This invention relates to endoprosthetic bone implant devices for joint replacement, particularly for total hip replacement.

Surgical removal of the natural joint and replacement with an artificial joint is practised in the treatment of severely diseased joints, for example, arthritic joints. The procedure which is carried out routinely in total hip replacement surgery involves resecting the femoral head and neck, shaping the femoral canal using reamers and rasps and introducing an endoprosthetic femoral component. This component generally consists of a metallic stem which locates down the medullary canal of the femur, and includes a head and neck portion. The ste is fixed into the canal using a self-curing acrylic cement, or the stem is pressed into place, relying on a good mechanical fit. Numerous designs of femoral components have been proposed in the past, some intended for cemented fixing and others for press-fitting. Cemented stems have generally had a good clinical record with only a small percentage of outright failures at a 10 year follow-up in old and generally inactive patients. However, for longer periods, or when the patients are younger and/or more active, failures are more common. The modes of failure include bone reεorption at the cement-bone interface due to high stresses, and cement cracking due to fatigue and stress

concentrations. Uncemented press-fit stems have been used most often in the younger active group. A number of problems have been encountered with these patients. An accurate fit has been difficult to achieve due to the complex curvatures of the intramedullary cavity of the femur and the variety of shapes and sizes which occur. This lack of fit is believed to have been in large measure responsible for the incidence of pain on activity and the eventual loosening due to bone resorption. Loosening has also led to subsidence of the stem within the canal which has led to reduced range of motion and even to dislocation if the shortening is too great. Another problem has been an incidence of splitting of the femur either at surgery or on activity, because of a wedge-shaped metallic stem causing hoop stresses in the bone. The splitting has led to subsidence with similar effects. Finally, over a long period of time, the canal of the femur expands in diameter, a process of aging, which leads to loosening of both cemented and press-fit stems.

The present invention is directed to the alleviation of the problems discussed above and to the provision of endoprosthetic joint devices which have other advantageous eatures.

One approach to the solution of the above listed problems involves a method of determining the optimum shape and dimensions of a skeletal implant by following a

series of defined steps.

A second approach is to define specific design features of a skeletal implant which have been found to have important practical advantages.

According to one aspect of the present invention therefore, there is provided an endoprosthetic joint device, especially a femoral joint component, having a stem adapted to be located within a canal of a bone forming part of a joint, said stem including a proximal portion which is a close fit in the bone canal and a distal portion which is tapered and an intermediate portion, shaped to provide clearance between the stem and the bone canal .

According to a second aspect of the invention, there is provided a method of determining the shape and dimensions of a femoral implant which comprises the following steps:-

(a) establishing the inner and outer contours of an average femur by measurements taken from a significant number of specimens, (e.g. at least 15 to 20), and feeding the data into a computer as a series of x, y and z coordinates,

(b) simulating on the computer the surgical resection of the neck and head of the femur and reaming a canal lengthwise of the representation of the femur,

(c) producing the proximalmoεt section of the ste by drawing a lateral circle on the resected top cf the representation of the femur having a diameter which corresponds approximately with the distal end of the bone canal, an anterior circle of the same diameter and a medial circle drawn as a best fit on the medial aspect of the resected top and expressing the outline of the proximalmoεt section of the stem of the implant to fit in the resected top by drawing mutual tangents to the lateral, anterior and medial circles, and

(d) representing the exterior contour of the stem of the implant by a cubic curve which joins the proximalmoεt section to the distal half cylindrical part of the stem, and which is a close fit proximally to the bone canal, while the distal half is a εliding fit distally.

The present invention deriveε in part from the realisation that it is highly desirable to design the prostheεis εo that in uεe the bone iε loaded in a εimilar way to the normal condition in the body. Thus, for example, in the case of a hip implant, the prosthesis should transmit the load to the bone from the upper part of the femur downwards in such a way that there iε a gradual load transmission from the upper part of the stem downwardly to the bone in the lower part. Overall loading of the bone iε important becauεe bone which is unloaded will tend to reεorb. Converεely, if the implant

transmits a high load concentration to a particular area of the bone, εtresεes are set up which cause bone damage. The aim iε to produce a profile of the upper part of the εtem which iε a good fit in the proximal bone canal and a diεtal portion which applieε a gradual and uniform load diεtribution to the femur in the region of the diεtal canal. In order to achieve the beεt approximation to thiε objective, to avoid εtreεε concentrationε and to design a prosthesis which iε capable of being εurgically inserted, the intermediate portion of εtem iε εhaped to provide clearance in the bone canal and the diεtal portion of the εtem is tapered over at least part of its length.

