GB2208800A

GB2208800A - Prosthetic implant with porous pad

Info

Publication number: GB2208800A
Application number: GB8814268A
Authority: GB
Inventors: Arden R Zolman; Philip J Andersen; Steven T Bayne
Original assignee: Bristol Myers Co
Current assignee: Bristol Myers Co
Priority date: 1987-08-24
Filing date: 1988-06-16
Publication date: 1989-04-19
Anticipated expiration: 2008-06-16
Also published as: AU1814288A; JPH0614943B2; DE3822154C2; JPS6456050A; IT8821727A0; AU613922B2; CA1300811C; FR2619707B1; IT1226850B; DE3822154A1; FR2619707A1; GB8814268D0; GB2208800B

Description

2 j f PROSTHETIC IMPLANT WITH WRAPPED POROUS SURFACE L'e_, 0 c) The

present invention relates to a prosthetic implant device and more particularly to such implants including porous surfaces thereon. This invention is particulary suitable for use as a femoral component for a hip prosthesis, although is not limited thereto.

Heretofore, various types of porous surfaces have been incorporated into prosthetic implants. Such porous implants are often implanted without the use of any type of bone cement material so that the porous surface will be in direct contact with the bone surface. After a period of time, bony ingrowth occurs in and around the porous surface to biologically affix or further secure the implant in the bone. Alternatively, an implant having a porous surface may be implanted with bone cement, such that the penetration of the bone cement into the porous surface of the prosthesis will enhance the fixation of the prosthesis.

The following US patents disclose prosthetic implants which include various types of porous surfaces:

US patent 3,906,550 to Rostoker et al discloses a porous fibre metal structure adapted for attachment to a prosthesis. The fibre metal material is moulded directly into the desired precise shape using dies and punches.

US patent 4,479,271 to Bolesky et al discloses a prosthesis including porous surfaces which are also either molded and compressed directly into shape or compressed and cut directly into shape.

US patent 4,536,894 to Galante et al discloses a prosthesis including porous surfaces in which the porous surfaces are porous pads which are compressed and cut directly to the desired shape and then adhered to substantially flat surfaces.

US patent 4,570,271 to Sump discloses a prosthesis with a porous surface in which the porous coating is preformed directly into the desired shape which corresponds to the preselected surface of the prosthesis. The preform porous coating is then overlaid onto the preselected surface, compressed, and heated to adhere the preform to the prosthesis. The preform may be produced by conventional multi-layer knitting, braiding, winding, weaving, or other continuous wire processes.

i - 2 US patent 4,589,883 to Kenna discloses a prosthesis including a porous surface in which the porous coating is comprised of spherical particles which appear to have been directly bonded to the prosthesis stem.

US patent 4,636,219 to Pratt et al discloses a prosthesis including a porous surface comprises of a layered metal mesh structure. The mesh may be bonded to a thin substrate (on the order of 0.010 to 0.012 inch) (0. 254 to 0.305mm) which thin substrate can then be cut or formed and applied to the body of a prosthesis on a flat surface as in Figure 3 of Pratt et al or contoured into specific shapes by processes such as creep forming.

US patent 4,660,755 to Farling et al discloses a method for constructing a surgical implant in which a porous layer is bonded to a substrate in the absence of a furnace, the bonding occurring via an electrode.

UK patent application GB2059267A to Ducheyne discloses a type of porous metal wire mesh suitable for use on a prosthesis.

UK patent application OB2142830A to Brown et al discloses a prosthetic implant having a porous surface provided by a perforated micro-contoured sheet.

UK patent application GB2153233A to Lee et al discloses a prosthetic implant which utilizes wedges to wedge between the bone cavity wall and the prosthesis stem in which the wedges may include a porous surface.

European patent application EP0178650A2 to Ducheyne discloses a porous flexible metal fibre material for implantation in conjunction with a prosthesis in which one or more layers of the porous sheet material are pressed against the bone surface with a layer of bone cement then between the-implant and the porous sheet material.

This invention provides a prosthetic implant which includes a porous pad separate from the prosthesis stem which is adapted for wrapping around the stem for attachment thereto.

In a preferred form, the invention provides a prosthetic implant which includes a porous pad separate from the fixation surface base portion such that the porous pad has a first preliminary precontoured 9 1 shape. The porous pad is adapte d for wrapping about the fixation surface base portion in a second and final shape corresponding to the shape of the fixation surface base portion about which it is wrapped.

