Note: Descriptions are shown in the official language in which they were submitted.
101520CA 02264283 l999-02- 19WO 98/07393 PCT/NZ97/00100A JOINT REPLACEMENT PROSTHESISFIELD OF INVENTIONThe invention comprises a joint replacement prosthesis, a method of fitting a jointreplacement prosthesis, and tools for use in fitting a joint replacement prosthesis.BACKGROUND OF INVENTIONHip joint replacement has been a very successful procedure for the treatment of elderlypatients suffering from osteoarthritis. On the femoral side the prosthesis consists of ametal ball supported by a short metal neck attached to a stem that is inserted into themedullary cavity of the femur. The implantation of these devices therefore requires theremoval of a large part of the proximal femur (head and neck) and the reaming ofcancellous bone from the medulla. The procedure not only relieves pain but restoresmobility. Where the acetabulum is also resurfaced, the acetabular component of a hipreplacement prosthesis typically comprises an acetabular cup fabricated in ultra highmolecular weight polyethylene (UHMWPE), which runs against a metal or ceramic ballof the femoral component.Recent improvements in stem design and cementing techniques have significantlyreduced the incidence of loosening of the femoral component. However, for youngpatients there is still a high incidence of implant failure. For patients less than thirtyyears of age, loosening of at least one component (femoral or acetabular) is far more101520CA 02264283 l999-02- 19W0 98l07393 PCT/NZ97/00100likely to occur.As well as providing maximum resistance to loosening, another requirement for a jointreplacement prosthesis is that it must be wear resistant at the joint interface and alsobio-compatible at the bone interface.SUMMARY OF THE INVENTIONThe invention provides an improved or at least alternative joint replacement prosthesis,and method of fitting a joint replacement prosthesis, and also tools for use in fitting ajoint replacement prosthesis.In broad terms in a first aspect the invention comprises a joint replacement prosthesisto replace a superior joint surface, comprising a conical base part and a hemisphericalor part hemispherical top providing a hemispherical or part hemispherical replacementbearing surface, the prosthesis being shaped and sized to be a wedge fit in a conicalrecess formed in the head or neck of a human femur or humerus without removal of allof the neck of the femur or humerus.The angle of the sides of the conical base part may be in the range 40 to 120 degrees,and preferably 50 to 90 degrees.The prosthesis may be a one part prosthesis or may be a two part prosthesis comprisinga conical base part and a hemispherical or part hemispherical top part which are coupled101520CA 02264283 l999-02- 19WO 98/07393 PCTINZ97/00100together. The conical base part or the hemispherical top part of a two part prosthesismay incorporate a protrusion from the base part or the top part which engages into arecess in the top part or the base part to couple the conical base part and thehemispherical top part to form the prosthesis, for example.Two part prostheses of the invention preferably comprise at least one access holethrough the base part to enable a screw or other fastening device to pass through thebase part to assist in securing the base part in the conical recess in the femur orhumerus. A one part-prosthesis may also optionally comprise at least one access holethrough the prosthesis to enable a screw or similar to pass through the prosthesis toassist in securing the prosthesis in place.Preferably the conical base of the prosthesis comprises a pin extending from the apex ofthe conical base to further assist in locating the conical base in the conical recess in thefemur or humerus.Preferably the external surface of the conical base comprises a roughened surface, toencourage bone regrowth into the roughened surface after initial fitting of the prosthesis.Preferably the conical base may have a roughened surface plus additionally have acoating of biological material e.g. hydroxyapatite, calcium, phosphate, or a boneenhancing material.Preferably the conical base comprises one or more fins extending radially from the101520CA 02264283 1999- 02 - 19WO 98/07393 PCT/NZ97/00100conical base to engage into bone to further assist in locating the prosthesis.Preferably the conical base includes a number of ribs on the exterior of the conical base.Preferably the conical base part of a two part prosthesis is formed from a metal or metalalloy, such as titanium or titanium alloy or other biocompatible metal. Thehemispherical top part is preferably formed from a biocompatible wear resistant materialsuch as alumina, an