ARTICULATION SURFACES IN THA FROM HISTORY UNTIL TODAY, WHAT WAS THE PROGRESS AND WHAT IS PROVEN?

Abstract

Among the many complications that lead to revision surgery of a Total Hip Replacement, aseptic loosening is still the most frequent one. Taking into account the role of wear debris in the event cascade leading to aseptic loosening, many different technical approaches have been developed thus far to minimize the volume of wear debris from THR bearings. Taking into account the role of material hardness in the wear of the system, a considerable effort has been devoted e.g. to improve by irradiation the polymeric chains crosslinking in UHMWPE, to optimise the distribution of carbides in Cobalt-Chromium alloys, to increase the hardness of different metallic alloys by the formation of hard ceramic layers by chemical or physic treatments.

Nevertheless, it is noted that the wear behaviour of ceramic bearings made out pure alpha-alumina remained unsurpassed thus far, and that it is a reference for THR bearing wear since 38 years of clinical use.

Today, hip replacement has become a rather frequent treatment that has been extended to many young and active patient. This has made more challenging the design of implants and of bearings because of the extension of the lifespan of the replacements and of the higher activity level they have to face. Moreover, the appropriate selection of the bearing couple is becoming more and more relevant, as it is noted that a relevant number of implant are operating in off-normal conditions. These are including verticalized sockets, presence of third bodies in the bearing gap, microseparation of the bearing components during the gait cycle, joint sub-luxations and impingemets due to patient activity either to ligament laxity. In these of-normal conditions not all the bearings perform in the same way, giving rise to conditions that may give rise to acute or chronic complications, in some cases of still unknown consequences e.g. surface layer spallation or delamination – leading to UHMWPE catastrophic wear, or sustained increase of Cr, Co, Ni ions in the body.

In Ceramic-on-Ceramic bearings a typical complication is stripe wear, that has been documented in pure alpha-alumina in case of verticalized sockets either following the repeated shocks due to joint microseparation. Other complication specific to ceramic are the fractures of the components due to severe traumas (like e.g. in road accidents) either inlay chipping that may take place intraoperatively.

To overcome these limits, the favourable properties of zirconia have been exploited to improve strength and toughness of alumina in a composite material, reinforced further by platelets nucleated in-situ during sintering. The mechanical properties of the new Alumina Matrix Composite (AMC) are superior to the ones of both alumina and of yttria-stabilized zirconia, without the handicap of Low Temperature Degradation (LTD) due to hydrothermal reactions with the biologic environment. Nevertheless, this do not mean that the material has unlimited performances, and care is demanded in its use especially in devices more challenging in design.