AU646814B2

AU646814B2 - Femoral joint component

Info

Publication number: AU646814B2
Application number: AU64087/90A
Authority: AU
Inventors: Peter Stanley Walker
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-09-15
Filing date: 1990-09-13
Publication date: 1994-03-10
Anticipated expiration: 2010-09-13
Also published as: GB8921008D0; EP0491787A1; WO1991003992A1; JPH05500177A; CA2064779A1; GB2236679B; GB9020035D0; AU6408790A; GB2236679A

Description

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

Surgical removal of the natural joint and re,)lacement 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 stem 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 O 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 resorption at the cement-bone interface due to high stresses, and cement cracking due to fatigue and stress concentrations. Uncemented press-fit stems have been tt# L 3LFr7E 0~ 2 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 features.

One solution to the above listed problems involves the definition of specific design features of a femoral implant which have been found to have important practical advantages.

According to the present invention therefore, there I 1 o 3 is provided a femoral joint component having a one-piece stem adapted to be located within the femoral bone canal of a resected femur, said stem comprising a proximal portion which is a close fit in the bone canal, a distal portion and an intermediate portion shaped to provide clearance between the stem and canal. wherein the distal portion is restrained against micromotion substantially radially of the stem by means of a sleeve member on said distal portion, permitting axial movement of said distal portion in said sleeve.

Advantageously, the shape and dimensions of a femoral implant of the invention are determined by the following steps:establishing the inner and outer contours of an average femur by measurements taken from a significant number of specimens, at least 15 to 20), and feeding the data into a computer ac a series of x, y and z coordinates, simulating on the computer the surgical resection of the neck and head of the femur and reaming a cipo canal lengthwise of the representation of the femur, S S WO 91/03992 PCT/GB90/01411 4 producing the proximalmost section of the stem 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 proximalmost 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 representing the exterior contour of the stem of the implant by a cubi" curve which joins the proximalmost 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 sliding fit distally.

The present invention derives in part from the realisation that it is highly desirable to design the prosthesis so that in use the bone is loaded in a similar 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 is a gradual load transmission from the upper part of the stem downwardly to the bone in the lower part. Overall loading of the bone is important because bone which is unloaded will tend to resorb. Conversely, if the implant 5 transmits a high load concentration to a particular area of the bone, stresses are set up which cause bone damage. The aim is to produce a profile of the upper part of the stem which is a good fit in the proximal bone canal and a distal portion which applies a gradual and uniform load distribution to the femur in the region of the distal canal. In order to achieve the best approximation to this objective, to avoid stress concentrations and to design a prosthesis which is capable of being surgically inserted, the intermediate portion of stem is shaped to provide clearance in the bone canal and the distal portion of the stem is tapered over at least part of its length.

Avoidance of micromotion of the tip of the stem radially of the bone canal is a feature of the implants of the invention. Radial micromotion can be contained by inserting a sleeve (open-ended or in the form of a cap) into the distal canal, the sleeve being a tight fit in the distal-most part of the canal and the distal end of the stem being held within the sleeve.

As will be described hereinafter, the invention includes a specific design of sleeve which not only limits micromotion but spreads the load generated by e~as micro-swinging motion of the prosthesis. Preferably, the sleeve, which preferably comprises a plastics material, has a non-uniform bore having a minimum diameter intermediate its ends so that any micro-movement of the -6distal-most end of the stem in a radial direction, causes spreading of the resulting radial load transmitted to the bone canal lengthwise of the sleeve.

The sleeve is preferably dimensioned so as to be a tight fit in the bone canal which the distal-most part of the stem is capable of some sliding movement axially in the sleeve.

For ease of installation of the sleeve into the bone canal, the sleeve may be retained on the distal-most part of the stem. Preferably, the sleeve is retained by means of a resorbable material which after installation is resorbed, thus permitting some axial movement of the stem in the sleeve.

In the preferred form, the femoral implant has a head and neck connected to the stem, and a pseudo collar formed in the region of the junction between the stem and the neck, said collar providing a ledge extending upwardly and outwardly from the stem. By ensuring that the ledge extends part circumferentially of the stem, sinkage of the stem into the bone canal is inhibited but .1°o not totally prevented.

