WO2007098549A1

WO2007098549A1 - Multi function hammer

Info

Publication number: WO2007098549A1
Application number: PCT/AU2007/000246
Authority: WO
Inventors: Ronald Sekel
Original assignee: Portland Orthopaedics Limited
Priority date: 2006-03-03
Filing date: 2007-03-03
Publication date: 2007-09-07
Also published as: AU2007219725A1

Abstract

A slap hammer assembly comprising ; a primary shaft having first and second ends, the first end having engagement rneaαs to enable engagement with an associated member. The second end has a handle grippable by an operator of the slap hammer, and intermediate the ends a weight capable of movement relative to the shaft away from and in the direction of the handle. The assembly further comprising an auxilHiry member at least partially mountable on the primary shaft and having a first end which engages an implant member responsive to an impact load applied by the weight and a second end which absorbs energy generated by the weight. The shaft is connected to the stem of a prosthesis while the implant member is located.

Description

MULTI FUNCTION HAMMER

BACKGROUND

The present invention relates to insertion tools and more particularly relates to tool assemblies which are capable of use in insertion and/or extraction of surgical prostheses. More particularly, the invention relates to a multi function slap hammer assembly which allows insertion of one component of a prosthesis while anchored to another component of the prosthesis. The invention further provides a slap hammer assembly which includes means to allow insertion of a prosthesis component and means to allow extraction of a prosthesis component.

PRIOR ART

Insertion of bone prostheses requires a variety of precision tools which perform an active function on a prosthesis for insertion. Specialised tools also exist for use in withdrawal of such prostheses in the case for instance, of a revision hip procedure when a stem must be removed.

Hip prostheses are commonly employed in hip replacements in orthopaedic surgical procedures. These procedures require specialized tools for preparation of the site, insertion of the prosthesis and in the case of a revision operation, extraction of the old prosthesis. Separate tools are usually required for reaming, insertion and extraction.

One generally known and widely used hip prosthesis typically comprises a distal shaft or stem having a gradual taper along its full length and terminating proximally in a neck which, mates with the head of the prosthesis via a Morse taper. The stem/shaft is inserted into the intra medullary cavity of the femur. In a total hip replacement, the surgical procedure will involve use of a reamer for reaming of the acetabulum and reaming of the proximal medullary cavity of the femur. The prosthesis stem is fitted after the surgeon has reamed out the medullary cavity to an extent conducive to the production of tight iuterfitting between bone and stem once the stem is placed into position. An insertion tool will be required to insert a distal part of the prosthesis in position into the medullary cavity. Another tool is required to attach a neck component once correct ante version of that component is established.

Slap hammers are commonly used as extraction tools for withdrawal of hip components and particularly the distal stem as part of revision hip procedure. This may be achieved typically by first engaging the proximal stem with a distal end of a slap hammer assembly and urging a moveabie slap weight proximally against a stationary handle or slap weight. This is intended to break the bond at the stem and bone interface and eventually withdraw the prosthesis by application of sufficient axial load. Engagement between the known slap hammer extraction tool and stem is preferably via co operating threads, hi the event of a screwed stem, an extraction tool may engage via a hex nut and withdraw by opposite rotation of the stem.

One of the problems encountered with stem insertion is unwanted axial movement of the stem distally when the neck component which receives the head component is hammered into engagement with the stem. Insertion of the neck into engagement with the stem component requires downward axial load on the neck, which is according to current methods transmitted direct to the $tem which must rely on its own engagement with bone to offer a reactive force against the significant axial neck insertion force. The danger is that the force required to insert the neck component can dislodge the stem and bone compromising the bond strength between stem and bone and consequently the correct elevation of the neck - i.e. the stem could end up at a lower elevation than it was originally set by the surgeon. This could cause misalignment between the femoral head component and acetabular cup necessitating further revision.

To date there has been no satisfactory means using a hammer for resisting this unwanted distal axial travel of a stem occasioned by impact load transfer during insertion of a neck component. Ia practice, the reaming followed by sizing with the prosthesis may be carried out a number of times i.e., reaming followed by inserting the prosthesis until there is a small distance, of travel of the shaft left near the neck of the femur to enable final positioning to ensure tight interfitting between prosthesis and bone and therefore a surgically satisfactory fit. In the final stages of this procedure, when the stem is hammered home, care must be taken by the surgeon to avoid loading the femur beyond the modulus of elasticity of the bone. The tolerable limits of bone elasticity are gauged mainly by the experience and fee), of the surgeon.

