PRECISION OF HIGH SPEED CUTTERS – ONE POSSIBLE BENEFIT FROM CAOS

Abstract

Modern hip-replacement requires fixation of the femoral component, the stem, in the proximal femur. After resection of head and neck, the surgeons prepare the shaft in order to make room for the stem. Cemented fixation of the stem requires over-reaming, because the surgeon needs to provide space for the cement mantle, usually between 2 and 4 mms wide. Reaming for cemented fixation means removal of (cancellous) bone stock. Precision of reaming is not of utmost importance, as cement will fill gaps and will provide close contact between implant and bone. Cementless fixation on the other hand requires rather precise reaming, as for the biological fixation to occur, a close contact between implant and bone is crucial. There are two ways to achieve such contact: ream the bone to the precise negative form of the implant, or compress the cancellous bone into this shape. Compressing is technically easier and is regarded by some as the better option: the supposedly weak cancellous bone is compressed and provides a firm contact surface for the implant. The other option is precise reaming of the surface, sparing the scaffolding of the cancellous bone to provide biological support for the implant. It is difficult though to achieve this precise cutting with traditional tools: an animal experiment conducted by the author showed fractured and destroyed bone in the hand broached group, resulting in defects and lasting atrophy in the periphery, due to inadequate load transfer. These results coincide with a cadaver study performed by v.Hasselbach et al in 1996. The alternative to traditional hand broaching in both studies was using a high speed cutter with 70,000 rpm. As such a cutter can not be applied by hand due to the high torque; surgery was performed in both studies using a robot guiding the cutter. Cuts were performed according to a preoperatively established plan.

In the animal experiment, histological examination after one year showed no signs of atrophy in the high speed cutter group, whilst atrophy was still present in the hand broached group. These results coincide with significantly better performance in the postoperative force plating.

Conclusion: Application of navigation systems has helped to solve the problems in orientation of both cup and stem. Yet the preparation of the interface of the stem remains an unaddressed issue both in navigated and minimal invasive surgery. The use of high speed cutters (which prove to be helpful also in total knee replacement – Acrobot and Robodoc) seems an option that should not be neglected. The interface between bone and implant is the location where the fate of the implant is decided.