CN112281037A

CN112281037A - Degradable magnesium alloy femoral internal fixation screw and preparation method thereof

Info

Publication number: CN112281037A
Application number: CN202011181359.5A
Authority: CN
Inventors: 陈民芳; 王亮; 由臣
Original assignee: Tianjin University of Technology
Current assignee: Tianjin University of Technology
Priority date: 2020-10-29
Filing date: 2020-10-29
Publication date: 2021-01-29

Abstract

The invention provides a degradable magnesium alloy femoral internal fixation screw and a preparation method thereof, wherein a pure magnesium ingot, zinc particles, magnesium-calcium intermediate alloy and magnesium-manganese intermediate alloy are used as raw materials, Mg-Zn-Ca-Mn alloy with excellent mechanical property and lower degradation rate is obtained by smelting, solution treatment, hot extrusion and other preparation methods, and the magnesium alloy material is used for designing and processing the femoral internal fixation screw.

Description

Degradable magnesium alloy femoral internal fixation screw and preparation method thereof

Technical Field

The invention relates to an internal fixation device in orthopedics in a medical appliance, belongs to the field of auxiliary biomedical appliances, and particularly relates to a biodegradable internal fixation femoral implant appliance which is a bone screw prepared from a degradable magnesium alloy.

Background

The bone implant device is required to have excellent biocompatibility and stable biosafety, and commonly used bone implant materials include metals, high molecular polymers, artificial tissue engineering materials, ceramics and the like. Titanium alloy, stainless steel and the like are typical representatives of medical implant metal materials, and the materials have enough strength and inactive chemical properties and are used for tooth reshaping, bone injury repair and the like; the high molecular polymer is used for non-bearing part fracture, but the elastic modulus of the high molecular polymer is too large, a stress shielding effect can be generated, the high molecular polymer needs to be taken out by a secondary operation, the pain and the economic burden of a patient are increased, and the mechanical property of the high molecular polymer is insufficient, so that the use requirement of more clinical fracture cannot be met. The magnesium alloy is a representative of medical degradable metal materials due to excellent mechanical properties and physical properties closer to natural bones, has potential for being used as a bone implant material for bearing parts due to biocompatibility, osteoconductivity and complete degradability, and can promote the deposition of calcium, stimulate the callus generation at fracture ends and induce osteogenesis. However, the application of the magnesium alloy is limited by the defects of too fast corrosion rate and mechanical property of the magnesium alloy in vivo, and the bearing capacity of the implant material is improved and the corrosion rate is reduced by means of magnesium alloy component design, surface treatment, structural design of bone implant instruments and the like, so that the magnesium alloy better meets the requirements of clinical orthopedic treatment.

In the prior art, a chinese patent (CN106676357B a high-plasticity magnesium alloy and a preparation method thereof) discloses a high-plasticity magnesium alloy and a preparation method thereof, which comprises the following components: zn 3-4%, Ca0.1-0.6%, Mn0.05-0.8%, and the balance Mg. The elongation percentage of the alloy with optimized components can reach 25-29.8%, but the yield strength is only 160-205 MPa, the tensile strength is 270-289 MPa, the clinical use requirement that the yield strength of the internal fixation material, namely bone, is more than 200MPa cannot be met, the corrosion resistance is unknown, and the application is unknown.

In the prior art, chinese patent (CN103845110B a multifunctional bone screw for poor bone conditions) discloses the following technical solutions: the self-locking type bone screw is composed of an outer screw and a self-locking type opening rod, wherein the outer screw is formed into a whole by a screw body and a base, a cylindrical hollow part is arranged in the screw body, a slit which is cut along the longitudinal axis of the screw body is arranged at the tip part of the screw body, round gradually-changed interface strengthening holes are symmetrically arranged on the slit along the pitch interval of an external thread, the interface strengthening holes are communicated with the cylindrical hollow part, the aperture of the gradually-changed interface strengthening holes is sequentially reduced backwards from the tip part of the screw body, however, the bottom end of the bone screw is opened, secondary damage to bones is easily caused, in addition, the situation that the secondary damage to the damaged bones is very difficult is avoided, and the.