Stems which form a distal component of a hip may typically be cemented, press fit or screwed, arcuate or straight. Failures in hip prostheses have occurred due to loosening at the cement / bone interface and at the prosthesis /cement interface. Hammering the neck component risks weakening of the femoral bone or cement mantle shortly after insertion and this could potentially contribute to major or minor subsidence or rotation of the prosthesis leading to an unsatisfactory surgical result and at worst a stem failure later in life.

One feature of the existing prostheses is a series of formations moulded into the bone engaging surfaces of the distal shaft which are often bevilled in order to encourage and stimulate hone growth therein. This bone ingrowth which is a relatively slow post operative process assists in holding the prosthesis firmly in position and also provides a keying and locking effect thereby lessening the possibility of rotational failure and/or unwanted axial subsidence of the prosthesis. At the time an insertion is performed, the anchorage of a stem is potentially at its weakest as there is no established bone ingrowth and in the case of a cemented stem, curing of the stem cement may be incomplete.

It is essential to avoid even micro axial movements of tile stem during final coupling of a neck component irrespective of whether they are screwed or cemented, as even small unwanted movements could result in misalignment or weakening.

The procedure to replace a failed prosthesis, known as a revision hip replacement, necessitates full extraction of the failed prosthesis from the medullary cavity. One way this is effected is by use of a tool known as a slap hammer. Slap hammers used in orthopaedic surgery are well known in the prior art. One example is disclosed in United States Patent 5,788,701- This teaches an instrument system for knee prothesϊs implantation with universal handle or slap hammer . The patent discloses a universal hand piece such as a stationary handle or a slap hammer with a universal quick release connector which attaches and detaches to various components used in a knee prosthesis implantation surgical procedures. Instrument end pieces to which the hand piece may attach may include, for example, punches, inserters, extractors, irapactots, or other instruments.

As before, oeck insertion requires the surgeon to impact the neck into the stem. If this is performed by conventional methods, that is, with an uncontrolled blow from a surgeon's mallet, the impacting force is likely to be transmitted through to the stem, and possibly into the bone of the patient. This may cause subsidence of the stem, or worse, bone breakage. Additionally, the neck and stem may not be joined in the desired manner, creating an inferior implant.

There is a long felt want in the field to provide a satisfactory solution to the problem of unwanted stem subsidence during coupling of a neck component and also to provide a tool capable of both coupling the neck component without subsidence of the stem and including means to withdraw an implanted but no longer wanted component.

INVENTION

The present invention provides a multi function slap hammer assembly which is capable of performing the dual roles of withdrawal of a stem In the conventional way and also insertion of a neck component of a prosthesis while engaged with another component of the prosthesis, to enable redistribution of or at least resistance to neck insertion loading away from the stem and bone. The invention further provide? a slap hammer assembly which includes means to allow insertion of a neck component while restraining a stem from unwanted distal moveroent The multi-function slap hammer is capable of diverting most of the transmitted load away from a stem, thereby preventing possible subsidence or bone breakage. Furthermore, the hammer stroke, and therefore the impact load, is limited by the geometrical constraints of the design. This prevents a surgeon .from applying excessive force and but does not compromise the integrity of the joint after implantation.

The multi function hammer assembly to be described herein is primarily used to insert a neck component of a hip prosthesis into the stem component. However, it can also perform its traditional role of removal of a stem from the medullary cavity of a patient's femur.

In addition to providing significant advantages over the prior art techniques, the present invention seeks to eliminate the problem of unwanted distal movement of a stem during insertion of a neck component. The invention provides an assembly which retains the conventional functions of the slap hammer and provides a new use including a role in insertion of a neck,

hi its broadest form the present invention comprises: a slap hammer assembly comprising ; a primary shaft having first and second ends, the first end having engagement means to enable engagement with an associated member, the second end having a handle grippable by an operator of the slap hammer, and intermediate said ends a weight capable of movement relative to the shaft away from and in the direction of said handle; the assembly further comprising an auxiliary member at least partially mountable on the primary shaft ,md having a first end which engages a member responsive to an impact load applied by said weight and a second, end. which absorbs energy generated by said weight

In one broad form the present invention comprises: an assembly capable of use in insertion of a neck component of a prosthesj 5 of the type comprising a stem insertatøe in bone and a detachable neck; the assembly comprising; a shaft having a leading end with means to detachably engage the stem so that the leading end is retainable by the stem; a handle for manual, retention of the assembly; and a member moveable relative to the shaft having a leading end which advances and retracts such that when it advances it urges the neck into engagement with said stem while the leading end of said shaft is retained by said stem; the engagement between said leading end of said shaft and said stem providing restraint against axial movement of the stem during insertion of said neck by said moveable member.