Preferably the pad is wrapped about at least two adjacent sides.

The invention also provides a process for wrapping a porous pad about a prosthesis stem in which the pad has first preliminary, substantially flat precontoured shape which is subsequently wrapped about the stem into a second and final shape for attachment to the stem.

The invention also provides a method of constructing a prosthetic implant comprising the following steps:

(a) providing a base implant including a stem portion having a plurality of sides; (b) precontouring a separate porous pad having a first desired configuration; and (c) wrapping the pad around the stem portion for attachment thereto in a second configuration conforming to the shape of the stem about which the pad is wrapped.

In one form of the invention, the method further includes the step of forming the pad, to correspond to the shape of the stem portion to which it is to be attached by placing the prosthetic implant in a fixture having a plurality of forming jaws, each in a retracted position away from the prosthetic implant; placing the porous pad in a desired location between the jaws and the implant, then activating the jaws against the fibre metal and the implant in a predetermined sequence to form the porous pad about the implant, conforming the pad to the shape of implant.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a femoral component for a hip prosthesis according to the present invention; Figure 2 is an anterior side elevational view thereof; 2; Figure 3 is a lateral side elevational view thereof; Figure 4 is a medial side elevational view thereof; Figure 5 is a cross-sectional view taken along lines 5-5 of Figure Figure 6 is a crosssectional view taken along lines 6-6 of Figure 2; Figure 7 is a side view of a femoral component according to the present invention located in a forming fixture with the fixture shown in a cross- sectional view taken along lines 7-7 of Figure 8c; Figure 8 is a cross-sectional view of the forming fixture taken along lines 8-8 of Figure 7; Figures 8a, 8b and 8c are cross-sectional views taken along lines 8-8 of Figure 7 of the forming fixtures illustrating various jaws in activated positions about the femoral component stem; Figure 9 is a top view of a pressed and prebonded sheet of porous material; Figure 10 is a top view of the sheet of Figure 9 illustrating a cut out portion; Figure 11 is a top view of the porous pad formed from the cut out of Figure 10; Figure 12 is a side view of a femoral component according to the present invention located in a bonding fixture with the fixture shown in a cross- sectional view taken along lines 12-12 of Figure 13; Figure 13 is a cross-sectional view of the bonding fixture taken along lines 13-13 of Figure 12; Figure 14 is an anterior side elevational view of an alternate embodiment of a femoral component; Figure 15 is a posterior side view of the component of Figure 14; and Figure 16 is an alternate embodiment for a porous pad to be used in conjunction with the component of Figure 14.

Figures 1-13 illustrate a particularly advantageous embodiment of a prosthetic implant according to the present invention. The invention will be described with reference to a femoral component 10 of a hip prothesis and is particularly suitable as such. However, it is understood that the principles of the invention may be suitable for other implants having elongated fixation stems. In addition, it is noted for reference that the particular femoral component 10 described herein is designed for implantation in a right femur, while a corresponding femoral component (not shown) for implantation in a left femur would be a mirror image of the femoral component 10 shown for the right femur.

The femoral component 10 of Figures 1-6 includes a distal end 12 and a proximal end 14. As is well known, the femoral component 10 is intended to fit within the intramedullary canal of a femur (not shown) such that the proximal end extends outwardly from the intramedullary canal of the femur to cooperate with an acetabulum or acetabular prosthetic member via a ball or the like carried at the proximal end 14. A distal portion 16 may include a plurality of grooves 18 extending longitudinally. A proximal portion 24 includes a porous surface or pad 26 encircling the femoral component 10. The distal portion 16 and the proximal portion 24 comprises the stem portion 20 of the femoral component 10. A neck 28 extends from the proximal portion 24 and is adapted to carry a ball 30 shown in phantom lines in Figure.1. An aperture 31 adjacent the neck 28 accommodates a tool for removing the femoral component 10 from the intramedullary canal should removal be necessary. The porous pad 26 is designed to extend outwardly from the proximal portion 24 albeit a small extension of about 0.5mm past the adjacent smooth surface of the proximal portion 24.

Figures 2. 3 and 4 illustrate a chamfer 32 on the anterior side of the femoral component which may be included to avoid impingement of the distal end 12 into the bone surrounding the intramedullary canal at that location.