Avoidance of micro-motion of the tip of the εtem radially of the bone canal iε alεo highly deεirable. In the caεe where the proεtheεiε of thiε invention iε intended to be a presε-fit in the bone canal, micro-motion can be contained by inεerting a plaεtic εleeve or cap into the diεtal canal which iε a tight fit in the diεtal-most part of the canal and in which the diεtal end of the εtem iε tightly held.

As will be described hereinafter, the invention includes a specific design of εleeve which not only limitε micro-motion but εpreadε the load generated by micro- εwinging motion of the prostheεiε.

In the caεe of preεε-fit proεtheεiε, the diεtal-moεt portion iε preferably cylindrical.

Where the prostheεiε iε intended for cemented fixing in the bone canal, the preciεe εhape of the distal portion has wider latitude since the εtem will be spaced from the interior of the bone canal by a cement mantle. However, correct load distribution iε juεt as important and it is an important feature of the implantε of the preεent invention that they can be employed in both cemented and cementleεε fittingε. In general, the method of determining the εhape and configuration of the implant iε the εame for cemented aε for presε-fit implants. However, in the case of the former, the bone is sized initially for a presε-fit prosthesis and then downsized to provide a sufficient clearance overall to allow for a cement mantle but with the εame relative dimenεionε. Thiε approach iε illustrated in Figures 6A and 6B of the accompanying drawingε.

According to a further aεpect of the preεent invention, therefore, there iε provided an endoproεthetic femoral joint implant which compriεeε a εtem adapted to be located and cemented in place in a femoral bone canal, εaid εtem including a proximal portion which iε εhaped to conform with the bone canal but with a predetermined εmall clearance intended to be filled with cement and a diεtal portion which iε tapered progressively from the proximal portion so aε to provide in the cemented implant a gradually thicker cement mantle between the implant and

the bone canal and a load distribution, when said implant is inserted from the proximal end, which is concentrated in the proximal end of said femoral bone.

The invention also includeε improved methodε of fitting bone implantε into bone canalε, in which a cannulated εtem iε uεed to enable the stem to be guided into the canal on a guide rod or to enable a presε-fit plaεtic εleeve to be located over the distal end of the εtem and thereby reduce micromovement between the stem and the bone.

The present invention is intended to be applied mainly to femoral joint componentε, but the principleε and deεignε deεcribed hereinafter are adaptable to other jointε.

Other aεpectε and featureε of joint componentε conεtructed in accordance with thiε invention will be apparent from the following detailed deεcription and accompanying drawingε, in which:-

Figure 1 iε a repreεentation of an average femur taken from meaεurementε of 26 εpecimenε.

Figure 1A iε a view taken in the direction of the arrow A in Figure 1.

Figure 2 iε a plan view taken in the direction of arrow B in Figure 1.

Figure 3 iε a top plan view of the εtem of a hip proεthesiε εhowing alεo the outline of the bone canal.

.Figures 4A and 4B are lateral and posterior viewε showing the fit of the stem in the proximal femur.

Figureε 5A, 5B and 5C illustrate the manner of forming a symmetrical version of the stem of the proεtheεiε.

Figureε 6A & 6B εhow reεpectively anterior-poεterior (A-P) views of cemented and preεε-fit proεtheεiε in accordance with the invention.

Figureε 7A & 7B εhow A-P and lateral views of a second embodiment of the invention.

Figureε 8A, 8B, 8C & 8D εhow variouε perspective views of the proximal part of the stem of a further embodiment having a pseudo collar.

Figureε 9A, 9B & 9C show a sequence of stepε for the introduction and fixing of a cemented proεtheεiε.

Figureε 10A & 10B illustrate the steps of fitting a presε-fit proεtheεiε with cannulated εtem in accordance with the invention.

Figure 11 iε a view of a presε-fit proεthesiε in a bone canal having a modified diεtal εleeve for εpreading the diεtal load diεtribution.

Figureε 1, 1A and 2 are repreεentationε of an average femur taken from measurements of 26 adult specimens. It will be appreciated that more or less specimenε may be taken, e.g. 15 or 30, εo long aε the number is enough to provide a representative average. The inner and outer

contourε of the average femur were produced by meaεurementε taken of εliceε made from εpecimenε and plotted uεing the computer-aided technique deεcribed by Walker et al in Clinical Orthopaedicε and Related Reεearch, No. 235, October 1988, page 25. The inner and outer contourε were digitiεed into a computer and εplined to determine 40 uniformly spaced points. The coordinates were scaled according to the total length of the femur and then corresponding point numbers were averaged. Thiε produced an average femur which had a natural appearance and in which each section was aligned εmoothly to adjacent sections aε shown in Figureε 1 and 1A. Using the computer, the surgical εtepε of reεecting the neck and head of the femur and reaming the canal were εimulated. Thiε is illustrated in Figures 1 and 1A, in which the crosε-hatched area repreεentε the inner contour of the bone canal of the femur which remains after εurgical reεection. The dimenεions of the resulting average femur were as follows:-

