WO1993005734A1

WO1993005734A1 - Prosthetic joint components

Info

Publication number: WO1993005734A1
Application number: PCT/GB1992/001745
Authority: WO
Inventors: Bernard Francis Meggitt; Arthur Edward Timbs
Original assignee: Bernard Francis Meggitt; Arthur Edward Timbs
Priority date: 1991-09-24
Filing date: 1992-09-23
Publication date: 1993-04-01
Also published as: GB2259859A; GB9220127D0; GB9120328D0

Abstract

The invention relates to a prosthetic joint component which provides load distribution approximating to that occuring naturally. The prosthesis comprises a separable head (10), a necked collar (20) and a stem (30), the stem (30) being fitted into the bone cavity. Either the stem (30) or the collar (20) carries a longitudinal spigot (31) which can slide within a corresponding bore (23) in the adjacent element in such a way that compressive load is transmitted through the calcar and not through the stem (30). A bolt (34) on the end of the spigot (31) prevents total separation of the elements, whilst flats (32) on the sides of the spigot (31) prevent rotation relative to the bore (23). The invention finds particular application in hip joint endoprostheses.

Description

PROSTHETIC JOINT COMPONENTS

This invention concerns prosthetic joint components and more particularly, but not exclusively, such components for replacement of the femoral head in the hip joint.

Femoral head replacement is a long-established procedure and commonly involves a component having a ball-shaped head connected by way of a necked portion to the wider end of a tapered stem. The necked portion very often diverges towards the stem to form a collar, but this is not always the case for such a component.

In use, the natural head and neck of the femur are excised for substitution by the corresponding parts of the component, with the component stem being entered into the medullary canal to position the ball and neck appropriately. The collar, when present, is engaged on the cut surface of the remaining bone.

Stabilisation of the component positioning is sought in one of several ways. Conventionally this involves the use of bone cement as a gap-filling medium between the stem and the surrounding bone. In a. more recent alternative approach, stabilisation is sought by way of a direct press fit of the stem into the bone. Generally speaking the current practice is for bone cement to be used for older patients, of 60 years age and above, say, but with the. press fit approach being increasingly used with younger patients. In any event, the components under discussion have been used very extensively to combat the symptoms of arthritic conditions with clear beneficial effect in terms of pain relief and increased mobility. However, this situation is equally clearly open to improvement in that the component position very often becomes unstable, or otherwise inappropriate, due to loosening in the femur over a varying period of time, with consequent failure of joint function even though the component may otherwise remain viable.

An object of the present invention is to improve this situation and to this end there is provided a prosthetic joint - 1 - component of collared form comprising an articulatory head portion leading through a collar portion to a stem, the stem being connected to the remainder of the component in longitudinally sliding manner, and a means being provided for preventing longitudinal separation of the stem from the remainder of the component.

The rationale for this new component is that it allows closer replication than is otherwise the case of the load pattern found in a natural healthy joint and without which bone resorpt on and component loosening occurs. More specifically this natural pattern is found to involve high longitudinal components of compression and low transverse components of a tensile nature in the calcar. In the case of a prior component form and cement stabilisation, with or without a collar, load transfer occurs predominantly through the lower stem, by-passing the upper femur. This results in low longitudinal compression and high radial loading in the calcar. In the case of a press fit a similar pattern occurs ^'when no collar is present. There is less reduction in calcar compression in the presence of a collar for this case, but there is still a high radial loading.

With the proposed new component the stem can be cemented or a press fit to stabilise the remainder of the component, when connected, in a radial sense. However, stabilisation in a longitudinal sense will be through the collar. In these circumstances the calcar is subject to a load pattern of relatively natural form.

The head portion and the collar portion may be a single, integral part, but in a preferred form the component includes a separable head portion. This allows the component to be deployed in a modular manner for use with heads of different size and/or material to suit varying requirements. Similarly, to the extent that the stem can initially be separate from the remainder of the component, other variations can also be made in the size and/or form of sub-components such as a neck/collar and a stem within a modular component range. Preferably the head portion is connected directly to the collar portion, the connection between the two portions being by way of a tapered interference fit.

Preferably in the proposed component the stem connection is such as to prevent relative rotation between the stem and the remainder of the component.

In a preferred form, such relative rotation is prevented by cooperating planar longitudinal surfaces associated respectively with the stem and the remainder of the component, the cooperation of the surfaces permitting relative longitudinal sliding therebetween.

The sliding connection may be formed by providing a spigot on the proximal end of the stem and a complementary bore in the collar portion to receive the spigot in longitudinal sliding engagement.

