WO2009071937A1

WO2009071937A1 - An instrument

Info

Publication number: WO2009071937A1
Application number: PCT/GB2008/051130
Authority: WO
Inventors: Chris Hunt; Gary Moore
Original assignee: Depuy International Ltd
Priority date: 2007-12-08
Filing date: 2008-11-28
Publication date: 2009-06-11
Also published as: GB0724020D0

Abstract

An instrument for preparing the end of a bone to receive a joint prosthesis component forming a ball component of an artificial ball and socket joint. The instrument comprises a shell for shaping bone around the end of a bone. The shell comprises a cavity defined by a shell wall and extends along a longitudinal axis of the instrument. The cavity is arranged to receive the end of the bone. The shell wall comprises at least one formation directed inwardly and arranged to engage a generally axially extending outer surface of an end of a bone received in 'the cavity to form a prepared outer surface of the end of a bone to receive the joint prosthesis. The at least one formation is arranged to form at least one indentation in the prepared outer surface of the end of the bone.

Description

An Instrument

The present invention relates to an instrument. In particular, but not exclusively, the present invention relates to a surgical instrument for preparing an end of a bone prior to implanting a resurfacing implant to replace an articulating surface at an end of a bone which forms the ball component of a ball and socket joint. The resurfacing implant may be used to replace the articulating surface of, for instance, the femoral component of a hip joint. More particularly, the instrument is intended to reduce the amount of contact area between the prepared bone and the inner surface of an implant, in order to assist in securing the implant into position.

Conventional orthopaedic joint prostheses for replacing ball and socket joints comprise an implanted socket prosthesis component and an implanted ball prosthesis component. Implantation of the ball prosthesis component typically involves removal of a large portion of the ball end of the bone, including all of the ball component and much or all of any neck at the end of the bone, in a process known as resection. The ball prosthesis component comprises a stem which is inserted into the open medullary canal of the resected bone. The stem terminates at a proximal end at a neck portion coupled to a head component formed as a partial sphere. The head component comprises a bearing surface which articulates within the implanted socket component.

It is known to replace only the articulating surface at the end of a bone in preference to removing and replacing a significant portion of the bone. In such a "resurfacing" joint prosthesis, the bone tissue that provides the ball component of a ball and socket joint (for example the humeral component of a shoulder joint or the femoral component of a hip joint) is prepared to receive a cap-like component having an outer bearing surface which can be received in the socket component of the joint prosthesis. Preparation of the bone involves shaping the bone tissue of the ball component of the natural joint so that its external shape is approximately the same as the shape of a cavity within the cap-like component. The cavity is rotationally symmetrical. Preparation further involves drilling a bore in the end of the bone in which a central locating implant pin of the cap-like component can be located. The resurfacing implant can then be implanted over the prepared bone end and secured in position using an impaction force, to create a press fit, or using bone cement.

It is known to use a reamer to prepare a bone for implantation of a component of an orthopaedic joint prosthesis. Conventionally, a reamer comprises a tool for enlarging holes, which may be used in particular in metalworking. In the present context, a reamer includes cutting teeth which can be moved against the bone surface to cut the bone. The reamer can be moved reciprocally along an axis to cut the bone, when the cutting teeth are arranged so that they face along that axis. In such a case the cutting teeth may be provided on the end of a wall of a reamer body similar to the teeth provided on a saw. A reamer may also have cutting teeth defined by slits in the wall of a reamer body. The material of the reamer body wall is deformed out of the plane of the reamer wall at each slit to provide a cutting tooth (alternatively referred to as "slit-like cutting teeth").

In order to prepare an end of a bone to receive a resurfacing component of a joint prosthesis, it is known to use a reamer assembly to remove the articulating surface of the bone so that the bone can receive the cap-like component. The reamer may comprise a reamer guide and a reamer shell with a sleeve portion which is a sliding fit upon the reamer guide.

Such a reamer assembly for preparing the ball component of a natural ball and socket joint for a resurfacing joint prosthesis is described in EP-1549230B1 ("A Reamer Assembly", DePuy International Limited). The reamer assembly is described as particularly suitable for preparing the ball component of a hip joint for resurfacing. The reamer guide comprises an elongate member which can be implanted within a bore cut into the end surface of the natural ball component of the joint along an axis defined by the sphere of the bearing surface. The reamer shell is arranged to cut bone around the reamer guide to a desired shape. The reamer guide comprises a sleeve portion at a proximal end which is arranged to slide over the reamer guide and a shell wall which extends distally from the sleeve in a generally cylindrical or conical fashion to a cutting edge at the distal end of the shell which engages and cuts the bone as the shell is rotated about the reamer guide. Slit like cutting teeth may also be provided in the shell wall to remove portions of the ball component of the bone, in particular those portions of the bone at the natural bearing surface of the joint.

