The Anatomy of the Patella

It protects the knee joint from damage

In This Article

The patella, most commonly referred to as the kneecap, is the largest sesamoid bone in the body. A sesamoid bone is one that is embedded in a tendon and, in the patella’s case, it exists within the quadriceps tendon. This tendon helps hold the patella in place along with other muscles found in and near the quadriceps so it can do its most important job, protect the knee joint.

Anatomy

The patella bone goes through the process of ossifying (turning into bone) between the ages of three and six years old.

In its whole, complete form the patella is a flat, triangular-shaped bone that is fairly dense.

The apex, or point, of the patella points downward towards the shin while the base of the bone sits upward towards the thigh. The base is also what attaches to the quadriceps tendon.

The back of the patella forms a joint with the femur, otherwise known as the thighbone. This makes the femur become a joint facet, as it also connects with a medial and lateral condyle of the femur. The patella is also attached by the vastus lateralis and medialis, which are muscles in the thigh.

From the apex of the patella comes the patellar ligament, which connects to the front of the tibia (shin bone). The middle of the patella consists of openings for arteries to supply blood to the kneecap. Muscles in the thigh, parts of the femur, and a tendon in the knee, referred to as medial patellar retinaculum and that crosses the knee joint in the middle of the patella, all work together to keep the patella stabilized.

Location

The patella sits between the femur and tibia, not only protecting the knee joint but connecting muscles in the front of the femur to the tibia. Under the patella and the at the end of the femur is articular cartilage, which makes it possible for the patella and femur bones to move alongside each other. This cartilage offers protection on top of added mobility with any knee movements.

Anatomical Variations

Most of the anatomical variations of the patella have to deal with the shape of the bone itself. These include:

  • Hypoplasia of the patella (often referred to as patella parva), where the patella is smaller than it should be or, in rare cases, completely absent.
  • Hyperplastic properties, where the patella grows larger than it should be for the space it lives in (known as patella magna).
  • Hunter’s cap patella, where the lateral side of the patella takes up the anterior surface of the bone.

There are three types of anatomical variations in a healthy patella based on the size and symmetry of the patella itself.

Measured by the Wiberg classification system, it's found that 10% of people have a type I patella (nearly symmetrical), 65% of people have a type II patella (a flat medial facet that’s much smaller than the lateral facet) and 25% of people have a type III patella (a small medial facet that is also curved).

Function

The main job of the patella is to help with knee extension and movement, while offering protection for the knee joint.

In the case of knee extension, this movement happens by the patella offering leverage that the quadriceps tendons it's attached to can put on the femur. In terms of protection, the patella gives the front of the knee joint support during activities like exercise or even from daily wear and tear from walking and stepping.

Associated Conditions

Patella injuries can be relatively common, particularly in those who are extremely active and put a lot of extra stress or pressure on the bone through running, exercising, and competitive sports. Two of the most common conditions related to the patella are patellar tendonitis and patella dislocations.

Patellar Tendonitis

Patellar tendonitis is also commonly referred to as jumper’s knee, as it is more likely to occur in sports or activities that require a lot of jumping like basketball.

Those who may be suffering from patellar tendonitis will experience pain in their patella, which will start when doing selected activities and may increase to interfere with daily movements, such as walking and going up and down stairs. This pain is caused by tiny tears happening in the patellar tendon which causes inflammation and weakening.

Patella Dislocations

For a patella dislocation, the patella will slip outside its grooved position inside the leg. This causes pain and swelling, as well as possibly tearing the ligaments that hold the patella in place from popping out of its location.

Some people may not have a complete patella dislocation, but they can experience a patellar subluxation. This occurs when the patella doesn't dislocate from its groove entirely but does have difficulty moving within its groove tracking which causes pain and swelling.

Rehabilitation

For patellar tendonitis, there are a number of ways to treat and rehabilitate the patella. Depending on the severity of the pain and tendonitis options range from over-the-counter medications like ibuprofen to physical therapy to help stretch and strengthen the muscles and tendons surrounding the patella.

For more serious cases, your doctor may opt for a corticosteroid injection to help relieve pain or even surgery if repairs to the tendons surrounding the patella need to be made.

For a patella dislocation, the only way to remedy this condition is to relocate the patella back into its groove. This may happen quickly on its own shortly after dislocation or with the assistance of a medical professional.

With dislocation, tendon tearing and fragments of bone may come loose. While tendon tears usually repair on its own, bone fragments will likely need to be removed surgically. If there are no fragments resulting from dislocation the usual treatment consists of immobilizing the knee until the swelling decreases (roughly three to six weeks) along with taking non-steroidal anti-inflammatory medications (NSAIDs) to help with any pain or discomfort.

For those who are experiencing patellar subluxation, treatment ranges from physical therapy to braces and tape to help guide the patella. Surgery may also be required depending on how severe the misalignment is or if it leads to frequent patellar dislocations.

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Article Sources

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  1. Cox Chandler. Hubbard John. Anatomy, Bony Pelvis and Lower Limb, Knee Patella. Treasure Island, FL: StatPearls Publishing; 2019.

Additional Reading

  • American Academy of Orthpaedic Surgeons. Patellar (Kneecap Fractures). Updated January 2017.

  • Kenhub. Patella. (n.d.)

  • Mayo Clinic. Patellar Tendinitis. Updated April 2018.

  • Teach Me Anatomy. The Patella. Updated December 2017.

  • University of Connecticut Health. Patellar Dislocation. (n.d.)