Acetabular prosthetic techniques

Abstract: A cement-retaining shield for use when fitting an acetabular prosthetic implant (12) comprises a plurality of leaf-like sections (18,20) displaceable outwardly (eg by a cam 46) substantially to cover the acetabular recess (10).

Description: This invention relates to acetabular prosthetic techniques, and in particular to apparatus for fitting an acetabular prosthesis, and to acetabular prosthetic systems. A common failure mode of acetabular components of total hip replacement prostheses is loosening between the bone cement/acetabulum interface. The bone cement/acetabulum interface could be improved if the bone cement is made to penetrate and fill interdigitions generated in the subcondral bone of the acetabulum. A number of attempts have been made to improve the pressurisation of bone cement in the acetabulum. The most successful has probably been the Charnley Ogee acetabular cup which features a fixed flange attached to the cup. This flange is trimmed in the operating theatre to fit the patient's acetabulum, with the intention of thereby reducing egress of the uncured cement when it is compressed by pressure on the acetabular prosthesis. The disadvantage of this system is the difficulty of templating the acetabulum and the inherent high cost of the flanged component. Another acetabular cup is shown in US-A-4563778. In this device a liner of the cup has a lip or flange which is shown as occupying most of the area between the cup and the acetabulum. This is achieved by reaming the acetabulum to fit the cup accurately, which is an undesirable complication in the surgical procedure. An object of the present invention is to provide an acetabular prosthetic technique in which these disadvantages are overcome or at least reduced. In one aspect, the invention provides an expandable cement retaining shield for covering an acetabular cavity during cementing of an acetabular prosthesis therein, the shield comprising a plurality of overlapping sections displaceable relative to each other whereby the size of the shield is variable according to the size of the cavity to be covered. The overlapping section may be displaceable outwardly from the centre of the shield. In a preferred form the shield is a substantially planar disc, the sections being slideably and pivotally connected to each other. In a second aspect, the invention provides an acetabular prosthesic system comprising an acetabular prosthesis adapted to be placed in uncured bone cement in a subcondral acetabular recess, in combination with a shield as set forth above. There may be locating means enabling the shield and the prosthesis to be located relative to each other. In a third aspect, the system according to the invention comprises adjusting means for adjusting the size of the shield and compression means for compressing the cement. The adjusting means may comprise a rotatable cam adapted to enter the centre portion of the shield. The locating means may comprise apertures in the shield through which fixed structure of the adjusting means engages the prosthesis. The compression means may include means for rotating the cam and for applying force to the shield and the prosthesis via the fixed structure of the adjusting means. There preferably are means for separating the adjusting means from the prosthesis and the shield after compression of the cement. The separating means may comprise a part moveable away from the fixed structure towards the shield and the prosthesis, said part also limiting rotation of the cam. The invention will now be described, by way of example only, with reference to the accompanying drawings, wherein Figure 1 shows an acetabular prosthesis cemented into a subcondral acetabular recess; Figure 2 shows the prothesis of Figure 1 in side elevation with a shield according to the invention disposed above it; Figure 3 is a plan view of the shield of Figure 2; Figure 4 is a scrap section on line B-B of Figure 3; Figure 5 shows the shield of Figure 3 in an expanded condition; Figures 6, 7 and 8 show tools for use in fitting the acetabular prosthesis, Figure 7 being an elevation on arrow 7 of Figure 6. In Figure 1, a subcondral acetabular recess 10 in pelvic bone 11 has received therein a prosthesis 12 having a socket 14 to accommodate a femoral prosthetic joint, as known in the art. The prosthesis 12 is cemented into the recess 10 by