Alternatively, the spigot may be provided on the distal end of the collar portion and the bore formed in the stem.

In a preferred form according to the embodiments including the sliding connections as specified above, a restraining element is provided on the spigot to prevent separation of the stem from the remainder of the component, this element preferably comprising a bolt fixed in a longitudinal bore in the end of the spigot.

In order that these and other features of the invention may be better understood, the same is described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows in sectional view an embodiment of a femoral component according to the invention; and

Figure 2 shows a similar view of a second embodiment according to the invention.

The component illustrated in Figure 1 has three principal sub-components, namely, a ball-shaped head 10, a necked portion 20 and a tapered stem 30. The head is connected with the proximal end 21 of the necked portion. The distal end of the necked portion diverges to form a collar 22 and is longitudinally slidably connected with the stem adjacent the wider end of the latter. The anatomical context of the component in use is indicated by outline illustration of the femur at 40 following excision of the natural head and neck. The head and necked portion are separably connected by way of a tapered interference fit. For this purpose the head is formed with a tapered socket 11 and the proximal end of the necked portion is formed to a complementary shape to engage the socket. The connection between the necked portion and stem is also separable. The necked portion is formed with a longitudinal bore 23 and the stem has a complementary spigot 31 projecting from its wider end to slidably engage the bore. The spigot has two diametrally opposed flats 32 along its proximal end portion, and the bore 23 is formed with corresponding flats 24, with these flats being mutually engaged to prevent rotation about the longitudinal direction.

Although this last connection is separable, it is held against separation during use. For this purpose the proximal end of the spigot has a threaded longitudinal bore 33 for receipt of a bolt 34 and associated washer 35 following connection of the necked portion, but before application of the head. The bolt is fitted such that a clearance remains between the proximal annular face of the necked portion 20 and the opposing annular face of the washer 35, this clearance of, for example, 5 mm, allowing free longitudinal sliding between the spigot 31 and the bore 23 of the necked portion. As an alternative to the bolt and washer, a bolt with an oversized head can be employed, dispensing with the need for the associated washer 35.

Use of the illustrated component is largely self-evident from the preceding description above. The femur is transected to excise the head and neck, the medullary canal reamed for receipt of the stem, and other site preparation effected in a generally conventional manner. The stem is then secured in the bone by cementing or press fit as appropriate, the wider end of the stem being set below the surface to be engaged by the necked portion. The necked portion is thereafter engaged over the spigot and seated on the bone, the bolt and washer applied to render the portion captive on the spigot, and lastly the head is fitted. The situation is then such that, subsequently, load applied to the head will pass to the bone predominantly as compression longitudinally through the calcar. The stem serves essentially to stabilise the position of the necked portion and head. This last function may involve the transmission of shear forces at the head by way of transverse components through the lower stem to the bone, but these can correspond to components found in the natural load pattern.

It may be advantageous to further restrict the relative longitudinal movement between the necked portion and the stem and to this end it is envisaged that a plastics sleeve (not shown), preferably of high density polyethylene, may be inserted between the spigot 31 and the bore 23 on assembly of the component elements. If the dimensions are appropriately selected this will increase the resistance to movement between the elements and would thus reduce or eliminate undesirable longitudinal sliding. In particular, such undesirable longitudinal sliding might arise during walking as the leg in which the prosthesis is implanted is lifted from the ground and a resulting tensile force, due to the weight of the leg, is exerted on the joint. With insufficient resistance to longitudinal sliding there is likely to be some small movement between the spigot and the bore and, in consequence, a gap will appear between the collar of the prosthesis and the resected neck of the femur. As the leg then impacts with the ground the gap will close and the collar and calcar will make dynamic contact. The resultant hammering in the joint could lead to wear damage and further bone resorption. However, with the plastics sleeve insert, the resistance to sliding will prevent the gap from forming, whilst the compressive force on the joint when subjected to body load will be sufficient to overcome the resistance to movement between the spigot and bore, thus ensuring that the load is still passed substantially to the calcar and that the collar is maintained in contact with the resected femur. Over a period of time the plastics sleeve is likely to absorb body fluid and expand to cause an increase in the resistance between the spigot and bore, but as the body weight will generally tend to urge the collar downwards towards the stem some relative longitudinal movement will still be permitted, both by the compressive force overcoming this resistance and through creep of the plastics material.

