Objective: In 2017, the substitute anteroposterior (sAP) line connecting the medial border of the patellar tendon at the articular surface level and the medial intercondylar tubercle was proposed for an AP reference in medial unicompartmental knee arthroplasty (UKA) and shown to be almost perpendicular to the surgical epicondylar axis (SEA). However, the AP alignment of the components implanted using this line in actual UKA procedures has not been known.

Methods: Postoperative computed tomography datasets from 60 lower limbs in 60 Japanese patients underwent fixed-type medial UKA, including 30 limbs using sAP line as AP reference and 30 limbs using medial intercondylar ridge (MIR), were used. First, the angles between the SEA and AP directions of the tibial and the femoral components were measured. Next, the anteroposterior rotational mismatch angles between the tibial and femoral components were measured.

Results: The tibial and femoral components implanted using sAP line were externally rotated 90.6°±3.4° and 92.1°±7.9° relative to the SEA, respectively; using MIR were 95.1°±8.5° and 91.1°±7.6°, respectively. The externally rotation angle of the femoral component relative to the tibial component when using sAP line and MIR as AP reference was 0.3°±8.7° and −3.9°±6.8°, respectively.

Conclusion: Using sAP line as the AP reference in medial UKA procedures makes it possible to implant the tibial component almost perpendicular to the SEA and to reduce the rotational mismatching between the femoral and tibial components.