Conventional radiography cannot replace CT scanning in detecting tibial tubercle lateralisation

Patellar instability can be caused by an excessive lateral distance between the anterior tibial tubercle and the trochlear groove (TT-TG). This study was designed to compare the TT-TG in reformatted computed tomography to the TT-TG on a 30 degrees axial conventional radiograph (CR) using lead markers to visualize the tibial tubercle and epicondyles. This is the first report on the use of lead markers for determining the TT-TG. Seven symptomatic knees in five patients (mean age 25 years, standard deviation 8.0 years) were investigated. Results showed that the tibial tubercle could be detected on 30 degrees axial CR by a lead marker. Determining the TT-TG however proved to be difficult. A good intra- and interobserver reliability (ICC >0.86) but large measurement error for the axial CR compared to CT was measured (Limits of Reproducibility as quantification of the measurement error was 18 mm for axial CR and 4 mm for CT). Because of the large measurement error for axial CR, the study was terminated after seven symptomatic knees. Positioning of the patient and markers, especially the tibial tubercle marker, probably are important factors leading to the large measurement error. Therefore, axial CR cannot replace CT to detect a pathological tubercle trochlear groove distance.