WO2005039425A2

WO2005039425A2 - Fracture nail from a shape memory alloy

Info

Publication number: WO2005039425A2
Application number: PCT/EP2004/012034
Authority: WO
Inventors: Wolfgang Franck; Ferdinand Haider; Wolfgang Biegel
Original assignee: Universitaet Augsburg; Friedrich Alexander Universitaet Erlangen Nuernberg
Current assignee: Universitaet Augsburg; Friedrich Alexander Universitaet Erlangen Nuernberg
Priority date: 2003-10-24
Filing date: 2004-10-25
Publication date: 2005-05-06
Anticipated expiration: 2006-04-24
Also published as: DE10349571A1; WO2005039425A3

Abstract

The invention provides a fracture nail for the surgical treatment of bone fractures, which comprises shape memory material in at least one section. After activation of the shape memory material, the nail forms a fixation means. Furthermore, a method for manufacturing a fracture nail is provided, which comprises a shape memory material in at least one section, wherein the nail is deformed in such a manner that a fixation means is formed, the shape formed by the deformation is held mechanically and the nail is heated for fixing the shape. Additionally, a holding means is provided for fixing the shape forming the fixation means during the heating.

Description

Fracture nail from a shape memory alloy

The present invention relates to a fracture nail or bone peg for the surgical treatment of bone fractures, especially of fractures of tubular bones, like upper arm and forearm.

As a common alternative to plasters, interlock nails or plates are used for the surgical treatment of fractures, in particular fractures of the upper arm. In order to achieve a secure and permanent positioning of the fragments relative to each other, it is necessary to fix the naii rotationally stable within the longitudinal axis of the bone. A plurality of nail systems for the treatment of fractures of the upper arm, so called humerus nail systems having various fixation mechanisms in the bone are known.

The access to the shaft of the humerus can be achieved either through the head of the humerus or through the elbow region. However, the access through the head of the humerus is critical, as it is necessary to split the rotator cuff which can result in calcification or heavy scar formation which results on the other hand in painful restrictions in the mobility of the shoulder joint. Therefore, this access should only be considered when there are injuries in the elbow region, which do not allow nailing in this region.

Massive titanium nails available in various diameters and various lengths are used for the surgical treatment of fractures of the upper arm. The ends of these nails comprise interlock holes for the receipt of screws for the fixation of the nail within the bone. In order to facilitate the entry of the screws, these holes have troughs at their edges. The implant is weakened thereby which may result in the failing of the implant. Furthermore, due to the small diameter of the nail which is necessary to be able to introduce the nail into the medullary space of the upper arm, the interlock holes are also small. Therefore, it is technically demanding to hit the small interlocks with the screws during the operation. Thus the operation time prolongs which in turn results in long x-ray screening times connected with high esposure of the patient and the personnel during the operation. Furthermore, as a consequence of the high rigidity of the system the risk to cause, in case of a narrow shaft and a small diameter of the medullary space, a fracture in the distal area is elevated compared with more elastic systems. In this system, the entire force is transmitted between the nail and the bone via the attachment screw. In osteoporotic bones, the screw bearings are often too soft, which leads to the attachment screws being loosened. They are moving backwards following the thread and the entire construction becomes instable.

A further humerus nail system is the so-called Marchetti nail (see Figures 1a and b). This nail comprises a solid short part in which four to five spring nails originate. These are held together by a thin wire core. After the introduction of the nail into the bone, the core is removed and the spring nails spring apart from each other and allocate themselves within the medullary space. It is the essential advantage of this system that a freehanded interlocking at the nail's end is not necessary. Due to the flexibility, the risk to cause iatrogenic fractures by levering is minimal. A diasadvantage is the punctual contact of the spring nails which may cause perforations. As the spring nails have straight ends, telescoping, i.e., an extension of the bone at the fracture, may occur upon axial stress. Furthermore, only an elbow-near access is possible. In case of a highly wasted medullary space, these nails will not have a sufficient hold, as they will have contact in the narrow part but will not have contact in the upper part.

A further known system is based on a similar principle, which comprises a hollow nail, through the lumen of which two Kirschner wires are passed which exit at the top (see Figures 2a and b) (Haider humeral nailing system (manufacturer Corin)).

At the other, the lower end of the nail, the wires are bent and fixed with a small plate. In this type, too, there is no need to make freehandedly an interlocking at the nail's end. The nail is very rigid and can only be introduced from the elbow. The very spiky Kirschner wires include a considerable perforation risk at the head of the humerus. As with the Marchetti nail, in case of axial stress telescoping may occur or in case of a comminuted fracture in the middle of the shaft, the nail may perforate the head of the humerus if the arm is propped up. Furthermore, analogous to the titanium nail, it is true that the risk to cause an iatrogenic fracture is elevated.

