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 explain the clinical relevance of the structure, and provide a summary of key points at the end of the article.
The patellar tendon runs inferiorly from the patella bone to the tibial tuberosity of the femur. 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 a sesamoid bone is the pisiform carpal bone that lies within the tendon of flexor carpi ulnaris.
The patellar tendon originates from the patellar apex and attaches to the tibial tuberosity, which is a bony protrusion on the anterior aspect of the proximal tibia. The patellar tendon could also be referred to by a different name.
A tendon is connective tissue that connects a muscle to a bone. From the perspective of muscle action, the term patellar tendon is correct. Another perspective to consider is that because the patellar ‘tendon’ connects a bone to a bone (patella to tibial tuberosity), it could be referred to as the patellar ligament. Ligaments are in place to limit movement and the patellar ligament limits flexion of the knee based on its bony attachments.
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.
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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.
Osgood-Schlatter Disease- This is an apophysitis, i.e. inflammation of a growth plate, caused by excessive pulling of the patellar tendon on the tibial tuberosity. It occurs in sporty adolescents whose tibia has not fully ossified.
Patellar tendon rupture- There are two possible types of rupture: a complete tear and a partial tear. The patellar tendon often ruptures proximally, near the patellar origin. A complete tear leads to a high riding patella and requires surgical repair while a partial tear leads to tendinopathy and can be treated conservatively.
Patellar reflex- This is a reflex arc that occurs in a relaxed lower limb, when the patellar tendon is struck. This lengthens the muscle slightly, which sends a signal to the spinal cord (L2-L4), which then sends another signal to the agonist muscle (quadriceps femoris) to contract, as well as a signal to the hamstrings to relax. The result is a slight kick of the knee. If the individual has brisk reflexes it can be a sign of an upper motor neurone lesion. Absent reflex may be a sign of a lower motor neurone lesion.
ACL reconstruction using a patellar tendon graft- In cases of anterior cruciate ligament rupture, a section of the patellar tendon (ligament) can be used as a replacement. The ligament is passed through the tibia and the corresponding surface of the femur, in the same direction as the ACL (from a region just anterior to the intercondylar eminence of the tibial, to the posteromedial surface of the lateral femoral condyle).
- The patella is a sesamoid bone that lies in the tendon of quadriceps femoris.
- The patellar tendon arises from the apex of the patella as well as its anterior and posterior surfaces.
- The patellar tendon inserts onto the tibial tuberosity.
- The patellar ligament is usually around 5 cm in length, but its length varies with knee flexion.
- The patellar tendon should be named the patellar ligament, as it connects a bone to a bone (patella to tibial tuberosity), not a muscle to a bone as a tendon does.
- Osgood-Schlatter disease occurs when quadriceps femoris pulls on the growth plate of the tibial tuberosity. It occurs most frequently in adolescents.
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