Papers by Author: Anke Bernstein

Abstract: Many attempts had been made to improve the durability of artificial joint replacement and other orthopaedic implants by approaching the mechanical properties of bone and artificial material. The most joint prostheses used today are manufactured of metal alloys based on cobalt, chromium or titanium. The mechanical stiffness of these materials is much higher than that of natural bone resulting in adverse effects such as local overloading on one hand or stress shielding phenomena with the lack of adequate mechanical load on the other. Both mechanisms contribute to earl loosening and failure of implants. Polymer materials may deliver mechanical properties very similar to bone and their mechanical behaviour may be modified in a wide range during the process of manufacturing. First attempts to lower the stiffness of the implant material and to gain the stiffness range of natural bone were made in the seventies by R. Matthys with his concept of “isoelastic hip prosthesis”. In this prosthesis the femoral stem was manufactured of polyacetal, a thermoplastic polymer with very good biocompatibility and elastic properties which are much nearer to bone than common metal alloys. While the prosthesis showed good results during the mechanical testing the clinical use in vivo became a disaster. Shortly after implantation polyacetal was degraded in the body and broke down under the immense loading of the human hip joint. Later attempts to use polymer materials alone for load bearing implants also failed in clinical practice over a long time because the mechanical interlocking between bone and implant was not sufficient for the biological demand. To make the outstanding properties of polymer materials useable for load bearing implants they are backed with metal alloys (as polyethylene for hip joint cups) until the presence. Only recent developments of polymer science succeeded in the use of polymers for loaded implants. One of the most interesting materials seems to be the polyetheretherketone (PEEK) which is successfully used for spinal fusion cages [2] and computerdesigned individual implants for defect reconstruction in the skull [4] meanwhile. A pre-clinical study of a new anatomically shaped flexible acetabular cup reported satisfactory results recently [3].

Abstract: Bioactive ceramics such as β-tricalcium phosphate (β-TCP) promote and enhance biological fixation. Ceramics with a porous interconnected structure are suited for facilitation of bony ingrowth. An interconnected pore system with pore diameters in excess of 100 µm is required for cell penetration, tissue ingrowth, vascularization and nutrient delivery to the centre of the regenerating tissue. Human osteoblasts were cultured on the surface of a ceramic. In an in-vivo study, β-TCP samples with a porous interconnected structure were implanted into the femur of sheep and then investigated 6 weeks after operation. Histological analysis was performed on the area surrounding the implant. An indentation test was performed to complete failure of the bone/ceramic compound. Linear load, peak load and stiffness were recorded. All cylinders were found to be biocompatible and osteoconductive. Bone was more abundant in the outer ring than in the rest of the cylinder. The ceramic/bone compound was of low mechanical grade.

Abstract: Bioactive ceramics such as hydroxypatite (HA) promote and enhance biological fixation. There is still a discussion on the desired longevity of the coating. Stable coatings require an optimum between resorption rate, flexural strength and adhesive strength of the coating. Ceramic coatings containing fluorapatite (FA, Ca5(PO4)3F) and calcium zirconium phosphate (CZP, CaZr4(PO4)6) promise lower resorption rates than conventional HA coatings in the biological milieu. It is hoped that they can improve the long-term stability of implants by eliminating the detrimental resorption of coating material. For the in vivo studies plasma sprayed coatings were generated. The materials were implanted into the distal femur epiphysis of rabbits and investigated after 2, 4, 6, 12, and 24 weeks postoperatively. Histological analysis was preformed on the areas surrounding the implant. The amount of osseointegration was determined by using the automatically image analysis. The bonding strengths were compared with HA coating and uncoated titanium alloy. According to available data, there is inhibition of mineralization of bone at the interface of calcium zirconium phosphate ceramics of the described composition.

Abstract: The reaction of animal bone to Ti6Al4V pins coated with ceramics of Ca2KNa(PO4)2 or Ca10[K/Na](PO4)7 as main crystalline phases were tested. It is shown that the new materials possess a very high resorption in comparison with hydroxyapatite (HA).