A further embodiment of the invention is illustrated in Figure 2, but in this case the connection between the stem and the remainder of the component is effectively reversed. Elements of the component corresponding to those in Figure 1 are given the same reference numbers, but increased by 100. As the illustration shows, a spigot 131 is carried on the distal end of the necked portion 120 and a corresponding bore 123 is formed in the stem 130 of the component. Once again, sliding movement between the spigot 131 and the bore 123 can take place and longitudinal separation of the stem from the remainder of the component is prevented by a retention means, such as a bolt 13_.4 and associated washer 135, the bolt being screwed into a threaded axial bore 133 in the distal end of the spigot 131. The bore in the stem has a wider portion 141 to accommodate the bolt head and washer, separation of the components being prevented by contact between the washer 135 and an annular shoulder 142 formed at the transition between bore portions 123 and 141. Similar means for preventing relative rotation between the bore and the spigot as described above may be incorporated, and a plastics sleeve between the spigot and the bore may be included in the component. Access to the head of the bolt is, in this case, through the lateral port of the femur (not shown) and an appropriate method of excising the bone and plugging the resulting access opening is used. In this embodiment it is also possible to use a separable head 110 with a press fit or other connection to the necked portion 120, allowing a modularity of component elements. While the invention has been described with more particular reference to the illustrated embodiments, it is not intended to be limited thereby. For example, it has been indicated that the component can represent one of a variety of possible forms from a modular assembly of parts. Thus the head, necked portion and stem are each open to provision in plural forms which vary individually in size, shape, material or other characteristic, given that these parts are adapted for interchangeable interconnection in a common manner. Variation from the illustrated form is also possible in other facets of detail. For example, rotation of the connection between the spigot and necked portion can be prevented by the provision of a keyway, splined configuration, elliptical shaping or other arrangement. Also this connection could involve a circlip or other mechanism to act against separation during use. As an alternative method of transverse stabilisation of the stem within the medullary canal, a through bore may be provided in the stem, transfixed by a screw or bolt passing through the width of the femur. This allows the longitudinal load transmission to be through the calcar, but provides in addition some resistance to radial stresses acting in the bone material.

Variation in the invention is also possible in more general terms in that it can involve application to other components than those for femoral head replacement. The basis for this variation is that other joint bones have dense trabecular regions which serve, like the calcar, to transmit load by longitudinal compression therethrough from an adjacent articulation surface to the main body of the bone. This is commonly the case for long bones involved in other joints, such as the humerus and ulna at the shoulder and elbow, but not exclusively so, with the talus at the ankle being another example. In any event, it is considered that the invention is applicable to such situations more generally by the provision of a component comprising a main body defining an articular surface and a bone-engaging surface, and a bone-penetrating element projecting from the latter surface in sliding connection normally thereof with the body.

Claims

1. A prosthetic joint component of collared form comprising an articulatory head portion leading through a collar portion to a stem, the stem being connected to the remainder of the component in longitudinally sliding manner, and a means being provided for preventing longitudinal separation of the stem from the remainder of the component.

2. A prosthetic joint component according to Claim 1, wherein the component includes a separable head portion.

3. A prosthetic joint component according to Claim 2, wherein the head portion is connected directly to the collar portion, the connection between the two portions being by way of a tapered interference fit.

4. A prosthetic joint component according to any preceding claim, wherein a spigot is provided on the proximal end of the stem and a complementary bore is formed in the collar portion to receive the spigot in longitudinal sliding engagement.

5. A prosthetic joint component according to any of Claims 1 to 3, wherein a spigot is provided on the distal end of the collar portion and a complementary bore is formed in the stem to receive the spigot in longitudinal sliding engagement.

6. A prosthetic joint component according to Claim 4 or 5, wherein means are provided for preventing relative rotation between the spigot and the bore.

7. A prosthetic joint component according to Claim 6, wherein the rotation prevention means comprise cooperating planar longitudinal surfaces associated respectively with the spigot and the bore.

8. A prosthetic joint component according to any of Claims 4 to 7, wherein the means for preventing separation of the stem from the remainder of the component comprise a restraining element on the spigot.

9. A prosthetic joint component according to Claim 8, wherein the restraining element comprises a bolt fixed in a longitudinal bore in the end of the spigot.

10. A prosthetic joint component according to any of Claims 4 to 9 comprising a plastics sleeve between the spigot and the bore, the sleeve permitting longitudinal sliding movement between the spigot and the bore selectively in response to body load.

11. A femoral hip component of collared form comprising an articulatory head portion leading through a collar portion to a stem, comprising means for preventing longitudinal separation of the stem from the remainder of the component, and means for permitting longitudinal sliding between the stem and the remainder of the component selectively in response to body load.