Hip resurfacing and metal-on-metal total hip arthroplasty

Since the advent of modern metal-on-metal hip resurfacing in the early 1990s the number of such procedures performed worldwide has increased steadily, and registries have documented this trend. There has been recent concern regarding adverse reactions to metal debris associated with poor implant placement and specific manufacturing issues following hip resurfacing, and expertise is evolving in relation to the management of such problems (1). In this issue of Hip International, the Oxford group have addressed the issue of hip resurfacing and suggest that it still has a valuable role in a well-defined patient population (2). Many advances in technology have followed a similar path, initial enthusiasm being supplanted by more restricted indications once the scope of any given procedure became more clearly defined. However, metal-on-metal total hip arthroplasty (MOMTHA) is a distinct entity. The concept has existed since 1938, and favourable outcomes were noted following the use of a number of early designs (3). Where problems occurred, these related principally to fixation rather than soft tissue reactions. Moreover, no significant problems in relation to teratogenicity were noted in epidemiological studies (4), and initial experiences with 28mm articulations provided further reassurance (5). A new generation of MOMTHAs evolved, using the acetabular components employed in resurfacing arthroplasty combined with large modular metal heads. Initially the concept was applied to revision of failed hip resurfacing, allowing retention of the original cup, but in the early 2000s the concept rapidly gained popularity, particularly in North America. It was an easier operation than resurfacing with the same perceived rewards of increased stability and low wear making it apparently ideal for young high-demand patients. Ironically (in retrospect) surgeons may have been less exacting with respect to cup orientation because their fear of dislocation was reduced. It is now recognised that any large diameter MOM articulation loaded under adverse conditions (vertical cup placement, excessive anteversion and micro-separation) will produce edge wear which can in turn lead to catastrophic wear, conditions which standard simulation studies failed to predict. This can result in the soft tissue and bone destruction that is now well recognised. It would appear that the factor which sets MOMTHA apart from resurfacing is the taper junction which usually includes a sleeve and thus two interfaces. This junction and the problems it can produce are not well understood. Taper specification and surface finish vary between different designs and the terminology describing the geometry of this junction is confusing. Another apparently critical factor that evaded simulator studies was that under adverse tribological conditions, such as start up or edge wear, levels of frictional torque created by hard on hard bearings can be very significant with values up to the order of 20NM. In MOMTHA this torque can also affect the cup bone interface and the proximal part of the femoral stem. Another issue affecting the taper is the effect of ‘toggle’. This is exacerbated if the centre of rotation of the femoral head is offset from the centre of the taper junction. It would seem the combined effects of torque and toggle with large diameter MOMTHA can result in fretting and crevice corrosion, and additionally galvanic corrosion if the stem is made of titanium. In this situation the taper junction rather than the MOM bearing can be the wear generator. In contrast to resurfacing, MOMTHA using a large diameter head with a monobloc cup is an unproven concept, and at the time of writing cannot be considered successful, and while some designs may emerge unscathed in the coming years, surgeons are urged to exercise considerable caution until more detailed information is available. It is worth noting that in the Australian Joint Registry (6) the failure rate for MOM bearings is higher for all head diameters greater than 28mm. Clearly this also includes modular cups with metal on metal bearings. In conclusion there is a responsibility on us all; industry, engineers and surgeons to learn lessons from this situation and to put in place mechanisms that will prevent or at least provide an earlier warning of similar problems.