Robotic-assisted medial unicompartmental knee arthroplasty restores estimated pre-arthritic coronal limb alignment: A retrospective cohort study

Background: Robotic-assisted medial unicompartmental knee arthroplasty (UKA) aims to restore pre-arthritic (constitutional) limb alignment, by re-tensioning of the medial collat-eral ligament (MCL). This study aimed to determine whether pre-arthritic coronal align-ment was restored following robotic-assisted medial UKA in patients with medial compartment osteoarthritis.Method: A retrospective study was undertaken, including 102 patients with a unilateral robotic-assisted medial UKA and a contralateral unaffected knee. Both the validated arith-metic hip-knee-ankle angle (aHKA) and alignment of the contralateral unaffected knee were used to estimate pre-arthritic alignment. The aHKA is a radiographic method to esti-mate the pre-arthritic mechanical hip-knee-ankle angle (mHKA). To verify restoration of pre-arthritic alignment, postoperative mHKA was compared to the aHKA. Additionally, postoperative mHKA, joint line congruence (JLCA), and knee joint line obliquity (KJLO) angles were compared between the operative and contralateral unaffected knee. Equivalence between postoperative and pre-arthritic alignment was assessed through the two-one-sided t-test (TOST), using equivalence margins of +/- 2.0 degrees.Results: Postoperative mHKA was equivalent to the aHKA (mean difference-0.38 degrees, 90% CI-0.69 to-0.07; p < .001), with 93 knees (91%) restored within 3.0 degrees their aHKA. Postoperative mHKA, JLCA and KJLO were equivalent between the operative and contralat-eral unaffected knees, with mean differences of-0.65 degrees,-0.65 degrees, and-0.40 degrees, respectively; all p < .001.Conclusions: Postoperative and pre-arthritic coronal alignment were equivalent following robotic-assisted medial UKA, with 91% of knees restored within 3.0 degrees of their pre-arthritic mechanical axis. These results demonstrate that both mechanical alignment and joint line congruence are restored by MCL re-tensioning in patients undergoing robotic-assisted medial UKA for medial compartment osteoarthritis.(c) 2023 Elsevier B.V. All rights reserved.