Immunohistochemical analysis of the ligamentum mucosum is the key to understand its clinical usefulness

The ligamentum mucosum (LM) is a ligamentous structure located anteriorly to the anterior cruciate ligament (ACL) within the knee joint. It belongs to the synovial layer of the knee joint capsule and is hence sometimes better known as the infrapatellar plica. Typically, it originates from the intercondylar notch of the femur and is inserted into the infrapatellar fat pat (IPFP), called Hoffa’s fat pad(Gallagher et al., 2005; Norris et al., 2018). Therefore, this structure does not suit the definition of a true ligament which states that ligaments are anchored to bones at either end(Frank, 2004).

The LM is perceived as the vestigial remnant of the embryological development of the knee (Cothran et al., 2003, Ogata and Uhthoff, 1990). However, it is hypothesized that it provides stabilization against anterior subluxation of the IPFP(Bohnsack et al., 2004) which may be the source of knee pain and instability(Howell, 2008). It may also serve as a stabilizer of the patella(Bohnsack et al., 2005) and the knee joint in addition to the ligaments and the menisci(Vahlensieck et al., 2002). It has been also found that its resection may significantly alter patellar and knee kinematics(Kambhampati et al., 2021). These functions have been attributed to its ligamentous structure which is similar to other ligaments of the knee joint(Abreu et al., 2008).

The LM has significant clinical importance. However, most previous studies have focused on its role in anterior knee pain(Boyd et al., 2005; Gallagher et al., 2005), and comparatively little research has examined its potential proprioceptive and pathogenic role in the knee joint. As such its role in anterior knee pain is only hypothetical. Other studies have examined the influence of the IFPF on revascularization after replacement of the anterior cruciate ligament (ACL)(Arnoczky et al., 1982). Indeed Kohn et al. (Kohn et al., 1995) found the IPFP and the LM to have a rich arterial blood supply.

Although many studies have addressed the nature of the LM it remains unclear whether to perceive it as an irrelevant or clinically significant structure. The main problem with examining the LM is the lack of properly planned and uniform studies. Most of the conclusions are drawn based on small clinical trials, small studies of cadaver samples or different populations. Moreover there is inconsistency regarding its prevalence that has been estimated to range from above 85% (Kim et al., 1996; Vahlensieck et al., 2002), to below 70%(Jouanin et al., 2005; Abreu et al., 2008) based on differences in age, population type, methodology or materials used. As findings fail to answer many questions regarding the LM, the present study is intended to provide a more detailed picture of its morphology to assist surgeons and other medical professionals (Gonera et al., 2022b, Gonera et al., 2022a).

The aim of the present study is to determine the morphology of the LM based on immunohistochemical analysis. It also assesses the quality of LM tissues by arthroscopic visualization during routine arthroscopic suturing of the ruptured ACL, in cooperation with a senior orthopedic surgeon. We hope this knowledge may be useful for surgeons while planning surgical procedures or clinicians while diagnosing patients who suffer anterior knee pain.

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