HK1135008B - Shape memory locking device for orthopedic implants - Google Patents

Description

Shape memory locking device for orthopaedic implants

Technical Field

The present invention relates to mechanisms for connecting two or more implant elements using shape memory techniques. In one embodiment, the present invention relates to a mechanism for coupling spinal rods and bone elements in a spinal fixation system.

Background

This application claims priority from U.S. application s.n.60/844,237 filed on 13/9/2006.

The primary purpose of orthopaedic devices is to maintain the anatomical alignment of bone segments (anatomical alignment) by sharing the load acting on the bone, usually until bone fusion occurs. Current orthopaedic implant memory locking devices are variations of certain designs in which a nut of some type is used to lock the support portion into the head of the bone anchor to form the coupling. For example, in spinal instrumentation, various fastening mechanisms, such as threaded fasteners or rivet fasteners, are used to ensure that fixation of the bone anchors to the connecting structure is facilitated. These members provide the necessary stability in tension and compression to achieve fixation. However, it is well known that threaded fasteners loosen under the influence of cyclic loads common in the spine. As a result, the mechanical stability of the spinal implant may be reduced. Furthermore, wear (known as abrasion) between the support and the memory locking device is expected to occur, which abrasion can generate particulate debris believed to be associated with post-operative complications such as implant-induced osteolysis, pseudoarthrosis, subacute mild implant infection, post-operative surgical site pain, and abnormal metal concentrations in the serum. For example, see: bullmann et al, Spine, 2003, 28(12) phase, page 1306-1313; wang et al, Spine, 5/1/1999, phase 24(9), page 899; senaran et al, 8/1/2004, 29(15) period, 1618-; and Kasai et al, Spine, 2003, 28(12) phase, page 1320-.

U.S. patent nos.6,210,413 and 6,254,602 disclose shape memory locking devices for orthopedic correction. The mechanism of the locking devices of these two patents depends on the interaction of the compression element (20 in the '602 patent, fig. 1, and 24a and 24b in the' 413 patent, fig. 2) and the locking element (22 in the '602 patent, fig. 1, and 26a and 26b in the' 413 patent, fig. 2). Only these two elements are made of shape memory alloy and have shape memory effect and super-elasticity. The compression member and locking member generate a locking force to urge the coupling member (18 in fig. 1 of the' 602 patent). Heating the locking element and the compression element causes the coupler and the connecting portion to tighten. However, without the typical threaded fastening technique, there is no way to release the portions or to re-tighten the connecting material formed of a material such as nickel-titanium (nitinol or NiTi) shape memory alloy.

The shape memory phenomenon is: the material is capable of assuming one shape at low temperatures and another shape when heated to higher temperatures; see Liu et al, Materials Transactions 1996, 37(4), stages 691-. The material is in its original form at this higher temperature. When cooled to a lower temperature, the material retains its original shape but the structure changes to martensite (a stable phase at the lower temperature) at which point the material can be easily deformed into a different shape. Upon heating, the material changes back to austenite (the stable phase at higher temperatures) at which time the deformation is restored and the shape is restored (one-way shape memory). The alloy can also have two memories (two-way shape memory) that exhibit a reversible effect, and the change in shape due to heating can be recovered by cooling. See: liu et al, 1996 and Liu et al, Acta Materialia, 1999, 47(1), 199-.

Superelasticity (SE) is the property that a material can exhibit a constant restoring force when deformed at a temperature at or above the austenite transformation temperature. See Liu et al, actaMaterialia 1997, 45(11), stage 4431 and 4439.

It is therefore an object of the present invention to provide a mechanism for attaching orthopedic devices, particularly spinal fixation systems that are stable and less likely to generate particulate debris.

It is another object of the present invention to provide a mechanism for connecting orthopedic devices that can be tightened, released, and reconnected in situ without the use of a coupling element.

It is a further object of the present invention to provide a shape memory device in orthopedic surgery that prevents loosening and wear at the implant interface after implantation.

Disclosure of Invention

The present invention, without the use of typical threading techniques, is based on the shape memory effect and superelasticity of shape memory materials such as nickel-titanium (nitinol or NiTi) shape memory alloys, developing a mechanism to connect the bearings of an orthopaedic implant, for example, in the head, spine, upper and lower limbs, and thus prevent loosening and wear at the implant interface of the orthopaedic implant construct. The shape memory phenomenon is: the material is capable of assuming one shape at low temperatures and another shape when heated to higher temperatures. The material is in its original shape at higher temperatures. When cooled to a lower temperature, the material retains its original shape but the structure changes to martensite (a stable phase at the lower temperature), at which point the material can be easily deformed to a different shape at the lower temperature. Upon heating, the material changes back to austenite (the stable phase at higher temperatures) at which time the deformation is restored and the shape is restored (one-way shape memory).