The distal-most portion is preferably cylindrical.

1 The invention also includes features which facilitate the fitting of femoral bone implants into bone canals, in which a cannulated stem is used to enable the stem to be guided into the canal on a guide rod or to enable a press-fit plastic sleeve to be located over the distal end of the stem and thereby reduce radial 7 micro-movement between the stem and the bone.

Other aspects and features of joint components constructed in accordance with this invention will be apparent from the following detailed description and accompanying drawings, in which:- Figure 1 is a representation of an average femur taken from measurements of 26 specimens.

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

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

Figure 3 is a top plan view of the stem cf a hip prosthesis showing also the outline of the bone canal.

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

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

Figure 6 shows anterior-posterior views of a press-fit prosthesis in accordance with the invention.

Figures 7A 7B show A-P and lateral views of a second embodiment of the invention.

Figures 8A, 8B, 8C 8D show various perspective views of the proximal part of the stem of a further embodiment having a pseudo collar.

Figures 9A 9B illustrate the steps of fitting a press-fit prosthesis with cannulated stem in accordance with the invention.

8 Figure 10 is a view of a press-fit prosthesis in a bone canal having a modified distal sleeve for spreading the distal load distribution.

Figures 1, 1A and 2 are representations of an average femur taken from measurements of 26 adult specimens. It will be appreciated that more or less specimens may be taken, e.g. 15 or 30, so long as the number is enough to provide a representative average.

The inner and *n eAA 0 r 4~ WO 91/03992 PCT/GB9001411 9 contours of the average femur were produced by measurements taken of slices made from specimens and plotted using the computer-aided technique described by Walker et al in Clinical Orthopaedics and Related Research, No. 235, October 1988, page 25. The inner and outer contours were digitised into a computer and splined 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. This produced an average femur which had a natural appearance and in which each section was aligned smoothly to adjacent sections as shown in Figures 1 and IA. Using the computer, the surgical steps of resecting the neck and head of the femur and reaming the canal were simulated. This is illustrated in Figures 1 and 1A, in which the cross-hatched area represents the inner contour of the bone canal of the femur which remains after surgical resection. The dimensions 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 is to define the shape of the stem and in particular the proximal part which will give the WO 91/03992 PCT/GB90/01411 optimal fit in proximal portion of the resected femur.

It has been determined that optimal fit is achieved by implant-cortical bone contact an implant bone spacing of imm or less), between the cortex and the implant.

The shape and dimensions of the most proximal section of the prosthetic device is thus determined as illustrated in Figure 3. This shows the top section of the stem defined from three circular arcs and common tangents.

Referring to Figure 3, reference numeral 31 is the medial and 32 the lateral side 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 stem (or the average where the distal end is tapered), which is specified to allow a close but sliding fit in the bone canal. An anterior circle B is drawn with the same diameter as lateral circle A. A medial circle C is drawn as the best fit to the proximal end of the bone canal.

The outline of the proximal section of the device is then determined by the mutual tangents TI, T 2 and T 3 Figure 4 shows the preferred shape of the proximal portion of the stem 41 of the device. In Figure 4, the cross-hatched part 35 represents the cubic curve in the anterior and lateral views and the relationship of the device to the bone canal. As can be seen, the contour of the upper region 40 of the stem 41 is a close fit 11 proximally to the bone canal, which provides good load transfer. Upper section 40 merges into an intermediate section 42 in which clearance is provided from the bone in the area between the lesser trochanter and the mid-point of the length of the stem, (the distal end of which is not shown in Figure which is indicated by the arrow 43. This arrangement has two advantages.

First, loads are transmitted between the stem and the bone proximally at 40, which corresponds to the physiological situation. Secondly, it avoids the danger of hanging up of the stem in the region 42 as the stem is being introduced surgically. The next step in the design of the device is to add a neck portion (together, if desired, with a partial collar as described below) and a modular femoral head.

A symmetrical version of the stem is designed as illustrated in Figures 5A, 5B 5C by mirroring the coordinate data file and taking the least envelope.

