The native knee joint line is varus relative to the tibia and remains parallel to the floor during gait even with varus lower-limb alignment. We investigated the desired degree of frontal obliquity for positioning the tibial component during unicompartmental knee arthroplasty (UKA).
We retrospectively analyzed full-leg, standing, hip to ankle digital radiographs from 107 osteoarthritic knees. We measured the hip-knee-ankle (HKA) angle, the tibial joint-line orientation angle (JLOA), which indicates the angle of the joint line (tibial component) relative to the floor, and the medial proximal tibial angle (MPTA), which is the angle of the joint line (tibial component) relative to the tibial mechanical axis, before and after UKA in the coronal plane.
The preoperative HKA angle (mean 7.3°, standard deviation (SD) 3.0) was significantly higher than the postoperative HKA angle (mean 3.4°, SD 3.0, P < 0.0001). The preoperative MPTA (mean 4.8°, SD 2.1) did not significantly differ from the postoperative MPTA (mean 4.5°, SD 3.0, P = 0.47). The mean postoperative JLOA was parallel to the floor (JLOA; 0.2°, SD 3.6).
When the tibial component is positioned along the natural joint slope restoring pre-disease alignment of the overall lower limb in the coronal plane, the tibial component was positioned parallel to the floor. When UKA is indicated for the medial osteoarthritis patient, the surgeon should install the tibial component four to five degrees varus to the tibial mechanical axis to maintain joint-line parallelism.