WO2010129990A1

WO2010129990A1 - Hip prosthesis

Info

Publication number: WO2010129990A1
Application number: PCT/AU2010/000543
Authority: WO
Inventors: Lindsay Laird
Original assignee: Atlax Pty Ltd Atf Atlax Trust
Priority date: 2009-05-12
Filing date: 2010-05-11
Publication date: 2010-11-18

Abstract

A femoral head prosthesis is described comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a generally frusto-conical portion defining at least part of said interior cavity, the wider end of the frusto-conical section being of at least twice the diameter of the narrower end of the frusto-conical section.

Description

HIP PROSTHESIS

FIELD OF THE INVENTION

The present invention relates to prosthetic implants and in particular to prosthetic hip implants.

BACKGROUND OF THE INVENTION

A first hip replacement may involve fitting a prosthetic cup socket to the acetabulum and resurfacing the femoral head before fitting a femoral head prosthesis. A range of prostheses of different sizes are provided such that the surgeon can select the best fit.

Hip replacements can eventually 'wear out' or fail requiring a revision hip replacement operation. This may be due to softening of the bone over time. Bone softening can occur for several reasons. For example, a section of bone may be poorly exposed to stress such that adequate bone density is not maintained leading to aseptic loosening of the prosthesis. This can occur when regions of bone are shielded from stress by other sections of bone or by the prosthetic component itself in a process known as 'stress shielding'.

Further, during a resurfacing hip replacement operation, one or more machining operations are generally carried out to prepare the femoral head for fitting of the prosthesis. There is a danger of notching of the femoral neck leading to altered stresses and/or subsequent femoral neck fracture.

Still further, many currently available femoral head components include a central stem which fits into the femoral head, and partly enter the femoral neck. These stems alter the stresses on the neck which may also lead to later fractures.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a generally frusto-conical wall defining at least part of said interior cavity, the wider end of the frusto-conical section being of at least twice the diameter of the narrower end of the frusto-conical section. The narrower end of the frusto-conical section may terminate in a generally planar top wall.

The wider end of the frusto-conical section may terminate in an outwardly flared side wall.

The edges of the frusto conical portion may be inclined at an angle of about 45 degrees

The prosthesis may include a plurality of projections extending into the interior cavity for engagement with the femoral head when the prosthesis is fitted.

In a second aspect the present invention provides a femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a circumferential sidewall defining at least part of said interior cavity, said sidewall being flared outwardly and terminating at said opening.

In a third aspect the present invention provides a femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a central stem which extends into said cavity, the width of said stem being less than 20% of the width of the prosthesis.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in this specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of this application.

Throughout this specification the word "comprise", or variations thereof such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The features and advantages of the present invention will become further apparent from the following detailed description of non-limiting embodiments, provided by way of example only, together with the accompanying drawings. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 shows a cross section of a femur including a resurfaced femoral head bearing a femoral head prosthesis according to the invention; and

Figure 2 illustrates the femur of Figure 1 with dashed lines indicating the approximate direction of stress forces transmitted through the head of the femur and their relation to the angled interior walls of the prosthesis.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

In each figure, corresponding features are identified by like reference numerals.

Figure 1 illustrates a longitudinal cross section of part of a femur 10 showing the femoral head 12 resurfaced and fitted with a femoral head prosthesis 14 according to the invention. The femoral head includes two chamfer portions 16,17 which have been resurfaced to conform to the circumferential side walls of the interior cavity 18 of the femoral head prosthesis.

The interior cavity 18 is defined by a frusto-conical portion 22 and a side wall portion 24 which flares outwardly to terminate at the opening of the cavity 18. The narrower upper end of the frusto-conical portion 22 terminates at a generally planar top wall 30.

In the illustrated embodiment, the wider end of the frusto-conical portion has a diameter of 50 mm and the narrower end of the frusto-conical section has a diameter of 15mm, being the diameter of top wall 30. The side-wall portion flares out and are inclined by approximately 3 degrees. In other embodiments the side-wall portion may be inclined from between 1 to 5 degrees, and between 2 and 4 degrees. The frusto- conical portion meets the top wall 30 at an angle of about 45 degrees.

Both the top wall and the frusto-conical portion include a plurality of projections 30 extending into the interior cavity 18 and these projections intrude into the bone when the prosthesis is fitted. The projections help to anchor the prosthesis onto the head of the femur to improve initial stability. In some embodiments, these projections are omitted.

The top wall further includes a stem 32 which extends axially from the top wall into a surgically prepared guide hole within the femoral head. The width of the axial stem is dimensioned to fit within a guide hole drilled in the femoral head during the resurfacing operation as will later be described. The stem is approximately 3% of the width of the prosthesis. The stem assists the surgeon in positioning the prosthesis and may also help to anchor the prosthesis to the head of the femur. However, it will be appreciated that the stem is optional such that the prosthesis may be provided without a stem.

