Orthopaedic implant device

Abstract: An orthopaedic implant device is formed from, or defined by, a combination of different materials. These devices include a body metal component and a porous metal surface layer (16) for intimate contact with bone and a polymer in the form of a casing (24) that includes adhesive characteristics for attachment to the body metal component and the porous metal layer (16). The preferred polymer casing is polyaryletherketone.

Description: The present invention relates to an orthopaedic implant device, which is a hip joint prosthesis utilized to replicate joint articulation of the skeletal structure following implantation in a patient. In current orthopaedic practice it is known to provide metal orthopaedic implants to repair or reconstruct joint movement for a patient. These metal orthopaedic implants are commonly made from cobalt chrome, titanium and stainless steel. Moreover, with cobalt chrome implants it is possible to provide a porous layer of cobalt chrome beads for intimate contact with bone to accommodate bone ingrowth into the porous layer. In a similar manner, titanium implants are provided with titanium beads or fiber metal pads in the form of a porous layer for bone ingrowth. In contrast to the metal orthopaedic implants, United States Patent 4,750,905, issued to James Koeneman et al on June 14, 1988, teaches a composite hip prosthesis wherein non-metallic components are assembled with a carbon fiber core, a woven fiber sheath and a thermoplastic resin skin forming the outer contour of a hip prosthesis. A hybrid metallic/non-metallic orthopaedic device is taught in United States Patent 4,454,612, issued to John McDaniel on June 19, 1984, wherein a metal core is covered with a thin polymer coating and a polymer fiber layer is attached to the coating to accommodate bone ingrowth. FR-A-219967 (corresponding to US 3,964,473) discloses an implant with metal sleeve in which the polymer is used as insulator between the electrically charged sleeve and implant. The only properties described for such a polymer are for use as an insulating material. One of the inventors of the present invention has proposed a substantially non-metallic composite core with a porous metal surface embedded into the outer surface of the core to define a hybrid metallic/non-metallic orthopaedic implant device. The aforegoing prior art addresses the issue of stress transfer in orthopaedic devices so that the interface between the device and the remaining bone does not impart substantially different loads to the remaining bone than ordinarily imparted prior to resection of bone. If the stress applied to the bone is not controlled excessive stresses may fracture the remaining bone stock while on the other hand stress shielding may result in bone resorption. The present invention teaches an orthopaedic implant device with a polymer in the form of a casing to adhesively attach a body, or core to a porous metal surface. Thus the present invention provides a hip prosthesis device comprising a metal body (20), a polymer casing (24) attached to the body and a porous metal surface layer (16) attached to the casing and adapted for contact with bone to receive bony ingrowth into the pores, the porous metal surface layer being made from titanium and extending substantially from the proximal end to the distal end of the polymer casing to cover substantially all of the polymer casing, the metal body extending outwardly from a proximal end of the polymer casing and being terminated by a head (14) which is adapted for articulation relative to an acetabulum of a patient wherein the metal body (20) is made from cobalt chrome and extends outwardly from the distal end of the polymer casing (24) to form a metal cap (22) over said distal end. According to a preferred embodiment, the polymer adhesive in the hip prosthesis generates a substantial volume of the hip prosthesis so that the body or core is relatively small in size resulting a flexible hip prosthesis. By controlling the dimension of the body or core the flexibility of the hip prosthesis is substantially equated with that of the surrounding bone. In a preferred embodiment the polymer adhesive is polyaryletherketone typically Ultrapek KR4177 (Ultrapek is a registered trademark in certain jurisdictions) polymer as manufactured by BASF. This polymer manifested aggressive adhesive characteristics to the metallic surface of the cobalt chrome and the titanium following transformation to a heated state. It is an advantage of the present invention that the polymer adhesive not only adhesively couples the porous layer relative to the core hut also generates strength characteristics for the resulting orthopaedic implant device. The invention is illustrated in the accompanying drawings in which Fig. 1 is a side view of a hip prosthesis that is cut away longitudinally to illustrate the construction therefore; Fig. 2 is a view similar to Fig. 1 illustrating a second embodiment a hip prosthesis; The orthopaedic implant device in Fig. 1 illustrates a hip prosthesis 10 with a neck 12 at a proximal end adapted to fixedly receive a modular head 14. From the neck 12 to a distal end the hip prosthesis is provided with a porous surface or layer 16 