alloy of cobalt, or other biocompatible hard ceramic or metalmaterial.Preferably a one part prosthesis is formed from a metal or metal alloy such as titaniumor titanium alloy, covered over the hemispherical or part hemispherical top surface ofthe prosthesis with alumina or other biocompatible hard ceramic material to form thereplacement bearing surface.In broad terms in a second aspect the invention comprises a method of fitting a jointreplacement prosthesis, comprising:drilling a guide hole into the head of a femur or humerus bone coincident with the axison which a prosthesis providing the replacement bearing surface of the joint is to befixed,drilling a conical recess into the head of the femur or humerus coaxially with the guidehole using a cutting tool comprising a pin which enters into the guide hole to correctly101520CA 02264283 1999-02-l9WO 98/07393 PCT/NZ97/00100align the cutting tool, andfixing a prosthesis component comprising a conical base and a hemispherical or parthemispherical top surface into the conical recess in the bone.In broad terms in a third aspect the invention comprises a tool for use in fitting a jointreplacement prosthesis, comprising a contact surface at one end of the tool to pressindirectly against the end of a bone opposite to the end to which the joint replacementcomponent providing the superior surface of the artificial joint is to be fitted, andcomprising a contact surface at an opposite end of the tool to bear against the superiorsurface of a joint to be replaced on the other end of the bone, before removal thereof,and a hole through the tool part which bears against said superior surface through whicha drill bit can pass to guide drilling of a hole into the bone at a correct angle.The invention will be further described with reference to hip joint replacement in whichthe prosthesis component providing the superior joint surface is fitted to the femur anda cup component if fitted is optionally also fitted to the acetabulum if required, but theinvention may also have application to shoulder joint replacement where the prosthesiscomponent is fitted to the humerus.BRIEF DESCRIPTION OF THE FIGURESThe invention will be further described with reference to the accompanying drawings,and by way of example and without intending to be limiting. In the drawings:101520CA 02264283 l999-02- 19W0 98/07393 PCT/NZ97/00100Figure 1 shows in diagrammatic cross-section a typical prior art hip replacementprosthesis fitted to a hip joint,Figure 2 shows a preferred form of two part hip replacement prosthesis of the inventionsimilarly fitted to a hip joint,Figure 3 is a cross-sectional view through the preferred form two part hip replacementprosthesis, which provides the superior surface of the artificial joint,Figure 4 shows the hemispherical top part of the preferred form two part prosthesis ofFigure 3,Figure 5 shows the conical base part of the preferred form two part hip replacementprosthesis,Figure 6 is a view of the conical base part of the preferred form two part prosthesis inthe direction of arrow A in Figure 5,Figure 7 shows in diagrammatic cross-section a preferred form one part prosthesis of theinvention fitted to a femur,Figures 8a to 8d show steps in fitting a hip replacement prosthesis according to themethod of the invention,101520CA 02264283 1999-02-19WO 98/07393 PCT/NZ97/00100Figure 9 shows a preferred form of tool for use in drilling a pilot hole during fitting ofa hip joint replacement prosthesis according to the method of the invention,Figure 10 shows a cutter for use in a drill, for forming a conical recess in bone duringfitting of the joint replacement prosthesis,Figure 11 shows a slide hammer for use in fitting the conical base part of the prosthesis,andFigure 12 shows the preferred form of tool of Fig 8 fitted to a femur bone,Figure 13 diagrammatically shows in cross-section a prosthesis of the invention fitted toa femur bone illustrating how, over time after fitting, regrowth of bone into theroughened exterior surface of the conical base part of the prosthesis can occur to assistin locating the prosthesis.DETAILED DESCRIPTIONFigure 1 shows a typical prior art hip replacement prosthesis. On the femoral side theprosthesis consists of a metal ball 2 on a short metal neck 3, attached to a stern that isinserted into the medullary cavity in the femur 1. Installation of the device requiresremoval of a large part of the head and neck of the femur and reaming of cancellousbone from the femur. Where the acetabulum is also relined, an acetabular cup 4typically fabricated from ultra high molecular weight polyethylene (UHMWPE) is fixed101520CA 02264283 1999-02-19WO 98/07393 PCT/NZ97/00100in position with a