The Oxford unicompartmental knee replacement gives good results in patients with symptomatic osteoarthritis of the medial compartment. Previous studies have suggested that the presence of radiolucent lines (RLLs) does not reflect a poor outcome in such patients. However, the reliability and validity of this assessment have not been determined. Our aim was to assess the intra- and interobserver reliability and the sensitivity and specificity of the assessment of RLLs around both tibial and femoral components using standard radiographs.

Two reviewers assessed the radiographs of 45 patients who had loosening of the tibial or femoral component confirmed at revision surgery and compared them with those of a series of 45 asymptomatic patients matched for age and gender.

The results suggested that, using standard radiographs, tibial RLLs were 63.6% sensitive and 94.4% specific and femoral RLLs 63.9% sensitive and 72.7% specific for loosening. Overall intra- and interobserver reliability was highly variable, but zonal analysis showed that lucency at the tip of the femoral peg was significantly associated with loosening of the femoral component.

Fluoroscopically guided radiographs may improve the zonal reliability of the assessment of RLLs, but further independent and comparative studies are required. In the meantime, the innocence of the physiological RLLs detected by standard radiographs should be viewed with caution.