GB2492810A

GB2492810A - Prosthesis head and base with offset attachment means

Info

Publication number: GB2492810A
Application number: GB201112010A
Authority: GB
Inventors: Ian Alexander Trail
Original assignee: UPPER LIMB INNOVATIONS Ltd
Current assignee: UPPER LIMB INNOVATIONS Ltd
Priority date: 2011-07-13
Filing date: 2011-07-13
Publication date: 2013-01-16
Also published as: GB201112010D0

Abstract

A prosthesis for partial or total replacement of an anatomical structure having a base portion 2 for attachment to a surface of an anatomical structure and a head portion (3, Figure 3) which is attachable to the base portion 2 and, in use, acts as a replaced anatomical structure or a portion thereof, the base portion having at least one attachment means 7, for attaching the base portion 2 to the surface of the anatomical structure and wherein the at least one attachment means 7 is located offset from an axis 23 that passes through the centre of a surface of the base portion 2 that, in use, contacts the surface of the anatomical structure.

Description

Prosthetic Replacement of Anatomical Structures The present invention relates to prosthetic replacement of anatomical structures and specifically, but not exclusively, to full or partial prosthetic replacement of thehumeral head.

Rheumatoid arthritis and/or osteoarthritis in anatomical structures, e.g. a shoulder joi.nl, can cause sevcrc pain and limit the range of movement of the structure. In such cases, it is desirable for a suflerer to undergo anthroplasty surgery to partially or fully replace the damaged joint with a prosthetic that mimics the replaced joint, or a portion of the joint.

For example, it is known to partially or fully replace a damaged shoulder joint with a shoulder prosthesis.

A known shoulder prosthesis, used for hurn.eral. head replacement, includes a hemi-spherical head attached to one end. of an elongate stem. In order to impiant the prosthesis the humeral head is resected and the elongate stern is inserted into the humeral canal, tpieally half-way down the length of the humerus, to anchor the prosthesis in place. The hemi-spherical head of the prosthesis is mounted on the exposed surface of the humerus and acts as a prosthetic replacement for the resected humeral head.

Such a shoulder prosthesis, with a relatively long stem, suffers from a number of disadvantages. Specifically, insertion of the long stem of the prosthesis into the humera.l canal requires extensive bone removal. In addition, it is extremely difficult to anatomically align the head onto a humeral stein.

In an effort to try and overcome these problems, a stemless' shoulder prosthesis has been designed. The prosthesis comprises a hemi-spherical prosthetic head attached to one end of a screw. In order to implant the prosthesis, the humeral head of the shoulder joint i.s resected. A bore is then drilled into the exposed surface of the humerus and the screw of the prostheses is screwed into this bore until the flat surface of the prosthetic head is flush with the exposed surface of the humerus.

However, such a prosthesis is disadvantageous in that it relies on bone ingrowth, i.e. bone fixing directly to the prosthesis, to fix the prosthesis in place.

This process can take anything in the region of six to twelve weeks to consolidate, or even longer. During this time, there is a potential for loss of fixation of the prosthesis.

1.0 In addition, force exerted on the prosthetic head is transmitted directly into the anatomical structure through the screw. This can cause internal damage within the anatomical structure.

Embodiments of the present invention seek to overcome the above problems.

According to a first aspect of the invention there is provided a prosthesis for partial or total replacement of an anatomical structure comprising a base portion for attachment to a surface of an anatomical structure and a head portion which is attachable to the base portion and., in use, acts as a replaced anatomica) structure or a. portion thereot the base portion comprising at least one attachment means, for attaching the base portion to the surface of the anatomical structure and wherein the at least one attachment means is located offset from an axis that passes through. the centre of a surface of the base portion that, in use, contacts the surface of the anatomical structure.

The use of at least one attachment means located offset from sai.d central axis of the base portion is advantageous in that it adds strength of initial fixation of the prosthesis to the anatomical structure, thus ensuring that there is no loss of fixafion of the prosthesis during the time taken for sufficient bone ingrowth to occur.

