Medial Meniscus Posterior Root Shows More Severe Histologic Degeneration in Female Patients With Advanced Osteoarthritis and Varus Alignment: A Histopathologic Analysis of Human Menisci

Background Medial meniscus posterior root (MMPR) tears are more commonly observed in female patients with preexisting osteoarthritis (OA); however, the histopathologic basis for medial root predilection and female predominance remains unclear. Questions/purposes (1) Is histopathologic degeneration associated with knee OA that selectively affects the MMPR? (2) Is greater histologic degeneration of the meniscus root insertion observed in female patients compared with male patients? (3) Do patients with varus alignment and severe OA exhibit greater tissue degeneration at the meniscus root insertion? Methods This histologic study analyzed menisci harvested from patients with OA undergoing TKA between August 2024 and July 2025. Patients with a history of prior meniscal surgery, rheumatologic or gouty arthritis, or previous cruciate or collateral ligament injury and those who declined participation were excluded. Of 122 eligible patients, 1% (1) declined, the meniscal root could not be retrieved in 18% (22), and tissue quality was insufficient in 11% (13). The final cohort included 86 patients, yielding 138 posterior meniscal roots: 69 MMPR and 69 lateral (LMPR). Of those, 63% (54) of patients were female. Mean ± SD age did not differ between the MMPR and LMPR groups (72 ± 7 versus 73 ± 7 years; p = 0.28). The severity of OA, assessed by Kellgren-Lawrence (KL) grade, was similarly distributed (MMPR Grade III 35% 24 of 69, Grade IV 65% 45 of 69 versus LMPR Grade III 38% 26 of 69, Grade IV 62% 43 of 69; p = 0.85). Mean ± SD hip-knee-ankle angle was comparable (MMPR 7° ± 6° varus, LMPR 8° ± 6° varus; p = 0.33). American Society of Anesthesiologists (ASA) classification was similar, with 17% of patients with MMPR and 14% of those with LMPR classified as having ASA III, and the remainder as having ASA II. Sagittal root insertions were stained with hematoxylin and eosin and Masson trichrome. Meniscal degeneration was evaluated using the modified Pauli score, a semiquantitative histologic grading system ranging from 0 to 12, with higher scores indicating greater tissue degeneration. Paired comparisons were performed in a subgroup of 52 patients in whom both menisci were analyzed. Linear regression was used to assess associations between histologic degeneration, demographic and clinical variables, and lower-limb alignment within each root group. Results Histologic degeneration was more severe in MMPRs than LMPRs (mean ± SD modified Pauli score 6.6 ± 2.6 versus 2.7 ± 1.6; p < 0.001), particularly in female patients (6.3 ± 2.5 versus 2.9 ± 2.1; p < 0.001). In 52 patients with paired samples, medial roots again showed more severe degeneration (5.3 ± 2.6 versus 2.2 ± 1.9; p < 0.001), with the shiny white fibers of the medial insertion most severely affected, especially in female patients. A moderately strong correlation was observed between the degree of varus alignment and the severity of MMPR degeneration, with greater varus associated with more severe degeneration (r = 0.54; p < 0.001). After controlling for potentially confounding variables such age, severity of preexisting OA, and lower-limb alignment, we found that male sex was associated with less severe degeneration (β = -2.63 95% confidence interval (CI) -3.90 to 1.36; p < 0.001), whereas KL grade IV was associated with more advanced root degeneration (β = 1.87 95% CI 0.59 to 3.13; p < 0.001). Conclusion OA was associated with more severe MMPR degeneration, particularly within the shiny white fibers, with the greatest degree of degeneration observed in female patients with advanced OA and in varus-aligned knees. Clinical Relevance These OA-related histopathologic changes may contribute to the higher incidence of medial root tears in female patients presenting with preexisting OA and varus malalignment. Moreover, highlighting the histopathologic changes of root insertional degeneration in the presence of additional risk factors may help identify, prevent, and manage MMPR injuries more effectively. These findings may ultimately contribute to improved patient care and long-term success in meniscal root repair.