According to a preferred embodiment, the assembly further comprises an impact member disposed about the shaft between said leading end of said moveable member and said neck. The impact member has a first end which engages the neck and a second end which, transmits loads from said moveable member urging the neck into engagement with said stem. The impact member is disposed concentrically about the shaft and is secured thereto via fasteners to restrain relative movement between the impact member and the shaft. According to one embodiment Ihe shaft engages the stem via opposite gender profiled parts including threads . Preferably, the stem includes a female thread and the leading end of the shaft has a corresponding male thread.

The shaft is divisible into first and second shaft members by a releasable coupling . Preferably the releasable coupling comprises interfitting via opposite gender profile parts. According to one embodiment the first shaft member has a male threaded profile part which engages a corresponding female threaded profile part of a leading end of the second shaft member. Alternatively , the first shaft member has a female threaded profile part which engages a corresponding male threaded profile part of the leading end of the second shaft member.

The moveable member is a slidable hammer which is concentrically disposed about the shaft such that the shaft, hammer and impact member are in axial alignment.

According to a preferred embodiment the impact member is hollow and provides a sleeve surrounding the connection between said first and second shaft members.

A proximal free end of the second shaft member receives and threadably retains thereon a handle which allows an operator to manually control the slap hammer. The neck includes a through passage through which the shaft passes.

The present invention will now be described in more details according to a preferred but non limiting embodiment and with reference to the accompanying illustrations.

In another broad form the present invention comprises: a multi function hammer assembly capable of use ϊn insertion of a neck component of a prosthesis of the type comprising a stem insεrtable in bone and a detachable neck; the assembly comprising; a first shaft having a leading end with means to detadhably engage the stem so that the leading end is retatnable by the stem; a second shaft having connection means at a first end to enable detachable connection of the first shaft to the second shaft, a handle disposed at a second end of the second shaft for manual retention of the assembly; and a member moveable relative to the second shaft having a leading end which advances and retracts such that when it advances it urges the neck into engagement with said stem via an impact member, wbile the leading end of said shaft is retained by said stem; the engagement^' between said leading end of said shaft and said stem providing restraint against axial movement of the stem during insertion of said neck by said moveable member.

Tn another broad form the present invention comprises:

an auxiliary impact member for a slap hammer assembly comprising ; a primary shaft having first and second ends, the first end having engagement means to enable engagement with an associated member, the second end having a handle grippable by an operator of the slap hammer, and intermediate said ends a weight capable of movement relative to the shaft away from and in the direction of said handle; wherein, the auxiliary member is at least partially mountable on the primary shaft and has a first end which engages an implant member responsive to an impact load applied by said weight and a second end which absorbs energy generated by said weight on impact and transfers the energy to the implant member. In another broad form the present invention comprises: a multi function hammer assembly capable of use in insertion, of a neck component of a prosthesis of the type comprising a stem msertable in bone and a detachable neck; the assembly comprising; ^■ first and second sub assemblies; the first sub assembly including a first shaft haying a leading end with means to detachably engage the stem so that the leading end is retaiπable by the stem; connection means to enable connection of a second shaft thereto an impact member disposed about said first shaft retaining therein said connection means; the second sub assembly including; a second shaft having connection means at a first end to enable detachable connection of the first shaft to the second shaft, a handle disposed at a second end of the second shaft for manual retention of the assembly; and a member moveable relative to the second shaft having a leading end which advances and retracts such that when it advances it urges the neck into engagement with said stem via an impact member, while the leading end of said shaft is retained, by said stem; the engagement between said leading end of said shaft and said stem providing restraint against axial movement of the stem during insertion of said neck by said moveable member.