bone screw (see screw 9 in Fig 2) or is bonded onto the surface of theacetabulum 5 using a methyl-methacrylate or similar cement. Alternative methods as willbe known in the art may also be used.Figure 2 shows a preferred form two part prosthesis of the invention similarly fitted toa hip joint. Referring also to Figure 3, the preferred form two part prosthesis comprisesa conical base part 6 which is fitted into a conical recess cut in the head of the femur,and a hemispherical top part 7. Where the acetabulum is also relined, the prosthesisbears against an acetabular cup component 8.The conical base of the prosthesis is a wedge fit into the conical recess cut in the headof the femur which provides maximum resistance to loosening, and the size of theprosthesis is such that only a part of bone material of the head and neck of the femuris removed, and part of the head or at least all or a substantial part of the neck of thefemur remains. Because the prosthesis of the invention does not require that all of theneck of the femur be removed, the option of fitting a conventional prosthesis later in thelife of the patient remains if necessary. In a two part prosthesis as shown, screws willusually be used to assist in securing the prosthesis into the conical recess in the head ofthe femur in addition to the seating of the conical base into the conical recess, but in aprosthesis having a steeper conical base, the use of additional securing screws may notbe required, as will be further described.Referring particularly to Figures 3 to 6, the two part prosthesis comprises a conical basepart 6 and a hemispherical top part 7 as described. The conical base part 6 is typically101520CA 02264283 l999-02- 19W0 93/07393 PCT/NZ97/00100formed from a metal such as titanium or titanium-alloy. Titanium and its alloys are"bone friendly" and easy to machine (but are less preferred for bearing surfaceapplications). The conical exterior surface 6a of the conical base part is preferablyroughened to provide a surface into which bone regrowth will occur in the months afterthe prosthesis has been surgically fitted to a patient. The roughened surface is preferablyformed by electro-discharge machining more commonly known as spark erosion, whichinvolves the removal of material using an electrical discharge from an electrode heldabove the surface to be modified, in a bath that contains an electrolyte such as kerosene.The electrode is precisely supported above the surface to be eroded by electro-mechanical feedback control, and is the negative profile of the surface. In the sparkerosion process parts of the conical exterior surface of the conical base part are removedso that the surface is eroded to a complex shape. This provides a surface comprising amultitude of small cavities into which bone regrowth will occur to enhance fixation of theprosthesis and minimise loosening. Figure 13 shows diagrammatically a cross sectionthrough the top of a femur some months after fitting of a prosthesis component of theinvention showing bone regrowth into the spark eroded lower surface of the conical basepart 6. The depth of the spark eroded surface is enlarged for illustrative purposes.Spark erosion is a preferred technique for forming a roughened surface on the conicalexternal surface 6a of the base component 6, but other techniques for forming aroughened surface may be less preferably utilised such as chemical etching for example.The conical base 6 Comprises a number of holes l0 axially through the base componentto enable fastening devices such as screws to pass through the base part into bone tosecure the base part to bone (see also Figure 8c). In the preferred form the conical base101520CA 02264283 1999-02-19WO 98/07393 PCT/NZ97/00100part comprises six screw holes through the part whereas in general only three fixingscrews will be fitted when fitting the prosthesis, to give more choices of position forscrew placement by the surgeon. For example if on fitting the prosthesis part 6 in placeduring surgery, one of the screw holes 10 is found to be positioned in a weakened areaof bone then the surgeon can use an adjacent fixation hole. However it is possible thatmore or less than three screws may be utilised and that more or less than six screw holesmay be provided through the base part 4.A protrusion 6b extends from the opposite side of the conical base part and into a recess7a in the underside of the hemispherical top part 7, to couple the hemispherical top partand the conical base part. The protrusion 6b preferably has a slight taper to a reduceddiameter at its distal end, and the recess 7a in the hemispherical top part has a similartaper, to assist in holding the hemispherical top