The present invention has many advantages by making the entire device, not just the connector, from a shape memory alloy, and having shape memory effect and superelasticity. Heating the memory head of the device compresses the connection without the use of auxiliary means (for example coupling elements) so that the memory head can clamp the connection completely. Removing the coupling means will provide a greater, even more uniform compressive force on the coupling. The present invention provides additional advantages by utilizing a two-way shape memory effect in which the memory device can be closed or tightened by increasing the ambient temperature and re-opened by decreasing the temperature. The locking and unlocking temperatures may be adjusted by using a heat treatment protocol. Furthermore, the memory head has a re-tightening effect, i.e. if wear/tear occurs at the junction of the head and the connection, the head will further clamp the connection. This particular function can benefit from the superelasticity of the shape memory alloy and is also manipulated by its own heat treatment protocol. For example, in spinal applications, the memory locking device can automatically clamp the spinal rod in place once the mechanism is heated to or above the austenite phase transition starting temperature. The memory locking device can be automatically released if the mechanism is cooled below the austenite phase transition starting temperature. If superelasticity occurs, the bearing portions of the memory locking device can be further tightened to help prevent loosening and wear at the implant interface of the implant construct. The locking temperature can be adjusted by using various methods such as heat treatment and thermo-mechanical treatment. The memory locking means can be triggered by direct contact, indirect contact or heating by remote means. The memory locking device can prevent loosening and wear at the implant interface of an orthopaedic implant construct (e.g., a spinal rod in a spinal instrument). This tightening process is automatically triggered when the ambient temperature of the locking device reaches the austenite phase transition starting temperature. If loosening occurs, the re-tightening effect is automatically initiated by the mechanism, thus maintaining the mechanical stability of the implant as a whole. To reduce the post-operative complications associated with implant wear debris, the re-tightening effect may also reduce and/or prevent wear debris from being generated due to wear at the implant interface. The memory locking device can also be used with other internal or external surgical implants.

In a preferred embodiment, the shape memory locking device utilizes the shape memory effect and superelasticity of nickel-titanium alloys to prevent loosening and wear of the implant interface of an orthopaedic implant construct (e.g., a spinal rod in a spinal instrument). The tightening process can be triggered automatically when its ambient temperature reaches the austenite phase transition starting temperature. Furthermore, in the event of loosening, the re-tightening effect is automatically activated by the mechanism in order to maintain the mechanical stability of the implant as a whole. This phenomenon is observed if the ambient temperature exceeds the austenite transformation starting temperature of the mechanism made of the shape memory material. In addition, the re-tightening effect can also reduce and/or prevent the generation of wear debris due to wear occurring at the implant interface to reduce post-operative complications associated with implant wear debris.

The application of the memory locking device is not limited to spinal surgical implants, but it also applies to other internal or external implants, for example in head, spine, lower and upper limb implants.

The memory locking device can be made of any metal or polymer having shape memory effect and superelasticity. When an alloy is used, the alloy is preferably a shape memory alloy comprising nickel and titanium. Preferably, the nickel titanium shape memory alloy consists essentially of nickel and titanium. More preferably, the alloy contains 10% to 90% titanium and the balance nickel. The ratio of nickel to titanium is preferably in the range of 1: 9-9: 1, 2: 8-8: 2, 3: 7-7: 3, 4: 6-6: 4 or 5: 5. Alternatively, the alloy is a nickel-free shape memory alloy containing other metallic and non-metallic elements that can exhibit shape memory effects and superelasticity.

In one embodiment, the locking device includes a low profile design concept to avoid or reduce implant protrusion after surgical implantation. Alternatively, the locking device may include a side opening, thereby enabling the implanted portion to be grasped from the side. In another embodiment, the locking device may include a cutout at the bottom or side wall of the locking device to release the locking device.

