Referencing the substitute anteroposterior line of the tibia improves rotational alignment of the tibial component in medial unicompartmental knee arthroplasty

There is no consensus regarding how best to determine the tibial rotational alignment in unicompartmental knee arthroplasty (UKA). The purpose of this study was to clarify whether using the substitute anteroposterior (sAP) line of the tibia, as has recently been proposed, can improve tibial rotation.

The study included 57 consecutive medial UKAs. From May 2015 to September 2016, 28 knees in 28 patients underwent UKA using the medial intercondylar ridge (MIR) line as the tibial anteroposterior (AP) reference (MIR group). From October 2016 to March 2018, 29 knees in 29 patients underwent UKA using the sAP line (sAP group). In both groups, the external rotation angle of the tibial component relative to a line perpendicular to the surgical epicondylar axis was measured using computed tomography-based three-dimensional preoperative planning software for TKA and UKA.

The mean external rotation angles of the tibial component in the MIR and sAP groups were 5.2° ± 8.5° (range, − 12.4° to 20.8°) and 0.7° ± 3.2° (range, − 6.0° to 7.4°), respectively (unpaired t test, P = 0.014). The variation in the external rotation angle of the tibial component was significantly smaller in the sAP group than in the MIR group (F test, P < 0.0001), as was the number of the outliers with more than ± 5° error (Fisher’s exact test, P < 0.0001).

The use of the sAP line as the AP reference could improve and stabilize the rotational orientation of the tibial component in UKA procedures.