Discontinuous fibre composite implant prosthesis

Abstract: A composite implant prosthesis is constructed from discontinuous fibre reinforcement, to form a sandwich structure of layered stiffness. The method for constructing the composite implant prosthesis includes the steps of closed moulding technique of producing a core of discontinuous fibres, aligned predominantly longitudinally, and a first casing of predominantly circumferentially orientated discontinuous fibres. The two layers can be formed by means of either single closed moulding process, or multiple closed moulding process. To cater for the variation in the femoral canal geometry, an outer second casing in the form of a sleeve is formed using closed moulding technique, which can be bonded on to the implant prior to or during surgery.

Description: This invention relates to the form and structure of a composite implant prosthesis. Heretofore, patents WO-A-87-04961, EP-171884 and US-A-3-893-196 disclose composite implant prosthesis comprising a core of continuous fibers extending over the entire length of prosthesis formed by moulding and machining, over layed by a layer of transversely oriented continuous fibres and additional layer of polymer to allow disposition adjacent to bone. The most relevant prior art is described in document WO-A-93 19699, which document falls under the provisions according to Art. 54(3) EPC. There is disclosed composite implant comprising an elongated core formed of short filament fibres having a length of less than four millimetres embedded in thermoplastic polymer; and encasing the core, a sheath formed of elongated carbon filament fibres embedded in a thermoplastic polymer wound around the core in layers and moulded thereto. US patent no. 4,902,297 describes the above mentioned machining of core as detrimental. It discloses a composite implant prosthesis formed by a plurality of substantially unidirectional strands of fibre, each fibre extending substantially over the complete length of the core, an inner casing defined by braided material enclosing the core, and an outer casing enclosing the inner casing. The core and the inner casing defining a substantially uniform width from one end to the other end, while the outer casing defines an irregular width from one end to the other to accommodate disposition of the prosthesis adjacent to a bone. Here the core is subjected to a substantial portion of the load applied to the composite hip prosthesis. Since the dimension of the core and first casing are substantially constant along the implant length from one end to the other, variation in the stiffness along the length of implant can not be accommodated with this design. This shortcoming is due to the manufacturing process employed in the making of the core and first casing. Here the core and first casing are made into a single straight rod of uniform width using pultrusion. This rod is then placed into a forming press where heat and pressure are used to form or bend the rod to the shape of the implant. With the present invention it is possible to form a composite implant prosthesis comprises of a core and first casing which can vary in width from one end to the other. This allows the width of the core and first casing to vary along the implant length to achieve variation in the implant stiffness along the implant length from one end to the other and to define an overall irregular width to accommodate disposition of the prosthesis adjacent to a bone. These sections may, therefore, in one embodiment of the invention, be produced in the shape of the final profile of the implant producing a complete implant and, therefore, require no forming process to shape the core and first casing. The invention comprises a composite implant prosthesis with a core formed by reinforcement of a matrix using discontinuous fibres aligned predominantly unidirectional along the implant length, and a first casing enclosing the core, with discontinuous fibres predominantly orientated transversely to that of the core, to form a circumferentially reinforced first casing. A method for constructing the composite implant includes the steps of: a) forming a core of unidirectionally aligned discontinuous fibre reinforcement along the implant, using a closed moulding process. b) using the section produced in (a) as an insert in a second closed moulding process to allow over moulding of the core in (a), and hence producing the first casing encapsulating the core. The orientation of the fibre reinforcement in this casing will be predominantly transverse to that of the core, to produce circumferential reinforcement. The described manufacturing process allows the cross section of the unidirectional aligned core and circumferentially reinforced first casing to vary along the implant length as well as having different proportions of reinforcement. The overall width of the composite implant prosthesis can, therefore, be varied along the prosthesis length from one end to the other to accommodate disposition of the prosthesis adjacent to a bone. An alternative method of production is to manufacture the core and the first casing separately and then bond them together. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a side view of one embodiment of composite implant prosthesis according to the invention; Fig. 2 is a side view of another embodiment of a composite implant prosthesis having a collar; Fig. 3 is a cross section view taken along line A-A of Figs 1 and 2 of the composite implant prosthesis (without the out sleeve 3). Fig. 4 is a schematic illustration of the fibre orientation and elastic moduli along the implant prosthesis (without the outer sleeve 3); Fig. 5 is a schematic illustration of a re-entrant socket into a ball joint which may be provided on the implant prosthesis; and Fig. 6 is a schematic illustration of an alternative form of ball joint which is included in the closed moulding of the core 1 and first casing 2 to product a complete implant prosthesis. Turning to Figures 1 and 2 , the composite hip prosthesis constructed in accordance with the process described above, includes a central core 1, and a first casing 2. In the case of the collared composite hip prosthesis shown in Fig. 2, a sandwich structure is formed by extending the core (1) and first casing (2) into the collar (4). The stiffness of this sandwich structure is optimised by controlling the thickness of the core (1) and first casing (2) to optimise the load on the calcar femoral. To accommodate the variation in the femoral canal shape from patient to patient, a separate outer sleeve, (3) may be formed by a closed moulding process. This is bonded on to the prothesis stem (as shown in Figures 1 and 2) during or prior to surgery. The outer sleeve preferably comprises material with a porous surface to achieve fixation of the prosthesis by means of mechanical interlocking achieved through bone ingrowth. In a preferred embodiment, the radial dimensions of the core and the first casing as shown in Fig. 3, are such that a ≥ a1 and b ≥b1. The dimensions of the core (1) and the first casing (2) as shown in Fig. 3 are varied along the length of the implant to optimise the stiffness of the implant along its length and to control the overall width of the implant to accommodate disposition of the prosthesis adjacent to a bone. The arrangement of the reinforcement as designed in the present invention leads to an implant with layered stiffness. The stiffness in the longitudinal direction to the stem is reduced across the cross section of the implant, going from the longitudinally aligned core (1) to the transversely aligned first casing (2). Conversely the stiffness in the transverse direction to the stem is increased across the cross section of the implant, going from the aligned core (1) to the first casing (2). In a preferred embodiment, the Young's moduli, E, for the core (1) and first casing (2) shown in Fig. 4 will be such that E11c ≥ E11o and E22c ≥ E22o. The ball joint of the prosthesis (5) is manufactured using a separate closed moulding process. This can be attached to the implant neck either by bonding or by a form of re-entrant socket into the ball to form a mechanical fixation as shown in Fig. 5. Alternatively, as shown in Fig. 6, the ball joint (5) can be included in the closed moulding process of the core (10) and first casing (2) to produce a complete prosthetic implant.