As shown in Figures 1-4, the proximal portion 24 includes a lateral side 34. a posterior side 36, a medial side 38, and an anterior side 40. A porous pad 26, as shown in the embodiment of Figures 1-6, is circumferentially wrapped around the proximal portion 24 of the stem portion 20. The pad 26 may be formed of any suitable porous material that is adapted to be preliminarily pressed into a first shape and then subsequently wrapped about the stem portion 20 into a second shape 1 conforming to the shape of the stem portion 20. One such suitable material is the fibre metal structure disclosed in US patent 3,906,550 to Rostoker and Galante, although it is noted that the Rostoker et al patent teaches that the fibre metal material is molded directly into the desired shape, rather than providing a first preliminary shape which is then subsequently wrapped or formed about a prosthetic implant into the second and final shape in accordance with the present invention. It is understood that any suitable porous material, particularly fibrous (wire type) porous structures which are adaptable to be practiced in accordance with the present invention, may be utilized. The material for the porous pad may be titanium wire having a diameter of 0.010 inches (0.254 mm), and the material for the femoral component may also be titanium. Again it is understood that any suitable materials may be utilized.

The shape of the porous pad 26 may have any desirable configuration, although preferably the pad is first formed in or pressed into a substantially flat sheet 126 (see Figures 9-11). The outer boundary of the pad 26 may have any suitable contour. The porous pad 26 is separate from the stem portion 20. The pad 26 is adapted for wrapping around the stem portion 20 for attachment thereto. The porous pad 26 has a first preliminary precontoured shape as shown in Figure 11 and is then subsequently wrapped and/or formed about the stem portion 20 for attachment thereto in a second and final shape corresponding to the shape of the stem portion 20 as shown in Figures 1-4. The porous pad 26 is wrapped around at least two to three adjacent sides, although preferably is wrapped around all four sides 34, 36, 38 and 4o to form a continuous porous surface circumferentially about the stem portion 20.

The porous material, such as a kinked titanium fibre metal, is press formed into a sheet 126 of porous material. The sheet 126 may have any desired thickness or dimensions. The dimensions of the sheet 126 may be adapted to cut out a single pad 26 having a desired outer boundary or adapted to cut out multiple pads 26 from a single sheet. The sheet 126 is prebonded in a vacuum furnace by holding the sheet for a suitable length of time at a temperature sufficient to create metallurgical bonds at the points of contact of the fibres'of the porous material within the sheet 126. A porous pad 26 having the desired outer contour is then cut from the sheet leaving a corresponding cut out 226 in the sheet 126. Prebonding the fibre metal sheet prior to cutting it creates mechanical integrity in the sheet 126 so that the fibre material is more easily formable and holds its shape better.

The porous pad 26, shown in Figure 11 which is subsequently applied to the femoral component 10 of Figures 1-4, includes a medial pad side 138 to correspond to the medial stem side 38, posterior and anterior pad sides 136 and 140 each extending from or adjacent to the medial pad side 138 and adapted to correspond to the posterior and anterior stem sides 36 and 40, respectively, and first and second tabs 120 and 122 extending from the posterior and anterior sides, respectively. The two tabs together comprise the lateral pad side 134 which will correspond to the lateral stem side 34 of the femoral component 10. The pad 26 may be asymmetric as shown with the posterior porous pad surface 136 having greater surface area than the anterior porous pad surface 140 and with the medial pad surface 138 being longer than the lateral pad surface 134. However, the pad 26 could readily be made symmetrically, if desired, as any appropriate shape may be utilized for the pad 26.

The pad 26 is then ready to be wrapped about the stem portion 20. The proximal portion 24 of the stem portion 20 includes a recess 74 (see Figure 6) having a shape corresponding to the pad 26 and adapted to receive the pad 26 for subsequent attachment thereto. The pads 26 can be shaped to conform to any desirable and suitable implant stem or fixation surface configuration. The proximal portion 24 of the stem portion 20 of the femoral component shown has an asymmetric noncircular cross-section as shown in Figures 5 and 6.

The process for wrapping the porous pad 26 may be accomplished by placing the femoral component 10 (without pad 26) in a forming fixture 60 as shown in Figures 7 and 8. The fixture 60 has a plurality of forming jaws 61, 62, 63 and 64. The forming jaws are each operated by corresponding rod 161, 162, 163 and 164 respectively, which operatively connects the jaws to cylinders 261, 262, 263 and 264 respectively. The cylinders 261, 262, 263 and 264 may be air operated cylinders or hydraulic cylinders or any other suitable activation mechanism. The jaws, rods and cylinders may be made of tool steel, or any other suitable material strong enough to form the porous pad 26 about the femoial component 10.