part on the conical base part together.Alternatively the protrusion may extend from the underside of the hemispherical top partinto a recess in the conical base part. Further, instead of a single central protrusion itis possible that two or three smaller pins from the conical base part may extend intomatching holes in the underside of the hemispherical top part or vice versa.Preferably the hemispherical top part 7 is formed of a ceramic such as alumina of highdensity, the hemispherical or part hemispherical external surface of which is highlypolished, to provide a bearing surface with minimum friction and the longest possibleworking life. With the prosthesis of the invention the surface or part of the prosthesiswhich contacts bone may be formed of a "bone friendly" metal such as titanium ortitanium alloy, while the bearing surface of the prosthesis which should have minimum10101520CA 02264283 l999-02- 19WO 98/07393 PCT/NZ97/00100friction and maximum resistance to wear can be formed of a very hard material such asa high density ceramic material or a CoCrMo alloy or other hard bio compatiblematerial, polished to a smooth bearing surface.The curved external bearing surface of the hemispherical top part 7 may comprise asurface of the material from which the hemispherical top part is formed which has beenmachined and polished. Alternatively the external surface of the hemispherical top partmay first be coated with a harder material such as titanium nitride, by a technique suchas vacuum deposition or similar.The conical part 6 base may be provided with three fins 11 or more or less such fins,which during fitting of the prosthesis will engage into the bone surface to enhancerotational stability of the device. Further, the conical external surface 6a of the base partmay be provided with one or more ribs from the wider periphery of the base parttowards the apex or around the external surface 6a or in any other configuration, tofurther assist in locating the conical base part against rotation. The ribs may be presenton the external surface of the conical base part before spark erosion so that the conical,ribbed surface is roughened by spark erosion as previously described. The ribs or finsmay be replaced by other protruding features as will be known in the art. Features aspins for example could also be used although this should not be seen to be limiting. Theessential effect is to prevent rotation of the prosthesis when in place.In the preferred form the conical base part also comprises a pin portion 6c extendingfrom the apex of the conical base part which when the prosthesis is installed according11101520CA 02264283 1999- 02 - 19WO 98/07393 PCT/NZ97/00100to the method of the invention will further assist in locating the prosthesis.Figure 7 shows a preferred form one part prosthesis of the invention fitted to the headof a femur. The one part prosthesis comprises a conical base 6 which is similarly fittedinto a conical recess cut in the head of the femur, and a part spherical top surface 7.The one part prosthesis is typically formed from a metal such as titanium or titanium-alloy. The conical exterior surface of the conical base is preferably roughened to providea surface into which bone regrowth will occur as referred to previously, and is againpreferably formed by spark erosion. The top surface 7a of the prosthesis is preferablycoated with a harder material such as titanium nitride, by a technique such as vacuumdeposition or similar, and is machined and polished.The angle of the walls of the conical base of the one part prosthesis shown is steeper sothat the prosthesis is a wedge fit into the conical recess in the head of the femur, andadditional screw fixing or similar may not be required. However, in another form of aone part prosthesis of the invention a screw may pass through an access hole through thecentre of the prosthesis for example and into bone below the prosthesis. The head ofthe screw is recessed below the top surface 7 of the prosthesis. Such an additional fixingscrew may not be required however. The one part prosthesis may be provided with fins11 or similar, as described above for the two part prosthesis of the invention, andcomprises a pin portion 6c extending from the apex of the conical base of the prosthesiswhich assists in locating the prosthesis.Figures 8a to 8d show fitting of a prosthesis of the invention to a femur utilising the12101520CA 02264283 1999-02-19WO 98/07393 PCT/NZ97/00100method of the invention. First, a pilot or guide hole 12 is drilled into the femurcoincident with the axis on which the prosthesis is to be fixed. This guide hole 12 isdrilled through the centre of curvature of the superior surface of the