Restoration of the native joint line in total knee arthroplasty is important in restoring ligamentous balance and normal knee kinematics. Failure to achieve this could lead to reduced range of motion, patellofemoral maltracking and suboptimal outcomes. The purpose of this study was to analyse the clinical and functional outcome of patients who demonstrated joint line changes after computer-assisted (CAS) total knee arthroplasty (TKA).

A prospective study was conducted for 168 patients (168 knees) who underwent CAS TKA by two surgeons at a single institution with an average follow-up of two years. The final change in joint line was calculated from the verified tibial resection, distal and posterior femoral cuts. Group A patients had joint line changes of less than 4mm and Group B patients had joint line changes of more than 4mm. Postoperative Oxford scores, Knee scores, Function scores and SF-36 scores were obtained at six months, one year and two years post-TKA. The final range of motion and the mechanical alignment were documented.

There was significant linear correlation between joint line changes and Oxford scores (p = 0.05) and Function scores (p = 0.05) at six months and Oxford scores alone at two years with increasing joint line changes having poorer outcome scores. Group A compared to Group B patients have better outcomes in terms of Oxford scores (mean 20 vs 27, p = 0.0003), Function scores (mean 69 vs 59, p = 0.03), SF-1 (mean 63 vs 50, p = 0.03), SF-2 (mean 66 vs 43, p = 0.05), SF-5 (mean 75 vs 63, p = 0.04), SF-6 (mean 84 vs 59, p = 0.003), SF-7 (mean 96 vs 83, p = 0.02), SF-8 (mean 84 vs 73, p = 0.006) and total SF-36 scores (mean 603 vs 487, P = 0.003), at six months, and Oxford scores (mean 18 vs 23, p = 0.0007) at two years.

In this study, CAS is a useful intra-operative tool for assessing the final joint line in TKA. Outliers in joint line changes of ≥ 4 mm are associated with poorer clinical outcome scores.