Total Hip Replacement: A Successful Interaction of Biology, Mechanics, and Materials Science

Low friction and decreased wear rates are desirable characteristics of materials for articulating surfaces of prostheses for total hip replacement (THR). The Charnley low-friction hip arthroplasty has shown good pain relief and clinical function as well as reasonable longevity and wear characteristics. Continued research successfully has improved the mechanical properties as well as the biocompatibility of THR prostheses. However, a hip prosthesis represents a large foreign body, which is not fully incorporated with the surrounding biologic environment. One of the relevant problems is the difference between the modulus of elasticity of the materials used for THR and the surrounding bone. The deformation characteristics of bone are anisotropic and the elastic moduli vary from one site to the next. Therefore, complete fixation between the prosthesis surface and the surrounding bone is difficult without ongoing deformation and remodeling of the interface surfaces. Stress-induced adaptive remodelling of bone is one of the key properties that makes long-lasting fixation and compatability between the implant and bone possible.

Implant wear and the generation of particulate debris are related directly to periprosthetic osteolysis. Reduction of wear of the articulation, and ultimately all implant surfaces, currently is one of the technological challenges in THR. Improvement of the tribologic and other physical properties of the materials, as well as improved implant designs will undoubtedly extend implant longevity. Many unsolved problems remain-for example, the optimal diameter and shape of the bearings still are unsolved issues. Because loosening of THRs seems to depend to a great extent on biologic factors, pharmacologic modulation of periprosthetic osteolysis has become timely. Inhibition of osteoclastic activity and pro-inflammatory cytokines as well the potential use of growth factors and other bioactive molecules to enhance osseointegration and prevent implant loosening currently are being studied.

An improvement in the tribology of bearing surfaces is an effective means of increasing the longevity of THRs. To this end, testing of new bearing materials using a hip simulator is a valuable method of evaluation. The introduction of highly crosslinked polyethylenes to improve tribology of the bearing surfaces seems very promising. Ceramic bearings and metal-on-metal THRs recently have become more popular. Traditional materials such as stainless steel and Co-Cr alloy have been improved with new technologies such as surface coatings and other manufacturing processes. Implant longevity can be further improved with combined strategies related to implant design, material properties, and the biomechanical environment. The ultimate outcome of new innovations must be documented clearly with long-term clinical survivorship analysis, exemplified by the Scandinavian total joint registers.