In the prior art, a chinese patent (CN111616788A a bone screw with high anti-pulling capability) includes a nut head, a screw rod, the screw rod is a cylindrical hollow structure, a bottom opening is provided with a groove on the nut head, a bottom surface of the groove is provided with a threaded through hole communicated with an inner cavity of the screw rod; the screw is inserted into the screw through hole, the top of the screw is in threaded fit with the screw through hole, and the screw is of a hollow structure; the screw rod structure also comprises two inclined inner screws which are symmetrical along the axis of the screw rod. Although the bone screw has higher anti-pulling capacity, the structure is complex, the production and the processing are difficult, the hollow structure can not be used as the screw of the main bearing part, and the operation difficulty is higher. In many prior art, the structure of the screw is very complicated, and proper biological materials are not selected, so that the practicability is lacked.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and designs a degradable magnesium alloy femoral internal fixation screw, wherein a degradable magnesium alloy material with excellent mechanical property and low degradation rate is adopted. Meanwhile, considering that the strength of the magnesium alloy is far lower than that of the titanium alloy or stainless steel material, the finite element analysis method is adopted, and the structure and the size parameters of the internal fixation screw are redesigned and optimized so as to meet the requirements of bone fixation and low degradation rate.

The technical scheme of the invention is as follows:

a degradable magnesium alloy femoral internal fixation screw is made of a degradable Mg-Zn-Ca-Mn alloy material with excellent mechanical property and low degradation rate; the nail head is spherical, a plum blossom groove structure for screwing is arranged at the upper end of the spherical nail head, and the stress is more uniform and is not easy to damage relative to a straight groove, a cross groove and a hexagonal groove during screwing; the screw body is of a full-thread structure, so that screws with different lengths can be conveniently screwed according to the condition of a patient; the pin head and the pin body are provided with excessive round angles, the purpose is to avoid the connection position from being broken in the screw screwing-in process under concentrated stress, and the structure with the sharp round angles has better corrosion resistance.

The invention relates to a research method for simulating screw mechanical behavior and optimizing screw structure and size by means of a finite element tool, establishes all three-dimensional models required by simulation analysis by means of a three-dimensional modeling tool UG, and simulates possible stress states of a femoral internal fixation screw implanted into a human body by means of a simulation analysis tool ANSYS, wherein simulation experiments comprise three-point bending, twisting and pulling.

On the basis of simulating the stress distribution of the screw, the invention optimizes the parameters of the screw and designs the structure of the screw, wherein the parameters comprise the inner diameter of the screw, the thread pitch and the thread profile angle.

In the invention, the overall structure of the internal fixing screw comprises three parts, namely a nail head, a threaded nail body and an excessive fillet connecting the nail head and the nail body.

In the invention, the structural size of the screw is as follows: the diameter of the head of the nail head is 5.8-6.0 mm, the width of the plum blossom groove is 3.0-3.35 mm, the outer diameter of the screw thread part of the screw nail body is 3.5-4.0 mm, the inner diameter is 2.3-2.5 mm, the screw pitch is 1.10-1.25 mm, the width of the thread crest is 0.05-0.15 mm, the total length of the screw is 28-32 mm, the small profile angle is 3-8 degrees, and the large profile angle is 35-42 degrees.

The magnesium alloy material used in the invention adopts pure magnesium cast ingot with the purity of more than 99.99 percent as a magnesium raw material, zinc particles with the purity of more than 99.9 percent as a zinc raw material, magnesium-calcium intermediate alloy with the mass fraction of 25 percent as a calcium raw material, and magnesium-manganese intermediate alloy with the mass fraction of 10 percent as a manganese raw material, and the magnesium alloy containing 0.19 to 0.21 percent of calcium, 2.0 to 2.9 percent of zinc, 0.3 to 0.9 percent of manganese and the balance of magnesium and inevitable impurities is prepared.

