WO2006024840A1

WO2006024840A1 - Surgical instrument and method for improving orthopaedic implants

Info

Publication number: WO2006024840A1
Application number: PCT/GB2005/003354
Authority: WO
Inventors: Tim Drew
Original assignee: University Of Dundee
Priority date: 2004-09-01
Filing date: 2005-08-31
Publication date: 2006-03-09
Also published as: GB0419338D0

Abstract

An apparatus (31) and method for improving orthopaedic implants which is suitable for use in total hip replacement (THR) operations. A vibrator (38) is used to apply controlled vibrations to a cement-receiving cavity in a bone to increase the penetration of the cement into the bone. A coupler (41) may be attached to an orthopaedic implant to impart the vibrations. The apparatus improves the penetration of the cement into the lattice like structure of cancellous bone tissue inside long bones such as the femur.

Description

Surgical Instrument and Method for Improving Orthopaedic Implants

The present invention relates to an apparatus and method for improving orthopaedic implants and in particular but not exclusively to total hip replacement (THR) operations.

In orthopaedic surgery, it is often necessary to implant objects in order to replace or reinforce parts of the human skeleton for example, in a total hip replacement operation. In many orthopaedic surgery procedures, cement is used to fix the implant in position and a number of cement products including polymethylmethacrylate (PMMA) are suitable for this purpose.

Figures 1 to 10 illustrate the procedure involved in total hip replacement. Figure 1 shows a hip 1 and a femur 3 which has an arthritic femoral head 5 at the top of the femoral shaft 7. In normal hips, the femoral head 5 fits into the acetabulum 13 and operates as a ball and socket joint. Where an arthritic femoral head 5 has degraded to such an extent that the patient is in considerable pain and has lost mobility, the hip joint can be replaced with a prosthetic joint.

Figure 2 shows the cut femoral shaft 9 with the arthritic femoral head 5 removed. The next stage in the process is to ream the acetabulum 13 using reamer 11 (Figure 3) . This allows the acetabular component 15 to be placed in the acetabulum 13 of the hip 1 (Figure 4) . A femoral rasp 17 is then used to clean out the femoral canal inside the femur 7 (Figure 5) to allow a femoral implant or stem 21 to be introduced into the femur (Figures 6 and 7) .

Figures 8 and 9 show the femoral implant head 23 positioned at the top of the femoral implant or stem 21 and the location of this ball joint 23 inside the acetabular component 15. Figure 10 shows the position of cement 25 around the acetabular component 15 and around the femoral stem 21.

A major problem with cemented total hip replacement is aseptic loosening of the femoral stem. It is estimated that this problem counts for over 70% of revisions associated with total hip replacement.

Aseptic loosening may occur: 1) at the interface between the femoral stem and the cement; and 2) at the interface between the cement and the bone.

The purpose of the cement is to completely fill the space between the skeleton and the surface of the implant. In this way, the cement acts as a filler that forms a mechanical bond with the bone but does not form a chemically adhesive bond with the bone. In order to maximise the strength of the mechanical bond, an optimum interlock between the cement and the bone must be made. The bone tissue in the side of the femur is cancellous bone which is a highly porous or cellular form of bone that is found in the interior of a long bone.

Current surgical methods and apparatus that are used to insert the femoral stem into the bone cavity rely on well known surgical instruments and the manual skill of the surgeon. Typically, the femoral stem is introduced into the canal, properly aligned and then pushed into the bone cavity which has been pre-packed with cement. Sometimes a mallet is used to aid this process. The overall effect is to force some of the cement into the voids and cavities of the inner surface of the bone on an uncontrolled basis. As a result, not all of the lattice is filled and a less than optimum bond between the cement and bone is created.

US Patent No. 5015256 discloses a method for fixing a joint prosthesis that uses a cementless procedure. In cementless procedures, granules are added to the cavity within the bone such that the bone will grow into the granuals to hold the prosthesis in place. This document shows the use of vibration means to fluidise the solid granuals to prevent the granuals forming a solid plug in the cavity and to allow full insertion of the prosthesis to be completed. It will be appreciated by a person of ordinary skill in the art that cementless prosthesis fixing is a separate and distinct field. US 2005/0010231 discloses a method and apparatus for strengthening the biomechanical properties of implants. A mechanical stirrer is used to agitate cement within a cavity to remove bubbles before a prosthesis is introduced in order to remove air bubbles from the cement.

