The knee is the largest joint in the body. It is a specialised hinge joint made up of four main things: bone, cartilage, ligaments and tendons. The main movement that occurs in the knee joint is flexion and extension. It does however allow for a small amount of rotation and gliding on the surfaces.
The patella is the small bone located at the front of the knee joint also known as the kneecap. The role of the patella is to protect the knee joint and to enable the quadriceps muscle to work with more strength. At the front of the knee there is a groove (patellofemoral groove) that provides a space for the patella to sit. This groove allows the patella to glide up and down over the knee joint as the knee bends and straightens. The patella is held in position by tendons at its top and bottom surfaces. Ligaments on either side play important role in maintaining the alignment of the patella
Instability of the patella results from the patella sliding too far to either side of the groove or out of the patellofemoral groove altogether known as dislocation.
Patella instability can occur at any age. The most common causes of patella instability in children include:
- A sharp blow to the patella, such as a fall, may push the patella out of place. For children with normal knee structures, this is a common cause of patella instability. It is often the result of high impact sports such as football.
- Movements associated with sports or activities that involve sudden stopping such as dancing or twisting on the knee such as the motion of swinging a bat.
- The femoral grove in the femur, that the patella sits, in may be uneven or shallow. This leads to higher risk the patella will slip out of place.
- Ligaments in some children are loose, leading to instability and dislocation of joints. This is more common in girls than boys and can affect both knees.
- Children may be born with patella instability. This usually results in the patella dislocating without pain, however this is rare.
The most common symptoms of patella instability in children are:
- Visual appearance of the knee – often the patella spontaneously returns to the patellofemoral groove following dislocation, however, if not, the patella will appear to the side of the knee, most often the outer side.
- The knee buckles or gives way
- A popping sound when the patella dislocates
Where the patella is dislocated and has not spontaneously returned to the patellofemoral groove, the patella will need to be corrected. This is known as reduction and will normally be performed an experienced health professional.
Diagnosis of patella instability without visible signs of dislocation involve assessment of symptoms and clinical assessment of the knee’s movement.
An x-ray will assist your doctor to assess how symmetrically the patella sits in the patellafemoral groove.
A CT scan may be performed to look at how your bones are developing. This is often a useful scan for planning surgery if realignment of the bones is necessary.
A MRI may also be required to check for any injury to surrounding tissues following the initial injury.
Initial patella injuries should be treated with:
Where patella injury or dislocation occurs as a result of a once off event such as a sporting injury, the knee may be treated conservatively. Conservative measures may include:
- Bracing to immobilize
- Crutches to reduce weight bearing
- Physiotherapy to strengthen the leg muscles to hold the patella in place to prevent future dislocations.
An unstable patella that has not responded to non-surgical treatment may require arthroscopic surgery to repair or tighten structures within the knee to hold the patella firmly within the patella-femoral groove. Also, surgery may be required if the smooth cartilage of the femur or patella has been damaged at the time of the dislocation. Depending on the reason for surgery the exact procedure will differ, but your surgeon will discuss the the options with you.