BASE TO CENTRE 170.0 mm

HEAD DIAMETER 45.0 mm

HEAD OFFSET 43.0 mm

DISTAL SECTIONS 18.16 mm

PROXIMAL SECTIONS 4.54 mm

The next step iε to define the εhape of the stem and in particular the proximal part which will give the

optimal fit in proximal portion of the reεected femur. It haε been determined that optimal fit iε achieved by implant-cortical bone contact (i.e. an implant bone εpacing of 1mm or leεε), between the cortex and the implant.

The εhape and dimenεionε of the moεt proximal εection of the proεthetic device iε thuε determined aε illuεtrated in Figure 3. Thiε εhowε the top εection of the stem defined from three circular arcε and common tangentε. Referring to Figure 3, reference numeral 31 iε the medial and 32 the lateral εide of the femur, while 33 is the anterior and 34 the posterior side. In Figure 3, circle A represents the diameter of the distal end of the εtem (or the average where the diεtal end iε tapered), which iε specified to allow a cloεe but εliding fit in the bone canal. An anterior circle B iε drawn with the εame diameter as lateral circle A. A medial circle C iε drawn aε the beεt fit to the proximal end of the bone canal. The outline of the proximal εection of the device iε then determined by the mutual tangentε T^, T2 and T3.

Figure 4 shows the preferred εhape of the proximal portion of the εtem 41 of the device. In Figure 4, the croεε-hatched part 35 repreεentε the cubic curve in the anterior and lateral viewε and the relationεhip of the device to the bone canal. As can be seen, the contour of the upper region 40 of the εtem 41 iε a cloεe fit

proximally to the bone canal, which provides good load tranεfer. Upper εection 40 merges into an intermediate εection 42 in which clearance iε provided from the bone in the area between the leεεer trochanter and the mid-point of the length of the εtem, (the diεtal end of which iε not εhown in Figure 4), which iε indicated by the arrow 43. Thiε arrangement has two advantageε. Firεt, loads are tranεmitted between the εtem and the bone proximally at 40, which correεpondε to the phyεiological εituation. Secondly, it avoidε the danger of hanging up of the εtem in the region 42 aε the εtem iε being introduced εurgically. The next εtep in the deεign of the device iε to add a neck portion (together, if deεired, with a partial collar aε deεcribed below) and a modular femoral head.

A εymmetrical verεion of the stem iε deεigned aε illuεtrated in Figureε 5A, 5B & 5C by mirroring the coordinate data file and taking the leaεt envelope.

Specific featureε of the deεign of the εtem and collar (where present) and of the method of fitting the εtem into a bone canal are illuεtrated by Figureε 6A & B, 7A & B, 8A-D, 9A-C, 10A & B and Figure 11.

Referring to Figureε 6A & B, the left-hand figure (Figure 6A) εhowε the outline of the εtem 41, the canal 46 and the intervening cement mantle 47. In general, a tapered diεtal portion 44 iε preferred since thiε preventε

high εtreεε concentration at the tip. With a taper, load is tranεferred to the bone uniformly, rather than abruptly near the tip aε would occur if the εtem were of uniform thickneεs. Figure 6B, showε the preεε-fit embodiment, in which the extreme distal end part iε closer to a cylindrical εhape and the loading on the distal load iε εpread by uεe of a plaεtic εleeve S, having a cloεed lower end, which iε a preεε-fit in the canal. The diεtal end 48 iε a tight-fit in the plaεtic sleeve, which in addition to spreading the load greatly reduces or eliminates micro- motion between the εtem and the bone. Aε shown in the presε-fit embodiment of Figure 6B the proximal part taperε εomewhat to approximately the diεtal quarter of the stem.

Referring to Figure 11, thiε εhowε a modified form of the plaεtic εleeve for use with presε-fit implantε. Radial or torεional micro-motion, e.g. of up to about 50 micronε and angular movements of up to about 0.1° can be readily tolerated. However, spreading of the load which is transmitted to the bone iε nevertheleεε desirable. T iε spreading of the load is achieved in the embodiment shown in Figure 11 by internal shaping of the plaεticε εleeve. As illuεtrated i Figure 11, the plaεticε sleeve • 110 (which iε a tight-fit in the diεtal bone canal 111) haε a non-uniform bore and in εection haε a convex internal shape 112. The εituation iε exaggerated in Figure 11, but the internal εhape iε such that the diεtal

portion of the εtem 113 iε a tighter fit in the intermediate part of the bore in the εleeve 110 than at its ends. The practical effect iε that if the head of the implant iε loaded in the normal way in use, the radial or torsional micro-motion transmitted to the distal end will be transferred to the bone over subεtantially the whole length of the εleeve 110 rather than be concentrated in a εpecific point.