The preparation method of the magnesium alloy material comprises the following steps:

and smelting, namely pickling and drying the raw materials used by the magnesium alloy, putting the magnesium alloy into a graphite crucible, introducing protective gas (SF 6: N2: 0.4%: 99.6%), and keeping the smelting process under a protective atmosphere environment. And after the magnesium ingot and the intermediate alloy are melted, stirring the molten metal by using a handheld electric drill, adding the preheated zinc particles into the molten metal, and continuing stirring for 5-10 min by using the handheld electric drill. And after the temperature of the smelting furnace reaches 720-730 ℃, starting high-shear stirring, then carrying out ultrasonic dispersion, standing for 5-10 min, casting into a preheated mold after the temperature is reduced to 690 ℃, and cooling to obtain a cast ingot with the diameter of 60 mm.

And (3) solid solution treatment, namely placing the cast alloy sample in a box furnace, setting the temperature rise time of the box furnace to be 10 ℃/min, heating to 420 ℃, then preserving heat for 24 hours, and quenching with hot water at 65 ℃ to obtain a supersaturated solid solution.

And (3) hot extrusion, namely heating the extruder to 315 ℃, preheating the scalped alloy cast ingot half an hour before extrusion, and then hot extruding the alloy cast ingot at 315 ℃ into a bar with the diameter of 8mm, wherein the extrusion speed is 2-3 mm/s.

And (4) processing the screw, namely processing the extruded bar into the magnesium alloy screw with optimized design through a lathe.

The invention has the advantages and beneficial effects that:

the invention optimizes the components of Mg-Zn-Ca-Mn alloy, comprehensively considers the improvement of the mechanical property and the corrosion resistance of the alloy, and controls the content of alloy element Zn to be 2.0-2.9 wt% to obtain proper amount of second phase particles; and the alloy elements Ca and Mn refine alloy grains and reduce the influence of impurities, so that the Mg-Zn-Ca-Mn alloy with the yield strength of up to 300MPa and the degradation rate meeting the clinical requirements of the bone screw is obtained. The invention simulates the mechanical action which the femur fixing screw may bear after being implanted into a human body by means of a finite element analysis tool, optimizes the structural size of the screw on the basis of simulating stress, avoids a fussy experimental link by the finite element analysis method, greatly saves time in the process of designing and optimizing, and reduces cost. The optimized screw improves the stress concentration phenomenon after being subjected to acting force, improves the bearing capacity, and simultaneously, the optimized structure also shows better corrosion resistance.

Drawings

FIG. 1 is a photograph showing the microstructure of a magnesium alloy prepared in example 1;

FIG. 2 is a photograph of the microstructure of the magnesium alloy prepared in example 2;

FIG. 3 is a photograph of the microstructure of the magnesium alloy prepared in example 3;

FIG. 4 is a photograph of the microstructure of the magnesium alloy prepared in example 4;

FIG. 5 is a perspective view of a bone screw designed according to the present invention;

wherein, 1 a spherical nail head, 2 a transition fillet, 3 a nail body and 4 a plum blossom groove structure;

fig. 6 is a photomicrograph of the bone screw of example 5 taken after 11 days of immersion;

fig. 7 is a photomicrograph of the bone screw of example 6 taken after 11 days of immersion.

Detailed Description

The invention is described in further detail below with reference to the figures and specific examples.

Example 1:

the magnesium alloy of the embodiment comprises the following components in percentage by mass: zn2.0%, Ca0.19%, Mn0.3%, and the balance Mg and inevitable impurities.

