Quadriceps femoris is a four-headed muscle that inserts onto the tibial tuberosity. It extends the knee, and one head (rectus femoris) flexes the hip. The patella is a sesamoid bone that lies within the quadriceps tendon. The patellar tendon connects the apex of the patella to the tibial tuberosity, and improves the way the quadriceps muscle pulls on the tibia. In this article we will discuss the gross and functional anatomy of the patellar tendon. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The patellar tendon runs inferolaterally from the patella bone to the tibial tuberosity. The patella is a large sesamoid (a bone within a tendon) bone, with a triangular transverse cross-section, that lies within the quadriceps tendon. Another example of sesamoid bone is the pisiform carpal bone that lies within the tendon of flexor carpi ulnaris. The patellar tendon runs from the apex, adjoining margins, distal posterior surface and rough anterior surface of the patella to the tibial tuberosity, which is a small bony bump on the anterior aspect of the tibia. The patellar tendon is technically not named correctly. A tendon is connective tissue that connects a muscle to a bone, and the patellar ‘tendon’ in fact connects a bone to a bone (patella to tibial tuberosity). The correct name is therefore the patellar ligament. The patellar ligament is approximately 5 cm in length. However, its length is not constant and mostly increases from full extension to 30 degrees of knee flexion.
The medial and lateral parts of quadriceps femoris descend on either side of the patella and are inserted onto the upper anterior surface of the tibia. They merge into a continuous capsule, and form the medial and lateral patellar retinacula. The posterior aspect of the patellar ligament is separated from the knee joint by an infrapatellar fat pad and a synovial membrane. An infrapatellar bursa also separates the patellar ligament from the tibia.
The function of the patella is to increase the length of the lever arm of the patellar tendon and therefore allow quadriceps femoris to exert a higher moment around the axis of rotation of the knee for a given level of muscle contraction than in the absence of a patella. The patella, whose peak thickness is between 2 and 3 cm, sits against the femur at a location which depends on the degree of knee flexion. This increase in lever arm ensures that knee extension is more efficient, and the action of quadriceps femoris is clearly transmitted through to the tibia.