The neck 28 of the femoral component 10 is seated in a cylindrical bushing 66 within base plate 68 of fixture 60 to hold the femoral component 10 in place. A sliding locating pin 69 may be placed through the aperture 31.to keep the femoral component 10 from rotating while the forming is taking place. The bushing 66 may suitably be made from a nonmetallic material such as delrin so as not to scratch the neck 28. The base plate may be made of aluminum or steel. Other suitable means of holding the femoral component 10 securely in the forming fixture 60 could also be utilized.

The jaws 61, 62, 63 and 64 are'each in a retracted position about the femoral component 10. The precut substantially flat porous pad 26 is placed in position between the jaws and the implant 10 with the medial pad portion 138 aligned with the corresponding medial side 38 of the stem portion 20. The jaws, which have interfacing surfaces corresponding to the desired pad and stem configuration, are then activated in a predetermined sequence to form the pad 26 about the implant conforming the pad 26 to the shape of the implant 10 as shown in Figures 8a, 8b and 8c. In Figure 8a the first forming jaw 61 is activated from its retracted position to a position in which the first jaw 61 is pressed up against the medial porous pad surface 138 which is thus pressed against the corresponding medial side 38 of the stem portion 20 shaping and locating the respective portion of the pad 26 thereto. The first jaw 61 is then held in its activated position against the stem portion 20.

Alternatively, the medial porous pad surface 138 could be preformed to the shape of the medial side 38 by utilizing a correspondingly shaped separate forming dye (not shown) for just the medial side (rather than forming this medial side using the sequential fixture 60). The separate dye would include a substantially U-shaped depression such as that shown in the cross-section of jaw 61 in Figures 8a and corresponding U-shaped projection corresponding to the outer contour of the medial side of the femoral component, which when brought together with the pad 26 in i between would preform the pad 2 6 into the substantially U-shaped configuration, such as that achieved using the first jaw 61 of the sequential forming fixture 60. If such a separate dye is used, the pad 26 having the preformed (U-shaped) medial pad surface 138 would then be placed in position in the forming fixture 60 between the jaws and the implant 10 with the preformed medial pad 138 aligned with the medial side 38 of the stem portion 20. The jaw 61 would be activated to hold the pad 26 having the preformed metal pad surface against the stem portion 20. The remaining sequential activation of the jaws would then continue.

The second and third jaws 62 and 63 may be then activated simultaneously as shown in Figure 8b to press the posterior and anterior porous pad surfaces 136 and 140, respectively, against the posterior and anterior sides 36 and 40 of the stem portion 20 thus shaping the respective portions of the pad thereto. Jaws 61, 62 and 63 are then held in this activated position against the stem portion 20.

The fourth jaw 64 is then activated as shown in Figure 8c to press the first and second tabs 120 and 122 of the lateral porous pad surface 134 against the lateral side 34 of the stem portion 20 thus-shaping the respective portions of the pad thereto. The tabs 120 and 122 mate together on the lateral side 34 to form a flush interface 27, thus providing a continuous circumferential porous surface connecting the anterior, medial, posterior and lateral sides.

The forming jaws are then released from their activated positions, and thus retracted away from the stem portion 20. The femoral component 10 with the conformed pad 26 surrounding it is removed from the forming fixture 60. The femoral component 10 is then placed in an appropriate bonding fixture 80 such as in Figures 12-13 in which the porous pad is held securely in place a gainst the stem portion 20. The pad 26 is positioned securely in the recess 74 which corresponds to the wrapped shape of the pad 26. The porous pad 26 is then bonded to the stem portion 20 to securely attach it thereto. The bonding may be achieved by diffusion bonding the pad to the stem portion by holding the pad securely thereagainst at a sufficient temperature for a sufficient length of time to achieve secure bonding. However, it is understood 1 that other bonding processes such as sintering or other suitable bonding methods may be utilized.

