Background

Mechanical axis alignment has been shown to influence the long-term outcome of fixed-bearing medial unicompartmental knee arthroplasty (UKA). However, a consensus on the optimal postoperative alignment target has not been established. To better define the optimal alignment target for medial fixed-bearing UKA, this study compares the postoperative mechanical alignment of well-functioning UKAs against 2 groups of failed UKAs, including revisions for progression of lateral compartment osteoarthritis (“Progression”) and revisions for aseptic loosening or subsidence (“Loosening”).

Methods

From our prospective institutional database of 3351 medial fixed-bearing UKAs performed since 2000, we identified 37 UKAs revised for Progression and 61 UKAs revised for Loosening. Each of these revision cohorts was matched based on age at surgery, gender, body mass index, and postoperative range of motion with unrevised UKAs that had at least 10 years of follow-up and a Knee Society Score of 70 or greater without subtracting points for alignment (“Success” groups). Postoperative alignment was quantified by the hip-knee-ankle (HKA) angle measured on long-leg alignment radiographs.

Results

The mean HKA at 4-month follow-up for the Progression group was 0.3±3.6 degrees of valgus compared to 4.4±2.6 degrees of varus for the matched Success group (p<0.001). For the Loosening group, the mean HKA was 6.1±3.1 degrees of varus compared to 4.0±2.7 degrees of varus for the matched Success group (p<0.001). The Loosening group was revised at a mean of 3.3±2.9 years compared to 5.4±4.0 years for the Progression group (p=0.004).

Conclusions

Mechanical alignment in the coronal plane is an important factor in the setting of fixed-bearing, medial UKA. Patients with well-functioning UKAs at 10 years exhibited mild varus mechanical alignment of approximately 4 degrees, whereas patients revised for progression of osteoarthritis averaged more valgus and those revised for loosening or subsidence averaged more varus. The optimal mechanical alignment for medial fixed-bearing UKA survival with contemporary polyethylene is likely slight varus.