The locking force of the locking device can be varied by adjusting the composition of the alloy (e.g. the content of nickel and titanium). The locking force may also be varied by different heat treatments, thermo-mechanical treatments, radiation treatments, ternary alloying, and any combination thereof. These processes are typically carried out in an oxygen and pressure controlled environment. Alternatively, the locking force can be varied by adjusting the diameter, height, size and base thickness of the locking device. The locking process is triggered by heating in a direct or indirect manner, for example by conduction heating or heating using electromagnetic radiation. The locking/unlocking temperature of the locking device can be adjusted by changing the composition of the alloy (e.g., the contents of nickel and titanium). But may also be varied by different heat treatments, thermo-mechanical treatments, radiation treatments, ternary alloying, and any combination thereof. Typically, these processes are performed in an oxygen and pressure controlled environment. The locking/unlocking of the device can be triggered by any change in the ambient temperature. If the ambient temperature reaches the specified locking temperature, the locking device can be locked by itself. The locking device may be released if the ambient temperature reaches a specified unlocking temperature. The locking temperature represents the austenite transformation start temperature. The locking temperature refers to a temperature above the austenite phase transition starting temperature. The unlocking temperature refers to any temperature below the austenite phase transition starting temperature.

Drawings

FIG. 1A is a front view of one embodiment of a memory locking device as described herein;

FIG. 1B is a cross-sectional view of a retaining member of the memory locking device;

FIG. 1C is a perspective view from the top opening of a shape memory locking device according to another embodiment described herein;

FIG. 2A is a front perspective view of a portion of another embodiment of a memory locking device;

FIG. 2B is a side view of a portion of a memory locking device according to another embodiment of the shape memory locking device;

FIG. 2C is an enlarged cross-sectional side view of the stem (stem) of the memory locking device forming an anchor according to another embodiment of the shape memory locking device;

FIG. 3A is a front perspective view of another embodiment of a memory locking device as described herein;

FIG. 3B is a cross-sectional view of a memory locking device according to another embodiment of the shape memory locking device described herein;

FIG. 4A shows a front perspective view of a portion of a low profile full bar capture memory locking device;

FIG. 4B shows a front view of a portion of the low profile half bar capture memory locking device;

FIG. 5 shows a front view of a portion of a side-ported memory locking device;

FIG. 6 is a graph of retention force versus compression (N) for an axial compression test of a test specimen;

FIG. 7 is a plot of holding force versus torque (Nm) for an axial torsion test of a test specimen;

fig. 8 is a graph showing mechanical test readings for 40 degrees of axial rotation of a specimen measured in holding force versus angular displacement.

Detailed Description

Memory locking devices designed to connect bearing portions of orthopaedic implants can prevent loosening and wear at implant joints of orthopaedic implant structures. The memory locking device can be designed for use with any internal or external surgical implant. The memory locking device may be used in orthopaedic applications such as in the head, spine, lower extremities and upper extremities.

Design of memory locking device

The present device is designed according to known designs for use as a prosthetic implant for orthopedic correction. It is modified as described below to use a shape memory material.

FIG. 1A depicts a memory locking device 10 according to a preferred embodiment of the memory locking device 10. The memory locking device 10 includes a retaining element 11 and a stem 19. In the present embodiment, the shank 19 has a length of about 15mm and a diameter of about 5mm at the bottom of the holding element 11. The retaining element 11 is cylindrical as shown in fig. 1C and has a height 14 of about 14mm and a thickness 15 of about 4 mm. The retaining member 11 defines a hole 9 about a central location for retaining a spinal rod 12 implanted in a human bone. The holes 9 have a diameter of about 6 mm. Preferably, the retaining element 11 also defines a top opening 18 connected to the hole 9. The top opening 18 is generally circular (having a diameter of about 2 mm) or oval in cross-section (having an inner circumferential radius of about 1mm) and may include a slit 8 having a width of about 1.3mm for insertion of an opening device to extend over the top of the holding element 11 to open the holding element. The top opening 18 allows for the insertion of a tool to expand the hole 9 so that a spinal rod can be installed into the hole 9. In typical applications, the hole 9 also includes splines 16 on the other side thereof (i.e., opposite the top opening 18) to enhance the mechanical stability of the retaining member 11 and the stability of the retained spinal rod 12. The bore 9 also includes two recesses 17 at each side of the splines 16 to facilitate expansion of the bore 9. The cross-section of each recess 17 is substantially semicircular with a radius of about 0.5 mm.