Claims: A composite implant prosthesis comprising a core (1) having a matrix and a first casing (2) wherein: the core (1) formed by reinforcement of the matrix using discontinuous fibres aligned predominantly unidirectionally along the implant length, and the first casing (2) enclosing the core (1), with discontinuous fibres predominantly oriented transversely to that of the core (1) to form a circumferentially reinforced first casing (2). A prosthesis according to Claim 1 wherein variation in the stiffness of the composite implant prosthesis along its length, is controlled by means of varying the dimensions of the aligned core (1) and first casing (2) along the length of the implant. A prosthesis according to Claim 1 or 2 wherein the dimensions of the aligned core (1) and first casing (2) we varied to define an overall irregular width to accommodate disposition of the prosthesis adjacent to a bone. A prosthesis according to Claim 1, 2 or 3 wherein the core (1) and first casing (2) extend to form a structural collar (4) with a sandwich structure the stiffness of which is optimised by controlling the thickness of the core (1) and first casing (2) forming the sandwich structure at the collar. A prosthesis according to any preceding claim provided with a ball joint (5) moulded with the core (1) and first casing (2) to produce a complete implant. A prosthesis according to any of Claims 1 to 4 provided with a ball joint (5) attached thereto by bonding or by a re-entrant socket in the ball joint (5). A prosthesis according to any preceding claims wherein an outer sleeve (3) is provided to correct the shape of the prosthesis so as to accommodate variation in femoral canal shape form patient to patient. A prosthesis according to Claim 7 wherein the outer sleeve (3) comprises material with a porous surface to achieve fixation of the prosthesis by means of mechanical interlocking achieved through bone ingrowth.

Inventors: ILHAN ESAT IBRAHIM DR, ROOZBEH SHIRANDAMI DR, ILHAN, ESAT IBRAHIM, DR., ROOZBEH, SHIRANDAMI, DR.
Applicants: SHIRANDAMI ROOZBEH DR, SHIRANDAMI, ROOZBEH, DR.
Application year: 1993
Main classification: A61F2
IPC codes: A61F2, B29C70, A61L27