Does the type of tibial component affect mechanical alignment in unicompartmental knee replacement?

There are a number of factors responsible for the longevity of unicompartmental knee replacements (UKR). These include the magnitude of postoperative alignment and the type of material used. The effect of component design and material on postoperative alignment, however, has not been explored.

We retrospectively reviewed 89 patients who underwent UKR with robotic guidance. Patients were divided into two groups, according to whether they had received an all-polyethylene inlay component (Inlay group) or a metal-backed onlay component (Onlay group). We explored the magnitude of mechanical alignment correction obtained in both groups.

Mean postoperative mechanical alignment was significantly closer to neutral in the Onlay group (mean=2.8°; 95% CI=2.4°, 3.2°) compared to the Inlay group (mean=3.9°; 95% CI=3.4°, 4.4°) (R2=0.65; P=0.003), adjusting for gender, BMI, age, side and preoperative mechanical alignment (Fig. 2). Further exploration revealed that the thickness of the tibial polyethyelene insert had a significant effect on postoperative alignment when added to the model (R2=0.68; P=0.01).

Patients who received a metal-backed Onlay tibial component obtained better postoperative mechanical alignment compared to those who received all-polyethylene Inlay prostheses. The thicker overall construct of Onlay prostheses appears to be an important determinant of postoperative alignment. Considering their higher survivorship rates and improved postoperative mechanical alignment, Onlay prostheses should be the first option when performing medial UKR.