INTRODUCTION: 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.

MATERIAL AND METHODS: 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.

RESULTS: 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.

CONCLUSION: 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.