Pnfcrably said central axis is substantially pcrpcndicular to thc surface of the base portion that, in use, contacts the surface of the anatomica.l structure.

Preferably said surface of the base portion is substantially planar.

Preferably the at least one attachment means is provided towards the periphery of said surface of the base portion.

Preferably the at least one attachment means is or comprises an aperture for receiving a suitable fastener. The at least one attachment means may also comprise a. suitable fastener. Th.e fastener is preferably arranged to pass tiuough the. surface of the base portion that, in use, contacts the surface of the anatomical structure and into the anatomical structure. The fastener is preferably a screw, but any suitable fastener may be used.

Preferably the base portion comprises a plurality of said attachment means.

In this case, the attachment means are preferably distributed around the periphery of the base portion. This is advantageous in that the base is securely fixed around its periphery to the anatomica.l structure, thus ensuring that there is no loss of fixation of the prosthesis during the time taken for sufficient bone ingrowth to occur.

Preferably the head portion is attachable to the base portion by an interference fit therebetween, Preferably. th.e head portion is provided with a formation and the base portion is provided with a co-operating formation arranged to form an intcrference fit with said formation. -4 -

Preferably said formation is a protrusion and said co-operating formation is a bore, or vice-versa. The bore and protrusion arc preferably arranged such that, when the head portion is attached to the base portion, the respective longitudinal axes of the bore and protrusion are co-incident and parallel to each other.

The bore and protrusion are preferably tapered such that they form said interference fit. Preferably the bore and protrusion are tapered so as to form a morse taper fit.

Preferably, the longitudinal axis of the formation is substantially co-incident with and parallel to said central axis of the base portion, when the head portion is attached to the base portion.

Preferably, a longitudinal axis of the co-operating formation is gubstantially co-incident with and parallel to said central axis of the base portion.

The base portion is preferably provided with a projection which houses said co-operating formation. The projection is preferably arranged to be received within a bore in the anatomical. struelure to which the prosthesis is to he attached. The projection preferably extends from the surface of the base portion that, in use, contacts the surface of the anatomical structure. A longitudinal axis of the projection is preferably located substantially parallel to and coincident with said central axis of the base portion.

Where said co-operating formation is a bore, the bore is preferably provided within the projection.

The head portion preferably comprises a substantially planar surface bounded by a curved surface. Preferably the curved surface is contoured to engage a complimentary articular surface. Preferably the curved surface forms a concave surface. Preferably the planar surface has the general shape of a circle and the curved surface has the general shape of a hemi-sphere.

Preferably the prosthesis is for partial or total replacement of a humeral head. In this case, the head portion, in usc, acts as a replaced humeral head, or a portion thereof and the base portion is for attachment to a surface of the proximal humerus.

By virtue of the above arrangement, the head portion is not directly attached to the anatomical structure (e.g. an exposed surface of a humerus), but is instead attached via the base portion. Accordingly, a force exerted on the head portion is 1.0 not directly transmitted into the anatomical structure (e.g. a humerus), as with the known. prosthescs (which use a central stem or screw to fix the prosthesis to the anatomical structure), hut is spread across the area of the base portion. This reduces the level of force exerted within the anatomical structure to which the prosthesis is attached and so reduces tile risk of loss of fixation of the prosthesis and/or internal damage to tile anatomical structure.

All of the features described herein may be combined with any of the above aspects, in any combination.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figure 1 shows a perspective view from above a base portion of a prosthesis according to the present invention; -6.

