New measurement technique for restoration of the trochlear offset after image-based robotic-assisted total knee arthroplasty: a reliability study

Introduction: The new concepts in total knee arthroplasty (TKA) tend to improve the alignment and ligament balancing after TKA. Nevertheless, the assessment of the anterior compartment is difficult. The purpose of this study was to describe a new measurement technique of trochlear offset restoration on CT-scan after primary robotic-assisted TKA and assess its reliability and repeatability. Method: This monocentric study assessed the trochlear offset restoration on a CT scan after 20 robotic-assisted TKA. To evaluate the trochlear offset restoration, we measured the depth difference between the native and the prosthetic trochlea. Four sequential positions were assessed on the trochlea: at full extension, at 30 degrees, 70 degrees, and 90 degrees flexion. For each of these positions, we compared the highest point of the lateral native condyle and the lateral prosthetic condyle, the highest point of the medial native condyle and the medial prosthetic condyle, the deepest point of the native trochlear groove and the prosthetic trochlea. Two independent reviewers performed the measurements to assess their reliability. To determine intraobserver variability, the first observer performed the measurements twice. Results: The mean age was 67.3 years old +/- 8.3. Mean values of the trochlear offset restoration for the medial condyle, trochlear groove and lateral condyle were respectively: 1.0 mm +/- 1.6, 1.1 mm +/- 1.5, -2.7 mm +/- 2.3 in full extension; -3.5 mm +/- 1.7, -1.5 mm +/- 1.7, -3.9 mm +/- 3.9 at 30 degrees flexion; -5.1 mm +/- 1.8, 2.1 mm +/- 2.7, -3.8 mm +/- 1.8 at 70 degrees flexion; 2.0 mm +/- 1.4 and 3.1 mm +/- 1.5 for the medial and lateral condyles at 90 degrees flexion. The radiographic measurements showed very good to excellent intra-observer and inter-observer agreements with mean kappa values of 0.92 and 0.74. Conclusion: We present a novel measurement technique on CT scan for evaluating the restoration of the trochlear offset after TKA, demonstrating excellent inter and intra-observer reliability.