In another broad form the present invention comprises; a sub assembly for use in a multi function hammer assembly capable of use in insertion of a neck component of a prosthesis of the type comprising a stem insertable in bone and a detachable neck; the sub assembly comprising; a first shaft having a leading end with means to detachably engage the stem so that the leading end is retainable by the stem; connection means to enable connection of a second shaft thereto; wherein the leading end of said shaft is retained by said stem; the engagement between said leading end of said shaft and said stem providing restraint against axial movement of the stem during insertion of said neck. In another broad form the present invention comprises; a sub assembly for use in a muliϋ function hammer assembly capable of use in insertion of a neck component of a prosthesis of the type comprising a stem insertable in bone and a detachable neck; the sub assembly comprising; a secondary shaft having connection means at a first end to enable detachable connection to another shaft, a handle disposed at a second end of the secondary shaft for manual retention of the assembly; and a member moveable relative to the secondary shaft having a leading end which advances and retracts such that when it advances it urges the neck into engagement with said stem via an impact member.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will now be describe in more detail according to a preferred but nott limiting embodiment and with reference to the accompanying illustrations wherein;

FIG. 1 shows a long sectional view of an assembly according to a preferred embodiment.

FIG. 2 shows an enlarged long sectional view of the necjc, impact member concentrically disposed about the first shaft member.

FIG. 3 shows a long sectional view of the second shaft member with handle and slap hammer disposed thereabout.

FIG. 4 shows a long sectional elevation view of a stem and neck components with the shaft engaged with the stem and impact member. FIG. 5 shows a long section view of the assemblies described in figures 1 - 4 fully assembled.

FIG. 6 shows a long sectional elevation of the assembly used as an extraction tooL

FIG. 7 shows an enlarged side elevation view of the impact member according to a preferred embodiment.

FIG, 8 shows a perspective view of the assemblies described in figures 1 - 4 fully assembled and connected to a stem component via a neck component.

FIG. 9 shows a side elevation view of the assemblies described in figures 1 - 4 MIy assembled and connected to a stem component via a neck component.

FIG. 10 shows an enlarged elevation view of the assembly of figure 9 with the handle absent.

FIG. 11 shows a long sectional view of the assemblies described in figures 1 - 4 fully assembled and connected to a stem component via a neck component.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a long sectional view of a multi function hammer assembly 1 according to a preferred embodiment.

Assembly 1 comprises a shaft 2 having a leading end 3 with means to detachably engage a stem component 5 ( see figure 4) so that the leading end 3 is retainable by the stem 5. Assembly 1 further comprises a handle 6 for manual retention of assembly 1 by an operator and a slap hammer 7 moveable relative to a shaft member 8. Hammer 7 has a leading end 9 which advances and refracts such that when it advances it urges impact member 10 disposed about second shaft member $ into engagement with neck 11 ( see figure 4) thence neck 11 into engagement with, stem 5, while the leading end 3 of said shaft 2 j$ retained by stem component 5.

The engagement between the leading end 3 of the $haft 2 and the stem component 5 provides restraint against axial movement of the stem component 5 during insertion of said neck 11 by the slap hammer 7.

According to a preferred embodiment, neck impact member 10 is disposed about the second shaft member 8 between the leading end 9 of slap hammer 7 and said neck. The impact member 10 has a first end 12 which engages the neck 11 and a second end 13 which transmits loads from the slap hammer 7 urging neck 11 into engagement with the stem component 5. Following assembly, the first shaft 14 which is preferably smaller than second shaft 8, damping spring 18 and neck impaction member 10 are assembled to form a captive -unit. The larger second shaft member 8 and the stationary mass forming handle 6 are joined by means of a thread 19 and /or a fillet weld. The assembly in its entirely may then be assembled/disassembJed by the surgeon in the operating theatre.

Figure 2 shows a long sectional view of the neck impact member 10 concentrically disposed about the first shaft member 14 .

Impact member 10 i$ disposed concentrically about first shaft member 14 and is secured thereto via fasteners 15 and 16_» preferably grub screws which extend through the impact member 10 and engage boss 17 to restrain relative movement between trie impact member 10 and the shaft member 14. Grub screws 15 and 16 may be filed away so they remain flush with the surface of impact member 10 .

Figure 3 shows an enlarged long sectional view of the second shaft member 8 with handle 6 and slap hammer 7 dispo$ed thereabout

The shaft 2 is divisible into first and second shaft members 8 and 14 as previously described via a releasable coupling 20 ( see figure 2) The releasable coupling comprises interfitting via opposite gender profile part$. According to one embodiment the first shaft member 14 has a rrjale threaded profile part 21 which engages a corresponding female threaded profile part 22 of a leading end 23 of the second shaft member 8. Alternatively ., the first shaft member 14 has a female threaded profile part ( not shown) which engages a corresponding male threaded profile part ( not shown ) of the leading end 23 of the second shaft member 8.

