KNEE REPLACEMENT SURGERY WITH A MODIFIED COMPUTER-AIDED NAVIGATION TECHNIQUE

Abstract

There are many reports in the literature about the benefits of computer-aided surgery with regards to improved limb alignment, reduced blood loss and embolic events but surgeons remain sceptical about its routine use because of availability, cost and time implications. To maximise these benefits and overcome the distractions, a modified navigation technique has been developed after evaluation of the standard measurements.

The true varus/valgus angle of the distal femoral cut achieved with navigation is unknown but represents presumed accurate alignment with regards to the mechanical axis through the femoral head. With placement of the femoral tracker in the medial supracondylar region clear of the intramedullary canal, the navigated cut was correlated with the cut placement determined with the standard intramedullary jig in 10 patients undergoing knee replacement. In addition, jigged femoral rotation was checked with the tracker placement. Tibial slope, varus/valgus angle and rotation were determined using surgeon placement of an external alignment jig and confirmed with tracker placement.

The navigated distal femoral cut ranged from +3 degrees to −2 degrees when measured against the distal cutting block stabilised over an intramedullary rod. The femoral rotation was within 1 degree of the trans-epicondylar line as outlined by navigation when a 3 degree externally rotated jig was used. All of the tibial measurements were within 0.5 degrees of the navigated planned positions.

The femoral cuts are presumed to be accurately determined with navigation as judged from long-leg alignment x-rays but this study highlights the potential error if a fixed valgus cut angle with alignment jigs is used. Tibial preparation, however, was accurately predicted by the surgeon using a traditional external alignment jig. Bone preparation time was reduced to 4 minutes (modified technique) compared to 12 minutes (full navigation, p< 0.05).

With this information, computer-aided navigation is now routinely used to determine the distal femoral cut only and an external alignment jig is used for tibial preparation without navigation. The reduction in blood loss and embolic events and improved limb alignment is now achieved with a reduction in preparation time over full navigated techniques. Use of the pinless surface mounted femoral jig alone highlights these advantages further.