a suitable bone cement 13 eg. polymethylmethacrylate. Referring to Figures 2 and 3, the prosthesis 12 is shown before insertion in the recess 10. It is provided with interrupted flanges 14 which key into the cement, as do prepared interdigitions and other irregularities in the cavity 10. However, for effective keying the cement must be adequately compressed before it is cured. For this purpose, an adjustable shield 16 is located over the prosthesis 12 to retain the cement in the recess as described hereafter. The shield 16 consists of a first pair of generally semi-circular sections or leaves 18 and a second pair of similar sections 20. Each section 18, 20 is connected via snap-fit rivets 22 within slots 24 in each of the sections. The flanges 23 of the rivets 22 (see Figure 4) are spaced sufficiently to permit free sliding and pivoting of the sections 18, 20 relative to each other to the extent permitted by the slots. The slots 24 are positioned orthogonally to diameters of the shield lying at 45° to inner edges 28,34 of the sections 18, 20 such that when the sections are closed-up as in Figure 3 the slots lie on a circle centred on the centre of the shield. The sections 20 have radially-extending slots 26 on the diameter perpendicular to their straight inner edges 28 (best seen in Figure 5). The edges 28 have recesses 30 in their centre positions so as to define an aperture 32 when the edges 28 abut. The inner edges 34 of the sections 18 are relieved such that when the shield sections are closed-up the edges 34 are clear of the slots 26, and further arcuate cut-outs in the edges 34 also leave the aperture 32 exposed. When the shield 16 is placed over the prosthesis 12 the slots 26 are aligned with two blind holes 36 in the top surface of the prosthesis. As described later, pegs of a locating and adjusting tool engage the holes 36 through the slots 26, locating the shield relative to the prosthesis. A cam of the locating tool is introduced into the aperture 32 and upon rotation forces the sections 20 outwards. The rivet and slot connections 22, 24 of the sections 20 to the other sections 18 cause the latter likewise to move outwards, orthogonally to the movement of the sections 20. The size of the generally planar disc-shaped shield 16 thus may be adjusted, the maximum extension of the Figure 3 shield being shown in Figure 5. A tool 39 for adjusting and locating the shield 16 during fitting of the prosthesis 12 to an acetabulum is shown in Figures 6 and 7. The tool comprises a knurled handle 40 with an extension rod 42. At the end of the rod 42 is an angled plate 44 wherein is rotatably mounted a blade-like cam 46. The cam may be rotated by a male hexagonal boss 47 on the other side of the plate 44. Projecting from the plate 44 on the same side as the cam 46 are two pairs of pegs 48, 50. The cam and the pegs are positioned such that the pegs enter the slots 26 of the shield when the cam engages aperture 32 thereof. The pegs 48 engage only the slots 26, but the pegs 50 are longer and pass through the slots to enter the holes 36 in the prosthesis, locating the shield relative thereto. The two pegs 48, 50 in each slot 26 constrain the sections 20 to move radially outwardly along the axis of the slots when adjusted. The rod 42 is hollow and contains a shaft 52 which at its end 51 nearest the handle is biased by a spring 53 against a further cam 54 rotatably mounted about the axis of the handle 40. A knurled knob 56 is provided to facilitate rotation of the cam by the user's thumb whilst holding the handle 40. At its other end the shaft 52 carries a plate 58 at the same angle as the plate 44. The plate 58 has an aperture 59 through which the cam 46 and pegs 48, 50 pass. A land 60 projects into the aperture 59 such that one surface 62 thereof provides a stop for the cam 46 when it is oriented to enter the aperture 32. A further surface 64 of the land limits rotation of the cam 46 to approximately 90° at which rotation the shield sections 18, 20 have been fully adjusted outwards. Upon rotation of the further cam 54 the shaft 52 is extended, displacing the plate 58 away from the plate 44, as shown dotted in Figure 6, whereby to disengage the adjusting and locating tool from the shield and the prosthesis. Figure 8 shows a tool 65 for applying compressive force to the bone cement and for rotating the cam 46. The tool comprises a female hexagonal socket 