Healing Integrity Determines Meniscal Extrusion After Isolated Medial Meniscus Posterior Root Repair in Knees With No Lower Limb Malalignment

Purpose To investigate changes in meniscal extrusion and meniscal healing status following root repair, and the relationship between meniscal extrusion and healing conditions. Methods Patients who underwent isolated root repair between 2019 and 2023 and consented to undergo second‐look arthroscopy to evaluate their healing condition (1 year postoperatively) were included. The mean interval between root repair and second‐look arthroscopy was 12.2 ± 1.3 months. The healing condition of the repaired meniscus was evaluated based on morphological continuity and meniscal tension at the bony attachment by probing (non‐lax healing group, Group 1; lax healing group, Group 2). Meniscal extrusion was assessed in the coronal plane on magnetic resonance imaging preoperatively, immediately postoperatively, and at 1 year postoperatively. We analyzed the serial changes in meniscal extrusion according to the healing status observed during second‐look arthroscopy. Results A total of 51 patients were enrolled in the study (7 patients refused to undergo second‐look arthroscopy, and 5 patients were lost to follow‐up), with 30 assigned to Group 1 and 21 to Group 2. The mean age was 59.1 ± 6.2 years with 8 male and 43 female. Regarding postoperative clinical scores, the minimal clinically important difference (MCID) for Group 1 was as follows: Lysholm, 84.0 ± 7.4 (MCID: 3.55); WOMAC, 16.1 ± 8.1 (MCID: 4.67); and KOOS, 23.7 ± 6.6 (MCID: 5.69). Group 1 showed significantly better outcomes than Group 2, which had the following scores: Lysholm: 77.1 ± 7.8 (MCID: 3.78), WOMAC: 24.1 ± 9.2 (MCID: 4.41), and KOOS: 32.1 ± 9.2 (MCID: 6.03). All patients in both groups achieved the MCID for Lysholm, WOMAC, and KOOS (Group 1: 100% [30/30]; Group 2: 100% [21/21]). Immediately after surgery, meniscal extrusion decreased in both Group 1 (3.4 ± 0.8 to 2.0 ± 0.8) and Group 2 (3.2 ± 0.8 to 2.3 ± 0.8), with no significant difference between the groups. However, at the 1‐year follow‐up, extrusion increased again, with a greater increase observed in the lax healing group (2.8 ± 0.6 vs 4.5 ± 1.2, P < .001). Meniscal extrusion decreased by 0.6 ± 0.9 mm in the non‐lax healing group, whereas it increased by 1.2 ± 1.3 mm in the lax healing group, showing a significant difference between the groups. Conclusions These findings suggest that non‐lax healing is associated with reduced meniscal extrusion postoperatively, whereas lax healing showed no such improvement. Differences in healing quality, as identified using second‐look arthroscopy, appear to be closely associated with postoperative meniscal extrusion. Level of Evidence Level IV, retrospective case series.