IMPORTANCE OF SOFT TISSUE DISSECTION IN PLACEMENT OF REFERENCE MARKER PINS IN COMPUTER AIDED HIP ARTHROPLASTY.

Abstract

Introduction: Movement of the limb during computer aided arthroplasty may cause soft tissue impingement on the reference marker(RM) and consequently alter the spatial relationship between RM and bone with resulting inaccuracies in navigation. The purpose of this study was to investigate the effect of different degrees of soft tissue dissection on the stability of reference markers during limb movement.

Methods: The stability of both one- and two-pin RM systems inserted using three different levels of soft-tissue dissection was analysed in relation to a super-stable RM in fresh cadaver lower limbs. The spatial relationship of the two RMs was analysed using the VectorVision^® system (BrainLAB, Germany) during multiple repetitions of four predefined limb movements. All tests were done with RMs inserted in both the distal-anterior femur and distal-lateral femur.

Results: Analysis of movements of the test RM in relation to the super-stable RM showed that rotations of less than 0.15^o and translations of less than 0.4mm occurred in most test combinations. The combination that showed the greatest instability was when a stab incision was used to insert a pin in the distal/lateral femur (translation 0.73mm+/−0.05, rotation 0.25^o+/− 0.05)(p< 0.001). This instability occurred in both single and double pin RMs(p=0.21).

Conclusions: RM pins can be placed in the anterior distal femur through simple stab incisions without resulting in significant soft tissue impingement during limb movement. If pins are placed in the lateral distal femur through stab incisions, impingement may occur from the fascia lata. Release of the fascia lata 1cm either side of the pin prevents significant impingement. Wide skin incision is unnecessary in any location.