The patella is also known as the kneecap. It sits in front of the knee joint and protects the joint from damage. It is the largest sesamoid bone in the body, and lies within the quadriceps tendon. The kneecap is an example of a bone we are all familiar with, and which has a significant functional role. In this article we will discuss the anatomy and clinical relevance of the patella.
The patella is the largest sesamoid bone in the body and it lies within the quadriceps tendon in front of the knee joint. The bone originates from multiple ossification centres that develop from the ages of three to six, which rapidly coalesce. The patella is a thick, flat, triangular bone with its apex pointing downwards. The bone has a medial and lateral border, as well as its base which lies proximally.
The patella is a dense trabecular bone with a thin compact lamina coveing it. The attachment of the quadriceps muscle if found on the superior surface extends distally onto the anterior surface. The rough marking found at the lateral and medial borders of the patella represent the attachments of vasti lateralis and medialis, and those at the apex represent the attachment of the patellar ligament.
The apex of the patella gives rise to the patellar ligament, which inserts onto the tibial tuberosity on the anterior surface of the tibia. The middle third of the patella has various vascular openings that allow for arteries to penetrate and supply the bone.
The patella is stabilized by the horizontal fibers of vastus medialis, as well as the anterior projection of the lateral femoral condyle. The tension in the medial patellar retinaculum also helps in its stability.
The primary function of the patella is during knee extension. The fact that the patella sits atop the anterior surface of the femoral condyles, increases the angle at which the quadriceps tendon pulls on the shaft of the tibia. The patella also functions to allow for smooth movement of the knee in flexion and extension, and also protects the anterior surface of the knee joint.
The blood supply to the patella arises from the genicular arteries, branches of the popliteal artery. There is:
- a superior lateral and medial
- inferior lateral and medial
- a descending and anterior genicular artery
They form a peripatellar anastomosis and supply the patella and the knee joint.
The superior two thirds of the posterior articular surface of patella articulates with the anterior surface of the femoral condyles. The lateral articulating surface of the patella is usually larger than the medial articulating surface. However the medial femoral condyle is larger and projects further anteriorly than the lateral femoral condyle.
Sinding-Larsson Johansson Syndrome
This is tenderness of the apex (lower pole) of the patella after repetitive pulling by the patellar ligament. It is one of the juvenile osteochondroses, and causes anterior knee pain. It is commonest in children aged twelve to fourteen years of age, and among those children that play a lot of sports. The same kind of symptoms have been reported where the quadriceps inserts onto the base of the patella. A similar disease process occurs at the tibial tuberosity, and is known as Osgood-Schlatter disease.
Patellar reflex also known as knee jerk is the contraction of quadriceps femoris muscle resulting in the extension of knee joint. The myotome that causes knee extension is L2-4 i.e. the femoral the lower two divisions of the femoral nerve. The patient is asked to relax their knee, and the patella ligament (below the tibial tuberosity, and above the tibial shaft) is struck.
In normal patients the knee should extend slightly. The patellar reflex may be more pronounced or diminished in some individuals and is not always a sign of neurological disease. If the knee fails to react (absent reflex), it may be a sign of a lower motor neurone lesion. If the knee extends too much and rapidly or forcefully extends (brisk/exaggerated reflex), it can be a sign of an upper motor neurone lesion.
These dislocations are relatively common among athletes, particularly young female athletes. The patella most often dislocates laterally, as the medial femoral condyle projects further anteriorly than the lateral femoral condyle. The apprehension test is utilised to determine if the patient has previously had a patellar dislocation. The clinician will apply pressure to the medial surface of the patella, and if the patient has previously had a dislocation they will express concern. If they have not had a dislocation previously i.e. not experienced the pain of the dislocation, they are less likely to be concerned.
Patellar dislocation is also common in patients who have had prolonged bed rest. The patella is pulled medially by the vastus medialis, and laterally by the vastus lateralis and the vastus intermedius. When the patient is in bed for a prolonged period, the vastus medialis wastes more than the lateral pulling muscles, and hence the patella can be dislocated laterally, due to the pull of two muscles (vastus lateralis and vastus intermedius) against one medially pulling muscle (vastus medialis).
The patella sits on the anterior surface of the femoral condyles, and is hence relatively vulnerable. Direct trauma to the knee is the commonest cause of patellar fracture. Symptoms include swelling of the knee and pain in the region. Surgery is the most usual treatment.