A resurfacing implant is then implanted over the prepared bone. The implant comprises an external convex bearing surface and an internal cavity. The cavity is generally cylindrical or conical and is sized to receive the prepared bone end. A central locating implant pin within the cavity is received within the bore in the end of the bone and assists in accurately aligning the implant. The resurfacing implant is then driven home over the bone end by an impaction force.

For implanting conventional ball prosthesis components into a bore prepared in an open end of a medullary canal of a bone, it is known to use a broach. US-6045556 describes such a broach. The broach comprises a plug portion arranged to fit within a bore drilled within the medullary canal, which serves to keep the rest of the broach aligned with the axis of this bore. The broach further comprises a proximal portion on which a plurality of cutting teeth is arranged to cut the internal surface of the bore in order to widen the bore to match a broader proximal stem portion of a ball prosthesis component. The proximal portion is connected to the plug portion via an elongate connecting portion. The broach is moved relative to the bone along the axis of the bore such that the cutting teeth progressively engage the internal surface of the bore and abrade or cut away bone portions.

A broach conventionally comprises a series of progressively taller chisel points mounted on a single substrate and used to enlarge holes, particularly in metal working. Thus, for certain particular embodiments, the terms reamer and broach may both be used to describe the same device for enlarging holes, or more generally for removing material from a substrate, for instance a bone. Therefore, the term "reamer" is used hereinafter in a broad sense to refer to any device suitable for removing bone material. Certain embodiments of the present invention could alternatively be termed "broaches". Furthermore, the terms "reamer" and "broach" are generally known in the art of orthopaedic implants to refer to any instrument capable of removing bone material, including from an exterior surface of a bone. More generally, the invention comprises a surgical instrument. It is known to secure orthopaedic joint prostheses using an adhesive (also referred to as bone cement). Alternatively, it is known to secure joint prostheses using techniques that do not use cement. For instance, it is known that if the surface of the bone can be brought into close proximity to the implant then bone growth around and over the prosthesis can secure the prosthesis in position. Furthermore it is known that if at least the surface of the prosthesis comprises a porous structure then bone growth can be encouraged through the cavities of the porous structure firmly embedding the prosthesis within the bone. As the bone is embedded into the prosthesis, the interface between the bone and the prosthesis is merged increasing the joint strength.

Such a porous structure can be provided as a porous coating applied to the prosthesis. Alternatively, part or the entire prosthesis can be formed from a material that is itself a porous structure.

Various porous coatings have been found to be effective, for example a porous coating is described in US-3855638. One particularly effective coating is applied to prosthesis components available from DePuy Orthopaedics Inc. under the trademark Porocoat. Porocoat is a porous coating that comprises a beaded coating that may, for instance, be sintered onto the underlying material of the prosthesis. The porous coating consists of a plurality of small discrete particles of a metallic material bonded together at their points of contact with each other to define a plurality of connected interstitial pores in the coating. Typically, the particles are formed of the same metallic material as the material from which the substrate is formed. Bone regrowth through the interstitial pores fixes the implant in position.

A porous coating can be applied to the inside of the cavity of a resurfacing implant in order to secure the resurfacing implant into position.

In order to securely fix a resurfacing implant to the end of the bone it is important to ensure that the end of the bone is accurately prepared such that it is a close fit for the cavity of the resurfacing implant. How securely the implant is seated is greatly affected by the degree of interference between the prepared bone and the interior surface of the implant cavity. For implants that are to be fixed into position without cement, using a porous structure, the amount of interference between the prepared bone end and the porous structure significantly affects the resulting strength of the implant. If the prepared bone is poorly sized for the implant cavity, it may even be difficult to fully seat the implant to its final position at all.

Inaccurate preparation of the bone end can result in a range of problems. In addition to having difficulty in fully seating the implant, excess exposed cut bone may be left uncovered by the implant. It is desirable that as little bone is cut away as possible during implantation of a resurfacing prosthesis to preserve the integrity of the remaining bone and thereby increase its strength. Difficulty in seating the implant can result in excessive impaction forces being applied, risking further damage to the bone. This can occur if there is too much contact between the bone and the implant. A poorly positioned implant may not transfer loads correctly to the bone, risking further damage to the bone later on. Conversely, if the implant fit is loose, its position may vary over time risking further damage when the joint is loaded. Furthermore, if the fit is too loose then bone in-growth into the porous structure is inhibited, resulting in a weaker joint.

