WO2003030761A1

WO2003030761A1 - An assembly for extraction of broken medullary nails

Info

Publication number: WO2003030761A1
Application number: PCT/DK2002/000686
Authority: WO
Inventors: Christian Ulrik Maegaard; Poul Tornhøj FREDERIKSEN; Børge KREBS
Original assignee: IWM UDVIKLING
Current assignee: IWM UDVIKLING
Priority date: 2001-10-11
Filing date: 2002-10-11
Publication date: 2003-04-17
Anticipated expiration: 2004-04-11

Abstract

The present invention is a fixation shaft for catching medullary nail, e.g. for removing the nail from a femur bone. The shaft has a threaded end with a non-circular cross-sectional shape. The shaft can be fastened to the nail by screwing the threads into engagement with the nail. Due to the non-circularity of the cross-sectional shape, the shaft repetitively forces the nail to widen and contract whereby the threads are allowed to engage any part of the nail, also a part which is not provided with corresponding threads and even without forming shavings. The invention further relates to an extraction device for establishing a pulsating unidirectional force. The extraction device has a displaceable hammer which, during extraction of a nail, hammers against an end stopper and thus generates a pulsating withdrawal force for extracting the nail.

Description

AN ASSEMBLY FOR EXTRACTION OF BROKEN MEDULLARY NAILS

Field of the invention

The present invention relates to an extraction device for extraction of hollow medullary nails. More specifically the invention relates to a device capable of removing broken hollow medullary nails e.g. from a femur.

Background of the invention

Medullary nails are commonly used by surgeons during the surgery of broken bones such as broken femurs. Typically, the medullary nails have the form of a hollow bar or rod and often, they are made of surgical steel or made of similar stainless alloys. They normally form an open top-end and a closed and slightly connϊcally shaped tip adapted for insertion into the bone, e.g. into a femur. Typically, a slit extends from the open top-end in a longitudinal direction toward the tip of the nail. The slit is provided in order to make the nail more flexible or bendable. The nails are typically provided with threads or holes in the top-end of the nail. The threads or holes enable various insertion or extraction means to be attached to the nail for inserting or pulling the nail out of the bone. After a period of 1-5 month after a nail has been inserted into the bone, the bone and tissue will grove into a fixed engagement with the nail.

Sometimes, it happens that a nail at a later stage, e.g. several years later, has to be removed. Reasons for removing a nail could be that the patient senses phantom pains or that the nail has broken. Moreover, a patient's immunity reaction or a traumatic or preventive situation may motivate the removal of a nail. Finally, it may be noted that a nail can be detected e.g. in metal sensor devices, e.g. in airports, and for practical reasons, some patients prefer to have the naii removed after healing of the femur.

In general, devices for removing medullary nails exist. The existing withdrawal devices are adapted to be attached to the top part of the nail by use of the threads provided in this part of the nail. Other devices are adapted to engage holes or similar attachment means provided in the nail. However, it may happen that the nail is broken at an earlier stage due to stress or excessive wear caused by abnormal use during the healing period of the femur or it may even happen that the nail brakes during its removal.

If a nail breaks, the remaining pieces of broken femur nail may cause severe problems and infections in the femur and should, if at all possible, be removed. It may, however, be difficult, if not impossible, to use traditional extraction devices which are adapted to engage a specific part of the nail, since such devises can only be used for removing the part of the nail which is adapted for the engagement. As an example, most nails are provided with removal means in the top part of the nail. If the nail breaks, it can be very difficult to remove the part of the nail which is below the fracture of the nail without severe complications for the patient.

Description of the invention

It is an object of the present invention to provide means for the removal of a medullary nail, even broken nails and to overcome the above mentioned disadvantages of existing systems for the removal of nails.