A system being new on the market is the so-called Fixion system. In this system, the nail is folded prior to the implantation and thus has a trefoil shaped cross section. After implantation, it is filled with saline solution through a valve and thus expanded (see Figures 3a and b). Thus, a circular cross-section is formed having contacting reinforcements, which simultaneously have a stabilizing effect against rotational forces. Any kind of an interlocking by screws or wires is not necessary. In case of a sufficient contact area a high stability is achieved. The use is though considerably limited as it is necessary that there are intact bone segments of about 5cm both below and above the fracture, i.e., it is only possible to treat fractures of the middle of the shaft. The maintenance of the pressure within the nail throughout the healing period is still questionable. In case of an early pressure loss, there will not be sufficient stability any longer, so that the risk of an incorrect healing or a pseudo-arthrosis will then be very high. Furthermore, a sufficient jamming is not guaranteed in those cases in which the nail has to be selected in such a manner that it can pass the smallest diameter but that it cannot expand in the upper space - having a broad medullary space - in a manner that a secure frictional locking is achieved.

Another new system is the Trueflex nail, a titanium nail having a star-shaped profile with very sharp cut edges. During introduction, these edges cut through the spongiosa into the corticalis. An additional interlocking is thus not necessary. Using this system, however, there is a high risk that the nail gets jammed during driving it into the bone of the upper arm. In this case, it is especially true that the secure nail-bone connection is only achieved in the region of the narrowest diameter.

In summary, all nail systems available on the market have disadvantages such as, e.g., extensive operations, risk to damage the bone by the fixation mechanisms, deficient stability in the osteoporotic bone and long radiographical screening times during the operation.

It is an object of the present invention to provide a novel and improved fracture nail. The nail should provide an optimal fixation within the bone and together with a simple and rapid handling during the operation. Furthermore, it should be possible to treat optimally both the shaft fracture and the subcapital fracture, i.e., the fracture immediately below the head of the humerus.

These objects are achieved by the features of the claims.

The present invention is based on the main idea that the fracture nail comprises at least in one section a shape memory material. After the activation of the shape memory material a fixation means is formed by which the nail is securely held in the medullary space.

Preferably, the nail is tubular. By at least one slot, which is arranged at at least one end of the nail in longitudinal direction, the nail is divided into a predetermined number of blades at said end. For example, four blades can be provided by the arrangement of two cross-shaped slots, the blades can be expanded outwardly and thus form the fixation means. Here the expanded shape is the shape of the shape memory material after the activation. The exact shape or curvature of the blades after activation can here be determined during the production of the nail and thus the expanding geometry of the blades can be adapted to the interior of the bone. Furthermore, by an adequate selection of the wall thickness of the tubular nail, the force during the expanding can be determined.

In the embodiment of the nail according to the present invention, the shape memory material is activated by an increase in temperature. The transformation temperature of the shape memory material is preferably at least about 40°C so that the shape memory material is not activated during the introduction of the nail into the bone by the prevailing body temperature. On the other hand, the transformation temperature should not be higher than about 60°C in order to avoid damage to the bone by heat necroses at higher temperatures during the activation.

The increase in temperature after the introduction of the nail into the bone can be achieved by a direct current passage through the nail, wherein the expansion of the blades can be controlled separately by separately contacting the individual blades. The increase in temperature can also be achieved by a thermal contact, e.g., by a sheathed heating conductor. Other possibilities to activate the material are also possible, e.g., to increase the temperature by inductive heating by an electrical field or also, in case the transformation temperature of the shape memory material is below the body temperature, to increase the temperature by the body temperature. In this case, the nail has to be cooled prior to or during the introduction into the bone.

In a further embodiment, the nail comprises superelastic material, i.e., shape memory material the transformation temperature of which is below room temperature, i.e., below about 15°C. Thus, the nail is present in its expanded shape at room temperature. In order to be introduced into the bone, the nail has to be held folded by a suitable device, e.g., a holding wire or a nylon thread. After the introduction of the nail into the bone, the shape memory material is activated by removing the device and the nail expands and thus the fixation means is formed. Due to the specific properties of the shape memory material, the expansion of the nail after activation takes place with a constant force throughout the entire expansion step. While the force and the degree of the expansion between the folded shape and the expanded shape originally memorized by the shape memory material can be controlled by the increase in temperature in the first embodiment, the nail according to the second embodiment expands autonomously after removing the device for holding the folded form by relaxation and thus again with a constant force until acquiring the shape originally memorized by the shape memory material.