It is an object of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere. In particular, it is an object of embodiments of the present invention to provide an instrument suitable for preparing an end of a bone for receiving a resurfacing implant.

According to a first aspect of the present invention there is provided an instrument for preparing the end of a bone to receive a joint prosthesis component forming a ball component of an artificial ball and socket joint, the instrument comprising: a shell for shaping bone around the end of a bone, the shell comprising a cavity defined by a shell wall and extending along a longitudinal axis of the instrument, the cavity being arranged to receive the end of the bone; wherein the shell wall comprises at least one formation directed inwardly and arranged to engage a generally axially extending outer surface of an end of a bone received in the cavity to form a prepared outer surface of the end of a bone to receive the joint prosthesis; wherein the at least one formation is arranged to form at least one indentation in the prepared outer surface of the end of the bone.

An advantage of the first aspect of the present invention is that the instrument is operable to form at least one indentation in the outer surface of an end of the bone. The prepared end of the bone can be received in the cavity of a resurfacing implant. The indentation in the bone surface results in reduced surface contact between the implant and the bone, at least during the initial stages of fitting the implant on the bone.

Preferably, the shell cavity has a closed end arranged to limit the insertion of the end of the bone into the cavity. The shell is arranged to shape the exterior surface of the end of the bone to form the prepared outer surface when the bone is fully inserted into the cavity and the end of the bone contacts the closed end of the cavity.

The bone is slightly compressible, and the indentation assists in reducing the amount of force required to fully seat the resurfacing implant on the end of the bone, while still preserving a large area of contact between the interior surface of the implant cavity and the outer surface of the end of the bone. Varying the number and locations of the indentations allows fine tuning of the amount of force required to seat the implant, while still allowing for a strong join between the implant and the bone due to bone ingrowth into a porous structure within the interior of the implant.

The indentations cause a discontinuity in the outer surface of the implant, in the sense that the outer surface deviates from a circular cross section at the point of the indentation. The reduction in contact between the interior of the implant and the surface of the bone may be relatively small, or may form a significant proportion of the outer surface of the bone within the implant cavity. The depth of the indentations may be chosen such that as the implant is seated on the end of the bone the indentations are to a large extent smoothed out due to compression of the intervening relatively raised portions of the surface of the bone.

In particularly preferred embodiments of the present invention, the or each formation is operable to form indentations in a rotationally symmetrical pattern about the outer surface of the end of the bone. This ensures that the bone is compressed uniformly, such that the implant does not deviate from its required position relative to the axis of the bone during an implantation step.

The shell wall may comprise two or more formations operable to form two or more indentations in the surface of the end of the bone.

The or each formation may be operable to cut the surface of the end of the bone to form the indentation. The or each formation may thus comprise a slit like cutting tooth directed inwardly and operable to cut the surface of the bone. Alternatively, the or each formation may be operable to abrade the surface of the end of the bone to form the indentation. Alternativley, the or each formation may be operable to compress inwardly the surface of the end of the bone to form the indentation.

At least one formation may be operable to form an indentation in portions of the surface of the bone that are inclined to the axis of the instrument.

The instrument may further comprise an instrument guide which depends from the closed end of the cavity along the axis of the instrument, the instrument guide being operable to be received within a bore provided in the end of the bone.

The instrument may further comprise a handle operable to be manipulated to apply a rotational motion to the instrument about the axis of the instrument such that the or each formation is brought into contact with the surface of the bone in a direction at an angle transverse to the axis of the instrument. Alternatively, the instrument may further comprise an impaction surface operable to receive an impaction force thereby imparting an axial motion to the instrument such that the or each formation is brought into contact with the surface of the bone in a direction generally parallel to the longitudinal axis of the instrument.

At least one formation may be operable to form at least one indentation that is at least partly circumferential about the end of the bone. At least one formation may be operable to form at least one indentation that extends at least in part generally parallel to the axis of the instrument.

The cavity may be generally circular in cross section and diverge towards its open end. The shell wall may diverge from the axis of the instrument at an angle less than 5°.

The at least one formation may be arranged to form at least one indentation that is between 0.1mm and lmm deep.

The instrument may be for preparing a femoral head to receive a femoral resurfacing implant forming part of a hip joint prosthesis.

According to a third aspect of the present invention there is provided a method of preparing the end of a bone to receive a joint prosthesis component forming a ball component of an artificial ball and socket joint, the method comprising: inserting an end of a bone into a cavity of an instrument shell, the cavity being defined by a shell wall and extending along a longitudinal axis of the instrument, the shell wall comprising at least one formation directed inwardly; and bringing the or each formation into contact with a generally axially extending outer surface of the end of the bone to form a prepared outer surface of the end of a bone to receive the joint prosthesis; wherein the at least one formation is arranged to form at least one indentation in the prepared outer surface of the end of the bone.

