Hypertrophied medial parapatellar plica: a case of a medial plica anatomical variation with insertion to the inter-meniscal ligament in an adolescent athlete treated arthroscopically

Synovial plicae are normal folds within knee joint, which are present in about 50% of individuals [5]. The medial parapatellar plica is common to cause mechanical symptoms, depending on its position, size and elasticity [6]. The embryological development of the knee plicae is still controversial [2, 6]. An old theory describes the existence of a cartilaginous centre, which will be developed the articular cavity of the knee [4]. In this theory, the centre has three layers, two cartilaginous and one intermediate layer. The development of the intermediate centre is complex, creating the medial and inferior plicae, as well as the meniscus and cruciate ligaments [4]. However, the most common theory support that the formation of knee joint started as multiple cavities, patellofemoral, femoromeniscal and meniscotibial, in the 8th week of embryogenesis, with subsequent fusion of those cavities by the end of 12th week. Any incomplete resorption of those mesenchymal cavitations result in the knee plicae [6].

Multiple proximal attachments have been described in the literature for the medial parapatellar plica. The most common insertion is in the medial wall of the synovial pouch, under the medial retinaculum or to the genu articularis muscle. However, according to the literature the distal attachment is constant at the infrapatellar fat pad [1, 3] or may be continuous with the anterior margin of an infrapatellar plica [9]. From a phylogenetic point of view, infrapatellar plica is a remnant between the femoro-tibial and femorofibula articulations, which is present in reptiles and amphibians [9]. In the present case, the hypertrophied medial parapatellar plica typically arised from the medial wall of the synovial pouch, but it was extremely hypertrophied in the medial patellofemoral compartment and presented an atypical distal insertion into the inter-meniscal ligament. This anatomical variation could be characterized a combination of medial and infrapatellar plica, with alterations in separation and resorption during embryonic development. The origin of the infrapatellar plica is usually from the intercondylar notch in the femoral footprint of the anterior cruciate ligament and attaches onto the synovium around the infrapatellar fat pad [9].

Histologically, normal plicae are characterized by a lining of synovial cells, while pathologic plicae frequently show increased vascularity and appearance of inflammatory cells [9]. However, in the current case, histological findings revealed a lining of single or reduplicated synovial cells, the specific characteristics of a normal plica, on a stroma of generalized fibrosis, as well as inflammatory cells into the fibrous tissues indicating the sub-acute phase of the reaction.

Occasionally, medial plica becomes symptomatic, as a result of overuse or history of injury [5]. This can be caused during repetitive knee motion, between 30º to 60º of flexion, with an engagement of the plica to the medial femoral condyle [2]. Despite the fact that this condition can cause anteromedial patellofemoral cartilage damage in up to 80% of the patients [2], the present case report did not reveal any cartilage involvement maybe due to the young age of the patient and the more elasticity of the tissues. Some patients may present with a sensation of mild medial or/and anterior knee pain, while others may exhibit crepitus, snapping or popping, effusion and joint locking [2, 5, 7]. In our case, anterior knee pain and locking knee was the predominant symptoms, due to a twisting injury during handball training session.

In addition, it is crucial to differentiate medial plica syndrome symptoms from other pathologies mimicking this condition, such as meniscus tear and patellofemoral syndrome-like symptoms [2, 9]. The diagnosis of medial plica syndrome is therefore based on non-specific clinical symptoms, MRI findings and clinical suspicion. Also, MRI can determine if there are multiple pathologies that can contribute to this condition, while can help guide the diagnosis with a sensitivity and specificity of 93% and 81%, respectively [2]. In the current case, both a hypertrophied anatomic variation of medial parapatellar plica and a horizontal medial meniscus tear was present. Therefore, the primary source of symptoms was not clear. However, the uncommon hypertrophy of the plica along with its atypical course and distal attachment is a rare anatomical variant. Apart from the anatomical interest, radiologists and orthopaedic surgeons should be aware of this anatomical variation as it may be found in a knee MRI or arthroscopy.

Initial treatment management for the medial parapatellar plica is conservative approach, including rest, cryotherapy, anti-inflammatory medication, physiotherapy and modification of activities of daily living [2]. However, surgical treatment with arthroscopic excision should be considered after failed conservative treatment, when pain persists over a long period [2], or/and when the medial plica covers part of medial femoral condyle [5]. In such cases, surgical treatment reduces the potential for erosion of the cartilage surface [3]. In the present case, immense arthroscopic treatment was applied based on both clinical appearance and MRI findings of the patient. Post-operatively, complete resolution of symptoms and resume of daily and sports activities was achieved.