Figure 2 shows a perspective view from below the base portion of the prosthesis shown in Figure 1; Figure 3 shows a perspective view from below a head. port:ion of a prosthesis according to the present invention; Figure 4 shows a side elevational view of the head portion of the prosthesis shown U) Figure 3; FigureS shows a front perspective view of an upper portion of a humerus; Figure 6 shows a front perspective view of the upper portion of the humerus shown in Figure 5, bit with the humeral head resected and with a measuring disc placed onto the exposed surface of the humerus; Figure 7 shows a front perspective view of the upper portion of the humerus shown in Figure 6, after the r.easuring disc has been removed and a bore has been drilled into the humerus; Figure 8 shows a front perspective view of the humerus shown in Figure 7, with the base portion of the prosthesis of Figures 1 and 2 attached to the exposed surface of the humerus; and Figure 9 shows the front perspective view of Figure 8, but with the head portion of the prosthesis shown in Figures 3 and 4 attached to the base portion.

Referring to the above fires there is shown a prosthesis 1 for replacement of the humeral head 14 in a shoulder joint. The prosthesis I comprises a base portion 2 and a head portion 3.

Referring to Figures 1 and 2, the base portion 2 comprises a generally circular plate 4 provided with a projection in the form of a. cap 5 centrally disposed on the plate 4, The plate 4 is a generally thin planar element and has a generally circular horizontal cross-section. The plate 4 has first and second surfaces 21, 22. The plate 4 comprises an axis 23 which passes through the centre of the first surface 21 of the plate 4 and is perpendicular to said sutface 21 at the point of intersection of the axis 23 with the surface 21.

A plurality of apertures 7 are provided in the plate 4. The respective centres of the apertures 7 are offset from said central axis 23 and pass through the thickness of the plate. The apertures 7 are circurnferentially distributed around the periphery o. the plate 4. Each aperture 7 is for receiving a respective fastener 8 (see below).

In the current embodiment the fasteners are screws 8. 1-lowever. it will be appreciated that any suitable fastener may be used. Similarly, although four apertures 7 are used in. the current embodiment, it will he appreciated that the number of apertures may be varied, depending on the desired strength of fixation.

The capS is centrally disposed on the first surface 2! of the plate 4. The cap has a generally cylindrical shape. The longitudinal axis of the cap 5 is substantially co-incident with and parallel to said cena1 axis 23 of the plate 4.

A bore 6 extends from an aperture 19 provided on the second surface 22 of the plate 4 and terminates part way into the ca.p 5. The bore 6 has a generally circular cross section and tapers along its length such that the radius of the bore 6 decreases from its open end (ic. the aperture 19) to its closed end. The longitudinal axis of the bore 6 is substantially co-incident with, and substantially parallel to. said central axis 23 of the base portion 2. The bore 6 has a frusto-conical shape.

Referring to Figures 3 and 4, the head portion 3 of the prosthesis comprises a.

hemispherical cap 9 closed by a circular base 10. The radius of the base 10 is less than that of the cap 9 such that the base 10 is recessed within the cap 9 to form a. rim 11.

A stem 12 is centrally disposed on the base 10. The longitudinal axis of the stern 12 is substantially co-incident with the centre of the base 10 and is substantially perpendicular to the base 10. The stem 12 is an elongate member o.f circular cross-section that tapers along such that the radius of the cross-section of the stein decreases from the base 10 to the free end of the stern. The stein 12 has a frusto-conical shape.

The head portion 3 is attachable o the base portion 2 by insertion of the stem 12 of the head portion 3 into the bore 6 of the base portion 2 to form an interference fit. In this respect, the bore 6 in the base portion 2 is sized and dimensioned to receive the stem 12 of the head portion 3. In addition, the respective longitudinal axes of the stem 12 and bore 6 are coincident with, and parallel to, each other when the stem 12 is received within the bore 6.

The diameter of the rim 11 of the head portion 3 is slightly eater than the diameter of the plate 4 of the base portion 2 such that the rim 11 can fit over the plate 4. Accordingly, when the stem 12 is inserted into the bore 6, the rim Ii of the head portion 3 fits around the plate 4 of the base portion 2.

