Medial parapatellar plicae are found in up to 79.9% of patients undergoing knee arthroscopy but are asymptomatic in most cases [17]. Sakakibara proposed a four-type medial parapatellar plicae classification [23]. Type A, cord-like (8.89%); type B, shelf-like but without medial femoral condyle coverage (35.56%), type C, large shelf-like, which covers the anterior medial femoral condyle (51.11%); and type D, characterized by a double insertion (4.44%) [23].
Type D medial parapatellar plica is the most infrequent variation of medial plicae, but it accounts for 10% of the symptomatic cases [8, 10, 17, 20, 23]. Type C and D plicae, are more likely to be symptomatic, and they have been correlated with anteromedial patellofemoral cartilage damage in up to 82.9% of the patients, ranging from ICRS I to IV injuries [2, 7, 8, 10, 21]. The present case report portrays a symptomatic medial parapatellar plica type D with a characteristic medial femoral condyle cartilage "sanding-mark"-like ICRS grade I injury [8], a surgically confirmed cause of anterior knee pain.
The embryological origin of the knee plicae is still controversial [12, 23]. However, the most accepted theory establishes the development of menisci, cruciate ligaments, and a partitioned joint cavity in the eight-week fetus, with the subsequent fusion of the joint knee compartments and resorption of the synovial septum by the end 12th week. In cases of incomplete resorption of the synovial septum, residual mesenchymal tissues result in the knee plicae [5, 6, 12, 13, 19, 26].
The reduplicated medial parapatellar plica is a pair of oblique shelf-like synovial pleats. They extend from the genu articularis muscle to the knee joint medial wall to the infrapatellar pad with a double insertion. The plica presents two borders, medial and lateral; two extremities, superior and inferior; and two surfaces, anterior and posterior. The medial margin origins from the medial patellar process, and the lateral or free border is concave towards the center of the joint. The superior extremity arises from the suprapatellar plica or neighboring synovial tissues, while its inferior extremity expands to the infrapatellar fat pad. Thus, the plica lies between the medial patellar articular surface and the medial femoral condyle [5, 8, 9, 17, 23, 25]. Histologically, it shows fibrous and chronic inflammation characteristics, most probably due to repetitive trauma during knee motion [2].
Additionally, its thick and stiff consistency impinges through its free edges on the anterior facet of the medial femoral condyle and medial patella [17]. The plicae engage with the medial femoral condyle between 30–60° of knee flexion, starting with the inner flap of type D plica and even locking into the patellofemoral joint [10, 14]. This repetitive motion results in tenderness, swelling, snapping, locking sensation, potential chondral injuries, and further degeneration in the patellofemoral joint [5, 10, 12, 13].
Magnetic resonance imaging helps guide the diagnosis, showing sensitivity and specificity as high as 93% and 81%, respectively [10, 16, 24]. Our findings reflect the characteristic magnetic resonance findings, including the presence of a low-intensity cord or shelf on T2-weighted sagittal and axial images, especially during knee effusion [8]. Conservative treatment is the initial approach, including rest, pain and anti-inflammatory medication, physiotherapy, and corticosteroid injections, showing satisfactory outcomes, especially for younger patients with acute symptomatology and without cartilage involvement [1, 4, 15, 18].
The presence of a type C and D medial parapatellar plicae has been associated with conservative treatment unresponsiveness, longer duration of symptoms, and associated patellofemoral cartilage injuries such as the patient presented in the current case report [10]. A meta-analysis by Gerrard et al. [5] reported favorable patient outcomes in medial parapatellar plica syndrome patients undergoing arthroscopic excision after failed conservative treatment. In the present study, arthroscopic media plica excision led to complete symptoms resolution at one month postoperatively, and the patient was then allowed to return to full activity, with no recurrence at the last follow-up, one year postoperatively.
A certain limitation of the study is the absence of pre and post operative patient reported outcome scores to objectively document the improvement. However, the patient post-operatively reported complete symptoms resolution and full return to previous activities. Moreover, primary aim of the study was not the clinical outcomes but to document the clinical relevance of this intra-articular anatomical variation of the knee.
This case report is helpful for radiologists and orthopaedic surgeons to recognize this special plica type on MRI and discriminate from other types. Differential diagnosis of duplicated plica from other types is important for daily clinical orthopaedic practice during decision-making with patients. As this case showed, the duplicated plica is recalcitrant to conservative whereas arthroscopic resection provides definite treatment followed by a short rehabilitation timeline to resume pre-operative activities.