Although any appropriate bonding fixture or holding mechanism may be used for bonding the pad 26 to the stem portion 20, Figures 12 and 13 illustrate such a suitable fixture 80. The fixture 80 includes a support frame 86 with a plurality of holding or bonding blocks or jaws 81, 82, 83 and 84 which surround the porous pad 26 on the proximal portion 24 and which conform to the shape of the stem portion 20 about which they are positioned. The bonding blocks which hold the pad 26 in place against the stem portion 20 are suitably made of a carbon material,as is the frame 86. The bonding blocks 81. 82, 83 and 84 are placed under sufficient pressure evenly holding the pad 26 against the stem portion 20 via screws 88.

Alternatively, rather than forming the substantially flat first preliminarily shaped pad 26 about the stem portion 20 into its second and final contoured shape, the pad 26 can be formed about a mandrel (not shown) which has a shape which corresponds to the portion of the implant to which the pad is to be attached. This forming about a mandrel can be done with a fixture 60 such as that described above used for forming the substantially flat or medially preformed pad 26 about the implant itself. Then the formed pad 26 which has been shaped about the mandrel is removed from the mandrel and placed about the femoral component 10. The pad 26 can then be securely bonded to the stem portion 20 as described above.

An alternate embodiment for a femoral component 400 is shown in Figures 14-16 in which the porous pad 426 is adapted to fit in recess 474. Figures 14 and 15 show the component 400 without the porous pad 426 thereabout. The pad 426 is adapted to be wrapped about three adjacent sides including the lateral, medial, and anterior sides 36, 38 and 40 to form a continous porous surface interconnecting these three sides. Since the pad 426 does not completely surround all sides of the implant, the pad 426 may be conveniently formed from its substantially flat first preliminary shape (as shown in Figure 16) to its second and final contoured shape which corresponds to the shape of the recess 474 in the stem portion 20 either by using a correspondingly shaped 1 1 I- 1 (substantially U-shaped for the embodiment shown in Figures 14-16) separate single forming dye (not shown), or a forming fixture 60 with multiple sequentially operated forming jaws such as previously described. The formed pad 426 can then be bonded to the stem portion 20 in a similar manner as previously described for the pad 26 which circumferentially surrounds the stem portion.

The prosthetic implant of the present invention provides an implant having a wrapped porous pad and a method of wrapping such pad about a prosthetic stem portion. The present invention provides for a substantially flat pad having a predetermined outer boundary to be subsequently wrapped about a prosthesis stem to conform to the shape of the stem and to be attached thereto. While this invention has been described and exemplified in terms of particularly advantageous embodiments, those skilled in the art can appreciate that modifications can be made without departing from the spirit and scope of this invention.

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Claims

1.. A prosthetic implant including a stem portion having a plurality of sides and wherein the implant includes a porous pad separate from the stem portion, the pad adapted for wrapping around and conforming to the shape of a portion of the stem portion for attachment thereto.

2. The prosthetic implant of Claim 1 wherein the pad has a preliminary substantially flat precontoured shape which is subsequently wrapped around the stem in a non-flat second and final shape corresponding to the shape of the stem portion about which the pad is wrapped.

3. The prosthetic implant of Claim 1 or Claim 2 wherein the pad is wrapped about a portion of at least two adjacent sides.

4. The prosthetic implant of Claim 3 wherein the pad is wrapped about all sides forming a continuous circumferential porous pad surface about the stem portion.

5. The prosthetic implant of any preceding Claim wherein the porous pad is made from a fibre metal material.

6. The prosthetic implant of Claim 5 wherein the porous pad of fibre metal material is prebonded prior to wrapping of the pad about the stem portion.

7. The prosthetic implant of Claim 5 wherein the porous pad of fibre metal material is prepressed into a substantially flat blanket having a desired thickness and prebonded prior to wrapping of the pad about the stem portion.

8. The prosthetic implant of any preceding Claim wherein the stem portion includes a recess corresponding to the shape of the porous pad for placement of the pad in the recess.

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9. The prosthetic implant of any preceding Claim wherein the portion of the stem portion about which the pad is wrapped has a noncircular, asymmetric, cross-sectional configuration.

10. The prosthetic implant of any preceding Claim where in the implant is a femoral component for a hip prosthesis.

11. The femoral component of Claim 10 wherein the stem portion has a proximal portion and a distal portion and wherein the stem portion includes a medial. a posterior, an anterior, and a lateral side.

12. The femoral component of Claim 11 wherein the porous pad is located substantially in the proximal portion of the hip prosthesis.

