Computer-assisted total knee arthroplasty (TKA) is reported to improve the overall accuracy of positioning of the femoral and tibia components. However, an acceptable target for alignment remains a matter for debate. A mechanical axis within 3° of neutral axis has been used as the primary outcome measure in many clinical trials comparing computer-assisted- and conventional TKA. However, the evidence supporting this arbitrary value is unreliable because previous reports are limited by their small sample size, inadequate radiographs, short follow-up and lack of clarity when defining a margin of accuracy.