Focal intratendinous radiolucency: A new radiographic method for diagnosing patellar tendon ruptures

Diagnosis of patellar tendon rupture is usually based on clinical history and examination. In equivocal cases, imaging may be required. Lateral radiograph is a simple and cost-effective method for prompt diagnosis. However, no specific radiological sign has been described. Rather than utilising patella alta as an indirect measure of patellar tendon rupture, we hypothesise that a focal intratendinous radiolucency is another reliable and accurate radiological sign for diagnosis. Sensitivity and specificity analysis was undertaken to evaluate the diagnostic value of this radiographic sign.

Lateral radiographs of mid-substance patellar tendon ruptures from 19 patients were analysed. These were then randomised with another 19 normal knee radiographs from age-matched patients to create a pool of 38 radiographs for interpretation. Six independent interpreters who were blinded to the diagnosis were requested to indicate whether rupture was present or absent based on the visualisation of a focal intratendinous radiolucency in the patellar tendon. The Insall-Salvati (IS), Caton-Deschamps (CD) and Blackburne-Peel (BP) ratios were measured in the same radiographs. Sensitivity and specificity for each of the radiographic measurements were calculated. Inter- and intraobserver correlations were reported in kappa statistics.

The average sensitivity and specificity for focal radiolucency in the patellar tendon substance were 82.5% and 95.2%, respectively. Sensitivity and specificity for the IS ratio were 84.2% and 78.9%, for CD was 68.4% and 84.2% and for BP was 68.4% and 89.4% respectively.

The presence of a focal intratendinous radiolucency in the patellar tendon is both accurate and reliable in diagnosing patellar tendon ruptures.