The bore 6 and the stem 12 are tapered so as to achieve a morse taper fit with each other. This provides secure connection of the head portion 3 and base portion 2 while permitting relative rotatable movement between the two.

The method of installing the prosthesis 1, to replace an anatomical structure in the form of a. humeral head 14, will now be described with reference to Figures 5 to 9 An anatomical structure in the form of a proximal humerus 13 is exposed and the head 1.4 of the humerus 13 is removed. A measuring disc 15 is then place on the exposed surface 16 of the humerus 13. The measuring disc 15 comprises a generally circular disc provided with an aperture 17 at substantially its centre. The measuring disc 15 is used to mark the centre of* tle exposed surface 16 of the humerus 13 by locating the disc appropriately on the exposed surface of the humerus 13 and then applying a mark through the aperture 17 in the disc. 15.

A generally cylindrical bore 20, centred on the marked centre of the exposed surface 16, is then drilled through the exposed surface 16 into the humerus 13. The bore 20 is sized and dimensioned to receive the cap 5 of. the base portion 2 of the prosthesis 1.

The cap 5 of the base portion 2 is then inserted into the bore 20, such that the first surface 21 of the plate 4 contacts, and is flush with, the exposed surface 16 of the humerus 13.

The base portion 2 is then fixed in position by passing respective screws 8 through the apertures 7 distributed around the periphery of the plate 4 and screwing them into the humerus 13, through its exposed surface 16. Before the screws 8 are -10-screwed itho the humerus 13, holes for the screws are drilled, measured and tapped usin.g the apertures 7 in the plate 4 as a guide.

The head portion 3 of the prosthesis I is then fitted onto the base portion 2 by inserting the stem 12 of the head portion 3 into the bore 6 in the base portion 2.

The rim 11 of th.e head portion. 3 then fits around the plate 4 of the base portion 2, with the base 10 of the head portion 3 flush with the second surface 22 of the plate 4 of the base portion 2.

Various sized head portions 3 can be trial, fitted onto the plate 4 of the base portion 2, in order to determine the correct size of head portion 3 to be used for the particular patient.

Screwing the base portion of the prosthesis to the exposed surface 16 of the humerus 13 at points offset from the central axis 23 of the base portion 2 is advantageous in that it adds strength of initial fixation of the prosthesis 1 to the humerus, thus ensuring that there is no loss of fixation of the prosthesis 1 during the time taken for sufficient hone ingrowth to occur.

In addition, since the head portion is not directly attached to the anatomical structure, but is instead attached via the base portion, the level of force exerted within the anatomicHi structure to which the prosthesis is attached, from a force applied on the head portion 3, is reduced. This reduces the risk of loss of fixation of the prosthesis and/or internal damage to the anatomical structure.

The above embodiment is described by way of example. Many variations are possible without departing from the invention.

