Oxford medial unicompartmental knees display contact‐loss during step‐cycle motion and bicycle motion: A dynamic radiostereometric study

The Oxford medial unicompartmental knee is designed fully congruent, with the purpose of maintaining a large contact‐area throughout motion and minimize wear. No other study has investigated this design feature in‐vivo. We aimed to evaluate if contact‐loss was introduced between the articulating surfaces of the Oxford medial unicompartmental knee during bicycle‐ and step‐cycle motion, and whether this correlated with essential implant parameters, such as polyethylene (PE) wear, knee‐loadings, and clinical outcome. To study contact‐loss, 15 patients (12 males, mean age 69 years) with an Oxford medial unicompartmental knee (7 cemented, mean follow‐up 4.4 years) were examined with use of dynamic radiostereometry (RSA) (10 frames/s). PE wear was measured from static RSA and clinical outcome was evaluated with American Knee Society Score (AKSS) and Oxford Knee Score (OKS). Data on knee‐loadings were acquired from the literature. Contact‐loss was deteced in all patients during both exercises, and the trend of contact‐loss correlated with the knee‐loadings. Median contact‐loss was 0.8 mm (95%PI: 0.3; 1.5) for bicycle motion and 0.3 mm (95%PI: 0.24; 0.35) for step‐cycle motion, and did not correlate with the PE wear rate of mean 0.06 mm/year. Possible in‐congruency was seen in three patients. Clinical outcome scores correlated with contact‐loss during step‐cycle motion. In conclusion, contact‐loss was seen in all patients indicating a clinical tolerance during load. Contact‐loss followed the knee‐loadings, which could explain why no correlation was seen with PE wear, as an increase in load was acommadated by an increase in contact‐area (contact‐loss reduction). The size of contact‐loss may reflect clinical outcome.