femoral head asshown.To ensure correct alignment of the guide hole a tool such as that shown in Figures 9 and12 may be used. In Figure 12 the tool is shown for illustrative purposes fitted to acadaveric femur bone. The tool comprises a Contact surface, suitably formed by aï¬exible cup 13 or similar at one end, and a movable arm 14 at the other end. Themovable arm 14 is slidably coupled to the shank 15 of the tool and may be fixed inposition by a screw coupling 16 which may be tightened or clamped by other suitablemeans which will hold the arm 14 in position when locked. During a surgical procedurethe patientâs knee is bent and the cup 13 is fitted to bear against the knee. The arm 14is moved along shank 15 until metal l()cating cone 17 is fitted against the femoral head,and then the arm 14 is locked in position. The guide hole 12 into the femoral head isdrilled through a correctly angled hole in the end of the movable arm to guide drillingof the hole 12 into the femoral head at the correct angle. In Figure 12 a drill bit 18indicates the angle of the drilling guide hole through the end of the movable arm 14 andlocating cone 17. The angle of the locating cone 17 can be adjusted by the surgeon priorto beginning drilling of the guide hole 12.Referring again to Figures 8a to 8d, after the guide hole 12 has been drilled the tool isthen removed, and a conical recess 19 is then drilled into the femoral head, coaxiallywith the guide hole 12 previously formed, using a suitable cutting bit. One form of13101520CA 02264283 l999-02- 19WO 98/07393 PCT/NZ97I00100cutting bit which is fitted to a surgical drill (similar to a drill bit) is shown in Figure 10.The cutting bit comprises teeth 20 shaped similar to a countersink, so that the cutting bitwill form a conical recess as shown in Figure 8b, the angle of the side walls of whichmatches the angle of the conical exterior surface 6a of the conical base part 6 of theprosthesis. The cutting bit comprises a centre pin 20a which extends beyond the teeth20 of the bit. In use the central pin is initially inserted into the axial guide hole 12before cutting of the conical recess 19 begins, to ensure correct location and orientationof the conical recess. A collar 21 around the exterior does not rotate with the bit andcan be held by the surgeon.After the conical recess as shown in Figure 8b has been formed as described, theprosthesis is fitted in place. In the case of a one part prosthesis the prosthesis may betapped into place using a small rubber hammer for example. In the case of a two partprosthesis as shown, the conical base part 6 of the prosthesis is first fitted in place.When the conical base part is fitted the pin 6c on the apex end thereof will engage intothe remaining part of the guide hole 12, to further assist in correctly locating theprosthesis. A slide hammer may be used to knock the base part into position. forexample. Figure 11 shows a suitable form of slide hammer, comprising a shaft 22 witha sliding weight 23. In Figure 11 a conical base part 6 of a prosthesis of the inventionis shown on the end of the slide hammer. Such a slide hammer may be used to tap theconical base part of the two part prosthesis into position as shown in Figure SC, toengage the radially extending fins 11 and ribs if provided on the external conical surfaceof the conical base into the femoral bone. After the conical base part has been fittedinto the conical cavity in the femoral head as shown in Figure 8c, bone screws 24 are141015CA 02264283 l999-02- 19WO 98/07393 PCT/NZ97/00100fitted and screwed securely home to further fix the prosthesis part to the femoral head.The hemispherical top part 7 may then be fitted over the protrusion 6b of the conicalbase part, to complete fitting of the prosthesis, as shown in Figure 8d.Figure 13 shows enlarged and diagrammatically bone regrowth into the spark erodedroughened surface of the conical base part at the interface between the prosthesis andfemoral bone. Such regrowth will occur over a period of months following fitting of theprosthesis, and further locks the prosthesis in place to inhibit subsequent loosening. Asstated spark erosion is one preferred means for roughening the external surface of theconical base part of the prosthesis but the bone contact surface of the prosthesis may beroughened by other means, and/ or may incorporate hydroxyapatite, calcium, phosphate,or some other bone enhancing material.The foregoing describes the invention including a preferred form thereof. Alterationsand modifications as will be obvious to those skilled in the art are intended to beincorporated within the spirit and scope of the invention disclosed and as defined in theappended claims.15