The method may further comprise applying a rotational force to the instrument such that the at least one formation is brought into contact with the surface of the bone in a direction at an angle transverse to the axis of the instrument.

Alternatively, the method may further comprise applying an axial impaction force to the instrument such that the at least one formation is brought into contact with the surface of the bone in a direction generally parallel to the axis of the instrument.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically illustrates in cross section a conventional resurfacing joint prosthesis during implantation over a prepared end of a bone forming the ball component of a ball and socket joint;

Figure 2 schematically illustrates in cross section a reamer being used to form indentations in a bone surface about an end of a bone in accordance with a first embodiment of the present invention; and

Figure 3 schematically illustrates in cross section a reamer being used to form indentations in a bone surface about an end of a bone in accordance with a second embodiment of the present invention.

Referring to figure 1 , this schematically illustrates a conventional resurfacing implant 2 which forms part of an orthopaedic joint prosthesis. In particular, resurfacing implant 2 forms the femoral component of a hip joint prosthesis and provides a partially spherical articulating surface 4 that is received within an implanted acetabulum prosthesis (not shown). The resurfacing joint prosthesis 2 provides a hard wearing bearing surface 4 on the head of a femur 6

Figure 1 is illustrated in cross section, and further illustrates the prepared end of the femur 6. The prepared femur 6 is received within an axial cavity 8 formed within the resurfacing implant 2. The axial cavity 8 is generally cylindrical or conical about the axis of the implant 2. Figure 1 illustrates the implant 2 not yet fully seated upon prepared femur 6, such that the cavity 8 is still partially open.

The resurfacing implant 2 further comprises an axial implant pin 10 which is received within an axial bore 12 within the prepared end of the femur 6. The bore may be cylindrical or may taper toward its closed end. The implant pin 10 serves to ensure that the resurfacing implant 2 is correctly aligned with the prepared end of the femur 6 before the implant 2 is implanted. At least part of the interior surface of the cavity 8 is provided with a porous structure 14 such as a layer of porocoat. The porous structure 14 serves to secure the implant 2 into position as natural bone can grow through the pores to firmly bond the implant into position.

Conventionally, the preparation of the head of the femur 6 to receive the resurfacing implant 2 involves first forming a bore 12 along or at a desired angle to the axis of the femoral head to receive the implant pin 10. The external surface of the femur 6 is prepared by the formation of three rotationally symmetrical reamed surfaces 16, 18 and 20. A first surface 16 is generally aligned parallel or close to parallel to the axis of the bore 12. The second surface extends between the first surface and a third surface at about 135°. The third surface is generally formed as a plane normal to the axis of the bore 12. The surfaces 16-20 may be formed using a reamer as described above in the introduction.

Once the femoral head is prepared, the resurfacing implant 2 is positioned over the femoral head such that the implant pin 10 is aligned with the bore 12 and the implant 2 is driven onto the femur 6 using an impaction force provided along the axis of the implant pin, in the direction indicated by arrow 22. The resurfacing implant 2 is driven along the axis of the implant pin 10 until the implant 2 is fully seated upon the femur 6 (figure 1 illustrates an intermediate position in which the implant 2 has not yet been fully seated such that the axial cavity 8 is still partially open between the upper surface 20 of the femur 6 and the closed end of cavity 8).

However, as discussed above in the introduction, if the end of the femur 6 is not accurately matched to the axial cavity 8 of the implant 2 then problems can occur. If the end of the femur is too small then the implant may be loose, or become loose during normal use. This may be due to the presence of a gap between the femur and part or all of the porous structure 14, which inhibits bone in growth. Alternatively, if the femur is too large then it can be difficult to fully seat the implant 2 into its final position, such that either axial cavity 8 remains partially open, or excessive damaging force is required. Incorrect seating of the implant 2 can result in the implant not correctly transferring loads to the bone, resulting in increased wear of the implant and possible damage to the bone, which can result in the implant loosening.

Problems in sizing the prepared end of femur 6 are due in part to tolerance stack up between the reamer used to prepare the bone and the implant 2. This can result in there being too much or too little surface area contact between the bone and the implant.

In accordance with embodiments of the present invention a modified reamer is provided for preparing the exterior surface of the end of a bone in order to receive a resurfacing implant. The reamer may be used for the complete preparation of the end of the bone, or the reamer may be used for the final stages of the preparation of the end of the bone, with the end of the bone first approximately prepared using a conventional reamer of the sort described above.

