The Morphological & Biomechanical Characteristics of the Ligamentum Mucosum and its Potential Role in Anterior Knee Pain

Abstract

IntroductionThe ligamentum mucosum (LM), or infrapatellar plica, is a non‐isometric structure of the knee joint that traverses from the intercondylar notch of the femur to the infrapatellar fat pad. The LM is composed of dense regular connective tissue, of which four different morphologies have been identified: separate, vertical septum, split, and fenestrated. Microscopically, pathological LM appears fibrotic and calcified, which can lead to impingement, and ultimately may be implicated in the pathogenesis of anterior knee pain. The specific aims of this study are to investigate the anatomical relationship of the LM to neighboring structures, determine whether the histological characteristics of the LM further define its function and its potential role in proprioception, and lastly to perform an in vitro biomechanical analysis of the LM under normal physiological loading.MethodologyFresh‐frozen cadaveric knee specimens (n=14) were dissected to reveal the structures of the joint capsule and to identify the morphology of the ligamentum mucosum. With the knee at 90° flexion, four measurements were recorded in order to examine the important relationships within the joint capsule. The intraclass correlation coefficient was calculated to determine the consistency and reproducibility of the measurements recorded. Maintaining the proximal and distal attachments, two specimens were harvested for histological analysis using standard hematoxylin and eosin, Masson's trichrome for collagen, and mono‐clonal mouse anti‐human neurofilament protein to identify neurons and peripheral nerve endings.ResultsOverall, 64% of the knees had a LM present. With respect to morphology, 50% of the 14 specimens had a separate type, 14% had a vertical septum type, and the remaining specimens had no LM present, which is consistent with known classification systems. The histological analysis confirmed the LM to be ligamentous, thus being composed of dense regular connective tissue. Preliminary immunohistochemistry results are inconclusive at this time for evidence of peripheral nerves and nerve endings, and the biomechanical properties of LM have yet to be determined.DiscussionThe role of LM in anterior knee pain is not well documented; however, studies have shown that excising the LM can relieve idiopathic anterior knee pain. With a decrease in elasticity, pathological LM may result in patellar maltracking and instability, ultimately triggering anterior knee pain. Future biomechanical testing will utilize an Instron materials testing machine to load the ligaments under tension at a constant velocity in order to determine the force at which the specimen fails. Thus, the findings from this study can be implemented in models of anterior knee pain, more specifically models of patellar instability and maltracking, to determine the proprioceptive and pathogenic mechanisms of the ligamentum mucosum.