Specific features of the design of the stem and collar (where present) and of the method of fitting the i. stem into a bone canal are illustrated by Figures 6, 7A o 7B, 8A-D, 9A 9B and Figure Figure 6, shows a press-fit implant, in which the extreme distal end part is closer to a cylindrical sh~pe and the loading on the distal load is spread by use of a plastic sleeve S, having a closed lower end, which is a press-fit in the canal. The distal end 48 is a tight-fit in the plastic sleeve, which in addition to spreading the

IA

1 Ax 0 12 load greatly reduces or eliminates micromotion between the stem and the bone. As shown in the embodiment of Figure 6, the proximal part tapers somewhat to approximately the distal quarter of the stem.

Referring to Figure 10, this shows a modified form of the plastic sleeve for use with press-fit implants.

Radial or torsional micromotion, e.g. of up to about microns and angular movements of up to about 0.10 can be readily tolerated. However, spreading of the load which is transmitted to the bone is nevertheless desirable.

This spreading of the load is achieved in the embodiment shown in Figure 11 by internal shaping of the plastics sleeve. As illustrated in Figure 11, the plastics sleeve 110 (which is a tight-fit in the distal bone canal 111) has a non-uniform bore and in section has a convex internal shape 112. The situation is exaggerated in Figure 11, but the internal shape is such that the distal ,o l D 41 w ::0 WO 91/03992 PGcr/G B90/0141 I 13 portion of the stem 113 is a tighter fit in the intermediate part of the bore in the sleeve 110 than at its ends. The practical effect is that if the head of the implant is 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 substantially the whole length of the sleeve 110 rather than be concentrated in a specific point.

Figure 7A is a side elevation of the joint implant and Figure 7B is a view taken along the line A-A in Figure 7A. As shown in Figures 7A and 7B it is advantageous to provide grooves or recesses in the proximal region of the stem on the anterior and posterior faces, while the medial face provides a close fit to the bone canal as described above. The grooves or recesses X extend longitudinally of the stem and preferably have rounded upper ends. The major advantages of these grooves is that they allow preservation of a significant amount of strong cancellous bone, which would otherwise have to be reamed away were the stem to substantially fill the canal. As can be seen in the drawing, the grooves X begin just below the neck or collar and form two shelves Y which are capable of taking axial force and resisting sinkage. Some sinkage is desirable during the initial 'wearing-in' period of the implant since this ensures that the proximal part of the stem forms good bone contact. However, control of the 14 amount of sinkage is important to avoid the risk of dislocation of the artificial joint. The grooves are machined down the stem in a direction parallel to the axis of the stem so as to terminate at a point in the region of the mid-point of the siem or just above it. As a result of this direction of machining, the stem could be removed (in case of a future problem) by simply tapping it upwards, because of the stem divergence in the upper stem, which greatly facilitates a revision procedure. Grooves formed in this way also provide axial load support and reduced stem stiffness.

Figures 8A-D include several views of a pseudo (partial) collar which is another advantageous, preferred feature of the design. In the pseudo collar, a protrusion or ledge is provided, e.g. medially, anteriorly and posteriorly but not laterally. The pseudo collar is formed integrally between the upper section of the stem and the neck of the device. For press-fit stems, there is a dilemma regarding the provision or not of a standard collar. If a collar is provided to avoid sinkage and transmit loads more naturally, the stem in the canal can progressively loosen, leading to bone resorption and pain. On the other hand, without a collar, the surgery needs to be particularly accurate, and the wedging action cai lead to progressive sinkage and even splitting of the bone. The pseudo-collar is half-way between a standard collar, and no collar. It will transmit load to the upper part of the femur. It 15 will retard sinkage but not entirely prevent it, because some bone remodelling would still allow a limited amount of sinkage. The hoop stresses due to wedging of the shallow wedge-shape of the upper part of the stem will be reduced.

Figure 9A 9B illustrate the method of inserting press-fit stems in accordance with the invention.

Preferably, the stem is cannulated to receive a guide rod 91. This has a useful purpose for introducing the implant to the bone canal. If the press-fit plastic sleeve 95 is introduced with the stem, it will be forced to the top of the straight portion 96 of the stem, and not permit downwards micromotion. After the stem/sleeve is introduced, a rod 91 can be pushed down the stem and the sleeve advanced a required amount.