13. The femoral component of Claim 12 wherein the proximal portion has a noncircular, cross-sectional configuration.

14. The femoral component of Claim 12 wherein the porous pad is wrapped about a portion of the anterior, medial, and posterior sides.forming a continuous porous surface connecting these three sides.

15. The femoral component of Claim 12 wherein the porous pad is wrapped about a portion of the anterior, medial, posterior, and lateral sides forming a continuous porous surface connecting these four sides thus circumferentially surrounding the stem portion.

16. The prosthetic implant of Claim 7 wherein the separate porous pad has a predetermined outer boundary defining a desired contour for the porous pad.

17. The prosthetic implant of any preceding Claim wherein the porous pad is a single pad.

1..

18. A femoral component for a hip prosthesis having a fixation surface base portion having a plurality of distinct sides and wherein the implant includes a porous pad separate from the base portion, and wherein the porous pad has a first, preliminary precontoured shape, the porous pad adapted for wrapping about a portion of at least two of the plurality of sides for attachment thereto in a second and final shape.

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19. A method of constructing a prosthetic implant comprising the following steps:

20. The method of Claim 19 further including the step of bonding the pad to the stem portion while holding the pad against the stem portion is a suitable bonding fixture.

21. The method of Claim 19 further including the steps of press forming and prebonding a sheet of porous material prior to precontouring the pad.

22. A method of constructing a prosthetic implant comprising the following steps:

(a) providing a base metal implant including a stem portion having a medial, a posterior, an anterior, and a lateral side; t 1 1 1 (b) forming a sheet of porous fibre metal material; (c), prebonding the sheet to bond the fibre metal material together; (d) cutting a porous pad from the sheet to provide the pad with an outer boundary defining a desired contour; and (e) wrapping the pad around the stem portion and conforming the pad to the shape of the stem portion about which the pad is wrapped for attachment thereto.

23. The method of Claim 22 wherein the process for wrapping the pad around the stem portion further includes the step of forming the pad to conform to the shape of one side of the stem portion first and then sequentially forming the pad to correspond to the shape of the remaining sides.

24. The method of Claim 22 wherein the process for wrapping the pad around the stem portion further includes the step of forming.the pad. to correspond to the shape of the stem portion to which it is to be attached by placing the prosthetic implant in a fixture having a plurality of forming jaws,'each in a retracted position away from the prosthetic implant; placing the porous pad in a desired location between the jaws and the implant, thenactivating the jaws against the fibre metal and the implant in a predetermined sequence to form the porous pad about the implant, conforming the pad to the shape of implant.

25. The method of Claim 24 wherein the process for wrapping the pad around the stem portion further includes the step of providing four forming jaws to be operated in the following sequence:

(a) activating the first jaw to press the porous pad against one of the sides of the implant and maintaining the first jaw in this activated position; 1 1 (b) activating the second and third jaws to press the porous pad against the two side adjacent the one side of the implant and maintaining the second and third jaws in these activated positions; and (c) activating the fourth jaw to press the porous pad against the remaining side enabling the porous pad to be conformed to the shape of the stem portion and fully surround the stem portion.

26. The method of Claim 25 wherein the process further includes the steps of:

(a) releasing the jaws from their activated positions; (b) removing the stem with conformed pad surrounding it from the fixture; (c) placing the stem in an appropriate bonding fixture in which the porous pad is held securely in position against the stem; and (d) bonding the porous pad to the stem to attach it securely thereto.

27. The method of Claim 22 wherein the process further includes the steps of:

forming the pad to correspond to the shape of the stem portion to which it is to be attached by shaping the porous pad about a mandrel which has a shape which corresponds to the portion of the implant to which the pad is to be attached; (b) removing the formed pad from the mandrel; (c) placing the formed pad about the prosthetic implant; and (d) attaching the pad securely to the stem.

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28. A prosthetic implant having a nonflat fixation surface wherein the implant includes a porous pad separate from the fixation surface, and wherein the porous pad has a first preliminary substantially flat preformed shape. the porous pad adapted for subsequently forming about the nonflat fixation surface for attachment thereto in a nonflat second and final shape corresponding to the shape of the fixation surface to which it is attached.

1 Published 1988 at The Patent Office. State House. 6671 High Holborn. London WC1R 4TP Further copies may be obtained from The Patent Office.