The prosthesis is formed from titanium but may be formed from any suitable material used for surgical prosthetics as are known in the art. The prosthesis may also include a porous-ingrowth surface including hydroxy apatite to encourage osseointegration.

Figure 2 shows the femoral head 12 and prosthesis 14 of Figure 1 but also indicates the transmission of stress forces through the femoral head as represented by dashed lines. Stress forces A are in line with the trabeculae in the femoral head and indicate the force in line with the psoas tendon. These stresses continue on to form the medial cortex of the femur. Stress forces B are in line with the superior pelvic ramus and continue as the trabecular stress lines through the femoral head to form the lateral medial cortex. These lines illustrate the relationship between the transmission of stress forces at the edge of the femoral head and the angled interior walls. It can be seen that the stress lines intersect the frusto-conical portion 22 at approximately 90 degrees. These stresses are imparted to the bone within the femoral head which encourages increased bone density and thus improves strength of the replacement hip arrangement in keeping with Charnley's bone healing principle, and Wolffe's law.

A given femoral head prosthesis must be selected with regard to the size of a patient's femoral head. Accordingly, a hip replacement system will include a range of different sized prosthetic components. Accordingly, the prosthesis of the invention will typically be provided in a range of sizes to integrate with a range of different femoral heads, each different size of femoral head prosthesis having a bone receiving face which is of predetermined dimensions.

Use of a prosthesis according to the invention will now be described with reference to the following example of a first hip replacement operation.

Prior to fitting of prosthetic components, the surgeon examines the patient's femoral head 12 and selects an appropriately sized femoral head prosthesis 14. The femoral head is then resurfaced to conform to the shape of the bone receiving face of the selected femoral head prosthesis for an appropriate fit. This involves the creation of two chamfered portions 16, 17 on the outside of the femoral head.

The surgeon resurfaces the bone by cutting and/or grinding the femoral head using instruments that provide a template for resection of the bone end to a particular predetermined size corresponding to the bone receiving face of the selected femoral head prosthesis.

The surgeon begins by drilling a central guide hole into the top of the femoral head. This guide hole is used to align subsequent cutting operations. A rotary chamfer cutter is then used to cut chamfer portion 17 at an angle to correspond with the flared side wall 24 of prosthesis 14. By using a chamfer tool, the probability of inadvertently "notching" the femoral head, as can occur by making a cylindrical cut, is reduced.

Next, a second chamfer cut is made using a tool to cut chamfer portion 16 at an angle corresponding to the angle of frusto-conical portion 22.

The surgeon applies the femoral head prosthesis as a trial attachment before impacting the prosthesis onto the bone. When impacting the prosthesis onto the head, the plurality of projections 30 extending from the top wall 20 and the first circumferential side wall 22 intrude into the bone to aid in seating the prosthesis. Further, stem 32 becomes seated in the guide hole that was previously drilled. Bone cement may be used with the prosthesis to improve stability.

It can be seen that embodiments of the invention have at least one of the following advantages:

• Stress lines in the finished hip replacement intersect orthogonally with bone contacting faces of the prosthesis over a large area, encouraging improved bone density.

• By resurfacing the femoral head making only angled chamfer cuts, the probability of notching the femoral head is reduced.

• By use of a slender central stem, this can engage with the guide hole used for aligning the chamfer cuts to stabilise the prosthesis on the femur. No further bone is required to be removed or disrupted to accommodate a wide central stem and/or alter stress on the neck or head of the femur. It will be appreciated by persons skilled in the art that numerous variations and/or modifications can be made to the invention as shown in the specific embodiments without departing from the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a generally frusto-conical portion defining at least part of said interior cavity, the wider end of the frusto- conical section being of at least twice the diameter of the narrower end of the frusto-conical section.

2. A femoral head prosthesis according to claim 1 wherein the narrower end of the frusto-conical portion terminates in a generally planar top portion.

3. A femoral head prosthesis according to either of claim 1 or claim 2 wherein the wider end of the frusto-conical portion terminates in an outwardly flared side wall.

4. A femoral head prosthesis according to any preceding claim wherein the edges of the frusto conical portion are inclined at an angle of about 45 degrees.

5. A femoral head prosthesis according to any preceding claim including a plurality of projections extending into the interior cavity for engagement with the femoral head when the prosthesis is fitted.

6. A femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a circumferential sidewall defining at least part of said interior cavity, said sidewall being flared outwardly and terminating at said opening.

7. A femoral head prosthesis comprising an interior cavity with an opening for receiving a femoral head, the prosthesis including a central stem which extends into said cavity, the width of said stem being less than 20% of the width of the prosthesis.