Accordingly, the present invention, in a first aspect, relates to a threaded fixation shaft for catching a broken medullary nail, said shaft having an elongate body with a centre axis, the elongate body being provided with a first end portion and a second end portion, the end portions being defined in respective opposite ends of the fixation shaft, the first end portion having a non-circular cross sectional shape and being provided with fixating means adapted to establish a connection between the elongate element and the medullary nail, the fixating means comprising threads for establishing an engagement between the shaft and an arbitrary or undetermined part of the medullary nail or more specifically with any part of an inner surface of the medullary nail.

In order to remove a medullary nail from a bone, the first end portion of the fixation shah- is inserted into the top-end of the medullary nail. By slightly pressing the shaft towards the nail and simultaneously rotating the shaft, the threads will work their way down through the cavity of the hollow nail until firm engagement between the nail and the shaft is established. The shaft may either be cutting or just pressing its threads into the engagement with the nail. As the nails are typically provided with a slit extending from the open top-end in a longitudinal direction toward the tip of the nail, the rotation of the first end portion of the shaft, i.e. the end portion having a non-circular cross sectional shape, causes the nail to expand in a radial direction and contract in a radial direction, respectively. Therefore, the cutting threads do not cut a continuous path down through the nail and accordingly, the cutting threads leave less or substantially no metal chips inside the bone. In fact, the engagement between the cutting threads and the nail may be purely based on the impression force between the threads and the surface of the nail. This, however, requires that the fixation shaft is dimensioned in relation to the nail so that a large surface pressure between the threads and the surface of the nail develops during the insertion of the fixation shaft into the nail. Preferably, the pressure should be of a size causing the material of the nail to reach its yield point.

The cutting threads may be provided in any size that matches a nail. Depending upon the size of the nail, threads in the range of M3x0,5-M16x2 may be used - see table 1 under detailed description of the invention. Due to the non-circularity of the cross sectional shape of the first end portion, the threads are provided with a number of elevations. As an example, the first end portion may be provided with a quadrangular cross sectional shape and accordingly, the threads are provided with four elevated or high points per round. In this .context, cutting means any kind of depression of the threads into the surface of the nail, e.g. by a chip removing cutting process or by material yielding resulting from the pressure between the threads and the nail or by any other process. The threads may have the form of regular helical threads or they may have the form of concentric rings.

Preferably, the first end portion of the shaft is provided with a smaller cross sectional size than an intermediate portion of the elongate body. Between the first end portion and the intermediate portion, the elongate body may preferably be conically shaped so as to smoothen out the transition between the first end portion and the intermediate portion of the elongate body. Thereby, the intermediate shaft may be provided with a fairly large radial size compared with the first end of the shaft. Accordingly, the shaft may be provided with relatively good strength and at the same time, the first end may be small enough to catch nails even of the type having a small internal radial size. The first end portion may further be provided with a conical and/or smoothly rounded tip so that the shaft easily catches the open top-end of the medullary nail.

The conical part or parts of the shaft will, during the insertion of the shaft, push fragments of bone, marrow and similar substances radially outwardly towards the inner surface of the hollow bone. For that reason, the conical shape of the transition between the two portions of the shaft and the conical shape of the tip of the shaft eases the insertion of the shaft into the bone and into the nail. Moreover, the conical shape reduces the notch sensitivity of the shaft and thus strengthens the shaft.

Since the bone may have grown into a fixed engagement with the nail, it may sometimes be very difficult to pull the nail out of the bone. Accordingly, the second end portion may be provided with attachment means for attaching withdrawal means to the fixation shaft and for transferring a force from the shaft to the nail a long the centre axis of the shaft for withdrawing the nail from the bone. The second end portion may as an example be provided with a handle for supporting a firm grip in the shaft and thus for supporting the surgeon in pulling the shaft relatively strongly for removing the nail from the bone. As another example, the second end portion may be provided with attachment means for attaching an extraction device. Preferably, the shaft is made from a metallic material such as surgical steel or from similar stainless steel alloys, e.g. werkstoff 4043, aluminium, titanium or any alloy thereof. In order further to enhance the strength, durability and lifetime of the shaft, at least the part of the threaded fixation shaft provided with threads may be hardened to at least 52 RWC. After the hardening, the cutting threads may be sharpened so as to make the engagement between the threads and the nail easier and stronger.