Advantageously, reamers in accordance with embodiments of the present invention provide for a repeatable method of reducing the amount of contact area between the prepared end of the bone and the inner surface of an implant later applied over the end of the bone. This reduction in the amount of contact area reduces the force necessary to position the implant over the end of the bone, thereby helping to ensure that the implant is correctly and fully seated and reducing the risk that an excessive impaction force is applied to the end of the bone.

Reamers in accordance with embodiments of the present invention reduce the amount of contact between the surface of the bone and the implant cavity by creating indentations about the peripheral external surface of the end of the bone. That is, the indentations are formed in the outer surface of the bone, and not for instance in the interior surface of a bore cut into the end of the bone. Indentations may additionally be formed in the end surface of the bone. The indentations may be formed as steps, ribs, grooves or any other form of surface discontinuity. Furthermore, the indentations may run partly or fully circumferentially, or axially along the external surface of the bone. The indentations ensure that contact between the interior surface of an implant cavity and the exterior surface of the bone is discontinuous either in an axial direction or in a circumferential direction (or both).

As described above in connection with figure 1 , for conventional reaming technique for preparing the end of a bone to receive a resurfacing implant, the prepared bone and the implant cavity are generally circular in cross section in a plane normal to the axis of the bone and the implant. Furthermore, the prepared end of the bone tapers towards its free end and the implant cavity tapers towards its closed end. Inserting the end of the bone into the cavity and applying an impaction force to the implant along the implant axis defined by the partially spherical articulating surface locks the implant onto the end of the bone forming an interference fit. This locking step is partly assisted by the fact that the prepared end of the bone is slightly compressible. As the implant is forced over the end of the bone, the bone compresses and provides a lateral force exerted on the interior surface of the implant cavity.

For a bone prepared using a reamer in accordance with embodiments of the present invention the indentations about the peripheral external surface of the bone assist in ensuring that this lateral force is applied evenly around the circumference of the bone. It is preferable that the indentations formed in the surface of the bone are circumferentially symmetrical. That is, the indented surface of the bone has rotational symmetry of at least the second order about a longitudinal axis of the end of the bone. It may be that other portions of the prepared end of the bone are not circumferentially symmetrical (for instance a later step in the preparation of the end of the bone may affect the shaped of the end surface of the bone in a non symmetrical fashion).

Referring now to figure 2, this illustrates in cross section a reamer 30 suitable for preparing the surface of an end of a bone in accordance with a first embodiment of the present invention. Figure 2 further illustrates a partially or fully prepared end of the femur 6. In accordance with embodiments of the present invention the femur 6 may be initially prepared using a conventional reamer as described above in order to approximately shape the femur 6 to be received within an axial cavity within the reamer. Reamer 30 comprises a reamer shell 32 formed from a shell wall 34 that defines a reaming cavity 36. Reaming cavity 36 is generally circular in cross section through a longitudinal axis defined by the reamer 30, which is parallel to the axis of the end of the femur. Reaming cavity 36 tapers towards its closed end, and thus matches the taper of femur 6 towards its free end.

Depending from the closed end of reaming cavity 36 is an axial reaming guide 38. Reaming guide 38 comprises a tapering elongate protrusion arranged to be received within an axial bore 40 within the end of femur 6. Reaming guide 38 serves to ensure that the reamer 30 remains aligned with the axis of the end of the bone defined by the bore 40. In accordance with certain embodiments of the present invention the reaming guide may be provided with cutting teeth at its distal end in order to cut bore 40 into the end of the bone. However, particularly for embodiments in which the end of the femur 6 is first prepared using conventional reamers, it may be that the bore 40 is already provided in the end of the femur 6, and as such the reaming guide 38 may comprise a smooth taper.

The interior surface of reaming cavity 36 is provided with formations 42 that are directed inwardly and arranged to shape the surface of the end of the femur. The formations 42 thus form indentations 44 about the surface of the end of the femur. As described above, preferably said indentations are distributed about the end of the femur such that the indentations at the prepared end of the femur are rotationally symmetrical.

The formations 42 may comprise slit like cutting teeth of a conventional form as described above. Such slit like cutting teeth are adapted to cut surface portions of the bone where they contact the end of the femur, and moved relative to the femur. Alternatively, formations 42 may comprise any formation that can modify the surface profile of the bone by abrading or cutting the bone surface.