As shown in fig. 1B, the locking strength of the memory locking device 10 can be varied by adjusting the diameter 13 (i.e., the diameter of the retaining member 11) and the thickness 15 of the retaining member 11. In a preferred embodiment, the diameter 13 is about 12mm and the thickness 15 is about 4 mm.

The shank 19 of the memory locking device 10 can be formed as or can be combined with different types of bone anchors or bone engaging fasteners. As shown in fig. 2A-2C, the stem is formed as an anchor 20 having a length of about 13 mm. As shown in fig. 3A, the shank forms a screw 30. Alternatively, the stem can form any kind of connector, such as a rod, shaft, wire, etc., for connection with different types of bone anchors.

The memory locking device can be manufactured in a low profile design where the height 14 of the retaining element is reduced to such an extent that the top opening 18 is not present and a portion of the bore is exposed outside the retaining element. Fig. 4A depicts a low profile full rod capture memory locking device 40 in which the height 14' of the retaining element 11 is only sufficient to capture at least 4/5 of the entire circumference of the implanted spinal rod 12. Fig. 4B depicts a low profile half-rod capture memory locking device 42 wherein the height 14 "of the retaining element 11 is further reduced so that only the 2/3 circumference of the implanted spinal rod 12 can be captured. The reduced height of the retaining element 11 helps to prevent or reduce protrusion of the orthopaedic implant on the skin of the patient. Therefore, the patient does not feel uncomfortable after implantation due to the protrusion of the implant.

The figures herein only show designs for full and half rod grasping, but similar designs may be used to grasp portions of the circumference of the implanted rod 12, such as a half circumference or a three-quarter circumference.

Figure 5 shows the design of a side-ported memory locking device 50. The side-open memory locking device 50 has a retaining element 51 defining a bore 58 and an axis L, which is generally symmetrical about the axis L. A side opening 56 is provided on one side of the hole 58 for capturing an implant such as spinal rod 12. At the other side of the hole 58, the two recesses 52 are arranged on opposite sides of the axis L, so that the holding element 51 can be opened as wide as possible. Upper splines 53 and lower splines 54 are provided at each side of side opening 56 to enhance the stability of the captured implant. The stem 55 can be formed as different types of bone anchors, such as hooks, screws, or any connector for connecting with a bone anchor, or the stem 55 can be combined with such bone anchors or connectors.

It will be appreciated that while the retaining element of the illustrated device is cylindrical, other retaining element shapes may be employed, such as rectangular posts, hexagonal posts, elliptical posts, or any other shape as will occur to those of skill in the art. It is also to be understood that the dimensions given in the description are merely illustrative of particular embodiments and that a person skilled in the art will be able to vary them as required.

Material for forming device

The memory locking device can be made of any alloy, polymer or shape memory material having shape memory effect and superelasticity. There are two general classes of materials that can be used to make the device: shape memory metal alloys and polymers.

Shape memory alloys are well known and their properties have been described in detail. For example, see: liu et al, Materials Transactions, 1996, 37(4) p.691-; liu et al, Acta Materialia, 1999, 47(1) stage 199-; liu et al, Acata Materialia, 1997, 45(11), p.4431-4439; and Yeung et al, Materials Science and Engineering A, 2004, 383(2) phase, pp 213-218. When an alloy is used, the alloy is preferably a shape memory alloy of nickel and titanium. Preferably, the alloy contains 10% -90% titanium and the balance nickel. The ratio of nickel to titanium is preferably in the range of 1: 9-9: 1, 2: 8-8: 2, 3: 7-7: 3, 4: 6-6: 4 or 5: 5. Alternatively, the alloy is a nickel-free shape memory alloy containing other metallic and non-metallic elements capable of exhibiting shape memory effects and superelasticity.

Shape memory polymers, either degraded or non-degraded, are known and can be used to make the present devices. Suitable materials are described in U.S. patent nos.6,720,412, 6,388,043, and 6,160,084 to Lendlein and Langer. The polymers are commercially available, for example, from the company MnemoScience GmbH, germany.

Method of operation

In a preferred embodiment, the memory locking device is made of near equiatomic nickel titanium (NiTi) alloy. The memory locking device is subjected to a heat treatment after processing. By this heat treatment, the memory locking device can be automatically closed by raising the ambient temperature to a temperature at or above the locking temperature, which is a temperature at or above the austenite phase transition starting temperature (As) (locking temperature). However, if the ambient temperature drops to an unlocking temperature below the austenite phase transition starting temperature, the memory locking device can automatically reopen again. The locking process can be triggered by direct or indirect heating, but the unlocking process can also be triggered by direct or indirect cooling.