During use of the shaft, there is a risk that small cracks in the metal develop into severe fatigue causing breakage of the shaft (metal fatigue). It is therefore an aspect of the present invention to provide a shaft with a safe and permanent identification tag, e.g. by means of an identification insignia embossed into the shaft. By means of the permanent identification tag, the surgeon may keep a record of each shaft. The shafts may either be disposed of, or inspected after a certain time period or after a certain number of uses.

The shaft may comprises vibrating means, e.g. an ultrasound transducer, for vibrating the nail to which it is attached. The vibrations can support the removal of a nail in particular in the event that the bone and tissue have grown into a fixed engagement with the nail.

According to a second aspect, the present invention relates to an extraction device for establishing a pulsating unidirectional force comprising:

an elongate tube with a proximal end adapted to connect the extraction device to a fixation shaft and a distal end, the proximal end and the distal end being defined at opposite ends of the elongate tube, the elongate tube being closed in the proximal end by a proximal stopper and the elongate tube being closed in the distal end by a distal stopper,

- a displaceable striking member adapted to slide along an axis between the proximal stopper and the distal stopper inside the elongate tube, the axis being substantially parallel to a centre axis of a medullary nail when the extraction device is connected to the nail by means of the fixation shaft,

- flexible or bouncy means being provided between the proximal stopper and the displaceable striking member, and

- means for displacing the striking member along the axis so as to make the displaceable striking member bounce against the flexible means and the distal stopper.

Preferably, the means for displacing the striking member is power driven, e.g. driven by compressed gas, electricity or by any other means of energy. Therefore, the device may further be provided with control means adapted to control the speed of the displacing of the striking member. The control means may further be adapted to control the impact of the displaceable striking member when it strikes the flexible means and the distal stopper respectively.

The flexible means may either be a spring provided between the displaceable striking member and the proximal stopper, a soft rubber layer, a layer of foam rubber or a layer of a similar material. The flexible means could also be an amount of gas being confined between the displaceable striking member and the proximal stopper. As the gas is compressed, the air pressure raises and the displaceable striking member softly changes its direction without causing any hard impact with the proximal stopper

According to a third aspect, the present invention relates to a n assembly for extraction of medullary nails, said assembly comprising

- a threaded fixation shaft for catching the medullary nail, said shaft having an elongate body with a centre axis, the elongate body being provided with a first end portion and a second end portion, the end portions being defined in respective opposite ends of the fixation shaft, the first end portion being provided with fixating means adapted to establish a connection between the elongate element and any part of an inner surface of the medullary nail, the fixating means comprising cutting threads with a non-circular cross sectional shape, and the second end portion being provided with means for establishing a connection between the elongate element and an extraction member, and

- an extraction device for establishing a pulsating unidirectional force comprising:

- an elongate tube with a proximal end adapted to connect the extraction device to the fixation shaft and a distal end, the proximal end and the distal end being defined at opposite ends of the elongate tube, the elongate tube being closed in the proximal end by a proximal stopper and the elongate tube being closed in the distal end by a distal stopper,

- a displaceable striking member adapted to slide along an axis between the proximal stopper and the distal stopper inside the elongate tube, the axis being substantially parallel to a centre axis of the medullary nail when the extraction device is connected to the nail by means of the fixation shaft,

- flexible means being provided between the proximal stopper and the displaceable striking member, and means for displacing the striking member along the axis so as to make the displaceable striking member bounce against the flexible means and the distal stopper.

Both the threaded fixation shaft and the extraction device may have some or all of the aforementioned features.

Since it may cause unnecessary troubles for the surgeon to firstly attach the shaft to the nail and then secondly to attach an extraction device to the shaft, the threaded fixation shaft may be an integrated part of the proximal stopper of the extraction device.

According to a fourth aspect, the present invention relates to a support pin having a proximal end and a distal end, the proximal end being provided with an annular support member adapted to support the medullary nail during the attachment of an extraction device to the nail.