Figure 2 further illustrates the reamer comprising a handle 46 attached to the top surface of shell 32. The handle 46 may comprise a "wingnut" arrangement, adapted to be manipulated by hand by rotating the reamer 30 about the reaming axis defined by reaming guide 38 in either direction as indicated by arrow 48. Thus, reamer 30 is arranged to be rotated while being moved in an axial direction such that the end of the femur 6 is progressively received in reaming cavity 36. In an alternative embodiment of the present invention it may be that the reamer 30 is provided with a spigot allowing connection to a power tool for driving the rotation of the reamer. For embodiments in which the formations 42 comprise slit like cutting teeth, the cutting teeth are arranged such that they engage and cut bone when moved in a generally circumferential direction. The direction of rotation for cutting bone is determined by the direction of the open end of the cutting teeth.

Formations 42 therefore cut or abrade the surface of the bone in continuous circumferential indentations 44, which appear as grooves or steps. While three such steps 44 are illustrated, it will be readily appreciated that this is merely exemplary and there may in fact be any number of such steps 44, in any spatial arrangement along the axial wall of the end of the bone.

As the reamer 30 is rotated over the end of the bone, indentations 44 are formed around the whole of the circumference of the end of the bone, such that the indentations are identical at all angular positions. Each indentation 44 may be formed from a single formation 42 on the interior surface of the reaming cavity 36, or may be formed from the combined actions of multiple similar formations at spaced apart positions around the circumference of the shell wall 34. The reaming cavity 36 has a closed end arranged to limit insertion of the end of the bone into the cavity. The final prepared outer surface of the bone is formed when the end of the bone contacts the closed end of the reaming cavity.

Figure 2 illustrates the surface of the end of the bone generally tapering towards its free end (illustrated still inserted into cavity 36). The angle of general taper is such that it matches the tapering cavity of a resurfacing implant to be positioned over the end of the bone. During the final stage of positioning of the resurfacing implant it may be that there is some or total deformation of the indentations 44, particularly as relative raised portions between indentations are compressed.

Advantageously, an end of a bone prepared using a reamer of the form illustrated in figure 2 provides for a more controlled required force to seat a resurfacing implant. The amount of contact between the surface of the bone and the interior surface of the implant can be controlled and varied more accurately than for a bone prepared using a convention reamer.

In alternative embodiments of the invention, the reamer illustrated in figure 21 may be modified by replacing the slit like cutting teeth with raised compacting surfaces adapted to compress portions of the surface of the bone inwards to form the indentations.

Referring now to figure 3, this illustrates in cross section a reamer 30 suitable for preparing the surface of an end of a bone in accordance with a second embodiment of the present invention. Figure 3 further illustrates a partially or fully prepared end of the femur 6. In accordance with embodiments of the present invention the femur 6 may be initially prepared using a conventional reamer as described above in order to approximately shape the femur 6 to be received within an axial cavity within the reamer.

Reamer 30 comprises a reamer shell 32 formed from a shell wall 34 that defines a reaming cavity 36. Reaming cavity 36 is generally circular in cross section through a longitudinal axis defined by the reamer 30 parallel to the axis of femur 6. Reaming cavity 36 tapers towards its closed end, and thus matches the taper of femur 6 towards its free end.

The interior surface of reaming cavity 36 is provided with formations 42 that are directed inwardly arranged to shape the surface of the end of the femur. The formations 42 thus form indentations 44 about the surface of the end of the femur. Similarly to the embodiment of the invention described above in connection with figure 2, preferably said indentations are distributed about the end of the femur such that the indentations at the prepared end of the femur are rotationally symmetrical. The reaming cavity 36 has a closed end arranged to limit insertion of the end of the bone into the cavity. The final prepared outer surface of the bone is formed when the end of the bone contacts the closed end of the reaming cavity.

The formations 42 comprise raised compacting surfaces adapted to compress portions of the surface of the bone inwards. Indeed, formations 42 may comprise any formation that can modify the surface profile of the bone by compressing the bone surface inwardly to form indentations.

Figure 3 further illustrates the reamer comprising a flattened upper surface 60. Flattened upper surface 60 is adapted to be subject to an impaction force along the axis of the reamer 30 and the femoral bore 40, as indicated by arrow 62. The impaction force may be provided by a hand operated impactor, such as a mallet, or a powered impactor. Thus, reamer 30 is arranged to be moved relative to the end of the bone in an axial direction such that the end of the femur 6 is progressively received in reaming cavity 36.

Formations 42 therefore deform the surface of the bone in axially extending indentations 44, in the surface of the bone which appear as grooves or steps. Figure 3 illustrates the indentations 44 being formed as continuous circumferential grooves or steps, corresponding to continuous banded circumferential formations 44 about the interior surface of cavity 36. However, it will be readily apparent that this need not be the case. The formations 42 may be configured so as to form axially extending grooves along part or all of the longitudinal extent of the portion of the end of the bone received in cavity 36 (the start and finish points of such grooves being dependent upon the axial position of the formations 42 within cavity 36). Alternatively, circumferential grooves may be provided that extend around only part or multiple discrete parts of the bone surface.