to accommodate bone ingrowth in uncemented surgical procedures. However, the porous layer 16 could also allow for bone cement infiltration if a cemented surgical procedure is followed. The neck 12 includes a collar 18 forming a lower boundary for the neck 12. An internal core or body 20 extends distally from the neck 12 to an integral end cap 22. A polymer casing 24 is adhesively secured to the core 20 and porous layer 16 in a manner to be described hereinafter to securely connect the porous layer 16 with the core 20. The core 20 and neck 12 are made from cobalt chrome which is sufficiently sized to permit a reduced diameter for the core 20 over most of its length in comparison to the neck 12. The diameter of the core 20 is reduced uniformly in a distal direction up to but not including the end cap 22. The polymer casing is preferably made of Ultrapek KR4177 which is a polyaryletherketone (PAEK) polymer sold by BASF. Ultrapek is a tradename of BASF for a partially crystalline, thermoplastic polycondensation resin. Experiments with this polymer casing indicated that substantial adhesion forces can be obtained when the polymer casing 24 is injection molded to the core 20. Preferably the polymer layer is 0.51 mm (.020 inches) thick. The porous surface layer 16 is made from titanium fiber metal as disclosed in United States Patent 3,906,550 issued to Rostoker and Galante. With titanium fiber metal as the porous layer, it is possible to melt the porous layer partially into the polymer casing 24 so that adhesion and mechanical interlock secure the porous surface layer 16 to the polymer casing 24. In order to construct the hip prosthesis of Fig. 1, the core 20 is placed within a die of an injection molding machine. The neck 12 and end cap 22 are used to center the core 20 within the die. The polymer is injection molded into the die around the core 20 at a temperature of about 410°C (770°F) to form the polymer casing 24. The metal core may also be preheated to a temperature of about 427°C (800°F) prior to the injection molding of the polymer. During heating in the injection molding machine, the core 20 is surrounded by nitrogen argon or subjected to vacuum to minimize oxidation of the surface of the core 20 to enhance adhesive bonding between the core 20 and the polymer. The core 20 and polymer casing are removed from the die and cooled to room temperature. Next, the porous surface layer is heated to about 410°C (770°F) and pressed into engagement with the polymer casing. The heated porous layer 16 melts the outer surface of the polymer casing 24 to penetrate therein. When a portion of the porous layer 16 is embedded into the polymer casing 24, further penetration is halted so that the polymer casing 24 and the porous surface layer 16 are cooled together. With the temperature reduced, the polymer casing 24 remains adhered to the porous layer 16 which is also embedded into the polymer casing 24. The polymer casing is rigid at room temperature to also physically retain the porous layer connected thereto. A test of the adhesion characteristics of Ultrapek KR4177 polymer was designed to compare ultimate tensile strength for cobalt chrome and titanium metals. A pair of cylindrical rod samples with 3.87 sq cm (0.6 square inch) faces were adhered together with Ultrapek KR4177 fully engaging the faces. The Ultrapek KR4177 polymer was melted at about 410°C (770°F) in a furnace with the faces engaging the melted polymer for 45 minutes. After heating the samples and polymer were cooled to room temperature. With the pair of samples adhered together by the polymer, a separation force was applied to the samples to measure what force was required to separate the polymer from either sample. For cobalt chrome with clean faces the tensile strength was 2.63 K sq cm (17 Ksi) which is the ultimate tensile strength of the polymer. The cobalt chrome faces where cleaned, glass bead blasted and passivated before engagement with the polymer. Similar testing of titanium resulted in a tensile strength of 1.08 K sq cm (7 Ksi). However if the titanium samples are titanium nitride coated or surface reacted by means of thermal exposure to nitrogen before the polymer is adhered thereto the tensile strength is increased to 1.55 K sq cm (10 Ksi). In addition, if the samples of titanium and polymer are heated by induction for 4 minutes as opposed to a furnace, the tensile strength is increased to 2.01 K sq cm (13 Ksi). This latter increase is believed to result from a thinner oxide layer on the titanium surface in the induction heating process as contrasted to a thicker oxide layer formed in the furnace. The oxide layer formed on the titanium surface with induction heating is reduced because of the short time (4 minutes) required to fully adhere the polymer to the sample faces, as contrasted to the 45 minutes required in the furnace heating process. In the alternative embodiment of Fig. 2 the core 12 is substantially uniform in diameter with a slight increase in diameter at the proximal region 122 of the hip prosthesis 110. The polymer casing 124 extends from an end cap 123 to a neck 112 so that no collar is provided. The porous surface layer 16 is also embedded into the polymer casing and adhesively secured thereto.