A NiTi shape memory alloy (Nitinol) is preferably used as shape memory material. Long-term experiences of the biological compatibility of this material are present.

As a further aspect, the present invention provides a method for manufacturing a fracture nail, which at least in one section comprises a shape memory material. Here, the nail is deformed in such a manner that a fixation means forms, mechanically fixed in this shape and heated in order to fix the shape, wherein the temperature during the heating is preferably between about 200°C and 500°C. After cooling the nail to room temperature, the nail is almost brought into the condition before the deformation. In case the transformation temperature of the shape memory material is above the room temperature, the nail can be introduced into the bone and the shape originally memorized by the shape memory material during the heating step can be achieved by an increase in temperature above the transformation temperature. In case the transformation temperature is below room temperature, the nail is kept together or closed by a holding device and the fixation means is formed by detaching the device after introduction into the bone.

The nail according to the present invention can be used for the surgical treatment of fractures, especially for stabilizing fractures of the upper arm and forearm, and as support implant in spine surgery or as pin for the fixation of screws in osteoporotic bones.

As a further aspect, the present invention provides a holding means, which is suitable for holding the shape forming a fixation means of a fracture nail according to the present invention during the heating step for fixing said shape. This holding means is preferably designed in such a manner that the blades of a tubular nail according to the present invention, during heating, are held in a shape expanded circularly outwardly or in a shape adapted to the interior of the bone. Preferably, the holding means consists of stainless steel, e.g., chrome nickel steel.

The fracture nail according to this invention has on the one hand the advantage that a freehanded interlocking of the nail's end during the operation, for example, by screws is not necessary. Furthermore, by using shape memory materials, the forces exerted onto the bone during the expansion can be controlled. In particular, the advantage of using superelastic shape memory material resides in the considerably flatter stress-strain-curve of said material. While a similar design from steel, as, e.g., in the known Marchetti-nail, exerts very high possibly harmful forces to the bone if the medullary space is narrow but may not exert sufficient forces to the bone if the medullary space is wide, the forces exerted in case of shape memory materials remain almost constant over wide ranges. Due to the controlled expansion, damage to the bone can largely be avoided while, nevertheless, a secure fixation of the nail within the bone can be guaranteed.

The present invention is described with regard to the attached drawings in more detail:

Figurel shows a known Marchetti nail a) in folded and b) in expanded condition; Figure 2 shows a known fracture nail with Kirschner wires (Haider system); Figure 3 shows a nail of the known Fixion system a) in folded condition and b) in expanded condition; and Figure 4 shows a graphic simulation of an x-ray of an upper arm bone with an introduced fracture nail according to the present invention a) before activation and b) after activation of the shape memory material.

For the production of a fracture nail according to the present invention, the shape memory material Nitinol is used in form of a tube having a length of about 200 mm to 280 mm length and a diameter of about 6 mm to 10 mm, corresponding to the typical size of the upper arm's bone shaft, said tube having been provided with slots at its end. Such a material is sold, e.g., by the company Euroflex, Pforzheim, Germany.

The basis of the shape memory effect is the martensitic transformation, a phase transformation without diffusion. In a martenisitic transformation, there is no considerable movement of the atoms relative to each other, rather whole lattice planes shear with respect to each other. The conditions being part of the martensitic transformation are called austenite phase (high temperature phase) and martensite phase (low temperature phase). The temperature at which the transformation from the martensite to the austenite phase starts is called the lower austenite temperature. However, there is no complete transformation after the lower austenite temperature has been reached. A prolonged maintaining of a temperature above the lower austenite temperature does not lead to a further transformation, only in case of a further increase in temperature the transformation continue. The temperature, at which the entire material has been transformed is called upper austenite temperature. The slotted tube is deformed, i.e., the blades are expanded, the thus produced shape is fixed in a specific mold from stainless steel and the material is then heated to about 300°C to 400°C for several hours, for the purpose of enabling the material to memorize the desired shape during the austenite phase, i.e., the high temperature shape, which means in the present case the shape of the fixation means of the fracture nail. The transformation temperature, i.e., the lower and the upper austenite temperatures, can be adapted by varying the temperature and the duration of the heating procedure. After cooling to room temperature, the nail is stable in the desired shape.

