Restoration of neutral mechanical alignment is traditionally considered as one of the prerequisites for successful total knee replacement. The purpose of this study was to investigate whether a certain bias towards undercorrection exists with conventional total knee arthroplasty (TKA) instruments.

A cohort of 456 consecutive patients, who underwent the same standardised TKA with restoration of neutral mechanical alignment as target, was studied. Based on the preoperative alignment, patients were stratified into three categories: valgus, neutral and varus. Component and limb alignment were compared between these groups.

The mean post-operative hip–knee–ankle angle was −0.7° (SD 2.5) in valgus knees, 0.2° (SD 1.9) in neutral knees and 2.4° (SD 3.9) in varus knees (p < 0.001). About 39.8 % of the varus knees remained in >3° of varus post-operative and 20.2 % of the valgus knees remained in <−3° of valgus. A systematic unintentional undercorrection was noted in varus knees, which was proportional to the preoperative varus deformity and which was caused by varus positioning of both the femoral and tibial components. In valgus knees, the undercorrection was caused almost exclusively by valgus bias of the femoral component’s position.

This study showed that conventional TKA instruments are associated with a systematic unintentional bias towards undercorrection of the pre-existing deformity. The clinical relevance of this study is that intentionally aiming at slight undercorrection of the deformity may lead to excessive undercorrection in reality in case the surgeon does not recognise the automatic bias that already exists with standard instruments.

Therapeutic study, Level III.