This study evaluates the effect of lower limb post-operative mechanical axis on the long term risk of revision surgery following primary total knee arthroplasty (TKA). The study is relevant because many recent clinical trials have evaluated the optimal surgical technique for accurately aligning components in TKA, despite little evidence that alignment may effect the long-term clinical outcome.

The data used in this study was collected prospectively as part of a randomized control trial comparing the long term survival of cemented versus uncemented TKA. The trial included 501 press-fit condylar posterior cruciate ligament-retaining prostheses performed by the senior author (PJG) or under his direct supervision. The post-operative mechanical axis alignment of the lower limb was measured following TKA using standard AP weight-bearing long leg alignment radiographs. A comparison was made between a well-aligned group with a mechanical axis alignment within 3° of neutral; and a poorly-aligned group whose mechanical axis alignment deviated greater than 3° from neutral. Survival analysis used revision surgery, with exchange of any of the three originally inserted components (femoral, tibial, polyethylene insert), as the endpoint.

There was no loss to follow-up in this study. The minimum follow-up of TKAs in this study was 5.8 years. In the population of TKAs that were followed up at 10 years, 6% (17/270) required revision surgery. There was a significant difference in the rate of revision surgery between the well-aligned group 5% (11/227) and the poorly-aligned group 14% (6/43 p< 0.05)

This study shows that post-operative lower limb mechanical axis alignment is an important determinant of the need for revision surgery at 10 years. Surgeons performing TKA should pay particular attention to the placement of the tibial and femoral components, so that a mechanical axis within 3° of neutral is achieved.