It is an object of the present invention to overcome or lessen the incidence of weak mechanical bonding between bone, cement and prosthesis that are inherent in present methods thus improving the durability of the surgical implant and lengthening the period between adjustments and further replacements.

In accordance with the first aspect of the present invention, there is provided a surgical instrument for improving cementation in orthopaedic implants, the apparatus comprising vibration means adapted to apply controlled vibrations to the orthopaedic implant during the positioning of the orthopaedic implant in a cement receiving cavity in a bone to controllably increase the penetration of the cement into the bone.

Preferably, the coupling means is adapted to be coupled to an orthopaedic implant.

Preferably, the vibration means provides vibrations at a variable frequency.

Preferably, the vibrations have a frequency of greater than 50 Hertz. Alternatively, the vibrations have a frequency of less than 50 Hertz.

Preferably, the vibrations have a frequency of 19 Hertz.

Preferably, the amplitude of the vibrations is of variable amplitude.

Preferably, the amplitude of the vibrations is less than 10mm.

Most preferably, amplitude of the vibrations is 4mm.

Preferably, the vibrations caused by the vibration means are longitudinal vibrations.

Optionally, the vibrations are transverse vibrations.

Preferably, the coupling means is shaped to conform to the orthopaedic implant.

Preferably, the coupling means is fastenable to the orthopaedic implant.

Optionally, the coupling means is a sleeve adapted to fit around the orthopaedic implant.

Optionally, the coupling means is a plate that may be pressed onto the orthopaedic implant.

Optionally, the coupling means is a pin or bar.

Optionally, the coupling means is a socket. In accordance with the second aspect of the invention, there is provided a method for improving cementation in orthopaedic implants, the method comprising the steps of: introducing cement into a cavity of a skeleton; applying a controlled vibration to an orthopaedic implant as it is being introduced into the cement to controllably increase the penetration of the cement into the cavity.

Preferably, the orthopaedic implant is introduced whilst the controlled vibration is applied.

Preferably, the vibrations are applied using a coupling means.

Preferably, the coupling means is coupled to the orthopaedic implant.

Preferably, the vibrations are provided at a variable frequency.

Preferably, the vibrations have a frequency of greater than 50 Hertz.

Alternatively, the vibrations have a frequency of less than 50 Hertz.

More preferably, the vibrations have a frequency of less than 50 Hertz.

Preferably, the vibrations have a frequency of 19 Hertz.

Preferably, the amplitude of the vibrations is of variable amplitude. Preferably, the amplitude of the vibrations is less than 10mm.

Most preferably, amplitude of the vibrations is 4mm.

Pref erably, the vibrations are longi tudinal vibrations .

Optionally , the vibrations are transverse vibrations .

Preferably, the coupling means is shaped to conform to the orthopaedic implant.

The present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 shows an arthritic hip;

Figure 2 shows the removal an arthritic femoral head;

Figure 3 shows the acetabulum of a hip being reamed;

Figure 4 shows the acetabular component of an artificial hip being fitted;

Figure 5 shows a femur being reamed;

Figure 6 shows a femoral stem being inserted into a femur;

Figure 7 shows a femoral stem in position with a femur;

Figure 8 shows a complete artificial hip; Figure 9 shows an artificial femoral head;

Figure 10 shows the position of cementation in an artificial hip joint;

Figure 11 shows a first embodiment of the present invention;

Figure 12 shows a second embodiment of the present invention;

Figure 13 shows a third embodiment of the present invention;

Figure 14 shows a fourth embodiment of the present invention;

Figure 15 shows a fifth embodiment of the present invention;

Figure 16 shows a sixth embodiment of the present invention;

Figure 17 shows a seventh embodiment of the present invention attached to a femoral stem; and

Figure 18 shows the improvement in the spread of a cement under the action of vibration.

Figure 11 shows a first embodiment of the present invention. The apparatus 31 comprises a handle 33 and a body 37 and coupling means 39. The body 37 contains a motor which provides oscillatory motion to a head 38 such that the head vibrates.