Furthermore, although the deforming action occurs as the reamer 30 is forced onto the end of the femur in an axial direction, it may be that the indentations are the result of two or more sequential applications of the reamer 30, with the reamer completely removed and repositioned at a new angular position prior to being reapplied. That is, the or each formation 42 within the reaming cavity 36 may result in two or more separate indentations 44 in the surface of the bone.

While three circumferentially extending steps 44 are illustrated, it will be readily appreciated that this is merely exemplary, and the precise pattern of indentations is dependent upon a range of factors, such as the properties of the bone being removed, the precise shape of the resurfacing implant and the degree of bone compression and lateral force when the resurfacing implant is ultimately placed over the end of the femur. While the precise pattern of indentations may vary, it is particularly preferred that the pattern of indentations remains circumferentially symmetrical, that is having rotational symmetry of at least the second order. This circumferential symmetry ensures that bone compression is even about the bone and implant axis, resulting in a correctly positioned implant that is able to correctly transfer loads to the femur from the hip joint.

As for the embodiment of figure 2, each indentation 44 may be formed from a single formation 42 on the interior surface of the reaming cavity 36, or may be formed from the combined actions of multiple similar formations at spaced apart positions around the circumference of the shell wall 34, or positions spaced apart along the shell wall 34 in the axial direction.

Figure 3 illustrates the surface of the end of the bone generally tapering towards its free end (illustrated still inserted into cavity 36). The angle of general taper is such that it matches the tapering cavity of a resurfacing implant to be positioned over the end of the bone. During the final stage of positioning of the resurfacing implant it may be that there is some or total deformation of the indentations 44, particularly as relative raised portions between indentations are compressed.

Advantageously, an end of a bone prepared using a reamer of the form illustrated in figure 3 provides for a more controlled required force to seat a resurfacing implant. The amount of contact between the surface of the bone and the interior surface of the implant can be controlled more accurately than for a bone prepared using a convention reamer.

In accordance with alternative embodiments of the present invention, in place of the raised compacting surfaces illustrated in figure 3, the formations may comprise slit like cutting teeth of a conventional form as described above in connection with figure 2, while the reamer continues to be applied by applying an impaction force axially forcing the reamer onto the end of the bone. Such slit like cutting teeth are adapted to cut surface portions of the bone where they contact the end of the femur and moved relative to the femur.

In order to ensure a close interference fit between the femur and the axial cavity of an implant it is preferable that the general taper of the axial implant cavity wall and the prepared femur relative to the axis of the implant pin is less than 10°. More preferably, the taper is less than 5°, for instance about 3°. Consequently, it is preferred that the general angle of the interior surface of the reamer is within these ranges. The degree of compression of the bone when an implant is fully seated upon femur is a key factor in determining how firmly the implant is attached. The smaller the angle of the walls of the cavity and the prepared femur to the axis of the implant pin, the tighter the interference fit will be. However, the tightness of the fit is reduced if the surface of the bone that is indented in increased. This is due to there being a smaller surface area of bone within an implant cavity that must be compressed. Thus reamers in accordance with embodiments of the present invention allow for greater control of the fit of an implant over the end of a bone.

Advantageously, reamers in accordance with embodiments of the present invention provide for a closely controlled fit between the bone and the implant obviating the problems identified above relating to a poor size match between the femur and the implant. In particular, the improved reaming allows the implant to be fully seated, thus encouraging bone ingrowth into the porous structure on the interior surface of the implant resulting in a stronger implant. As the implant is fully seated, the possibility of cut bone being left exposed is reduced. The depth of indentations generated in the surface of the bone may very according to the size of the formations on the interior surface of the reamer shell. However, it is preferable that the indentations are less than 5mm deep, for instance less than 2mm deep. Most preferably, the indentations are less than or equal to lmm deep. If the indentations are excessively deep then bone material may not be in contact with the whole of the exterior surface of the end of the bone when the implant is fully seated on the end of the bone. It is preferable that the indentations are at least 0.1mm deep in order to obtain a significant reduction in the impaction force required to fully seat the implant onto the end of the bone.

While the present invention has been predominantly described herein in the context of the preparation of the end of a femur in order to receive an implanted resurfacing implant onto the head of a femur, it will be readily apparent the invention is not limited to this. The present invention may be readily applied for preparing a bone in any situation in which as prosthesis is to be applied to the end of a bone, for instance a resurfacing humeral component of a prosthetic shoulder joint. Further advantages and applications of the present invention will be readily apparent to the appropriately skilled person without departing from the scope of the appended claims.

