A697. USE OF COMPUTER-BASED TEMPLATING IN PREDICTING IMPINGEMENT OF TIBIAL STEM AUGMENTS IN KNEE ARTHROPLASTIES AFTER OPENING WEDGE OSTEOTOMY

Abstract

High tibial valgus osteotomy is now well established in management of medial knee osteoarthritis. While conventional closing osteotomies are usually within 2 cm of the knee joint, opening wedges typically pivot more distally from the joint line; theoretically the same angular correction will cause greater linear shift of the tibial plateau away from the tibial long axis. We hypothesise that this may lead to an increased incidence of problems with future knee replacement where tibial stem augments are needed, and to evaluate this we used a computerbased templating system with web-based component templates for sizing and implant position planning.

We studied 10 knees that had undergone opening wedge osteotomy. Pre-operative and postoperative mechanical and anatomical axes, and corrections achieved, were measured radiologically. Computer-based knee arthroplasty templating was then performed with the TraumaCad digital templating software (Orthocrat, Israel), using Depuy PFC tibial component templates with 75 mm stem augments. Cases were analysed for impingement of tibial stem augments when added to a well-placed tibial tray, and conversely for the need for tibial tray downsizing to avoid tray overhang if stem augments were placed centrally.

Results: Mean pre-operative mechanical axis was 10.6o varus (1.6o to 22.3o). Mean osteotomy to joint line distance was 25.7mm (21.0mm to 33.1mm). In four knees, the addition of a 75 mm tibial stem augment to a well-placed tibial component caused stem impingement on cortex. In these four cases, central placement of the stem augment in the canal led to medialisation of the tibial component, necessitating downsizing of tibial tray by one to two sizes to avoid medial overhang and resulting in sub-optimal coverage of the cut tibia. These four cases all had valgus corrections of over 11o (11.5o to 19.6o). Conversely the six cases that did not have impingement or sizing problems all had corrections under 9o (3.0o to 8.2o). Our early results suggest that higher degrees of valgus correction with opening wedge osteotomy may lead to problems with future knee replacements requiring tibial stem augments. We are in the process of recruiting more cases to determine threshold levels for different makes and models of implants, using the same templating software system.