The inner diameter of the ring should be large enough to fit around the end of a broken nail and, when the extraction device is forced into the nail, the nail will expand until the ring on the supporting pin prevent further expansion.

The use of supporting pin provides extra safety when the tip of a broken nail is damaged severely so that the extraction device is prevented from getting a good grip in the nail. Supporting pins are preferably made with inner diameters of different sizes, e.g. varying with one mm in the range of 6 mm to 20 mm.

The tip could be damaged e.g. when the fracture of the nail is angled more than 30° - 40° or when the nail has small cracks. Under these circumstances, the nail may break if a supporting pin, during the fastening of the extraction device, does not support it.

When the extraction device is being used for removing a nail, the supporting pin and the ring is removed simultaneously with the nail.

Detailed description of the invention

An embodiment of the invention will now be described in details with reference to the drawing in which:

Fig. 1 is a picture showing an assembly of extraction devices and nails,

Fig. 2 shows a detailed view of a fixation shaft, Fig. 3 shows an assembly comprising a shaft and a handle,

Fig. 4 shows an assembly comprising a shaft, an end stopper and a displaceable striking member,

Fig. 5 shows a detailed drawing of an alternative fixation shaft,

Fig. 6 shows a detailed drawing of another alternative fixation shaft,

Fig. 7 shows an extraction device,

Fig. 8 shows a diagram of a pneumatic setup for the extraction device,

Fig 9 shows an embodiment of the extraction device wherein the striking member is driven electrically, and

Fig. 10 shows details of the attachment of a fixation shaft to an extraction device.

Fig. 1 shows a picture of two fixation shafts 1,2, a femur nail 3 and a broken femur nail 4. In Fig. 1, the nails and fixation shafts are of different size. Fig. 1 further shows a displaceable member 5 for providing an extraction force along the longitudinal axis of the nail. Fig. 1 further shows four different support pins 6 for supporting the upper part of the nail during the removal of the nail from a bone.

The inner diameter of the ring should be large enough to fit around the end of a broken nail. When the extraction device is forced into the nail, the nail expands until the ring on the supporting pin prevents further expansion. Supporting pins are preferably made with inner diameters of different sizes, e.g. varying with one mm in the range of 6 mm to 20 mm.

The use of a supporting pin improves the safety and is of particular importance e.g. in cases wherein the tip of a broken nail is damaged in a way that will not allow the extraction device to get a sufficient grip in the nail.

For an easier insertion of the fixation shaft in the nail, the fixation shaft is provided with a handle 10. While a nail is arranged in a bone, the bone tissue may grow into a fixed engagement with the lower portion of the nail, and therefore the extraction of the nail can be difficult. For easing the removal of the nail, the displaceable member 5 may be movably arranged on the fixation shaft so that the member can be shifted back and forth along the fixation shaft to beat against the handle 10, and to thus beat out the nail.

Referring to Fig. 2, the fixation shaft according to the present invention comprises a first end portion 11 provided with threads. The shaft is further provided with attachment means 12 for attaching a handle for easing the withdrawal of the nail or for attaching an end stopper against which the displaceable member may beat. In the shown embodiment, the attachment means is provided in the form of a slightly curved groove in the second end portion of the shaft. The radius of curvature of the groove may preferably be selected in the range of R4-8 such as in the range of R5-7 such as in the size of R6. The use of a curved shaped groove enables a handle or end stopper, e.g. in the form of a circular rod or bar, to be fastened to the shaft without creating a notch sensitive area of the shaft. Fig. 3 shows an assembly comprising a shaft and a handle and Fig. 4 shows an assembly comprising a shaft, an end stopper 14 attached to the curved groove by means of a locking pin 13. Fig. 4 further shows a displaceable striking member 5.

Again referring to Fig. 2, the shaft has a conically shaped tip. The conical shape of the tip eases the insertion of the shaft into the bone and into the top part of the nail.