Claims

CLAIMS:

1. An instrument for preparing the end of a bone to receive a joint prosthesis component forming a ball component of an artificial ball and socket joint, the instrument comprising: a shell for shaping bone around the end of a bone, the shell comprising a cavity defined by a shell wall and extending along a longitudinal axis of the instrument, the cavity being arranged to receive the end of the bone; wherein the shell wall comprises at least one formation directed inwardly and arranged to engage a generally axially extending outer surface of an end of a bone received in the cavity to form a prepared outer surface of the end of a bone to receive the joint prosthesis; wherein the at least one formation is arranged to form at least one indentation in the prepared outer surface of the end of the bone.

2. An instrument according to claim 1, wherein the at least one indentation extends around at least part of the circumference of the prepared outer surface of the end of the bone.

3. An instrument according to claim 1, wherein the at least one indentation extends along the prepared outer surface of the end of the bone in the same plane as the axis of the instrument.

4. An instrument according to any one of claims 1 to 3, wherein the shell cavity further comprises a closed end arranged to limit the insertion of the end of the bone into the cavity.

5. An instrument according to claim 4, wherein the shell is arranged to form the prepared outer surface of the end of the bone when the end of the bone is fully inserted into the cavity and in contact with the closed end of the cavity.

6. A instrument according to any one of claims 1 to 5, wherein the instrument is operable to form indentations distributed about an end of a bone such that the indentations have rotational symmetry of at least the second order about a longitudinal axis of the end of the bone.

7. A instrument according to any one of claims 1 to 6, wherein the shell wall comprises two or more formations operable to form two or more indentations in the surface of the end of the bone.

8. A instrument according to claim 2, wherein the or each formation is operable to cut the surface of the end of the bone to form the indentation.

9. A instrument according to claim 8, wherein the or each formation comprises a slit like cutting tooth directed inwardly and operable to cut the surface of the bone

10. A instrument according to claim 3, wherein the or each formation is operable to compress inwardly the surface of the end of the bone to form the indentation.

11. A instrument according to any one of claims 1 to 10, further comprising an instrument guide which depends from the closed end of the cavity along the axis of the instrument, the instrument guide being operable to be received within a bore provided in the end of the bone.

12. A instrument according to claim 2, further comprising a handle operable to be manipulated to apply a rotational motion to the instrument about the axis of the instrument such that the or each formation is brought into contact with the surface of the bone in a direction at an angle transverse to the axis of the instrument.

13. A instrument according to claim 3, further comprising an impaction surface operable to receive an impaction force thereby imparting an axial motion to the instrument such that the or each formation is brought into contact with the surface of the bone in a direction generally parallel to the longitudinal axis of the instrument.

14. A instrument according to claim 2, wherein the interior surface of the cavity is generally circular in cross section and diverges towards its open end such that the angle between the interior surface of the cavity and the axis of the instrument is less than 5°.

15. A instrument according to any one of claims 1 to 3, wherein at least one formation is arranged to form at least one indentation that is between 0.1mm and lmm deep.

16. A instrument according to any one of claims 1 to 3, wherein the instrument is for preparing a femoral head to receive a femoral resurfacing implant forming part of a hip joint prosthesis.

17. A method of preparing the end of a bone to receive a joint prosthesis component forming a ball component of an artificial ball and socket joint, the method comprising: inserting an end of a bone into a cavity of an instrument shell, the cavity being defined by a shell wall and extending along a longitudinal axis of the instrument, the shell wall comprising at least one formation directed inwardly; and bringing the or each formation into contact with a generally axially extending outer surface of the end of the bone to form a prepared outer surface of the end of a bone to receive the joint prosthesis; wherein the at least one formation is arranged to form at least one indentation in the prepared outer surface of the end of the bone.

18. A method according to claim 17, wherein the at least one indentation extends around at least part of the circumference of the prepared outer surface of the end of the bone.

19. A method according to claim 18, further comprising applying a rotational force to the instrument such that the at least one formation is brought into contact with the surface of the bone in a direction at an angle transverse to the axis of the instrument.

20. A method according to claim 17, wherein the at least one indentation extends along the prepared outer surface of the end of the bone in the same plane as the axis of the instrument.

21. A method according to claim 20, further comprising applying an axial impaction force to the instrument such that the at least one formation is brought into contact with the surface of the bone in a direction generally parallel to the axis of the instrument.

22. A method according to any one of claims 17 to 21, further comprising inserting the end of the bone into the cavity until the end of the bone contacts an end wall of the cavity to form the prepared outer surface of the end of the bone.