The preparation method comprises the following steps:

the material selection is as follows: the method is characterized in that a pure magnesium ingot with the purity of more than 99.99 percent is used as a magnesium raw material, zinc particles with the purity of more than 99.9 percent are used as a zinc raw material, a magnesium-calcium intermediate alloy with the mass fraction of 25 percent is used as a calcium raw material, and a magnesium-manganese intermediate alloy with the mass fraction of 10 percent is used as a manganese raw material.

Smelting: the raw materials used by the magnesium alloy are washed with acid and dried, then are put into a graphite crucible, protective gas (SF 6: N2: 0.4%: 99.6%) is introduced, and the smelting process is kept under the protective atmosphere environment. And after the magnesium ingot and the intermediate alloy are melted, stirring the molten metal by using a handheld electric drill, adding the preheated zinc particles into the molten metal, and continuing stirring for 5-10 min by using the handheld electric drill. And after the temperature of the smelting furnace reaches 720-730 ℃, starting high-shear stirring, then carrying out ultrasonic dispersion, standing for 5-10 min, casting into a preheated mold after the temperature is reduced to 690-700 ℃, and cooling to obtain a cast ingot with the diameter of 60 mm.

Solution treatment: and placing the cast alloy sample in a box furnace, setting the temperature rise time of the box furnace to be 10 ℃/min, heating to 420 ℃, then preserving heat for 24 hours, and quenching with hot water at 65 ℃ to obtain the supersaturated solid solution.

Hot extrusion: heating the extruder to 315 ℃, preheating the scalped alloy cast ingot half an hour before extrusion, and then carrying out hot extrusion at 315 ℃ to obtain a bar with the diameter of 8mm, wherein the extrusion speed is 2-3 mm/s.

Example 2:

the magnesium alloy of the embodiment comprises the following components in percentage by mass: zn2.3%, ca0.2%, mn0.5%, and the balance Mg and inevitable impurities, and the preparation method is the same as example 1.

Example 3:

the magnesium alloy of the embodiment comprises the following components in percentage by mass: zn2.5%, ca0.2%, mn0.7%, and the balance Mg and inevitable impurities, and the preparation method is the same as example 1.

Example 4:

the magnesium alloy of the embodiment comprises the following components in percentage by mass: zn2.9%, ca0.21%, mn0.9%, and the balance Mg and inevitable impurities, and the preparation method is the same as example 1.

And (3) performance testing:

1. and (3) testing the microstructure: fig. 1, fig. 2, fig. 3, and fig. 4 are SEM photographs of the extruded magnesium alloys of examples 1, 2, 3, and 4, respectively, and it can be seen from fig. 1 to 4 that the average grain sizes of the four alloys are 3.0 μm, 1.9 μm, 1.3 μm, and 2.0 μm (measured by SEM scale), respectively, and the recrystallized grains have a gradual thinning tendency as the Mn content increases, because dynamic recrystallization occurs during the extrusion process, Ca2Mg6Zn3 and Mn particles capable of pinning grain boundaries and thinning grains are formed, and the second phase gradually increases, and the thinning effect is more obvious.

2. And (3) testing mechanical properties: the extruded bar was processed into a standard tensile specimen and the mechanical parameters of the material measured by room temperature tensile test are shown in table 1.

TABLE 1

Alloy number	Yield strength (Mpa)	Tensile strength (Mpa)	Elongation (%)
				Example 1	210	274	14.1
Example 2	302	327	13.9
				Example 3	275	314	15.7
Example 4	245	312	14.7

Comparing the four alloys with each other, it was found that when the Mn content in the alloy is higher than 0.5 wt.%, the strength of the alloy is reduced, presumably because the number of dislocations in the dislocation clusters increases due to the deterioration of the grain refining effect and the stress concentration increases, thereby reducing the strength of the alloy.