66 to engage the boss 47, carried by a shaft 68 terminating in an enlarged end 70 which the user may grasp to turn the cam 46. The end 70 also is shaped, with a curved upper surface 72 so that the user may apply pressure via the shaft 68 to the boss 47 and thence through the fixed plate 44 to the shield and the prosthesis. In use, the subcondral acetabular cavity 10 is prepared in known manner and an appropriate quantity of bone cement is introduced therein. The shield is placed on the cam and pegs 46, 48, 50 of the tool 39 followed by the prosthetic implant 12. The surgeon then introduces the prosthesis into the cavity 10, holding the tool 39 in one hand. Whilst holding the prosthesis in position with the tool 39, he attaches the tool 65 to the boss 47 and expands the shield 16 until it covers the edges of the acetabular cavity 10, and preferably also presses into surrounding soft tissue to form a seal. The bone cement is thus effectively contained in the cavity, and the surgeon then compresses it as necessary by applying axial force to the tool 65. Preferably, the force is maintained until the cement is sufficiently cured for the prosthesis to be firmly held. The tool 65 is then detached and the tool 39 disengaged from the shield 16 and prosthesis 12 by rotation of the cam 54. The consequent extension of the plate 58 steadies the shield and prosthesis, so that withdrawal of the cam 46 and pegs 48, 50 does not disturb the adhesion of the cement. If desired the surgeon can apply further pressure to the prosthetic implant 12 by insert the end 66 of the tool 65 directly into the socket 14 of the prosthesis, the surface 74 of the end 66 being shaped appropriately for this purpose.

Claims: An expandable cement retaining shield for covering an acetabular cavity during cementing of an acetabular prosthesis therein, the shield (16) comprising a plurality of overlapping sections (18, 20) displaceable relative to each other whereby the size of the shield is variable according to the size of the cavity to be covered. A shield as claimed in Claim 1, wherein the overlapping sections (18, 20) are displaceable outwardly from the centre of the shield. A shield as claimed in Claim 2, wherein the shield (16) is a substantially planar disc, the sections (18, 20) being slideably and pivotally connected to each other. An acetabular prosthetic system comprising an acetabular prosthesis (12) adapted to be placed in uncured bone cement in a subcondral acetabular recess, in combination with a shield (16) as claimed in any preceding claim. A system as claimed in Claim 4, wherein the shield comprises locating means enabling the shield and the prosthesis to be located relative to each other. An acetabular prosthetic system comprising a shield (16) as claimed in any of Claims 1, 2 or 3, in combination with adjusting means (39) for adjusting the size of the shield and compression means (65) for compressing the cement. A system as claimed in Claim 4 or 5, further comprising adjusting means (39) for adjusting the size of the shield and compression means (65) for compressing the cement. A system as claimed in Claim 6 or 7, when dependent from Claim 2, wherein the adjusting means (39) comprises a rotatable cam (46) adapted to enter the centre portion (32) of the shield. A system as claimed in Claim 6, 7 or 8, wherein the locating means comprises apertures (26) in the shield through which fixed structure (50) of the adjusting means engages the prosthesis. A system as claimed in Claims 8 and 9, wherein the compression means comprises means (70) for rotating the cam and for applying force to the shield and the prosthesis via the fixed structure (44) of the adjusting means. A system as claimed in Claim 10 comprising means (54, 58) for separating the adjusting means (39) from the prosthesis (12) and the shield (16) after compression of the cement. A system as claimed in Claim 11 wherein the separating means comprises a part (58) moveable away from the fixed structure (44) towards the shield and the prosthesis, said part also limiting rotation of the cam (46).

Inventors: BUCKLEY NEIL, SHEEHAN JAMES MICHAEL, BUCKLEY, NEIL, SHEEHAN, JAMES MICHAEL, SHEEHAN, JAMES MICHAEL C/O ZIMMER LIMITED
Applicants: SHEEHAN JAMES MICHAEL, SQUIBB BRISTOL MYERS CO, ZIMMER LIMITED, BRISTOL-MYERS SQUIBB COMPANY, SHEEHAN, JAMES MICHAEL
Application year: 1993
Main classification: A61F2
IPC codes: A61F2