Preferably, the length of the first end portion is in the range of 25-75 mm, such as in the range of 35-65 mm, such as in the size of 50 mm. The shaft may be made with a total length 15 of the shaft which matches the individual sizes of nails on the market. However, a total length in the range of 400-800 mm. such as in the size of 500 mm. is a suitable length which supports withdrawal of most of the known nails. The second end portion 16 of the nail may be made in the length between 10-30 mm. such as in the length of 15 mm. The shaft does not necessarily have to have a circular cross sectional shape. However, a circular cross sectional shape of all other parts than the first end portion would be preferred, e.g. with a diameter in the size of 5-15 mm. such as 7-13 mm. such as in the size of 10 mm. The cross sectional shape of the first end portion 11 should be non-circular and preferably quadrangular or at least substantially quadrangular. By rotation of the shaft, with the first end portion inserted into the nail, the nail will be forced to flex radially in and out while the shaft is forced into the nail by slightly pushing the shaft in the longitudinal direction towards the nail.

The threads 18 should preferably be metric. As an example, M12xl,25, M10xl,25 or M8xl or even M4x0,7 may be adapted for withdrawal of nails of various sizes - see table 1, below for suggested thread types.

Table 1

# No specific type recommended

Figs 5 and 6 shows two alternative embodiments of the shaft. The shaft shown in Fig. 5 is provided with a conical part 20. The conical part may have an angle 21 to the longitudinal axis of the shaft in the range of 10-20 degrees such as in the size of 15 degrees. The diameter 22 of the intermediate portion of the shaft may preferably be 2-6 mm. such as 3- 5 mm- such as 4 or 5 mm. larger than the diameter 23 of the second end portion of the shaft. The shaft shown in Fig. 6 is provided with another conical part 24. The conical part may have an angle 25 to the longitudinal axis of the shaft in the range of 10-20 degrees, such as in the size of 15 degrees.

The shaft may be machined from a metal bar which - at least in the first end - has been hardened to a sufficient mechanical hardness, e.g. to a hardness above 52 RWC. Preferably, the shaft is made without any notches in order not to weaken the shaft unnecessarily.

The shaft may preferably be made from various metallic or non metallic materials such as stainless steel 18/10, titanium, aluminium - e.g. with hardened steel threads or threads made from a hard ceramic material or the shaft may be made from a carbon fibre reinforced resin such as epoxy or polyurethane - again with threads made from a hard steel or ceramics material.

Fig. 7 shows an extraction device for providing an axial force for beating out the nail. The extraction device comprises an elongate tube 27 closed in a first proximal end 28 with a proximal stopper 29 adapted for connection of the extraction device to a fixation shaft 30. At the opposite, distal end 31, the tube is closed by a distal stopper 32. A displaceable striking member 33 is adapted to slide along an axis between the proximal stopper and the distal stopper inside the elongate tube 27. In order to release air from the area between the proximal stopper and the striking member, the pipe 36 is arranged in the centre of the elongate tube. The striking member is provided with a centre hole so that the member may slide along the pipe 36. The device further comprises flexible means in the form of a spring 34 adapted to dampen the impact of the displaceable striking member against the proximal stopper.

The device further comprises an inlet connector 35 attached to the distal stopper for connecting pressurised air to the device. During operation, the pressurised air is periodically, blown into the area between the distal stopper and the displaceable striking member and released from the area between the distal stopper and the displaceable striking member, respectively. Accordingly, when air is blown into the area, the displaceable striking member is forced to move towards the proximal stopper. As the striking member has compressed the spring, the air is released and due to the reaction from the compression of the spring, the striking member returns towards the distal stopper. As the striking member moves back and forth, the air confined between the proximal stopper and the striking member is allowed to freely pass in and out through the internal pipe 36 extending from the proximal stopper to the distal stopper. The result is a pulsating beating in the direction towards the distal stopper. The beating is transferred to the nail, whereby the nail is "hammered" out of the bone.