As shown in figure 3 a sub assembly 24 is formed by second shaft member S, slap hammer 7 and the connection of the stationary weight 6 to the shaft 8. The initial connection is provided via a thread 25. A circumferential fillet weld 26 then secures this joint further. The slap hammer 7 is positioned on the shaft 8 prior to joining the shaft 8 to the sub assembly 27 as shown in figure 4.

These sub-assemblies 24 and 27 ate separated from one another for a number of reasons. Firstly, separating the two assemblies reduces the weight of the instrument, making it more manoeuvrable. Secondly, the two sub-assemblies 24 and 27 have the potential to play different roles. Upon connecting the two sub-assemblies, the multi-function hammer assembly may be used to impact the neck 11 into the stem 5. If the stem needs to be τemoved from the patient, only sub assembly 24 will be required as shown in figure 6^" . Impact member 10 includes a cavity 28 which receives and retains therein a spring ( not shown). The purpose of the spring is two-fold. In the first instance the spring permits the neck impaction member 10 to travel along the $mall shaft 14. This allows the entire range of neck sizes to be used with the same instrument Secondly, the spring acts as a damping mechanism. Without the spring, the neck impaction member 10 would rebound from the neck and possibly fail to mate with the neck again upon return. The spring serves to minimise the rebound of the neck impaction pad.

Figure 4 shows a long sectional elevation view of sub assembly 27 comprising stem 5 and neck component 11 with the shaft 14 engaged, with the stem S and impact member 10. Slidable hammer 7 is concentrically disposed about the shaft 8 such that the shaft 8, hammer 7 and impact member 10 are in axial alignment

Shaft member 14 engages stem 5 via opposite gender profiled parts including threads at junction 30 . Preferably, the stem 5 includes a female thread and the leading end 3 of the shaft 14 has a corresponding male thread. According to a preferred embodiment the impact member 10 is hoJlow and forms a concentric sleeve surrounding the boss 17 forming the connection between the first and second shaft members 14 and 8 respectively,

The fully assembly 1 comprises a large shaft 8, small shaft 14 connected to the shaft 8, an impaction member 10 disposed about a connection between the large and small shafts. Cavity 28 is formed to receive a biasing spring which allows axial movement in the impact member 10 to accommodate necks of different sizes- To complete assembly T stationary weight forming handle 6 is connected to a distal end of shaft 8.

Figure 5 shows a long section view of the assemblies described in figures 1 - 4 fully assembled and with corresponding numbering. The slap weight is only aligned axially; it is free to move up and down the large shaft. The surgeon holds the stationary mass with one hand, and with the other moves the slap weight up and down to impact the neck into the stem. The number of impactions required is at the surgeon's discretion.

Figure 6 shows a long sectional elevation, of the assembly used as an extraction tool. In some situations it may be necessary to remove the stem 5 from a femoral shaft. The multi function hammer assembly described herein is primarily used to insert the neck into the stem. However, it can also perform its traditional role of removal of the stem from the medullary cavity of a patient's femur.

Only one part of the assembly 1 is required namely sub assembly 24. The sub- assembly 24 consists of the large shaft 8, hammer 7 and handle 6. Neck impaction member 10 is not required for extraction. Instead, the other sub-assembly consisting of the large shaft, slap weight and stationary mass is used. The large shaft 8 is inserted directly into the stem 5 (the neck 11 has already been removed by this stage). The surgeon may now apply force to the stationary mass handle 6 to remove the stem from the femur using hammer 7 urged in the direction of arrow 36 as required.

Figure 7 shows an enlarged side elevation view of the impact member

10 according to a preferred embodiment and with corresponding numbering. Leading end 40 includes a cylindrical portion 41 which transforms into a conical region 42. An intermediate cylindrical region 43 includes a cleaning slot 44 for a spring housed in cavity 28 and cleaning slot 45 for grub screw slot . An additional cleaning slot 46 is provided at end 13. Oαce the neck 11 has been impacted into stem S, the sub- assemblies 24 and 27 must be removed. The removal process of the small shaft 14 of sub-assembly 27 has been aided by the addition of a knurled finish to the largest diameter of the neck impaction member 10. A second method for removal is the use of the additional cleaning slot 46. If required, the surgeon may place an appropriate instrument into this slot and use it to create torque, thereby removing the shaft 14 from the stem 5.

