There are many methods for analysing wear volume in failed polyethylene acetabular components. We compared a radiological technique with three recognised ex vivo methods of measurement.

We tested 18 ultra-high-molecular-weight polyethylene acetabular components revised for wear and aseptic loosening, of which 13 had pre-revision radiographs, from which the wear volume was calculated based upon the linear wear. We used a shadowgraph technique on silicone casts of all of the retrievals and a coordinate measuring method on the components directly. For these techniques, the wear vector was calculated for each component and the wear volume extrapolated using mathematical equations. The volumetric wear was also measured directly using a fluid-displacement method. The results of each technique were compared.

The series had high wear volumes (mean 1385 mm³; 730 to 1850) and high wear rates (mean 205 mm³/year; 92 to 363). There were wide variations in the measurements of wear volume between the radiological and the other techniques. Radiograph-derived wear volume correlated poorly with that of the fluid-displacement method, co-ordinate measuring method and shadowgraph methods, becoming less accurate as the wear increased. The mean overestimation in radiological wear volume was 47.7% of the fluid-displacement method wear volume.

Fluid-displacement method, coordinate measuring method and shadowgraph determinations of wear volume were all better than that of the radiograph-derived linear measurements since they took into account the direction of wear. However, only radiological techniques can be used in vivo and remain useful for monitoring linear wear in the clinical setting.

Interpretation of radiological measurements of acetabular wear must be done judiciously in the clinical setting. In vitro laboratory techniques, in particular the fluid-displacement method, remain the most accurate and reliable methods of assessing the wear of acetabular polyethylene.

Introduction The ABG I acetabular insert is an ultra high molecular weight polyethylene (UHMWPE) component used in primary hip arthroplasty. Studies have shown early osteolysis and aseptic loosening of the ABG I uncemented cup compared with other implants. Theories advocate that loosening is initiated by the biological response to insert wear debris; wear volume and the distribution of particle size are considered to be important parameters. This study analysed explanted plastic inserts to identify any mechanical properties that may have contributed to early failure.

Materials and Methods 21 ABG I acetabular components were revised due to aseptic loosening over a 16 month period. Silicone casts of the insert sockets were made and volumetric analysis performed using a shadowgraphing technique and a coordinate measuring machine (CMM). The UHMWPE inserts were divided into uniform pieces with a diamond-tipped microsaw and analysed for hardness, wear, stress and strain properties using a microhardness tool, pin-on-plate analysis and small punch testing. We performed identical tests on explanted inserts from other manufacturers.

Results We present the findings of the above tests and provide suggestions as to why these particular implants are more prone to early failure when compared with other common implants. We also discuss the results of volumetric analysis by shadowgraphing compared with CMM.

Fatigue fractures which originate at stress-concentrating voids located at the implant-cement interface are a potential cause of septic loosening of cemented femoral components. Heating of the component to 44°C is known to reduce the porosity of the cement-prosthesis interface.

The temperature of the cement-bone interface was recorded intra-operatively as 32.3°C. A simulated femoral model was devised to study the effect of heating of the component on the implant-cement interface.

Heating of the implant and vacuum mixing have a synergistic effect on the porosity of the implant-cement interface, and heating also reverses the gradients of microhardness in the mantle.

Heating of the implant also reduces porosity at the interface depending on the temperature. A minimum difference in temperature between the implant and the bone of 3°C was required to produce this effect. The optimal difference was 7°C, representing a balance between maximal reduction of porosity and an increased risk of thermal injury. Using contemporary cementing techniques, heating the implant to 40°C is recommended to produce an optimum effect.