The proximal stopper and the distal stopper may preferably be attached to the intermediate part of the elongate tube by a threaded connection. Thereby, it will be easy to disassemble the device completely, e.g. for the purpose of cleaning or sterilising it. Preferably, all parts are made from surgical steel or a similar stainless steel alloy, aluminium, carbon or glass fibre reinforced epoxy or polyester or from a similar material which preferably is durable towards autoclaving.

The displaceable striking member 33 may preferably be provided with piston rings 37 for sealing the gab between the inner surface of the elongate tube and the displaceable member so that air does not pass from the area between the displaceable member and the distal stopper to the area between the displaceable member and the proximal stopper.

Instead of using a spring to bounce the impact against the proximal stopper, the speed of the striking member may be reduced by not releasing all the air from the area between the striking member and the proximal stopper. The compressibility of the air will thus alleviate the impact between the striking member and the proximal stopper.

In order to control the speed of the striking member, the device may be provided with a 3/2 way valve, e.g. a frequency controlled valve, for repetitively letting air into the tube and releasing the air from the tube, respectively. In order to reduce the time it takes for the air to blow into and out of the area between the distal stopper and the displaceable striking member, the 3/2 way valve may advantageously be placed directly adjacent the inlet to this area.

Fig. 8 shows a pneumatic diagram of the pneumatic setup for controlling the movement of the striking member. The device is started by pushing the start button 40. Air will then be guided to the valve 42. Since the valve 42 is in its open position, air will be guided to the area between the striking member and the distal stopper of the device 46. Simultaneously, air will be guided to the valve 44 and as the air pressure after a certain time period has pressed down the valve 44, the valve 42 will be released and the provision of air to the area between the striking member and the distal stopper will be interrupted. After a while, the valve 44 returns to the top position and the stroke repeats itself. The valve 47 allows air to be blown into the area 48 between the distal stopper and the displaceable striking member and released from the area between the distal stopper and the displaceable striking member, respectively. The air which is blown out of this area, leaves the valve through the valve port 49. The valve is inserted in the flowline 50 close to the device 46. This way of arranging the valve enhances the efficiency of the device since the air which is blown out of the area, only has to flow through a confined flowline for a very short distance and thus the resistance against the displaceable striking member as it strikes from the proximal stopper towards the distal stopper is reduced.

Fig. 9 shows an alternative embodiment of the extraction device, wherein the striking member is electrically driven. The device comprises a controller for controlling the frequency of the device. The device is coupled in a "Switch Mode" coupling which enables the device to be connected to any electrical power source AC or DC irrespective of the supplied voltage and/or frequency.

The device comprises a converter for provision of a correct AC/DC current for driving the device. The controller turns the current with a selectable frequency.

An on/off switch provides the beating upon a command from the operator.

In order to improve the safety, the electrically driven device may be securely grounded and made with magnetic anti-inductance.

Fig. 10 shows details of the attachment of a fixation shaft to the extraction device. The curved groove 50 of the fixation shaft is blocked by a locking pin 51 screwed into engagement with the fixation pin through the extraction device 52. The pin may preferably be provided with a handle 53 easing fast attachment and detachment of a fixation shaft.

Claims

1. A threaded fixation shaft for catching a broken medullary nail, said shaft having an elongate body with a centre axis, the elongate body being provided with a first end portion and a second end portion, the end portions being defined in respective opposite ends of the fixation shaft, the first end portion being provided with fixating means adapted to establish a connection between the elongate element and the medullary nail, the fixating means comprising threads with a non-circular cross sectional shape for engagement with any part of an inner surface of the medullary nail.

2. A shaft according to claim 1, wherein the first end portion has a smaller cross sectional area than an intermediate portion of the elongate body and wherein, in a first transitional zone between the first end portion and the intermediate portion, the elongate body is conically shaped in order to smooth out the transition between the first end portion and the intermediate portion of the elongate body.

3. A shaft according to claim 1 or 2, wherein the first end portion is provided with a smoothly rounded conical tip.

4. A shaft according to any of claims 1-3, wherein the second end portion is provided with attachment means for attaching withdrawal means to the fixation shaft and for transferring a force in the direction of the centre axis of the shaft to the nail for withdrawal of the nail from the bone.

