Tibial component rotation during the unicompartmental knee arthroplasty: is the anterior superior iliac spine an appropriate landmark?

Purpose: No “ideal” landmark for tibial component rotation in medial unicompartmental knee arthroplasty (UKA) has been suggested by a biomechanical and clinical study. The aim of this study was to investigate whether the anterior superior iliac spine (ASIS) could provide a consistent rotational landmark of the tibial component during mobile-bearing medial UKA using computed tomography (CT). Methods: During sagittal tibial resection, we utilized the ASIS as a rotational landmark. In 47 knees that underwent postoperative CT scans after medial UKA, the tibial component position was assessed by drawing a line tangential to the lateral wall of the tibial component. Rotation of the tibial component was measured using two reference lines: a line perpendicular to the posterior cortical rim of the tibia (angle α) and Akagi’s line (angle β). Instant bearing position and posterior cruciate ligament (PCL) fossa involvement were also evaluated. External rotation of the tibial component relative to each reference line was considered positive values. Results: The mean α and β angles were 8.0° ± 6.1° (range −4.0 to 24.3) and 8.7° ± 4.8° (range 1.9–25.2), respectively. Fourteen knees (29.8 %) showed PCL fossa involvement of the tibial resection margin. One bearing showed complete 180° rotation at 2 weeks postoperatively. Conclusion: Due to the wide variation and inherent difficulty of identifying the ASIS during the operation, it is not recommended for guidance of sagittal tibial resection during medial UKA. In cases of inappropriate tibia component rotation, risk of PE bearing spinning and iatrogenic PCL injury should be reminded after medial UKA. Level of evidence: IV.