3. Electrochemical testing: electrochemical polarization experiments and impedance experiments were performed in Simulated Body Fluid (SBF) at 37 ℃ using a Zennium electrochemical workstation, respectively. Using the test sample as a working electrode, graphite as an auxiliary electrode and saturated calomel as a reference electrode to form a three-electrode assembly (the working area of the sample is 0.8 cm)²). All sample surfaces were smoothed with 3000# water sandpaper and soaked in SBF for 30min to ensure potential stabilization prior to Electrochemical Impedance Spectroscopy (EIS) and polarization experiment (IE) testing, followed by testing. The frequency range of an EIS experiment is set to be 0.1Hz-100kHz, the scanning rate of the IE experiment is set to be 1mV/s, 3 parallel samples are arranged in each group to ensure accurate precision and stability, and the measured electrochemical data of the material are shown in a table 2.

TABLE 2

Alloy number	Ecorr(V)	Icorr(μA/cm2)	βc(v/dec)	βa(v/dec)	Rp(kΩ·cm2)
						Example 1	-1.502	30.65	0.24	0.09	0.94
Examples2	-1.509	32.25	0.18	0.12	0.97
						Example 3	-1.556	48.17	0.17	0.15	0.70
Example 4	-1.582	35.03	0.14	0.18	0.95

As the Mn content increases, the polarization potential gradually decreases, representing a weaker charge suppression effect, and the corrosion product film layer formed is less stable, eventually leading to a decrease in the protective effect.

Example 5:

optimized pre-screw machining

The specific dimensions of the screw are: the head diameter of the nail head is 5.8mm, the width of the plum blossom groove is 3.0mm, the outer diameter of the screw thread part of the screw nail body is 3.5mm, the inner diameter is 2.4mm, the thread pitch is 1.25mm, the width of the thread crest is 0.1mm, the total length of the screw is 30mm, the small tooth form angle is 3 degrees, and the large tooth form angle is 35 degrees.

The screw comprises a screw head and a screw body with threads, wherein the screw head 1 is spherical, a plum blossom groove structure 4 for screwing is arranged at the upper end of the screw head, and the screw is more uniformly stressed and is not easily damaged relative to a straight groove, a cross groove and a hexagonal groove when being screwed. The nail body 3 is full thread structure, and the screw of different length is conveniently twisted according to the patient condition.

And (3) performing stress relief annealing treatment on the processed screw at 170 ℃ for 1h in a box furnace to eliminate residual stress generated by machining, and performing surface cleaning and sterilization treatment on the annealed screw by using absolute ethyl alcohol.

Example 6:

after optimization, the screw is machined, as shown in fig. 5, and in addition to the difference between the size and the size before optimization, the screw is designed with an excessive fillet 2 connecting the head and the body of the screw, which is the same as that in embodiment 5.

The specific dimensions of the screw are: the head diameter of the nail head is 5.9mm, the width of the plum blossom groove is 3.25mm, the outer diameter of the screw thread part of the screw nail body is 3.5mm, the inner diameter is 2.5mm, the thread pitch is 1.15mm, the width of the thread crest is 0.1mm, the total length of the screw is 30mm, the small tooth form angle is 7 degrees, and the large tooth form angle is 40 degrees.

And (3) testing the mechanical property of the screw: three-point bending, twisting and pulling tests were performed on the screws, respectively.

Three-point bending: the screw is tested according to the national standard GB/T232-2010 metallic material experiment method, and the loading rate is 1mm/min until the screw is broken. The maximum bending force of the screw after the design optimization is improved by 60N compared with the maximum bending force of the screw before the optimization from the three-point bending test.

Torsion test: the test speed was 10deg/min and the twist angle was 5 deg. The torque of the screw after the design optimization is 319N/mm, the torque of the screw after the design optimization is 389N/mm, and is improved by 70N/mm.

Pull-out experiments: according to YY/T1504-2016 (Experimental method for axial extraction force of surgical implant metal bone screws), screws are screwed into polyurethane with the density of 0.2 g.cm < -3 >, the screws and the polyurethane are respectively fixed on a chuck of a testing machine to carry out an axial extraction test, and the loading speed is 1 mm/min. The maximum extraction force of the screw after the design optimization is improved by 3N compared with that of the screw before the design optimization.

