Alar folds

From the medial and lateral borders of the articular surface of the patella, extensions of synovial membrane covering the infrapatellar fat pad project into the interior of the joint. These form two fringe-like folds termed the alar folds (or plicae):

• The medial alar fold is a longitudinal fold just medial to the patella. It is an inconsistently present fold that is wide and very soft. Triangular at the cross-section, it is thought to be the folding of synovium in the relaxed position of retinaculum. The medial alar fold has also been called the plica alaris elongata, plica alaris, alar ligament, semi-lunar fold, mediopatellar pseudomeniscus, and patellar meniscus.

• The lateral alar fold is inconstant, located on the lateral part of the patella.

The alar folds must not be confused (https://ajronline.org/doi/10.2214/ajr.177.1.1770221#FIG15) with the medial and lateral plicae. These structures are inconstant synovial folds (cordlike elevation in the synovial wall on MRI):

• Medial patellar plica: This plica is located on the medial side of the knee and is present in about 72% of knees. It can vary in size and thickness and is the most commonly symptomatic plica due to its potential to impinge on the medial femoral condyle or patella.

• Lateral patellar plica: This is the least common type, found in only about 1.3% of knees. It is located on the lateral side of the knee and is rarely symptomatic.

Below, the alar folds converge and are continued as a single band, the infrapatellar synovial fold (ligamentum mucosum), to the front of the intercondyloid fossa of the femur.