5. A shaft according to any of claims 1-3, wherein the second end portion is provided with gripping means for transferring a force in the direction of the centre axis of the shaft to the nail for withdrawal of the nail from the bone.

6. A shaft according to claim 1, made from a stainless steel alloy steel.

7. A shaft according to claim 1 or 2 wherein at least the part of the threaded fixation shaft provided with threads is hardened to at least 52 RWC.

8. A fixation shaft according to any of the preceding claims, wherein the threads have been sharpened after the hardening.

9. A fixation shaft according to any of the preceding claims, wherein an identification insignia has been embossed into the shaft.

10. An extraction device for establishing a pulsating unidirectional force comprising:

an elongate tube with a proximal end adapted for connection of the extraction device to a fixation shaft and a distal end, the proximal end and the distal end 5 being defined at opposite ends of the elongate tube, the elongate tube being closed in the proximal end by a proximal stopper and the elongate tube being closed in the distal end by a distal stopper, a displaceable striking member adapted to slide along an axis between the proximal stopper and the distal stopper inside the elongate tube, 10 - flexible means being provided between the proximal stopper and the displaceable striking member, and means for displacing the striking member along the axis.

11. A device according to claim 10, wherein the means for displacing the striking member 15 is power driven.

12. A device according to claim 10 or 11, wherein the power driven means is driven by compressed gas.

20 13. A device according to claim 10 or 11, wherein the power driven means is driven by electricity.

14. A device according to claim 13, being driven by an AC or DC motor.

25 15. A device according to claim 10-14, further comprising control means adapted to control the speed of the displacing of the striking member.

16. A device according to claim 15, wherein the control means is further adapted to control the impact of the displaceable striking member when it strikes the flexible means and the

30 distal stopper respectively.

17. A device according to claim 15 or 16, wherein the control means comprises a pneumatic setup comprising at least two valves interconnected so as to periodically provide air and release air from an area confined between the displaceable striking

35 member and the distal stopper, respectively.

18. A device according to claim 10-17, wherein the flexible means is a spring provided between the displaceable striking member and the proximal stopper.

19. A device according to any of claims 10-18, wherein the axis is substantially parallel to a centre axis of a medullary nail when the extraction device is connected to the nail by means of the fixation shaft.

20. An assembly for extraction of medullary nails, said assembly comprising

- a threaded fixation shaft for catching the medullary nail, said shaft having an elongate body with a centre axis, the elongate body being provided with a first end portion and a second end portion, the end portions being defined in respective opposite ends of the fixation shaft, the first end portion being provided with fixating means adapted to establish a connection between the elongate element and any part of an inner surface of the medullary nail, the fixating means comprising threads with a non-circular cross sectional shape, and the second end portion being provided with means for establishing a connection between the elongate element and an extraction member, and

- flexible means being provided between the proximal stopper and the displaceable striking member, and - means for displacing the striking member along the axis so as to make the displaceable striking member bounce against the flexible means and the distal stopper.

21. An assembly according to claim 20, wherein the threaded fixation shaft is an integrated part of the proximal stopper of the extraction device.

22. An assembly according to claim 20 or 21, wherein the means for displacing the striking member is power driven.

23. An assembly according to claim 22, wherein the power driven means is driven by compressed gas.

24. An assembly according to claim 22 or 23, wherein the power driven means is driven by electricity.

25. An assembly according to claim 24, being driven by an AC or DC motor.

26. An assembly according to any of claims 20-25, further comprising control means adapted to control the speed of the displacing of the striking member.

27. An assembly according to claim 26, wherein the control means is further adapted to control the impact of the displaceable striking member when it strikes the flexible means and the distal stopper respectively.

28. An assembly according to any of claims 20-27, wherein the flexible means is a spring provided between the displaceable striking member and the proximal stopper.

29. A support pin with having a proximal end and a distal end, the proximal end being provided with an annular support member adapted to support the medullary nail during the attachment of an extraction device to the nail.