3-D landmarking repeatability emphasizes challenges in scan positioning during weight bearing CT of the knee

Maintaining repeatability of positioning during knee weight bearing CT (WBCT) acquisition can be challenging. We evaluated a manual bone landmarking system and automatically derived angulation metrics to advance understanding of joint positioning repeatability.

Bilateral knee WBCTs for 14 recruited individuals taken at a mean ± SD interval of 14.9 ± 8.1 days using a frame and laser guidance for positioning were used in this technical study. One observer manually placed 33 bony landmarks at each knee, verified by a second observer who also placed landmarks at each end of the scan range in the centre of the bone. Landmark repeatability error was the mean ± SD absolute distance (mm) between baseline and follow-up. Automated methods for knee flexion angle (KFA), valgus and vertical positioning alignment were developed from the landmarks.

Most repeatable landmarks were around the patella and proximal fibula with best performance at the proximal fibula tip (1.3 ± 0.5 mm). Least repeatable landmarks were along shallow curves, e.g. central tibial margins and around the trochlea, with worst performance at the tibial tuberosity (5.5 ± 4.7 mm). Mean repeatability for 85% of placed landmarks was in the 1-4 mm range. Valgus alignment absolute repeatability error ranged from 0.1-2.1° and for KFA was 0.3-15.5°, neither dependent on KL grade severity. Worst vertical scan range placement error was ∼60 mm.

Despite guidance systems, repeatability of KFA and vertical positioning could be optimised at the time of acquisition. This would provide a robust platform for evaluation of metrics such as meniscal extrusion and 3-D joint space width from WBCT that have shown promise in the evaluation of knee OA.