When the temperature is below the locking temperature, the locking device is in the first configuration. In the first configuration, as shown in FIGS. 1A-1C, because the shape memory material is in its martensitic state, the diameter of hole 9 is slightly larger than the outer diameter of spinal rod 12. Thus, when the retaining member is held in this first configuration, the spinal rod 12 is permitted to move within the hole 9.

When the temperature of the memory locking device is raised above the locking temperature, the shape memory material transitions from its martensitic state to its austenitic state. In the austenitic state, the locking device is reconfigured to the second configuration. In the second configuration, the diameter of the hole 9 is reduced to contact the spinal rod 12 such that the spinal rod 12 is not permitted to move within the hole 9. In another embodiment, the locking means may comprise a cut-out at its bottom or side wall in order to release the locking means. These devices are designed to automatically re-tighten if they come loose in any event, such as wear and tear. The re-tightening effect is based on the superelasticity and shape memory effect of the shape memory material.

The locking force of the locking device can be varied by adjusting the alloy composition (e.g. the content of nickel and titanium). The locking force may also be varied by different heat treatments, thermo-mechanical treatments, radiation treatments, ternary alloying, and any combination thereof. These processes are typically carried out in an oxygen and pressure controlled environment. Alternatively, the locking force may be varied by adjusting the diameter, height, size and base thickness of the locking device.

The locking process is triggered by direct or indirect heating, for example by conduction heating or using electromagnetic radiation. The locking/unlocking temperature of the locking device can be adjusted by changing the alloy composition (e.g., the contents of nickel and titanium). It can also be modified by different heat treatments, thermo-mechanical treatments, radiation treatments, ternary alloying, and any combination thereof.

The locking/unlocking of the device can be triggered by any change in the ambient temperature. If the ambient temperature reaches the locking temperature, i.e. the austenite phase transition starting temperature, the locking device is able to lock itself. The locking means can be released if the ambient temperature reaches the unlocking temperature, i.e. any temperature below the austenite phase transition starting temperature.

The device can reduce or prevent wear debris due to wear occurring at the implant interface to reduce post-operative complications associated with implant wear debris.

The invention will be further understood by reference to the following non-limiting examples. The memory locking device is described below with reference to the drawings. In the following discussion, the same or similar elements are denoted by the same reference numerals, and redundant description is omitted.

Example one: preparation of the samples

The shape memory locking devices described herein are fabricated from a near-equiatomic nickel titanium (Ti-50.8 wt% Ni) alloy. The facility then receives a special heat treatment protocol ranging from 250 ℃ to 800 ℃ and from 30 minutes to 60 minutes according to the process described in Materials Science and engineering A2004, 383(2) stage 213-218, Yeung et al. Locking temperature of memory locking device (higher than A)_s) And unlocking temperature (below A)_s) Programmed accordingly. When the ambient temperature is higher than the locking temperature, the memory locking device is closed; and when the ambient temperature is lower than the unlocking temperature, the memory locking device can be loosened.

For example, the shape memory locking device is first treated at 800 ℃ for 1 hour, followed by furnace cooling, then treated at 500 ℃ for 0.5 hour, followed by water quenching. Finally, the locking temperature is programmed to 35 ℃ or above and the unlocking temperature is 35 ℃ or below. The present heat treatment protocol is a non-limiting example of various heat treatment protocols developed for adjusting the locking and unlocking temperatures.

Example two: comparative testing

The memory locking device described in fig. 1A-1C was tested in comparison to four conventional spinal couplers, which are characterized by: 1. TSRH manufactured by Ti-6Al-4V alloy (Medtronic, Sofamor Danek Co.); 2. made of Ti-6Al-4V alloy (DePuy Spine, Johnson)&Johnson corporation) manufactured by Moss Miami; 3. CD-H manufactured by Ti-6Al-4V alloy (Medtronic, Sofamor Danek Co.); 4. made of Ti-Al6-Nb7 alloy (B)Company) manufactured AO USS. The locking torque provided to secure the coupling device to the spinal rod is in accordance with the manufacturer's recommendations.

