Restoration or relative overcorrection of pre‐arthritic coronal alignment leads to improved results following medial unicompartmental knee arthroplasty

PURPOSE: A pre-arthritic alignment strategy for medial unicompartmental knee arthroplasty (UKA) aims to restore a patient's native lower limb alignment which may translate into improved outcomes. This study aimed to assess whether patients with pre-arthritically aligned knees versus patients with non-pre-arthritically aligned knees demonstrated improved mid-term outcomes and survivorship following medial UKA. The hypothesis was that pre-arthritic alignment in medial UKA would lead to better postoperative outcomes. METHODS: A retrospective study of 537 robotic-assisted fixed-bearing medial UKA was conducted. During this procedure, the surgical goal was to restore pre-arthritic alignment guided by re-tensioning of the medial collateral ligament (MCL). For study purposes, coronal alignment was retrospectively evaluated using the mechanical hip-knee-ankle angle (mHKA). Pre-arthritic alignment was estimated through the arithmetic hip-knee-ankle (aHKA) algorithm. Knees were grouped according to the difference between postoperative mHKA and estimated pre-arthritic alignment (i.e., mHKA - aHKA) as Group 1 (pre-arthritically aligned: mHKA restored within 2.0° of the aHKA), Group 2 (mHKA > 2.0° overcorrected relative to the aHKA), or Group 3 (mHKA > 2.0° undercorrected relative to the aHKA). Outcomes included the Knee Injury and Osteoarthritic Outcome Score for Joint Replacement (KOOS, JR), Kujala, proportions of knees achieving the patient acceptable symptom state (PASS) for these scores, and survivorship. PASS thresholds for KOOS, JR and Kujala were determined using a receiver operating characteristic curve method. RESULTS: A total of 369 knees were categorized as Group 1, 107 as Group 2, and 61 as Group 3. At 4.4 ± 1.6 years follow-up, mean KOOS, JR was comparable among groups, while Kujala was significantly worse in Group 3. The proportion of knees achieving the PASS for Kujala (76.5 points) was lower in Group 3 (n = 32; 59%) compared to Group 1 (n = 260; 74%) (p = 0.02). 5-year survivorship was higher in Group 1 and Group 2 (99% and 100%, respectively) compared to Group 3 (91%) (p = 0.04). CONCLUSION: Pre-arthritically aligned knees and knees with relative overcorrection from their pre-arthritic alignment following medial UKA demonstrated improved mid-term outcomes and survivorship compared to knees with relative under correction from their pre-arthritic alignment. These results encourage restoring or relatively overcorrecting pre-arthritic alignment to optimize outcomes following medial UKA, and caution against under correction from the pre-arthritic alignment. LEVEL OF EVIDENCE: IV, case series.