Aiding in this instruments ability to be used for the entire range of neck sizes is the grub screw slot 45. The geometry of slot 45 has been designed so that the appropriate range of motion may be obtained, but restricts unwanted movement and keeps the sub-assembly 27 in which it is used captive.

Since the sub-assemblies 24 and 27 are to remain captive, cleaning is a relevant consideration. The sub-assembly 24 consisting of the large shaft 8 aud stationary member 6 and slap weight/hammer 7 is not as susceptible to difficult cleaning access as sub-assembly 27 , which ha$ internal springs and other moveable elements. This need for access has been addressed by the use of cleaning slots 44, 45 and 4$ in the neck impaction member 10.

Figure 8 shows a perspective 3D view of the assemblies described wjth reference to figures 1 - 4 fully assembled and connected to a stem component via a neck component. Figure 9 shows a side elevation view of the assembly of figure 8. Figure 10 shows an enlarged elevation view of the assembly of figure 9 with the handle absent and figure 11 shows a long sectional view of the assemblies described in figures 1 - 4 fully assembled and connected to a stem component via a neck component

Set out below are parameters for the assembly according to a preferred but non limiting embodiment.

U 1.1 Calculations

1.1.1 Mass Distribution

1. Distance from end of distal taper to bottom of slot for the smallest and largest neck:

2. Stationary weight is preferably 75% of existing slap hammer weight (including shaft). Existing Slap hammer weight - 0.97 + 0.4 = 1.4kg

Stationary weight -= 0.75(1.4) = lkg

3. Slap weight is approximately 75% of stationary weight (i.e. 0.75kg).

4. Make slap weight dimensions equal to stationary weight dimensions. This gives:

* 0.75kg for slap weight

• 0.75kg + 0.25kg (for shaft without a hole) = lkg for stationary

1.1.2 Thread Calculations

The thread calculations need only be performed on the smallest thread diameter. This is the most likely place at which failure will occur. Several assumptions were made in determining the stresses in the threads, including:

• Take F=10,0OON (the approximate mallet blow is 7,000N)

• For an M6 thread, D_major = 6mm; D_minor = 4.77mm

• Shafts will not shear threads or break in tension (all values taken from Norton¹)

¹ Norton, R.L., Machine Design, Art Integrated Approach, Third Edition, Prentice Hall, 2006 1.1.2.1 Tensile Stress

The shear stress will occur at the major diameter, that is, at the internal thread. The shaft will not shear before the tapered hole due to the material.

It is reasonable (and conservative) to assume that the first two threads take the entire load. Therefore,

The threads will not fail in tension or shear.

It will be recognised by persons skilled in the art that although the present invention is described with respect to particular embodiments and features and uses, numerous variations or equivalents are possible without subtracting from the spirit or scope of the claimed invention.

Claims

THE CLAIMS DEFINTNfG THE INVENTION ARE AS FOLLOWS:

1 A slap hammer assembly comprising ; a primary shaft having first and second ends, the first end having engagement means to enable engagement with an associated member, the second end having a handle grippable by an operator of the slap hammer, and intermediate said ends a weight capable of movement relative to the shaft away from and in the direction of said handle; the assembly further comprising an auxiliary member at least partially mountable on the primary shaft end having a first end which engages a member responsive to an impact load applied by said weight and a second end which absorbs energy generated by said weight.

2 A slap hammer assembly according to claim 1 wherein the weight is slidable along the primary shaft .

3 A slap hammer assembly according to claim 2 wherein the weight is concentrically mounted on said primary shaft. 4 A slap hammer assembly according to claim 3 wherein the auxiliary member is an open ended sleeve mounted at least partially about said shaft.

5 A slap batnmex assembly according to claim 4 wherein the primary shaft includes an extension member having a first end including means to enable detachable engagement with a femoral stem and a second end eπgagable with another part of the shaft via a coupling.

6 A slap hammer assembly according to claim 5 wherein the auxiliary member is concentrically mounted about said primary shaft at a location approximating a location of said coupling .

7 A slap hammer assembly according to claim 6 wherein the first end of the extension member is formed to engage a recess in a neck component of a hip prosthesis.

75 8 A slap hammer assembly according to claim 7 wherein the auxiliary member has an inner passage which receives therein the coupling between the extension member and the remainder of the primary shaft.

9 A slap hammer assembly according to claim 8 wherein the second end of the auxiliary member has a bearing surface which receives impact loads from said slidable weight

10 A slap hammer assembly according to claim 9 wherein the inner passage at the first end of the auxiliary sleeve member has a narrow diameter and the jnner passage at the second end of the auxiliary sleeve member has a wide diameter.

