A prospective randomized study was performed to compare the clinical results of standard vs. minimally invasive TKA (MIS TKA), and to identify the preoperative factors that help to figure out the indications of MIS TKA.

Eighty patients with osteoarthritic knees were randomized to standard TKA and MIS TKA groups. The operations were performed by one senior author with the same TKA system. Preoperative evaluation included body mass index (BMI), range of motion (ROM), deformity, and WOMAC and Knee Society scores. Operative factors were evaluated, including length of skin incision, operation time, and operative complications. Postoperative evaluation was performed daily until discharge from hospital, at 1 month, 3 months, 6 months, and 1 year postoperatively, including blood loss, amount of epidural anesthesia, visual analog scale (VAS), ROM, possible day of active straight leg raising (SLR), possible day of discharge, complications, WOMAC and Knee Society scores, patient's satisfaction with the operative scar, and radiological evaluation.

There were statistically significant differences in daily VAS, operation time, length of skin incision, and possible day of SLR (p<0.05). There were no significant differences in other clinical aspects that were compared in this study. Complications occurred only in MIS group; 1 partial tear of MCL and 1 fracture of lateral femoral condyle. Preoperative ROM or BMI was not correlated with longer operation time. Significantly longer operation time was observed when both the preoperative fixed flexion contracture and varus deformity were greater than 10 °, or one or both of them was greater than 15 ° (p<0.05).

MIS TKA has the advantage of less pain, shorter skin incision, and earlier possible SLR in the early postoperative period, but it requires a longer operation time and careful surgical technique. The surgeon should select the MIS procedure by carefully considering its advantages and disadvantages. Both the preoperative flexion contracture and varus deformity greater than 10 °, or one of them greater than 15 ° seems to be the relative contraindication of MIS TKA from the result of this study.