Work carried out at the dept of orthopaedics Princes Royal Hospital

Case Report

A fit and healthy eighteen year old lady injured her right knee when moving from a squatting position to fully erect. The knee became swollen over the next 24hrs and she had difficulty walking due to medial knee pain. On initial presentation clinically the knee was found to have a large effusion with range of movement from 30-70 degrees only limited by pain. Radiographs taken at presentation demonstrated no bony injury. Initially the diagnosis was unclear and urgent physiotherapy was commenced. At clinic follow up the knee still lacked full extension with more localised anteromedial joint line tenderness and a clinical diagnosis of displaced meniscal tear was made. Acute arthroscopy was performed revealing a torn ligamentum mucosa (Fig 1) in an otherwise pristine knee with an intact ACL (Fig 2).

The torn ligamentum was debrided using a shaver (Fig 3).

Following surgery the block to extension resolved and the knee settled quickly with physiotherapy.

Figure 1

Figure 1: Ligamentum Tear

Figure 2

Figure 2: Intact ACL

Figure 3

Figure 3: Debridment of Ligamentum

Discussion

The ligamentum mucosa is also referred to as the ligamentum patellae, inferior patella plicae, infrapatella fold or septum. It is believed that the plicae of the knee are developmental remnants from the mesenchymal tissue that fills the notch during embryonic development and in fact there can be a complete septum between the two tibiofemoral joints of the knee. [₁]

Pathology and symptoms arising from the ligamentum mucosa are not well described in the literature. The presence of this ligamentum can make visualisation of the notch and ACL difficult during arthroscopic surgery. The ligamentum mucosa has been described as a cause of anterior knee symptoms and when thickened in chronic anterior knee pain its removal has been advocated. [₂, ₃] Indeed it can become an arthrofibrotic band after surgery, for example ACL reconstruction, giving sharp anterior knee pain limiting flexion.

There have also been cases of acute haemarthrosis following ruptured ligamentum mucosa alluded to in the literature but it has not to our knowledge been previously described as a cause of locking or mimicking a meniscal tear. [₂]

Genuine locked knees have a mechanical cause and arthroscopy has been advocated for the investigation and treatment of knee problems where definite clinical signs are demonstrated. [₄, ₅] Previous reports have demonstrated rare pathologies causing knee locking including pigmented villonodular synovitis, intra articular ganglion, gouty tophi and posterior cruciate ligament rupture. [₆, ₇, ₈, ₉, ₁₀]

Our case is of topical interest as it mimicked a meniscal tear, the resection of which lead to a resolution of symptoms with no long-term complications.