

*:. Commercially available devices have to be extensively tested before they are made available for use, as part of the device development process to ensure that repairs are : *** effective. Commercial organisations manufacturing these devices will be required to conduct many types of test to ensure the devices are fit for purpose and to satisfy regulatory authority's requirements for safety and efficacy. In regard to mechanical properties of surgically implanted devices, an important feature will be testing the implants in-vitro, to define the structural support relationship between implant and biological tissue.

To defme with accuracy, load-sharing properties of bone and implant, a materials that can be configured and applied at a fracture site, approximating the mechanical properties of biological bone callous material, from early formation phase to near-bone-like, can improve the understanding of bone healing processes for the benefit of patients and of health-care authorities. **.* * e ***S

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Description

The object of this present disclosure reveals a synthetic material, in the form of a composite that can be applied to simulate bone healing processes. Early bone-like material, referred to as callus, can be simulated to mimic early, pre-osteogenic bone, osteogenic bone and mature bone if the mechanical properties of the applied material can be adjusted.

Settable polymers are infused with fibres or particulates in various ratios and geometrical arrangements to facilitate a range of desirable mechanical properties, whose reaction to an applied load will yield a resultant reaction, appropriate to the biological status of a fractured bone, during healing processes.

Settable polymers, preferably epoxy resins, polyurethanes and silicone elastomers can be variously configured from having relatively stiff, to flexible or elastic properties Examples of this are; silicone or polyurethane elastomers of Shore hardness 3 0-90, (A) Other polyurethane compounds and structural epoxy resins have a Shore hardness between 60-120 (D) and tensile strength 20-80 Mpa.depending upon essential formulation, cross-linking arrangement and polymerisation regime. However, adding other materials in the form of particulates or fibres to a settable resin will facilitate desirable variations in compressive, tensile and shear moduli that are able to approximate the mechanical properties(') of developing bone callus material.

A composite is manufactured by adding to a settable resin, one or more polymer types, ceramics or glassy material in the form of fibres or particulates. These additions to the resin are preferably, but not limited to, compliant polymers including polyethylene, polyurethane, acronitrile, nitrile, latex, polyester, nylon. The polymers materials are advantageous to produce composites that have a stress-strain relationship or Young's modulus of 0.1 Gpa to 3OGpa. Other materials added to the settable resins such as glass-fibres, or ceramic fibres and ceramic particulates, can impart Young's modulus values 10-100 Gpa. It will be appreciated that the quantity of a material added to a settable resin, the shape of the added material and its orientation, isotropic or anisotropic, will strongly influence the mechanical properties of the disclosed composite material.

Alternatively, metal wires of a uniform or random length that have appropriate elastic properties, for example nickel-titanium, super-elastic shape memory alloy, austenitic * : stainless steel and medium-high carbon steel, heat-treated and tempered to a spring-like condition, are suitable materials to be encapsulated in a settable resins as a means : ** to alter mechanical properties. * S..

Particulate materials of a size range lO-lOOmicrons, including hydroxyl calcium : *.** phosphate, and said compliant polymers, are the preferred, particulate filler materials.

* es The ratio by volume, of particulate filler-to-resin is preferably 2%-40%. *..**

* A combination of resins type, filler type, volume density and geometrical arrangement of resin-to-filler, will facilitate a wide range of mechanical properties that are able to approximate early to mature bone callus material. The Young's modulus of callus formations, according to the literature('2) is 0.05 Cpa to 80 Cpa. The stiffer value given here is when callus has become essentially like the surrounding bone material and is capable of normal skeletal loading.

An example of an arrangement of filler-to-resin is described; this example may be favourably compared to a mature callus, formed around the site of a healing bone fracture: A commercially sourced epoxy resin is prepared by mixing epichiorohydrin (epoxy polymer) and bisphenol-A (catalyzing agent) of approximately 50% volume, per agent mix. The total volume is 20 millilitres. Added to this, hydroxyapatite in a powder form, the average size of the powder particulate being approximately 20- 25microns. The total volume of hydroxyapatite added to the epoxy/catalyst is 5 millilitres. The powder is mixed in using a small stainless steel spatula; it is advisable not to stir the mixture too vigorously to prevent entrapment of air pockets. When mixed, the mixture is allowed to stand for 30 minutes.

A model bone, of a suitable synthetic material, is arranged with a fracture, by sawing or cutting; for example a tibia with a mid-shaft fracture showing 3mm of separation.

An inter medullary nail is positioned in the medullary canal of the fractured bone to simulate the type of therapy that might be applied to a fracture in a long-bone.

Adhesive tape, or a formable, putty-like material such as "BlueTackTM" or other mastic, semi adherent agent is used to form a small enclosed trough or canal that extends incompletely around the outer surface of the bone, in close proximity to the edges induced fracture. The surface of the inserted inter medullary nail, exposed at the bone fracture, furnished with a protection to prevent callus mimic material form adhering.