Figure 12 shows a second embodiment of the invention in which a coupling means 39 is provided by a collar 41 with a fastening means 42 and a hinge such that the collar 41 may be opened and part of the orthopaedic implant enclosed within the collar 41 in order to cause vibrations to the collar to the implant. The coupling means 41 will, in use, be attached to an orthopaedic implant or alternatively to an area of the patient's body, in order to provide vibrations to a cavity within which the cement is enclosed. The frequency and amplitude of the vibrations is selectably controlled by the operator. The vibrations may be caused by simple harmonic motions within the oscillatory motion in the motor or by a more complex motion.

Figure 13 shows a coupling means 39 provided with a ball 43 designed to cooperate with the socket joint of the acetabular component 15 shown in Figure 4.

Figure 14 shows an embodiment of the present invention 60 which has a coupling means 39 comprising a flat plate 62 through which vibrations can be transmitted to an orthopaedic implant or part of the human body.

Figure 15 shows a further embodiment of the present invention 70 in which the coupling means is provided by a socket 72. This socket is adapted to conform to the shape of the femoral head 23 as shown in Figure 10. Figure 16 shows a further embodiment of the present invention 80 in which a pivot point 82 is provided such that the coupling means can be conveniently angled in use.

Figure 17 shows the embodiment of the present invention shown in Figure 11 attached to a femoral implant 21.

In use, the embodiments of the present invention allow vibrations to improve the manner in which the cement settles within the cavity of the skeleton and therefore improve the manner in which the cement enters the lattice like structure of the cancellous bone in the femur, for example.

In the case of total hip replacement, the femoral stem is held by a tool, preferably a hand held tool in accordance with the present invention, which is capable of imparting controlled vibration through a range of frequencies to the femoral stem. The femoral stem is introduced into and immersed in the cement already filling the bone cavity whilst it vibrates. The normal displacement which takes place and the excitation of the cement caused by the vibrating femoral stem then forces the cement into the bone lattice in an even distribution pattern ensuring that all or most of the lattice is filled with an appropriate measure of cement reducing air pockets and areas of inconsistency in the bond.

In addition, the excitation process improves adhesion between the cement and the femoral stem. Moreover, the amount of pressure that need be applied is reduced considerably or obviated entirely lessening the risk of any incidental strain and/or damage to the bone during the procedure.

Figures 18a and 18b illustrate the manner in which the cement may enter the lattice like structure of the cancellous through a femur.

A cavity containing a plurality of dimples was designed to model the behaviour of the cavity in a femur that is created during a total hip replacement operation. The cavity was fitted with cement and a prosthesis was introduced into the cavity. During introduction of the prosthesis, longitudinal vibrations having an amplitude of 4-5 min and a frequency of ≤ 50 Hz were imported to the prosthesis using a vibrating module and drill (not shown) . In this example a frequency of 19Hz and amplitude of 4mm was used.

Figures 18a and 18b show the resultant distribution of cement within dimples as derived from the mould.

The figures are each divided into three Gruen zones 92, 94, and 96. These zones provide an indication of the positional importance of cement loosening or inadequate cementation to joint failure.

In this illustration each dimple is categorised depending upon the amount of cementation that has occurred. The fully shaded dimple 98 shows a formed cementation within the dimple, a half shaded dimple 100 is a semi-formed cementation; both can be considered to provide good mechanical interlock between the prosthesis and the mould or bone. The quarter shaded dimple 102 and the un-shaded dimple 104 illustrate unformed dimples or instances where there was a dip in the surface of the cement cast. Both cases are examples of bad mechanical interlock.

As is apparent from a comparison of figures 18A and 18B, the use of the present invention provides an increased number of dimples exhibiting improved mechanical interlock (Figure 18B) in comparison with a straightforward introduction of a prosthesis as is known in the state of the art (figure 18A) .

In figure 18B, the number of dimples demonstrating good interlock is much higher in zones A,B and C. Therefore, the present invention provides an apparatus and method that improves mechanical interlock between a prosthesis and the dimpled cavity by ensuring the cement penetrates further into the dimples .

The present invention provides increased penetration of cement into the bone structure and therefore improves the mechanical bond between the bone and the cement and the cement and the implant. Furthermore, instead of relying on pressure and displacement caused by forcing the femoral stem into the cement within the bone cavity to drive the cement into the lattice, the invention uses controlled vibrations to improve penetration.

