Implant materials for hip endoprostheses: old proofs and new trends

Titanium - a Cementable Material for Endoarthroplasty

As materials for arthroplasty, titanium alloys exhibit the following advantages over conventional steel, cobalt chromium or chromium nickel alloys - good fatigue strength, excellent biocompatibility, low modulus of elasticity, and high corrosion resistance. The previous worse clinical outcome was most likely caused by crevice corrosion and led to reduced use. To warrant safe use, the design should be optimised (sufficient proximal diameter, proximal collar), in order to reduce unwanted deformation in the proximal part of the prosthesis. Additionally, a rough surface (Ra > 2.5 µm) should not be used. Further research in surface treatments (e. g. silicate-silane) could facilitate additional improvement.

[FE-analysis of surface stresses for the tribological system in total hip prostheses]

The implantation of a total hip prosthesis is an operation which is performed frequently due to advanced hip joint damage both in humans and in veterinary medicine in dogs. The long-term result of a hip prosthesis is mainly determined by aseptic loosening of the prosthesis; among other causes, abrasion particles of the tribological pairing are responsible for the loosening. For the analysis of the surface stresses with different tribological pairings, a finite element model was generated which was based on the CAD data of a commercial total hip prosthesis. After transmission of a physiological force in the components of the three tribological pairings ceramic/polyethylene, ceramic/ceramic and metal/polyethylene, stresses were calculated. Stresses in the ceramic/ceramic tribological pairings were conspicuously higher than in the other material pairings. In the future adapted prostheses have to be developed that ensure optimal friction and absorption characteristics of the components.

Medium-term results of a modern metal-on-metal system in total hip replacement

Since 1988, metal-on-metal articulation from cobalt-chromium-molybdenum alloy was reintroduced into hip arthroplasty as an alternative to metal-on-polyethylene or ceramic-on-polyethylene components. Modular joint surfaces were developed for the second generation metal-on-metal articulation using newly introduced and proven prosthetic implants. Since 1990, 78 patients with 78 uncemented total hip replacements were followed up in a prospective study. The mean followup was 60 months. Three patients were lost to followup. The average age of the patients at the time of surgery was 48.8 years. Thirty-three patients had been operated on previously. No early infections occurred; one late infection occurred after 3 years. Dislocation of the prosthesis occurred in one patient who was lost to followup. In two patients ectopic ossifications were removed 17 and 27 months postoperatively. At revision surgery no metallosis could be identified. At the last followup examination, the Harris hip score was 96.8 points on average. There was no evidence that the metal-on-metal articulation gave rise to new problems or complications. Metal-on-metal articulation reduced wear considerably in the authors' previous experience. It is hoped that foreign body reactions are reduced significantly so that an alternative for total hip replacement in younger and active patients will be available.

Biological reactions to wear debris in total joint replacement

The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, evidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an increasing need to implant hip prostheses in the younger, more active patient the need to understand the mechanisms of failure and to develop artificial hip joints using alternative materials have become major issues in the orthopaedic community. This review focuses initially on our current understanding of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosening. While the precise mechanisms of osteolysis induced by UHMWPE wear debris have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 micron) for macrophage activation. The review then considers whether the problem of wear-debris-induced osteolysis may be overcome with the use of new generation metal-on-metal or ceramic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of the wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the characteristics of the wear particles generated in vivo before predictions can be made. It is concluded that the pre-clinical testing of any new materials for joint replacement must include an analysis of the wear particle characteristics and their biological reactivity in addition to the usual assessment of wear.