The Imperial Joint Line Congruency Measurement is a valuable tool in total knee arthroplasty.

Background Preservation of joint line height is an important factor in post-operative function after Total Knee Arthroplasty (TKA). This is the first study investigating the reliability of the novel Imperial Joint Line Congruency Measurement (IJLCM) technique for the assessment of joint line height using plain radiographs. Methods The reliability of two techniques used to measure joint line height on pre-operative and post-operative plain radiographs is presented. 120 patients that underwent TKA from 6 different international centres were included. Measurements were performed using each technique by two senior orthopaedic surgeons at two different timepoints (test-retest). Two undergraduate medical students performed joint line measurements using the most reproducible of the two techniques on 40 pre-operative and post-operative images to establish the reliability of the measurement technique. Results The IJLCM demonstrated an average absolute difference of 1.83mm (CI 1.56–2.10mm) and excellent inter and intra-rater reliability between senior orthopaedic surgeons (>0.92 (CI 0.88–0.94) when measuring joint line height on plain radiographs. Overall Crohnbach’s alpha over 0.92 confirmed internal consistency. Measurements performed using the control technique as previously described by Figgie et al. had an average absolute difference of 5.75mm (5.17–6.32mm). Comparison of measurements by senior orthopaedic surgeons and medical students using the IJLCM technique with ANOVA and student’s t-test demonstrated acceptable agreement and inter-rater reliability of >0.92 (0.87–0.95). Conclusion This study shows excellent accuracy, precision, and reliability of the novel IJLCM technique. Furthermore, excellent agreement between senior orthopaedic surgeons and medical students when using the IJLCM could be shown. The IJLCM technique is reliable for joint line assessment.