It is envisaged that a vibration may be applied to the human skeleton such that, on initial introduction of the cement, vibration can be applied to settle out the cement and to allow it to penetrate into the bone prior to the introduction of the implant. Improvements and modifications may be incorporated herein without deviating from the scope of the invention.

Claims

1. A surgical instrument for improving cementation in orthopaedic implants, the apparatus comprising vibration means adapted to apply controlled vibrations to the orthopaedic implant during positioning of the orthopaedic implant in a cement receiving cavity in a bone to increase the penetration of the cement into the bone.

2. The surgical instrument as claimed in claim 1 wherein, the coupling means is adapted to be coupled to an orthopaedic implant.

3. The surgical instrument as claimed in any preceding claim wherein, the vibration means provides vibrations at a variable frequency.

4. The surgical instrument as claimed in any preceding claim wherein, the vibration means provides vibrations at a frequency of greater than 50 Hertz.

5. The surgical instrument as claimed in any of claims 1 to 3 wherein, the vibration means provides vibrations at a frequency of less than 50 Hertz.

6. The surgical instrument as claimed in claim 6 wherein, the vibration means provides vibrations at a frequency of 19 Hertz .

7. The surgical instrument as claimed in any preceding claim wherein, the vibration means provides vibrations of variable amplitude.

8. The surgical instrument as claimed in claim 7 wherein, the amplitude of the vibrations is less than 1Omm.

9. The surgical instrument as claimed in claim 7 or claim 8 wherein, the amplitude of the vibrations is 4mm.

10. The surgical instrument as claimed in any preceding claim wherein, the vibrations caused by the vibration means are longitudinal vibrations.

11. The surgical instrument as claimed in any preceding claim wherein, the vibrations caused by the vibration means are transverse vibrations.

12. The surgical instrument as claimed in claim 2 wherein) the coupling means is shaped to conform to the orthopaedic implant.

13. The surgical instrument as claimed any of claims 2, and 12 wherein, the coupling means is fastenable to the orthopaedic implant.

14. The surgical instrument as claimed any of claims 2, 12 or 13 wherein, the coupling means is a sleeve adapted to fit around the orthopaedic implant.

15. The surgical instrument as claimed any of claims 2, 12 or 13 wherein, the coupling means is a plate that may be pressed onto the orthopaedic implant.

16. The surgical instrument as claimed any of claims 2, 3, 13 or 14 wherein, the coupling means is a pin or bar.

17. The surgical instrument as claimed any of claims 2, 12 or 13 wherein, the coupling means is a socket.

18. A method for improving cementation in orthopaedic implants, the method comprising the steps of: introducing cement into a cavity of a skeleton; and applying a controlled vibration to an orthopaedic implant as it is being introduced into the cement to increase the penetration of the cement into the cavity.

19. A method as claimed in claim 19 wherein, the orthopaedic implant is introduced whilst the controlled vibration is applied.

20. A method as claimed in any one of claims 18 and 19 wherein, the vibrations are applied using a coupling means.

21. A method as claimed in claim 20 wherein, the coupling means is coupled to the orthopaedic implant .

22. A method as claimed in any one of claims 18 to 21 wherein, the vibrations are provided at a variable frequency.

23. A method as claimed in any one of claims 18 to 22 wherein, the vibrations have a frequency of greater than 50 Hertz.

24. A method as claimed in any one of claims 18 to 22 wherein, the vibrations have a frequency of less than 50 Hertz.

25. A method as claimed in claim 24 wherein, the vibrations have a frequency of 19 Hertz.

26. A method as claimed in any one of claims 18 to 25 wherein, the amplitude of the vibrations is variable.

27. A method as claimed in claim 26 wherein, the amplitude of the vibrations is less than 10mm.

28. A method as claimed in claim 27 wherein, the amplitude of the vibrations is 4mm.

29. A method as claimed in any one of claims 18 to 28 wherein, the vibrations are longitudinal vibrations.

30. A method as claimed in any one of claims 18 to 29 wherein, the vibrations are transverse vibrations.

31. A method as claimed in any one of claims 18 to 30 wherein, the coupling means is shaped to conform to the orthopaedic implant.