A plica is a common anatomical feature inside the knee joint, a fold in the synovial membrane that lines the joint capsule. This membrane produces the lubricating fluid for the knee, and plicae are remnants of the knee’s development before birth. Although they are present in many people and are considered a normal variation of anatomy, they generally do not cause issues. However, a plica can become irritated or inflamed, leading to a painful condition known as plica syndrome.
The Anatomy and Location of Plicae
A plica is a band of thick, fibrous synovial tissue that extends into the joint cavity. During fetal development, the knee is divided into three separate compartments by these membranes, which typically resorb to form a single joint space. The plicae are the leftover portions of these dividing walls.
There are four potential plicae in the knee: the suprapatellar, infrapatellar, lateral, and medial plicae. The medial plica, sometimes called the medial shelf, is the most common to cause symptoms, which is why plica syndrome is often referred to as medial plica syndrome. This specific fold is located on the inner side of the kneecap, running parallel to the thigh bone, where it is vulnerable to friction. The size of the plica can vary greatly among individuals, ranging from a thin ridge to a larger, more prominent band of tissue.
The Mechanism and Symptoms of Plica Syndrome
Plica syndrome occurs when the synovial fold becomes inflamed, thickened, and rigid due to repeated trauma or irritation. This irritation often results from overuse in activities like running, cycling, or using a stair-climbing machine. Direct blunt trauma to the knee or a sudden increase in activity level can also initiate the inflammatory process.
When the plica is swollen, it loses elasticity and can get pinched or caught between the patella and the femur, causing friction and pain. This mechanical impingement leads to localized, aching pain, typically felt on the inner side of the knee. The pain often intensifies with activities that increase load on the front of the knee, such as squatting, climbing stairs, or prolonged sitting.
A common symptom is a snapping or clicking sensation as the knee moves, particularly between 30 and 60 degrees of knee flexion. This occurs as the thickened band of tissue catches or rubs against the underlying bone. Patients may also experience a feeling of instability or a “catching” sensation in the knee when standing up after rest. Localized swelling in the joint and tenderness to the touch over the inner aspect of the kneecap are also signs of an irritated plica.
Diagnosis and Treatment Pathways
Identifying plica syndrome starts with a thorough physical examination and an analysis of the patient’s history. The physician will perform specific manual tests, such as the Hughston or Stutter test, to reproduce the clicking sensation and localized tenderness. Ruling out other potential causes of anterior knee pain, such as a meniscus tear, patellofemoral pain syndrome, or tendinitis, is essential.
Imaging techniques are used to exclude other pathologies. X-rays are helpful to look for bone abnormalities, while magnetic resonance imaging (MRI) can visualize soft tissues. However, MRI may not always clearly show a thin, irritated plica unless it is significantly thickened or there is fluid buildup in the joint. Arthroscopy, a minimally invasive surgical procedure, may be used for diagnosis, as it allows direct visualization of the fold.
Initial treatment focuses on conservative, non-surgical measures to reduce inflammation and strengthen the surrounding musculature. This includes a period of rest or modification of activities that aggravate the knee, and the use of non-steroidal anti-inflammatory drugs (NSAIDs) to manage pain and swelling. Physical therapy is a primary component, focusing on strengthening the quadriceps muscles and stretching the hamstrings to reduce mechanical stress on the knee joint.
If conservative treatments, including physical therapy, fail to alleviate symptoms after several months, a corticosteroid injection may be administered. Surgical intervention, which involves arthroscopic excision of the irritated plica, is considered a last resort. Recovery focuses on restoring full range of motion and strength through post-operative physical therapy.