In order to configure the fixation means, the individual blades can be expanded at its ends outwardly in form of circle segments. As it cannot be assumed that such a circular expansion provides the ideal adaption to the interior of the bone, various expanding geometries can be achieved by respective holding means during the heat treatment. Thus, it is possible to adapt the nail to the bone's anatomy and specifically to the shape of the medullary space in which the blades are arranged after expanding. After cooling to room temperature, the material can again be easily reformed to an almost closed tubular nail. Here, the slots by which the blades are formed can extend either over a length which essentially corresponds to the entire length of the nail, up to about half of the length of the entire nail, as shown in Figure 4, or only in the upper region, which corresponds to the region of the bone in which the medullary space is present. The number of blades is selected in correspondence with the application. However, it is preferred to use 4 or 6 blades, i.e., 2 or 3 slots. A thread is cut into the lumen of the nail's lower end, by which the interlocking screws can be angle-stably jammed by means of thread bolts. Thus, the stability in osteoporotic bones is increased.

In a first embodiment in which the deformation of the nail into the shape forming the fixation means, i.e., the high temperature shape, is achieved by heating the nail, the nail is produced from a Nitinol material having a transformation temperature, i.e., a lower austenite temperature of above room temperature at about 45°C. The nail is introduced into the fractured bone during the operation and, after it is optimally positioned, heated to above its transformation temperature by a suitable heating mechanism. Now the re-formation into the fixed, memorized shape starts in the Nitinol, i.e., the expansion of the blades and thus the fixation within the bone.

Various realization types for the heating mechanism of the nail are possible:

The heating of the nail can be achieved by a direct current flow through the Nitinol. After the introduction and positioning of the nail, a rod-shaped contact electrode is introduced into the tubular nail up to its tip and through said contact electrode, an electric current can be passed through the nail in longitudinal direction. The heat generated by the current flow through the nail heats the nail to the desired transformation temperature and the nail adopts its final shape when the temperature is increased up to the upper austenite temperature. By controlling the current, the degree of expansion of the nail can be intendedly controlled. Thus, it is possible to stop the expansion before the previously fixed shape is achieved in that, e.g., the nail is heated to a temperature which is below the upper austenite temperature. Furthermore, it is possible to adjust the extend of exerted forces via the temperature, while the position remains unchanged. Furthermore, it is possible to separately control the expanding of the individual blades by a respectively partitioned contact electrode, in order to achieve in this manner an optimal fixation even if inadvantageous geometries of the bone's interior are present.

Furthermore, it is possible to heat the nail with an introduced sheathed heating conductor. After the introduction and positioning of the nail, a suitably shaped sheathed heating conductor is introduced into the tube up to its tip. Sheathed heating conductors are commercially available and do not cause any shortage problems due to their interiorly insulated guidance of the current carrying lead. Through the sheathed heating conductor, an electric current is passed which heats the heating conductor. Subsequently, the nail is heated to the desired transformation temperature and adopts its final shape after the upper austenite temperature has been reached. Here the degree of expansion can be controlled by the control of the current, too. Contrary to the above described heating by direct current passage, it is in this manner not possible to control the single blades. But the necessity to create a direct electrical contact to the nail is avoided.

Furthermore, the heating of the nail can be achieved by an inductive coupling or by contact with a tube through which a heated fluid is flowing. Furthermore, it is possible to condition the shape memory material in such a manner that the upper austenite temperature is about 35°C, so that the blood heat activates the Nitinol.

By using a so-called "one-way shape memory material" it is guaranteed that the nail does not return to its initial shape after cooling to body temperature.

In a second embodiment, the shape memory material is activated by a relaxation of the superelastic Nitinol. In this case, a Nitinol material having a transformation temperature below room temperature, so-called superelastic material showing rubber-like mechanical behavior, is used for producing the fracture nail. In this case, the nail also adopts the expanded shape at room temperature. This shape is transformed into the shape of the closed nail by means, e.g., of a holding wire mechanism prior to the introduction into the bone. After the fracture nail has been introduced into the injured bone, the holding wire is removed whereon the nail expands. In this second embodiment, no external heating means are necessary for the transformation of the nail.

The advantage of using superelastic material is that the force during the relaxation, i.e., during the expansion of the nail, is in wide ranges essentially constant. If, however, the relaxation of conventional material is used, as it is already realized in the so-called Marchetti nail, the blades expand uncontrolledly after removing of the holding wire. In contrast to that, the expansion in case of superelastic material is controlled and the force is essentially constant during the entire expansion procedure.

