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

Background: 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.

Methods: 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.

Results: 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.

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