A small quantity of the viscous epoxy mix is then transferred by a spatula to the prepared fracture, filling a prepared trough placed at an appropriate site around a fracture.

Subsequent to a bone fracture, callus formation is not uniformly distributed around the fracture; it forms at a foci and progresses around the bone, to then form a continuous, annulus-like formation. The callus formation at this stage does not have sufficient resilience to allow normal physiological loads to be borne. The artificial callus made from, in this example, epoxy-hydroxyapatite mixture, is configured to reflect the geometrical arrangement of the early callus at this early stage. To further mimic bone fracture healing processes, other settable resins and filling agents are * employed in several stages; each stage is able to address the changing mechanical properties of biological callous material.

The application of differing callus materials over a period of, up-to six weeks is *. conducted by preferably removing the previous artificial callus and replacing it with a new mixture. During this process, the artificial bone is retained by clamps or other : ** suitable means, ensuring that no movement occurs between bone-parts, while removing callus mimic material. Alternatively, first callus material can be left in-situ * : * * and new material placed on top, in a laminate fashion. A further application of increasingly stiff callus is also, preferably by the removal of the last material, and replacement by new. This preferred technique is advised as in the biological situation, the first callus, of insufficient load bearing properties, is reconfigured by cell and protein activity into a stiffer, load bearing material: this is an analogue process, not incremental.

The example given of one form of settable resin mixed with a particulate in a specific ratio was demonstrated by experiment; the mixture was tested and found to approximate callus material; this information was found in the scientific literature giving mechanical properties of callus(1-3) The fibres or particulate materials proposed can be sourced from any suitable polymer, ceramic or metal type that will enable a wide variation or mechanically reactive forces to be available from said composite structure.

Fibres and particulates will be arranged in different orientations in order to achieve a range of structural properties, which will withstand the relevant physiological loading.

The elastic modulus of cancellous bone ranges from an approximate 0.05 GPa to 5 GPa. 0.05 GPa represents the very early bone tissue which is very spongy. These soft tissues gradually mineralise to hard bony tissue of about 18.2 GPa.

The use of synthetic materials, formed as a composite to facilitate a close approximation of mature or early development of hard tissue, as found in bones in the mammalian species, facilitates a desirable range of mechanical properties. It would be realised that this also has application in the production of complete artificial bones to enable accurate studies to be conducted on the strength of the skeletal structure or on single elements of the skeleton. Furthermore, the reaction of a modulus-matched artificial bone, when fitted with instrumentation such as trauma devices or therapeutic artefacts, such as articular joints, can also be more accurately defmed with a bone that closely resembles natural bone.

The resemblance to natural bone mechanical properties can be approximated in all of the phases of biological bone development and conditions of disease. For example, children's bones, well known to the clinician, are more flexible, compared to an adult.

Osteoporosis, a common problem, associated with the older age group, post menopausal women and associated with certain other diseases Variations in the mechanical properties of a cast bone can be realised by defining the required properties and making a suitable mixture of a settable resin and adding fibres or particulates in a random or arranged geometry. Equally, and with particular reference * to fibrous or wire-like materials, the form of each fibrous strand can be straight or * . . with a radius or combinations of both shapes. These aforementioned adjustments in a composite casting, gives further potential to vary mechanical properties. To construct : * an artificial bone, a mould is made, using a pattern from commercially available artificial or cadaveric bone. Silicone elastomers can be used to make this mould. The :. pattern bone is carefully removed from the mould shell, leaving a cavity. This cavity is then filled with one or more of the above mentioned composite arrangements. A : s femoral bone, for example is constructed of several types of bone material, including spongy bone, solid bone and forms of collagen. These have different mechanical * ** properties that can be invested in an applied composite. In the case of a mould for a femoral bone, the trochantric head is generated by applying two composites in separate layers; stiff composite layer, followed by a less stiff layer. This mimics. the hard bone outer shell and the partially spongy substrate. The medullary canal can be generated by inserting a sacrificial core; for example a hard wax, previously cast. The remaining parts of the bone are then cast by applying further composite materials in the mould. The composites are applied in manner similar to painting a surface, maintaining the geometric arrangements (when necessary) of fibres or particulates, is facilitated by inserting them in the settable resin, after a film is attached to the mould surface. a... *a..

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Fig 1 Shows a composite casting as a solid cylinder with randomly orientated fibres, encapsulated in a settable resin I. Settable resin solid cylinder 2. Multiple fibres of random shape encapsulated randomly in a settable, cast resin solid cylinder Fig 2. Shows a composite casting as a solid cylinder with linearly orientated fibres being nominally straight, encapsulated in a settable resin l.A cast, solid cylinder of a settable resin 2. Multiple straight fibres, orientated in the axes of the cast cylinder.