Mechanical testing

The samples were mechanically tested using a Material Testing System (MTS)852.02Mini Bionix (USA). Custom test stands for compression and torsion testing were attached to the MTS and connected to an external digital thermometer. The entire test stand was immersed in a water bath to control the test temperature. Ambient temperature is controlled by an external temperature controlled thermal cycler (Isotemp 2006S, Fisher Scientific, Pennsylvania, usa). The thermal cycler provides a stable temperature environment of 37.5 ℃ (± 0.5 ℃) for a test stand attached to a Materials Testing System (MTS). The actual water temperature was monitored using a digital thermometer (HI92801C, Hanna, portugal). The same arrangement is used in conventional spinal coupler testing.

The memory locking device described herein is embedded in the test rig with the stem of the device perpendicular to the compression axis. One end of the spinal rod is connected to the memory locking device along the compression axis and the other end is connected to a load cell of the MTS testing machine. The locking means of the memory device is triggered when the temperature of the water bath rises above 37 ℃. When the locking of the memory device was completed, the water bath temperature was reduced to 37 ℃.

The same arrangement is also used in the mechanical testing of one of the conventional locking devices (AO USS). One end of the spinal rod 101 is connected to the screw 102 by using a coupling device 103 and then the other end is attached to the load cell of the MTS. The coupler is then tightened to secure the pedicle screw 102 and spinal rod connection.

The test protocol is described in the ASTM F1789 standard and previous studies on mechanical testing of spinal instrumentation systems. See Yeung et al, Materials Science and engineering A, 2004, 383(2) phase, pp 213-218; magerl Clinical Orthopaedics, 1984, 189, pages 125-41; kotani et al, Spine, 7/15/1999, phase 24(14), page 1406; stanford et al, Spine, 2/2004, 15/29 (4) p.367-; balabaud et al, Spine 2002, stage 27(17), page 1875-1880; and Glazer et al, Spine 1996, 5/15/21 (10) phase, 1211-1222 page.

Axial compression test

The axial compression test includes two loading modes: dynamic compression and static compression. The dynamic compression was applied by pre-loading the coupling with a 350N compression load and then applying a dynamic load of ± 20N for five cycles. The static load is applied by applying a compressive load to the coupling device at a rate of 0.1 mm/sec until the load reaches 800N or a 1mm offset between the spinal rod and the coupling device is produced. The load cell then returns to its original position.

Axial torsion test

An axial torsion test is applied to the coupling device by a spinal rod attached to an MTS load cell. The test was conducted with angular displacement controlled at 20 to 40 degrees of rotation. During testing, the load cell rotated the spinal rod at a rate of 0.25 degrees/second. Once the load cell reaches a specified angle of rotation, the cell returns to the initial position at the same rate. All data in the compression and torque tests were recorded by the MTS machine. For significance, data was analyzed by using Student's t-test, and statistical analysis was performed using the SPSS program (SPSS for Windows, version 11.0.0).

Test results

FIG. 6 shows the results of a static compression test of one of the shape memory locking devices described herein ("SMAV 3") and the test results of four conventional spinal couplings. Retention is shown as mean ± standard deviation. The retention force of the memory locking device was found to be higher than 800N and prevented from slipping. For the conventional coupling, the average retention force of the Moss Miami sample was 800 without slip, while the average retention force of the AO USS sample was only 705N. The TSRH and CN-H samples were slipped at 540N and 728N, respectively. The test results show that the memory locking device locks more tightly than the TSRH, CD-H and AO USS samples during axial compression and compares to the Moss Miami (MOSS) samples.

FIG. 7 shows axial rotation test results for one of the memory locking devices and other conventional spinal couplers. Retention is shown as mean ± standard deviation. The maximum retention force of the memory locking device for the 40 degree axial rotation test results was 5.0 Nm. The test results of the conventional memory locking device showed that the maximum holding forces of the AO USS, CD-H, Moss Miami and TSRH samples were 3.0Nm, 1.8Nm, 4.25Nm and 2.7Nm, respectively. The axial torsion test result shows that the memory locking device is superior to all the traditional memory locking devices.

Fig. 8 shows the torque-time relationship for a 40 degree axial rotation test for one of the memory locking devices and a conventional memory locking device. Torque-time relationship of the coupling under the 40 degree axial rotation test. Line (a) represents the shape memory locking device (SMA V3) described herein at various angular displacements, line (b) represents the conventional spinal coupling CD-H, line (c) represents the conventional spinal coupling TSRH, line (d) represents the conventional spinal coupling AO USS, and line (e) represents the conventional spinal coupling MOSS.

