Abstract
INTRODUCTION
Thermal necrosis of the femoral head, due to heat generation during cement polymerization, is a concern in hip resurfacing. Bone necrosis could cause fractures and/or implant loosening. Some authors1 found an inverse relationship between the size of the femoral component and the risk of revision after hip resurfacing. We postulate that smaller implants contain proportionally more cement than larger ones and that this could explain the effect of implant size on revision rate. As such, we investigated the relation between implant size and both, the average cement mantle thickness and the cement-filling index (fraction of cement volume and total volume within the implant).
MATERIALS AND METHODS
Nineteen human femoral heads, collected during total hip arthroplasty, were machined for hip resurfacing with original ReCap (Biomet) instruments. The head sizes were chosen so we could implant two resurfacing heads for each even size between 40 and 56 mm, and one for size 58 mm. Each reamed head was provided with a number of anchoring holes proportional to the head size and was kept at 37°C. After pressure-lavage with water at 20°C, polymeric replicas of the original Recap implants were cemented according to a strict protocol. The exact amount of Refobacin Bone Cement LV (Biomet) needed to fill half the volume of the implant was pored into the resurfacing head and 2.5 minutes after starting cement mixing, the implant was manually impacted on the reamed femoral head.
Specimens were scanned with computer tomography from the distal border of the resurfacing head to the top of the dome and CT-images were analyzed with an adapted version of validated segmentation software2. Based on gray values we identified four different elements: the polymeric stem and the outer shell of the implant, the cement-free cancellous bone and the cement mantle. Both, the average cement mantle thickness and the cement-filling index were calculated as described previously3.
RESULTS
The average cement mantle thickness was 2.63 mm (SD: 0.86; 1.65–4.60), the average cement-filling index was 36.65% (SD: 10.81; 21.52–57.60). Cement mantle thickness was poorly correlated with implant size (Pearson's correlation coefficient: −0.12; p=0.628; fig. 1), whereas the cement-filling index had a moderate to good correlation (Pearson's correlation coefficient: −0.51; p=0.026; fig. 2)
CONCLUSION
Our results show that the cement mantle thickness is not related to implant size, but that smaller femoral resurfacing heads are easier to fill-up with cement than larger once. As such, we expect more thermal bone necrosis associated to the higher cement-filling index of smaller implants. This could explain their higher early revision rate.