Fig 3 Shows a composite casting as a solid cylinder with randomly placed, encapsulated particulates.

1. Settable resin solid cylinder 2. Multiple, randomly distributed particulates, encapsulated in a solid cylinder Fig 4 Shows a composite casting as a solid cylinder with linearly arranged particulates encapsulated in a settable resin 1. Settable resin solid cylinder 2. Multiple, linearly and aligned particulates, encapsulated in a solid cylinder Fig 5 Shows a section, through a fractured tibial bone with callus formation, in 3 development phases.

1. Medullary channel 2. Early callus formation impingining into the bone 3. Early callus formation, completely surrounding a tibial bone 4. Intermediate callus formation completely surrounding a tibial bone 5. Final callus formation, completely surround a tibia! bone * : . Fig 6 Shows a section through a fractured tibia showing the arrangement of an early callus formation, in 3 development phases : *** 1. Tibia! bone section at fracture site 2. Medullary channel * 3. First phase callus formation on exterior bone surfaces and impinging into bone.

4. Second phase callus formation : 5. Final, local stages of callus formation

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2 Medullary channel 3 First phase callus formation on exterior bone surfaces and impinging into bone.

4. Second phase callus formation Final phase, local callus formation.

Fig 8 Shows a a tibial bone side view and cross-section.The arrangement of a early stages of callus formation, bridging the fracture, is indicated.

1. Tibial bone part side view 2. Fracture site 3. Medullary channel 4. Early stage of callus formation Intermediate stage of callus formation 6 Final stage of local callus formation 7. Tibial bone part, separated by complete fracture 8. Shows columnar view of a tibial bone with a cross-section of formed callus at a fracture site. *S.. * * *S*. *

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Claims

Claims.1. A settable polymer in an initial liquid state has another reinforcing material added to the polymer to form a composite, the settable polymer then solidifies to form a solid structure.
2. As in claim 1), the volumes of a settable polymer, to the volume of a reinforcing material are adjustable to produce desirable properties, able to approximate the mechanical properties of compression, shear and/or torsion of proto-bone, immature or mature mammalian bone; in compression 0.1 Mega Pascals to 25 giga, Pascals, are the range of properties; in torsion, 0.2 megaPascals to 1.0 mega Pascal; shear 1.0 megaPascal to 4.0 gigaPascal
3. As in claim 1) and 2), A reinforcing material added to a settable polymer consists of fibrous or particulate or shavings or wire-forms of a suitable material to adjust the mechanical properties of a solid composite material.
4. As in claim 1) and 2) and 3), desirable filler materials are selected from metals, ceramics or polymers or biological materials or combinations thereof.
5. As claimed in claim 4), a filler is comprised of commercially available steel and its alloys, or titanium and its alloys or tin or chromium or aluminium or its alloys or magnesium or its alloys in a wire-form, cut or punched section or as powders.Particulate materials are comprised of calcium hydroxyl apatite, titanium, stainless steel aluminium oxide, silicon carbide, or zirconia. Biological materials consist of cellulosic or collagenous species such as plant fibres, hard-wood varieties or leather.
6. As claimed in all preceding claims, the ratio by volume of any filler material placed in a settable resin is another method whereby the mechanical properties of the solid composite, may be adjusted. The ratio can be adjusted within the range 100:1 to 1:1 filler-to-resin, by volume.
7. As claimed in all preceding claims, filler materials are randomly distributed and orientated in a settable resin.
8. As claimed in claims 1) through to claim 6) inclusively, wire-form and fibrous materials are orientated in a settable resin by forming progressive layers of the settable resin and manually or by machine, placing filler materials onto a setting resin. Further suitable layers are placed onto the first layer, the filler material volume and orientation being kept the same.
9. As claimed in all preceding claims, a settable resin is comprised of Epoxy resins or polyurethanes or silicon elastomers. Each resin is comprised of two components: a filler and a polymerizing or catalysing agent: in addition, viscosity adjusting agents such as an accelerating agent or an agent to retard polyimerisation can be added to enhance a composite manufacturing process
10. A composite material as claimed in all preceding claims is made from one settable resin type and combination of filler materials in a desirable orientation. The physical properties of a composite is able to approximate the physical properties of mammalian bone to facilitate compression, torsion or tension testing of healing bone fractures, in-vitro, mimicking the progressive development of natural bone material.
11. A composite material as claimed in claims 1 to 9, that is cast to form anatomically correct human bone, with properties that approximate compression, tension or torsion properties found in mammalian bone material.
12. As claimed in claim 11, a composite material used to cast or fabricate mammalian bone material in strata: each stratum being mechanically representative of cancellous, cortical, spongy and collagenous bone material.
13. As claimed in claim 12, an anatomically correct bone, made from layers and interpositions of composite material is used for in-vitro testing of mammalian bones