HIP International

Femoroacetabular impingement is a very common cause of secondary osteoarthritis (OA) in the young adult. It is an important co-factor in the better recognized prearthritic deformities such as residual hip dysplasia (RHD), Perthes disease and slipped capital femoral epiphysis (SCFE). Another subgroup of patients has isolated malrotation of the hip joint and/or reduced femoral head-neck offset causing femoroacetabular impingement and chronic hip joint pain. Special clinical tests and imaging modalities can identify these patients at an early stage when they have little or no OA. The common biomechanical pathway for deformities causing chronic femoroacetabular impingement is local damage of the capsular-labrum complex and the cartilage. Understanding the anatomy, biomechanics and pathophysiology of these conditions of the hip joint is a prerequisite for planning treatment.

Polyethylene wear debris induced osteolysis is a major cause of failure in artificial hip joints. Sub micrometre size particles are taken up by macrophages which are stimulated to release osteolytic cytokines such as TNFα. This leads to bone resorption, loosening and failure. In vitro cell culture studies have shown particles in the size range 0.1 to 1 micrometre to be at least six times more reactive than larger particles. Studies of historically used gamma irradiated in air polyethylene show increased wear rate with damaged femoral heads and with aged and oxidised polyethylene. The aged and oxidised polyethylene also produced a greater percentage of smaller particles leading to increased osteolytic potential. Combined tribological and biological simulation models have been developed for pre-clinical assessment of osteolytic potential of artificial hip joints.

There is now considerable clinical concern about the effect of polyethylene wear debris induced osteolysis in long term failure of hip replacements. This paper compares the wear of stabilised and crosslinked polyethylene to alternative hard on hard bearings. The volumetric wear rates of stabilised and moderately crosslinked polyethylene 50 to 35 mm³/million cycles were less than previously reported for historical gamma irradiated in air polyethylene, but still of a level that in the long term could cause osteolysis. The moderately crosslinked polyethylene produced less wear than non-crosslinked polyethylene, but particles were smaller and more reactive resulting in little change in the osteolytic potential. Alumina ceramic on ceramic produced substantially less wear and osteolytic potential. Metal on metal also produced less wear than polyethylene but the particles adversely influence cell viability.

This author's development of metal/metal hip resurfacing began in 1989, with the first patient implantation in February 1991. In the first three years a pilot study identified optimum fixation as hydroxyapatite coated uncemented cups and cemented femoral components. From March 1994 hybrid fixed components have been used. These implants have generally been satisfactory with respect to fixation but high wear of the bearing, metallosis and osteolysis have been seen with some components inserted during 1996, a period during which the metal microstructure was altered by the heat processes, hot isostatic pressing and solution heat treatment. The Birmingham Hip Resurfacing was developed taking account of experience with earlier resurfacing designs. This implant employs hybrid fixation with a porous ingrowth acetabular component and has an as-cast metal microstructure having had no post casting heat treatments. During the past 4 years the author has performed over 1,000 Birmingham Hip Resurfacingís with a total failure rate of less than 1.0%.