Spacer rotation technique allows precise evaluation of gap balance in total knee arthroplasty

The symmetry of the flexion and extension gap influences the functional and long-term outcome after total knee arthroplasty (TKA). Most surgeons check it by applying varus and valgus stress using spacers. This technique has limited accuracy and could be easily extended by rotational movement of the spacer. The objective was to determine the detection threshold and interobserver reliability of this technique. In an in vitro setting with a human cadaveric knee, gap asymmetries were simulated by different medially and laterally applied forces. Using an optical measurement system, the pivot point of the spacer was calculated as a function of the gap symmetry in the first part of the experiment. In the second part, the detection threshold and interobserver reliability of 4 surgeons were determined. For this purpose, gap asymmetries were adjusted to between 0 and 120N in a blinded trial. With a symmetrical gap, the centre of rotation of the spacer was located in the centre of the tibia. With increasing gap asymmetry, the centre of rotation of the spacer shifted to the tight side. This shift was approximately linearly dependent on the force difference. A perfectly balanced gap was detected by the examiners in 50% of the cases. From a force difference of 40N, all examiners identified the gap asymmetry in all cases (ICC = 1.0). The method of spacer rotation described is suitable for reliably detecting gap differences at ≥ 40N, independently of the examiner.