Besides the treatment of fractures of the upper arm, the fracture nail according to the present invention is also suitable for further uses: analogously to the humerus nail, nails having a smaller diameter can be produced for the stabilization of fractures of the forearm, too. The use of conventional nails is inhibited by the small diameter of the medullary space which makes the interlocking by screws at the end of the very thin nail technically quite difficult and faulty. Furthermore, the nail according to the present invention can be used as deformable longitudinal support for fixator-internal systems in spine surgery. Here, the erection of a fractured body of a vertebra is to be realized by the deformation of the longitudinal support in the system. Thus, it is possible to achieve a more uniform and more regulated erection as up to now with the use of long reposition levers. Furthermore, the nail according to the present invention can be used as pin for the fixation of plates in the bone. The fixation of plates in an osteoporotic bone is quite difficult as there is not sufficient hold for the screws in the bone. By means of a Nitinol pin which is inserted in the bore hole of the existing plate and which deforms in the medullary space and thus fixes automatically, a large-area stabilization and if at least two holes are used per fragment even an angle-stable construction alike a fixator-internal system can be achieved. Additionally a so- called mono-cortical stabilization becomes possible, i.e., the screw must not break through the opposite bone wall.

Claims

1. A fracture nail for the surgical treatment of bone fractures, wherein the nail comprises in at least one section a shape memory material, wherein a fixation means is formed after the activation of the shape memory material.

2. The nail according to claim 1 , wherein the nail is tubular.

3. The nail according to claim 1 or 2, wherein at least one slot is arranged at at least one end.

4. The nail according to claim 3, wherein the nail is divided into a predetermined number of blades by the at least one slot.

5. The nail according to claim 4, wherein the blades expand controlledly after the activation of the form shape material within the bone and thus form the fixation means.

6. The nail according to claim 5, wherein the expanding geometry of the blades is adapted to the interior of the bone.

7. The nail according to claim 5 or 6, wherein the blades expand each with a predetermined curvature and force.

8. The nail according to at least one of the preceding claims, wherein the shape memory material is activated by an increase in temperature.

9. The nail according to at least one of the preceding claims, wherein the transformation temperature of the shape memory material is between about 40°C and about 60°C.

10. The nail according to claim 8 or 9, wherein the increase in temperature is achieved by a direct current passage.

11. The nail according to claim 8, 9 or 10, wherein the increase in temperature is achieved by an introduced sheathed heating conductor.

12. The nail according to at least one of claims 1 to 8, wherein the shape memory material comprises superelastic material, the transformation temperature of which is below room temperature.

13. The nail according to claim 12, further comprising a means for holding the shape even at room temperature.

14. The nail according to claim 13, wherein the shape memory material is activated at room temperature by removing the means for holding the shape.

15. The nail according to at least one of the preceding claims, wherein the shape memory material comprises a NiTi shape memory alloy.

16. A method for manufacturing a fracture nail for the surgical treatment of bone fractures, particularly according to at least one of the preceding claims, wherein the nail comprises shape memory material in at least one section, comprising the steps deforming the nail so that a fixation means is formed; holding mechanically the shape formed by the deformation; and - heating the nail for fixing the shape.

17. The method according to claim 16, wherein the temperature during the heating is between about 200°C and about 500°C.

18. The method according to claim 16 or 17, wherein the nail is re-deformed almost into the condition prior to the deformation after cooling essentially to room temperature.

19. The method according to at least one of claims 16 to 18, wherein the nail is tubular.

20. The method according to claim 19, wherein the nail is slit for dividing it into a predetermined number of blades prior to the deformation.

21. The method according to claim 20, wherein the nail is deformed by expanding the blades and the fixation means is thus formed.

22. Use of the nail according to at least one of claims 1 to 15 for the surgical treatment, particularly for the fracture stabilization of bone fractures.

23. The use according to claim 22 for the treatment of fractures of the upper arm.

24. The use according to claim 22 for the treatment of fractures of the forearm as support implant in spine surgery and/or for fixing plates in osteoporotic bones.

25. A holding means for fixing a nail comprising shape memory material in at least one section, particularly a nail according to any one of claims 1 to 15, in a shape in which the nail forms a fixation means, during heating the nail for fixing the shape.

26. The holding means according to claim 25, wherein the mold comprises two sections wherein the first section comprises a rotationally symmetrically upwards tapered surface essentially circularly, concavely curved inwardly and the second section forms the convex counter part of the first section for fixing the nail.

27. The holding means according to claim 25 or 26, wherein the holding means consist of stainless steel.