Claims

<claim-text>Claims 1. A prosthesis for partial or total replacement of an anatomical structure comprising a basc portion for a.tta.cimi.ent to a surface of an anatomical structure and a head portion which is attachable to the base portion and, in use, acts as a replaced anatomical structure or a portion thereof, the base portion comprising at least one attachment means, for attaching the base portion to the surface of the anatomical structure and wherein the at least one attachment means is located offset from an axis that passes through the centre of a surface of the basc portion that, in use, contacts the surface of the (4 anatomical structure.</claim-text> <claim-text>2. A prosthesis according to claim 1 wherein said central axis is substantially perpendicular to the surface of the base portion that, in use, contacts the (4 surface of the anatomical structure.</claim-text> <claim-text>3. A prosthesis according to either of claims 1 or 2 wherein said surface of the base portion is substantially planar.</claim-text> <claim-text>4. A prosthesis according to any prcecding claim whei-ein the at least one attachment means is proded towards the periphery of said surface of the base portion.</claim-text> <claim-text>5. A. prosthesis according to any preceding claim wherein the at least one attachment means is or comprises an aperture for receiving a suitable fastener.</claim-text> <claim-text>6. A prosthesis according to claim 5 wherein the at least one attachment means also comprises a suitable fastener.</claim-text> <claim-text>7. A prosthesis according to claim 6 wherein the fastener is arranged to pass through the surface of the base portion that, in use, contacts the surface of the anatomical structure and into the anatomical structure.</claim-text> <claim-text>8. A prosthesis according to eithcr of claims 6 or 7 wherein the fastener is a screw.</claim-text> <claim-text>9. A prosthesis according to any preceding claim wherein the base portion comprises a plurality of said attachment means.</claim-text> <claim-text>10. A prosthesis according to claim 9 wherein the attachment means are distributed around the periphery of the base portion.</claim-text> <claim-text>11. A prosthesis according to any preceding claim wherein the head portion is r attachable to the base portion by an interference fit therebetween.</claim-text> <claim-text>12. A prosthesis according to claim 11 wherein the head portion is provided with a formation and the base portion is provided with a co-operating formation (\J arranged to form an interferencc fit with said formation.</claim-text> <claim-text>13. A prosthesis according to claim 12 wherein said formation is a protrusion and said co-operating formation is a bore, or vice-versa.</claim-text> <claim-text>14. A prosthesis according to claim 13 wherein the bore and protrusion are arranged such that, when the head portion is attached to the base portion, the respective longitudinal axes of the bore and protrusion are co-incident and parallel to each other.</claim-text> <claim-text>15. A prosthesis according to any of claims 12 to 14 wherein the longitudinal axis of the formation is substantially co-incident with and parallel to said central axis of the base portion, when the head portion is attached to the base portion.</claim-text> <claim-text>16. A prosthesis according to any of claims 12 to 15 wherein a longitudinal axis of the co-operating formation is substantially co-incident with and parallel lo said central axis of the base portion.</claim-text> <claim-text>17. A prosthesis according to any of claims 13 to 16 wherein the bore and protrusion are tapered such that they form said interference fit.</claim-text> <claim-text>18. A prosthesis according to claim 17 wherein the bore and protrusion are tapered so as to form a morse taper fit.</claim-text> <claim-text>19. A prosthesis according to any of claims 13 to 1 8 wherein the base portion is provided with a projection which houses said co-operating formation.</claim-text> <claim-text>20. A prosthesis according to claim J 9 wherein the projection is arranged to be received within a bore in the anatomical sthicture to which the prosthesis is to be attached.</claim-text> <claim-text>21. A prosthesis according to claim 20 wherein the projection extends from theN-surface of the base portion that, in use, contacts the surface of the anatomical structure.</claim-text> <claim-text>22. A prosthesis according to any of claims 19 to 21 wherein a longitudinal axis of the projection is located substantially parallel to and coincident with said central axis of the base portion.</claim-text> <claim-text>23. A prosthesis according to any preceding claim wherein the head portion comprises a substantially planar surface bounded by a curved surfacc.</claim-text> <claim-text>24. A prosthesis according to claim 23 wherein the curved surface is contoured to engage a complimentary articular surface.</claim-text> <claim-text>25. A prosthesis according to either of claims 23 or 24 wherein the curved surface forms a concave surface.</claim-text> <claim-text>26. A prosthesis according to any of claims 23 to 25 wherein the planar surface has the general shape of a circle and the curved surface has the general shape of a hemi-sphere.</claim-text> <claim-text>27. A prosthesis according to any preceding claim wherein the prosthesis is for partial or total replacement of a humeral head.</claim-text> <claim-text>28. A prosthesis according to claim 27 wherein the head portion, in use, acts as a replaced hurneral head, or a portion thcreof and the base portion is for attachment to a surface of the proximal humerus.</claim-text> <claim-text>29. A prosthesis substantially as described herein with reference to the accompanying drawings (4 r (4</claim-text>