Another method for preventing the plastic sleeve from being pushed to the top of the straight portion on introduction, is to fit a collar or ring made from a resorbable material, such as poly (L-lactide) or poly (L-lactide-co-glycolide) above the plastic sleeve. In a short period of time after introduction, the polymer will gradually resorb, allowing some space for downward movement of the stem within the plastic sleeve.

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Claims

1. A femoral joint component having a one-piece stem adapted to be located within the femoral bone canal of a resected femur, said stem comprising a proximal portion which is a close fit in the bone canal, a distal portion and an intermediate portion shaped to provide clearance between the stem and canal, wherein the distal portion is restrained against micromotion substantially radially of the stem by means of a sleeve member on said distal portion, permitting axial movement of said distal portion in said sleeve.

2. A joint component according to claim 1 wherein the sleeve member has a bore which is shaped so that the distal portion of the stem is a tighter fit in an intermediate part of the length of the bore of the sleeve member than at its ends.

3. A joint component according to claim 2 in which the sleeve member has non-uniform bore having a minimum dj meter at a point intermediate its ends so that any Micro movement of the distal end of the stem in a radial direction causes spreading of the resulting radial load transmitted to the bone canal lengthwise of the sleeve member.

4. A joint component according to any one of the i preceding claims wherein the sleeve member is closed at one end. A joint component according to any one of the preceding claims wherein the distal portion of the stem 1 (i J 17 is cylindrical.

6. A joint component according to any one of the preceding claims wherein the sleeve member comprises a plastics material.

7. A joint component as claimed in any one of the preceding claims wherein the sleeve member is dimensioned to be a tight fit in the bone canal while permitting downward micromotion of the stem in the sleeve member.

8. A joint component according to any one of the preceding claims wherein the sleeve member is retained on the stem by resorbable material which is arranged so as to maintain the sleeve member in position on the stem during installation of the component in the bone canal, the sleeve member after resorption of said resorbable material permitting some axial movement of the stem in the sleeve.

9. A joint component according to claim 8 wherein said resorbable material is a poly(L-lactide) or a poly(L-lactide-co-glycolide). A joint component according to any one of the preceding claims wherein the stem is formed with longitudinally extending grooves in a proximal portion of the stem, said grooves stopping short of the proximal end of the stem and providing for axial load bearing on preserved cancellous bone.

11. A joint component according to claim 10 wherein the internal surfaces of the grooves are vertical or splayed slightly outwardly to facilitate subsequent removal of o 18 the component.

12. A joint component according to any one of the preceding claims having a head and neck connected to said stem, wherein a pseudo collar is formed in the region of the junction between the stem and the neck, said collar providing a ledge extending upwardly and outwardly from the stem.

13. A joint component according to claim 12 wherein said ledge extends part circumferentially of the stem so as to inhibit sinkage of the stem into the canal under load but not to totally prevent sinkage.

14. A joint component according to any one of the preceding claims wherein the stem is longitudinally cannulated to facilitate installation of the component in a bone canal. A method of determining the shape and dimensions of a femoral implant as claimed in claim 1 which comprises the following steps:- establishing the inner and outer contours of an average femur by measurements taken from a significant number of specimens, at least 15 to 20), and feeding the data into a computer as a series of x, y and 1 *t z coordinates. simulating on the computer the surgical *0 'resection of the neck and head of the femur and reaming a canal lengthwise of the representation of the femur, producing the proximalmost section of the stem by drawing a lateral circle on the resected top of 19 the representation of the femur having a diameter which corresponids 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 proximalmost section of the stem of the implant by drawing mutual tangents to the lateral, anterior and medial circles, and representing the proximal-medial curve and the proximal-anterior curve of the stem of the implant by a cubic curve which joins the proximalmost 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 sliding fit distally.

16. A method according to claim 15 which includes the additional, step of determining the contour of an intermediate portion located between the proximal end and the distal end, said intermediate portion providing clearance between the stem and the bone canal.

17. A method according to claim 15 or 16 w ch includes the additional step of determining the contour of a partial collar located above the proximal end of the stem, said collar protruding beyond the outer contour of the proximal end of the stem only part-circumferentially of said stem so as to reduce the rate of sinkage of the implant in use. Dated this 5th Day of January, 1994 PETER STANLEY WALKER Attorney: LEON K. ALLEN Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS