Patella

the triangular sesamoid bone embedded within the tendon of the quadriceps femoris muscle at the knee. The patella articulates with the femur. Also, see patella.

2. Pathology

This bone may be involved in several pathologic conditions.

In some cases the patella may be partitioned into two or more segments (bipartite patella). Often this condition is bilateral and involves the superolateral aspect of the bone. On radiographs a bipartite patella may be mistaken for a patellar fracture.

Dorsal defect of the patella is a condition that may result from a variation in normal ossification of the bone; alternatively, it may be related to traction occurring in the insertion site of the vastus lateralis muscle. On radiographs it appears as a radiolucent area in the superolateral aspect of the bone.

Patella alta and patella baja are abnormalities of patella position occurring in various disorders. Patellofemoral instability occurs frequently and may also be related to the position or shape of the patella. The patella is pulled laterally from the trochlea and across the lateral femoral condyle, which may result in osteochondral injuries of the medial patellar facet or lateral femoral condyle. Sometimes a small avulsion fracture occurs at the patellar site of attachment of the medial retinaculum.

Chondromalacia patellae represents a syndrome of pain and crepitus over the anterior aspect of the knee resulting from loss of cartilage over one or more portions of the patella. MR imaging is perhaps the best noninvasive diagnostic technique for assessment of cartilage loss, but arthrography, particularly in association with CT scanning, can be valuable. Chondromalacia patellae may be associated with patella alta or osteoarthritis of the patellofemoral compartment.

Plicae are synovial remnants found in the knee that divide the joint into three compartments. Suprapatellar, medial patellar and infrapatellar plicae are commonly encountered and can be identified by various means, such as arthrography, computed arthrotomography, MR imaging or arthroscopy. These plicae may cause symptoms if they become thickened (plica syndrome) or persist in embryonic form (as complete septa).

In osteoarthritis, typical and severe abnormalities are observed on the posterior surface of the patella, most frequently the lateral facet. A degenerative enthesopathy occurs on the anterior surface of the patella, consisting of enthesophytes at the site of osseous attachment of the quadriceps apparatus, termed the tooth sign or whiskering.

Traumatic dislocation of the patella may result from a direct blow or an exaggerated contraction of the quadriceps mechanism. Predisposing factors include patella alta, deficient height of the lateral femoral condyle, shallowness of the patellofemoral groove, genu valgum or recurvatum, lateral insertion of the patellar tendon, muscular weakness and excessive tibial torsion. Osteochondral fractures of the medial patellar facet and lateral femoral condyle are important diagnostic clues.

Patellar fractures result from direct or indirect forces, the latter related to contraction of the quadriceps muscles. The majority are transverse fractures. Ischaemic necrosis is a complication of patellar fractures, involving the proximal fragment. Fractures should be differentiated from bipartite patellae, in which separate ossification centres develop in the superolateral aspect of the bone. In addition, fragmentation and separation of the lower pole of the patella is referred to as Sinding – Larsen – Johansson disease and represents a stress-related phenomenon.

Osteochondritis dissecans of the patella, although rare, may occur if the articular surface is exposed repeatedly to trauma or a tangential shearing force. Other conditions in which patellar abnormalities may occcur are Paget's disease, diffuse idiopathic skeletal hyperostosis (DISH) and haemophilia. Complications of total knee replacement may also affect this bone.

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