Slip was observed in all samples. The holding force of conventional mechanisms will remain at a substantially constant value as the slip occurs. In contrast, for the memory locking device as shown in the figures, the shape memory devices alternate in a saw tooth pattern. When sliding occurs, the mechanical behavior of the memory locking device is different from that of other conventional mechanisms because the holding force rises again immediately after sliding. These phenomena clearly demonstrate the ability of the memory locking device to re-tighten once loosening or wear has occurred.

Modifications and variations of the present invention will be apparent to those skilled in the art and are intended to be included within the scope of the appended claims. All references cited in this application are specifically incorporated by reference.

Claims (23)

1. A locking mechanism for an orthopaedic implant, comprising:

a retaining element defining a hole for retaining the orthopaedic implant and an opening at a surface of the retaining element for communication with the hole and defining at least one recess at a side of the hole that facilitates expansion of the hole;

a shank at the bottom of the retaining element for forming or connecting with a bone anchor;

wherein the locking mechanism is made of a shape memory material selected from the group consisting of metal alloys and polymers having at least two shapes in memory such that the locking mechanism allows movement of the orthopaedic implant when at a locking temperature and restricts movement of the orthopaedic implant when at an unlocking temperature;

wherein the locking temperature refers to a temperature above the austenite phase transition starting temperature, and the unlocking temperature refers to any temperature below the austenite phase transition starting temperature.

2. The locking mechanism of claim 1 wherein the shape memory material is a nickel-titanium alloy or a nickel-free alloy having shape memory and superelasticity.

3. A locking mechanism according to claim 2, wherein the locking mechanism comprises an alloy comprising 20-80% nickel and 80-20% titanium.

4. The locking mechanism of claim 1, wherein the orthopedic implant is a spinal rod.

5. The locking mechanism of claim 1 wherein the bone anchor is a hook, pin, rivet or screw.

6. The latching mechanism of claim 1, wherein said opening is located at a top surface of said retaining element.

7. The locking mechanism of claim 6 wherein the aperture includes splines at a side opposite the opening to enhance locking stability.

8. The latching mechanism of claim 1, wherein said opening is located at a side of said retaining element.

9. Locking mechanism according to claim 1, characterized in that a cut-out is provided at the bottom or side wall of the locking mechanism in order to release the locking mechanism.

10. The locking mechanism of claim 1, wherein the orthopaedic implant is for fixation of a head, spine, lower limb or upper limb.

11. The locking mechanism of claim 1, wherein the locking force of the locking mechanism is adjustable by selecting the composition, dimensions, or treatment of the locking mechanism.

12. The locking mechanism of claim 11 wherein the locking force is adjusted by selection of nickel and titanium compositions or a nickel-free metal composition.

13. Locking mechanism according to claim 11, characterized in that the locking force is adjusted by heat treatment.

14. The locking mechanism of claim 11 wherein the locking force is adjusted by thermal mechanical treatment, radiation treatment, ternary alloying, or combinations thereof.

15. The locking mechanism of claim 11, wherein the locking force is adjusted by selecting a diameter, a base thickness, or a height of the locking mechanism.

16. The locking mechanism of claim 1, wherein the handle is formed in a shape selected from the group consisting of a screw, a rod, a shaft, and a wire.

17. A method for manufacturing a locking mechanism for a prosthetic implant, the method comprising the steps of:

forming the locking mechanism of claim 1;

treating said shape memory material with a special heat treatment ranging from 250 ℃ to 800 ℃, ranging from 30 minutes to 60 minutes, to determine a locking temperature and an unlocking temperature;

wherein the locking temperature refers to a temperature above the austenite phase transition starting temperature, and the unlocking temperature refers to any temperature below the austenite phase transition starting temperature.

18. The method of claim 17, wherein the locking force of the locking mechanism is adjusted by selecting a composition of the shape memory material, a size of the locking mechanism, or a treatment thereof.

19. The method of claim 18, wherein the locking force is adjusted by selecting nickel and titanium compositions.

20. The method of claim 18, further comprising adjusting the locking force by heat treatment.

21. The method of claim 18, further comprising adjusting the locking force by irradiation treatment.

22. The method of claim 18, further comprising adjusting the locking force by thermo-mechanical treatment, conductive heating, electromagnetic radiation, ternary alloying, or a combination thereof.

23. The method of claim 18, wherein the locking force is adjusted by selecting a diameter, a base thickness, or a height of the locking mechanism.