Immersion test

The screw after cleaning and disinfection is subjected to a soaking experiment, a soaking solution adopts Simulated Body Fluid (SBF), and the result of soaking for 11 days shows that the screw before optimization has corrosion on local threads as shown in figure 6 and gradually expands, the optimized screw has no local thread corrosion as shown in figure 7 and tends to be uniformly corroded, and the designed transition fillet has stronger corrosion resistance than the sharp joint before.

Claims

1. A degradable magnesium alloy femur internal fixation screw comprises a screw head and a screw body and is characterized in that the fixation screw is made of a degradable magnesium alloy material with excellent mechanical property and low degradation rate; the nail head is spherical, the upper end of the spherical nail head is provided with a plum blossom groove structure for screwing, the nail body is of a full thread structure, and a transition fillet is arranged between the nail head and the nail body.

2. The degradable magnesium alloy femur internal fixation screw of claim 1, wherein the diameter of the head of the screw is between 5.8 and 6.0mm, the width of the quincunx groove is between 3.0 and 3.35mm, the outer diameter of the screw thread part of the screw body is between 3.5 and 4.0mm, the inner diameter is between 2.3 and 2.5mm, the pitch is between 1.10 and 1.25mm, the width of the thread crest is between 0.05 and 0.15mm, the total length of the screw is between 28 and 32mm, the minor profile angle is between 3 and 8 degrees, and the major profile angle is between 35 and 42 degrees.

3. The degradable magnesium alloy femoral internal fixation screw according to claim 1 or 2, wherein the magnesium alloy material adopts a pure magnesium ingot with a purity of more than 99.99% as a magnesium raw material, zinc particles with a purity of more than 99.9% as a zinc raw material, a magnesium-calcium intermediate alloy with a mass fraction of 25% as a calcium raw material, and a magnesium-manganese intermediate alloy with a mass fraction of 10% as a manganese raw material, and the magnesium alloy containing 0.19-0.21% by mass of calcium, 2.0-2.9% by mass of zinc, 0.3-0.9% by mass of manganese and the balance of magnesium and unavoidable impurities is prepared.

4. The preparation method of the degradable magnesium alloy femoral internal fixation screw is characterized by comprising the following steps:

1) smelting the magnesium alloy of claim 3Pickling and drying the used raw materials, putting the cleaned raw materials into a graphite crucible, introducing a protective gas SF 6: n is a radical of₂99.6 percent of the alloy powder in 0.4 percent, and the smelting process is kept to be carried out under the environment of protective atmosphere; after the magnesium ingot and the intermediate alloy are melted, stirring the molten metal by using a handheld electric drill, adding preheated zinc particles into the molten metal, and continuing stirring for 5-10 min by using the handheld electric drill; starting high-shear stirring after the temperature of the smelting furnace reaches 720-730 ℃, then carrying out ultrasonic dispersion, standing for 5-10 min, casting into a preheated mold after the temperature is reduced to 690 ℃, and cooling to obtain a cast ingot with the diameter of 60 mm;

2) solid solution treatment, namely placing the as-cast alloy sample in a box furnace, setting the temperature rise time of the box furnace to be 10 ℃/min, heating to 420 ℃, then preserving heat for 24 hours, and carrying out quenching treatment by using hot water at 65 ℃ to obtain a supersaturated solid solution;

3) hot extrusion, namely heating an extruder to 315 ℃, preheating the scalped alloy cast ingot half an hour before extrusion, and then hot extruding the alloy cast ingot at 315 ℃ into a bar with the diameter of 8mm, wherein the extrusion speed is 2-3 mm/s;

4) and (3) processing the screw, namely processing the extruded bar into the magnesium alloy screw which is optimized by the design in the claim 2 by a lathe.