Figure 7A iε a εide elevation of the joint implant and Figure 7B iε a view taken along the line A-A in Figure 7A. Aε εhown in Figureε 7A and 7B it is advantageouε to provide grooveε or receεεeε in the proximal region of the εtem on the anterior and poεterior faceε, while the medial face provideε a cloεe fit to the bone canal as deεcribed above. The grooveε or recesseε X extend longitudinally of the εtem and preferably have rounded upper endε. The major advantageε of theεe grooveε iε that they allow preεervation of a εignificant amount of strong cancelIOUE bone, which would otherwise have to be reamed away were the stem to εubεtantially fill the canal. Aε can be seen in the drawing, the grooveε X begin juεt below the neck or collar and form two εhelveε Y which are capable of taking axial force and resisting εinkage. Some εinkage iε desirable during the initial 'wearing-in' period of the implant εince thiε ensureε that the proximal part of the εtem formε good bone contact. However, control of the

amount of sinkage iε important to avoid the riεk of diεlocation of the artificial joint. The grooves are machined down the stem in a direction parallel to the axiε of the εtem so as to terminate at a point in the region of the mid-point of the stem or just above it. As a reεult of this direction of machining, the εtem could be removed ( in case of a uture problem) by simply tapping it upwardε out of itε cement bed (where present), becauεe of the εtem divergence in the upper εtem, which greatly facilitateε a revision procedure. Grooves formed in this way also provide axial load εupport and reduced εtem εtiffneεε.

Figureε 8A-D include εeveral viewε of a pεeudo (partial) collar which is another advantageous, preferred feature of the deεign. In the pεeudo collar, a protruεion or ledge iε provided, e.g. medially, anteriorly and poεteriorly but not laterally. The pεeudo collar iε formed integrally between the upper εection cf the εtem and the neck of the device. For preεε-fit εtemε, there is a dilemma regarding the proviεion or not cf a standard collar. If a collar iε provided to avoid εinkage and transmit loads more naturally, the εtem in the canal can progreεεively looεen, leading to bone reεorption and pain. On the other hand, without a collar, the surgery needs to be particularly accurate, and the wedging action can lead to progresεive sinkage. and even splitting of the bone. The pseudo-collar iε half-way between a εtan ard collar,

SUBSTITUTESHEET

and no collar. It will tranε it load to the upper part o the femur. It will retard sinkage but not entirely prevent it, because some bone remodelling would still allow a limited amount of εinkage. The hoop εtreεεeε due to wedging of the shallow wedge-shape of the upper part of the stem will be reduced.

Figureε 9A, 9B & 9C illuεtrate the methods of introducing cemented εtems and Figures 10A & 10B the method of inserting presε-fit εtemε in accordance with the invention. Preferably, the εtem is cannulated to receive a guide rod 91. Thiε haε a uεeful purpoεe for introducing both cemented and preεε-fit applicationε. In cemented application, it iε considered desirable to first plug the canal (at 92), juεt below where the diεtal tip of the εtem would locate. Then the canal iε filled with εoft cement 93, through a nozzle 94. The stem iε introduced over the guide rod 91 and the assembly held still until hardening of the cement takes place. The guide rod is then withdrawn aε εhown in Figure 9C. However, effective plugging iε sometimes difficult and slippage of the plug and leakage of cement may occur. By uεing a stem which iε cannulated, the plug can be held on a wire and the wire removed afterwardε. For press-fit stems, if the preεε-fit plastic εleeve 95 iε introduced with the stem, it will be forced to the top of the εtraight portion 96 of the εtem, and not permit downwardε micro-motion. After the

εtem/εleeve iε introduced, a rod 91 can be puεhed down the εtem and the εleeve advanced a required amount.

Another method for preventing the plastic εleeve from being puεhed to the top of the εtraight portion on introduction, iε to fit a collar or ring made from ε reεorbable material, εuch aε poly (L-lactide) or poly (L- lactide-co-glycolide) above the plaεtic εleeve. In a short period of time after introduction, the polymer will gradually resorb, allowing some εpace for downward movement of the εtem within the plaεtic εleeve.