WEAR PERFORMANCE OF A PROTOTYPE CALCIUM CARBONATE CONTAINING PMMA KNEE SPACER

Introduction Temporary use of antibiotic-impregnated polymethylmethacrylate (PMMA) bone cement spacers in two-stage revisions is considered to be standard of care for patients with a chronic infection of a joint replacement. Spacers should be wear resistant and load-bearing to avoid prolonged immobilisation of the patient and to reduce morbidity. Most cement spacers contain barium sulphate or zirconium dioxide as radio-opaque substrate. Both are quite hard materials that may negatively influence the wear behaviour of the spacer. Calcium carbonate is another radio-opaque substrate with lower hardness potentially increasing the wear resistance of the spacer materials. The purpose of the study was to compare a prototype PMMA knee spacer (calcium carbonate loaded) with a commercially available spacer (containing barium sulphate) regarding the wear performance and particle release in a knee wear simulator. Material and Methods Spacer K (TECRES, Italy) was used as barium sulphate (10%) containing spacer material. A prototype material (Heraeaus Medical, Germany) with 15% calcium carbonate was compared. Both were gentamicin impregnated, ready-made for clinical application (preformed) and consist of a tibial and a femoral component. Force-controlled simulation was carried out on an AMTI knee simulator. The test parameters were in accordance to ISO 14243–1 with a 50% reduced axial force (partial weight bearing). Tests were carried out at 37 °C in closed chambers filled with calf serum. Tests were run for 500,000 cycles at a frequency of 1 Hz. For wear analysis, gravimetric wear measurements according to ISO 14243–2 and wear particle analysis according to ASTM F1877–05 were performed. Results Fig. 1 presents the results of the gravimetric wear measurements. For the Spacer K cement a mean articular wear mass of 375.53±161.22 mg was determined after 500.000 cycles (femoral components: 149.55±17.30 mg, tibial components: 225.98±153.01 mg). The prototype cement showed lower mean total wear of 136.32±37.58 mg (femoral components: 74.32±33.83 mg, tibial components: 61.99±15.74 mg). However, a statistically significant lower wear rate was only seen for the femoral components (p=0,027). In Fig. 2 isolated PMMA wear particles are shown and the morphological characteristics are given in Tab. 1. Discussion and conclusion The prototype material showed better wear performance in terms of gravimetric wear and particle release. Thus calcium carbonate seems to be a promising material as radio-opaque substrate in PMMA spacers. Nevertheless, the wear amount released from both spacer materials is much higher as compared to conventional total knee replacements with polyethylene inserts. In this context biological reactions against PMMA particles and an increased release of cytokines have been reported in vitro [1] and furthermore, the promotion of osteolysis has been shown in vivo in the presence of PMMA particles [2]. As a clinical consequence we suggest excessive debridement during removal of the cement spacer components to reduce the risk of third body wear for the final joint replacement. Beside the wear performance further studies are essential to prove the mechanical stability and the antibiotic release kinetics for the prototype cement.