The patella is triangular in shape with a superior base and inferior apex. The posterior surface is smooth, composed of articular cartilage, and is divided into medial and lateral facets. The anterior surface is rough, for attachment of tendons and ligaments.
The ossification centers of the patella appear between 3 and 6 years. They fuse at puberty, with higher levels of activity.
The medial and lateral facets of the patella are covered in hyaline cartilage and articulate with the medial and lateral condyles of the femur, respectively, to form the patellofemoral component of the knee joint. Some authors describe the medial facet as having superior, middle, inferior and lateral portions and the lateral facet as having superior, middle and inferior portions (subfacets) 4. The lateral facet is larger than the medial, which allows for side identification when the bone is placed posterior surface down on a flat surface 5.
Musculotendinous and ligamentous
The patella serves for attachment of the quadriceps tendon (superiorly) and the patellar ligament (which attaches to the tibial tubercle, inferiorly), although few quadriceps tendon fibers are continuous over the anterior surface. The quadriceps tendon and patellar ligament are really the same structure with the patella (as a sesamoid bone) embedded with it.
The medial and lateral patellar retinaculum, which are condensations of fascia rather than true ligaments, attach the patella margins to surrounding fascia. The medial patellar retinaculum attaches to the vastus medialis / sartorius fascia and is often disrupted in lateral patellar dislocation. The lateral patellar retinaculum is attached to the fascia of vastus lateralis and iliotibial band 3.
The quadricep muscles pulls the patella obliquely and laterally in relation to the femur. There are factors that prevent such displacement: larger lateral condyle of the femur, tension in the medial retinacular fibers and direction of insertion of fibers of the vastus medialis muscle.
The medial patellofemoral ligament (MPFL) originates near adductor tubercle of the femur and inserts into the superomedial aspect of the patella. Its function is to prevent lateral patellar dislocation during knee extension.
- superior: common tendon of quadratus femoris, suprapatellar bursa
- inferior: patellar tendon, infrapatellar bursa, infrapatellar fat pad
- lateral: lateral patellar retinaculum
- medial: medial patellar retinaculum
- posterior: knee joint, femur
- anterior: prepatellar bursa
Arterial blood enters via the anterior surface of the patella and an anastomotic patella ring is formed supplied by the paired superior and inferior geniculate arteries as well as the anterior tibial recurrent artery 2.
- branches of nerves to vastus medialis and vastus lateralis 6
- bipartite patella
- multipartite patella
- absent patella
- variation in shape (see: Wiberg classification)
- dorsal defect of the patella (may occasionally be symptomatic)