11 A slap hammer assembly according to claim 10 wherein the outer surface of the auxiliary member tapers downward$ from the bearing axea at said second end the narrow diameter first end.

12 A slap hammer assembly according to claim 11 wherein the first end of the auxiliary member urges the neck member into engagement with a stem when an impact load is applied by said slidable weight. 13 A slap hammer assembly according to claim 12 wherein the first end of the extension member engages a stem while said weight is transmitting an insertion force to the auxiliary sleeve.

14 A slap hammer assembly according to claim 13 wherein the extension member engages said stem via a screw thread. 15 A slap hammer assembly according to claim 1,4 wherein the neck component has a through passage which receives the primary shaft thereby enabling the neck component to slide there along.

16 A slap hammer assembly according to claim 15 wherein the auxiliary member is slidable along the primary axis between the neck and first end of the weight. 17 A slap hammer assembly according to claim 16 wherein the first end of the extension member grips the stem during impact by the weight on the bearing surface of the auxiliary member, thereby allowing an operator to apply an opposite axial force on the stem as the weight engages the neck component. 18 A slap hammer assembly according to claim 17 wherein the coupling in the primary shaft with the extension member is effected by opposite gender interfitting.

19 A slap hammer assembly according to claim 18 wherein, the auxiliary member has openings into said inner passage which receive fasteners preventing relative movement between said auxiliary member ad said shaft.

20 An auxiliary impact member for a slap hammer assembly comprising ; a primary shaft having first and second ends, the first end having engagement means to enable engagement with an associated member, the second end having a handle grippable by an operator of the slap hammer, and intermediate said ends a. weight capable of movement relative to the shaft away from and in the direction of said handle; wherein, the auxiliary member is at least partially mαuntable on the primary shaft and has a first end which engages an implant member responsive to an impact load applied by said weight and a second end which absorbs energy generated by said weight on impact and transfers the energy to the implant member.

21 An auxiliary impact member according to claim. 20 wherein, the auxiliary member is an open ended sleeve mounted at least partially about said shaft.

22 An auxiliary impact member according to claim 21 wherein, the auxiliary member is concentrically mounted about said primary shaft at a location approximating a location of said coupling .

23 An auxiliary impact member according to claim 22 wherein the weight is slidable along the primary shaft .

24 An auxiliary impact member according to claim 23 wherein the weight is concentrically mounted on said primary shaft.

25 An auxiliary impact member according to claim 24 wherein the auxiliary member has an inner passage which receives therein the coupling between the extension member and the remainder of the primary shaft.

26 An auxiliary impact member according to claim 25 wherein the second end of the auxiliary member has a bearing surface which receives impact loads from said sHdable weight. 27 An auxiliary impact member according to claim 26 wherein the inner passage at the first end of the auxiliary sleeve member has a narrow diameter and the inner passage at the second end of the auxiliary sleeve member has a wide diameter.

28 An auxiliary impact member according to claim 27 wherein, the outer surface of the auxiliary member tapers downwards from the bearing area at said second end the narrow diameter first end.

29 An auxiliary impact member according to claim 28 wherein the first end of the auxiliary member urges the neck member into engagement with a stem when an impact load is applied by said slidable weight.

30 An auxiliary impact member according to claim 29 wherein the primary shaft about which the impact member fits, includes an extension member having a first end including means to enable detachable engagement with a femoral stem and a second end engagable with another part of the shaft via a coupling,

31 An auxiliary impact member according to claim 30 wherein the auxiliary member is slidable along the primary axis between the neck and first end of the weight.

32 An auxiliary impact member according to claim 31 wherein the first end of the extension member grips the stem during impact by the weight on the bearing surface of the auxiliary member, thereby allowing an operator to apply an opposite axial force on the stem as the weight engages the neck component. 33 An auxiliary impact member according to claim 32 wherein, the auxiliary member has openings into said inner passage.

34 An auxiliary impact member according to claim 33 wherein, tbe first end of the extension member engages a stem while said weight is transmitting an insertion force to the auxiliary member. 35 An assembly capable of use in insertion of a neck component of a prosthesis of the type comprising a stem insertable in bone and a detachable neck; the assembly comprising; a primary shaft having a leading end with means to detachably engage the stem of said prosthesis so that the leading end is retainabte by the stem; a handle for manual gripping of the assembly; and a weight moveable relative to the primary shaft having a leading end which advances and retracts such that when it advances it urges the .neck into engagement with said stem while the leading end of said shaft is retained by said stem; the engagement between said leading end of said shaft and said stem providing restraint against axial movement of the stem during insertion of said neck by said weight.