Claims: A hip prosthesis device comprising a metal body (20), a polymer casing (24) attached to the body and a porous metal surface layer (16) attached to the casing and adapted for contact with bone to receive bony ingrowth into the pores, the porous metal surface layer being made from titanium and extending substantially from the proximal end to the distal end of the polymer casing to cover substantially all of the polymer casing, the metal body extending outwardly from a proximal end of the polymer casing and being terminated by a head (14) which is adapted for articulation relative to an acetabulum of a patient characterised in that the metal body (20) is made from cobalt chrome and extends outwardly from the distal end of the polymer casing (24) to form a metal cap (22) over said distal end. A hip prosthesis device according to claim 1 wherein said metal body (20) is tapered from the proximal end towards the distal end of said polymer casing (24). A hip prosthesis device according to claim 1 or claim 2 wherein the casing (24) cooperates with the body (20) to vary the stiffness of the device along its length. A hip prosthesis device according to any preceding claim wherein the metal body (20) and porous metal surface layer (16) are made of different metals. A hip prosthesis device according to claim 4 wherein the metal body (20) and porous metal surface layer (16) are mechanically and chemically incompatible. A hip prosthesis device according to any preceding claim in which the porous metal surface layer is formed from titanium which is provided with a nitrided surface. A hip prosthesis device according to any preceding claim in which the surface of the cobalt chrome is passivated for secure adhesion to the polymer. A hip prosthesis device according to any preceding claim in which the surface of the cobalt chrome is roughened prior to adhesion to the polymer. A hip prosthesis device according to any preceding claim in which the casing (24) is substantially similar to a polyaryletherketone or is a polyaryletherketone. A hip prosthesis device according to any preceding claim in which the porous metal surface layer (16) is partially embedded into the casing. A hip prosthesis device according to any one of the preceding claims in which the thin layer of polymer is about 0.51 mm (.020 inches) thick. A hip prosthesis device according to any preceding claim in which the casing (24) defines a substantial volume of the hip prosthesis device so that the metal body (20) is relatively small, whereby the dimension of the metal body (20) is controlled, such that the flexibility of the prosthesis is substantially equated with that of the surrounding bone. A hip prosthesis device according to any preceding claim wherein the casing (24) including adhesive characteristics in one state to adhere to both the body (20) and the porous surface (16) and the casing including strength characteristics in a second state to transmit loads applied following implantation in a patient.

Inventors: CHU HELEN, CROWNINSHIELD ROY DOUGLAS, DEVANATHAN THIRUMALAI N C, PARR JACK ELDON, CHU, HELEN, CROWNINSHIELD, ROY DOUGLAS, DEVANATHAN, THIRUMALAI N.C., PARR,JACK ELDON
Applicants: SQUIBB BRISTOL MYERS CO, BRISTOL-MYERS SQUIBB COMPANY
Application year: 1991
Main classification: A61F2
IPC codes: A61L27, A61F2