Ligament of anterior horn of medial meniscus of knee joint: case reports and literature review.

BACKGROUND: The meniscus is essential for knee joint biomechanics, contributing to load distribution and stability. Variations such as the anterior medial meniscofemoral ligament (AMMFL) originating from the anterior horn of the medial meniscus (AHMM) are rare but significant. These variations may predispose individuals to medial meniscus injuries and knee dysfunction, though their biomechanical implications remain unclear. The AMMFL disrupts normal anchorage of the AHMM to the tibial plateau, increasing meniscal mobility and potentially altering load transmission and predisposing to degeneration or tears.

CASE PRESENTAION: Case 1: A 49-year-old woman presented with left knee pain without trauma. MRI identified a medial meniscus posterior root tear (MMPRT). During arthroscopy, an AMMFL was discovered connecting the AHMM to the intercondylar notch. The MMPRT was repaired with transtibial suture fixation, while the AMMFL was retained due to its non-impinging nature. Quantitative MRI revealed an AMMFL measuring 3 mm × 2.1 mm with low signal intensity. At 10-month follow-up, the patient reported significant pain relief and restored knee function (e.g., resolution of squatting difficulty, KOOS score improved from 48 to 84). Case 2: A 58-year-old woman with chronic right knee pain and osteoarthritis underwent unicompartmental knee arthroplasty. During surgery, a free AHMM attached to the intercondylar notch via a tendon-like AMMFL was observed and excised. Intraoperative measurement showed a 27 mm ligament-like structure, confirmed histologically as dense parallel collagen bundles without fibrocartilage. At 8-month follow-up, the patient walked freely; full weight-bearing achieved by 4 weeks (Oxford Knee Score improved from 19 to 39). Treatment differed based on impingement status and degenerative context: retention after root repair in Case 1 versus excision during UKA in Case 2. Both cases revealed below-average medial posterior tibial slopes (MPTS, case 1 is 4.49° and case 2 is 2.17°), questioning the biomechanical role of AMMFL in MMPRT development.

CONCLUSION: AMMFL is a rare anatomical variant with unclear biomechanical roles. The abnormal attachment of the AMMFL could potentially alter normal meniscal kinematics and load distribution. This anatomical configuration may be associated with posterior root tears even in knees with a low tibial slope, suggesting a possible biomechanical vulnerability that warrants further investigation. This potential biomechanical vulnerability-inferred from the anatomy and the presentation of MMPRT with low MPTS-highlights the importance of meticulously evaluating the posterior horn and root attachment in patients with an AMMFL on MRI. Although a causal link remains unproven, the association suggests these patients may be prone to tears despite the absence of other traditional risk factors. Awareness of this variant is crucial for accurate imaging diagnosis and arthroscopic evaluation, as it may identify knees at biomechanical risk for posterior root tears, warranting further investigation.