36^" An assembly according to claim 35 further comprising, an auxiliary impact member disposed about the primary shaft between said leading end of said moveable weight and said neck.

37 An assembly according to claim 36 wherein the impact member has a first end which engages the neck and a second end having a load bearing surface which transmits loads from said moveable weight urging the neck into engagement with said stem. 38 An assembly according to claim 37 wherein the auxiliary impact member is disposed concentrically about the shaft and is secured thereto vja fasteners to restrain relative movement between the impact member and the shaft.

39 An assembly according to claim 38 wherein the shaft engages the stem via opposite gender profiled parts including threads •.

40 An assembly according to claim 39 wherein the primary shaft includes an extension member divisible from the primary shaft by a releasable opposite gender coupling.

41 An. assembly according to claim 40 wherein the moveable weight member is a slidable hammer wliich is concentrically disposed about the primary shaft such that the shaft, hammer and auxiliary impact member are in axial alignment. 42 An assembly according to claim 41 wherein the impact member is hollow and forms a sleeve surrounding the coupling between said extension member and the primary shaft.

43 An assembly according to claim 42 wherein a proximal free end of the primary shaft member receives and threadably retains thereon a handle which allows an operator to manually control the slap hammer.

44 An assembly according to claim 43 wherein the neck component includes a through passage through which the primary shaft passes.

45 A iflultj function hammer assembly capable of -use in insertion of a neck component and withdrawal of a stem component of a prosthesis of the type comprising a stem insertable in bone and a detachable neck; the assembly comprising; a primary shaft having a leading end with means to detachably engage the stem so that the leading end retains the stem; an extension member having connection means at one end to enable detachable connection from the primary shaft, a handle disposed al an end of the primary shaft for manual retention of the assembly; and a weight member moveable relative to the second shaft having a leading end which advances and retracts such that when it advances it urges the neck into engagement with said stem via an auxiliary impact member, while the leading end of said primary shaft is retained by said stem; the engagement between said leading end of said shaft arid said stem providing restraint against axial movement of the stem during insertion of said neck by said moveable weight via said auxiliary member.

46 A multi function hammer assembly capable of use in insertion of a neck component of a prosthesis, of the type comprising a stem component insertable in bone and a detachable neck, while restraining the stem from axial movement; the assembly comprising; first and second sub assemblies; the first sub assembly including an extension shaft having a leading end with means to detachably engage the stem component so that the leading end retains the stem; connection means on the extension shaft to enable connection, to a primary shaft an auxiliary impact member disposed about said extension shaft retaining therein said connection means; the second sub assembly including; a primary shaft having connection means at a first end to enable detachable connection of the primary shaft to the extension shaft, a handle disposed at a second end of the primary shaft for manual retention of the assembly; and a weight member moveable relative to the primary shaft and having a leading end whicb. advances and retracts such that when it advances it urges the neck into engagement -with said stem via the auxiliary impact member, while the leading end of said extension shaft is retained by said stem; the engagement between said leading end of said shaft and said stem thereby providing restraint against axial movement of the stem during insertion of said neck by blows from said weight member.

47 Λ sub assembly for use in a multi function hammer assembly capable of use in insertion of a neck component of a prosthesis of the type comprising a stem insertable in bone and. a detachable neck; the sub assembly comprising; an extension shaft having a leading end with means to detachably engage the stem $0 that the leading end retains the stem; connection means to enable connection of the extension shaft to a primary shaft ; wherein, when the leading end of said extension shaft is retained by said stem; the engagement between said leading end of said extension shaft and said stem provides restraint against axial movement of the stetn during insertion of said neck by blows from a weight mounted on said primary shaft.

48 A sub assembly for me in a multi function hammer assembly capable of use in insertion of a neck component of a prosthesis of the type comprising a stem insertable in bone and a detachable neck; the sub assembly comprising; a primary shaft having connection means at a first end to enable detachable connection to an extension shaft of another sub assembly, a handle disposed at a second end of the primary shaft for manual retention of the assembly; and a weight rαoveabie relative to the primary shaft having a leading end which advances and retracts such that when the weight advances it urges the neck into engagement with said stem via an auxiliary impact member on said other sub assembly while the extension shaft retains the stem.