Does flexion of the femoral implant in total knee arthroplasty increase knee flexion: A randomised controlled trial

Prosthetic and operative modifications in total knee arthroplasty (TKA) have been proposed to maximise post-operative knee flexion as it is essential in routine functional activities.

We performed a double blind randomised controlled trial to compare clinical outcomes of primary cruciate-retaining TKA for osteoarthritis with the femoral component implanted in either 4° flexion in the sagittal plane (F) or in a neutral position (C). The primary outcome of knee flexion and secondary outcomes knee extension, quadriceps strength, WOMAC, SF-12v2, timed stand test, stair climb test and satisfaction were assessed at 1 year. Knee flexion and extension were also assessed intra-operatively. Implant flexion was measured from true lateral radiographs.

Thirty-nine participants (40 knees) were recruited, 20 knees per group. Three subjects from the control group and two from the flexed group were lost to 1 year follow-up but numbers were sufficient to satisfy the sample size calculation. Significant differences were found between the groups in knee flexion (F: 113.6 ± 8.8° pre-operative, 122.4 ± 6.0° intra-operative, 110.2 ± 7.5° 1 year, C: 117.4 ± 11.7°, 117.4 ± 7.6°, 103.5 ± 10.7°. p = 0.031) and mental component score of the SF12-v2 (F 53.3 ± 13.2, C 61.1 ± 7.3, p = 0.009) but there were no significant differences in other outcomes and patients were equally satisfied.

Flexing the femoral implant in this cruciate retaining TKA system provided a significant difference in knee flexion compared to a neutral position. The improvement appears to occur predominantly at surgery and was not associated with a clinical or functional benefit at 1 year. (ACTRN12606000325505). Level of evidence: Level 1; randomised controlled trial.