Sensitivity and specificity of post-operative interference gap assessment on plain radiographs after cementless...

Implant performance of cementless THA is often evaluated by radiolucency on plain radiographs, often classified as interference gaps on direct post-operative radiographs. However, the diagnostic performance is unknown. The aim was to evaluate the diagnostic performance of radiographic assessment of post-operative gaps after primary THA by comparing it with CT confirmed gaps, and secondary to define optimal cut-off criteria for assessing gaps on plain radiographs compared with CT.

Patients (N = 40) with a primary cementless THA performed between July 2015 and March 2016 were enrolled in the study. Radiolucency was assessed on post-operative AP pelvic digital radiographs by two observers independently. Maximum width and percentage of coverage per zone were reported. Gap volume was measured by manual segmentation on CT images.

When defining a gap as a radiolucency extending through >50% of a zone, the interrater agreement Kappa was 0.241. Sensitivity was 65.8% for observer 1 (Kappa = 0.432), and 86.8% for observer 2 (Kappa = 0.383). When defining a gap as a radiolucency with a width >1 mm, the interrater agreement Kappa was 0.302. Sensitivity was 55.3% and 50% for observer 1 and observer 2, respectively. The ROC-curve resulted in an optimal threshold of 0.65 mm (AUROC = 0.888) and 0.31 mm (AUROC = 0.961) for the two observers.

The diagnostic performance of observers detecting interference gaps on radiographs showed low sensitivity. Further on, the inter-rater agreement is too low to do a general recommendation about thresholds for defining gaps. Evaluating progression of radiolucency on radiographs should be performed in the light of these findings.