The Patella (knee cap) is a small bone in front of the knee and acts as a fulcrum for the joint. It is embedded in the quadriceps tendon and, normally, has to stay centered, within the trochlea (a groove in the femur bone), when we bend our knees. A dislocation of the patella occurs when the patella moves completely out of the trochlea. If the knee cap partially comes out of the groove then it is called a subluxation. Either way, the patella is unstable and this is called Patella Instability.
Patella dislocation is commonly observed in young athletes between 15 and 30 years of age but can occur at an older age. It can occur as a result of trauma or, more commonly, as a result of anatomy mal-alignment.
In a traumatic event the patella dislocates as a result of direct blow, such as a football helmet hitting the knee. Another traumatic circumstance would be twisting injury where a stabilizing ligament ( MPFL- Medial Patella Femoral Ligament) is torn.
When the patella dislocates as a result of mal-alignment it, essentially, the patients own anatomy that is not perfectly aligned which predisposes the patient to recurrently have the patella come out of the groove. The mal-alignment can be because the hips and legs are rotated outwardly too much. Another anatomical mal-alignment can be the result of the trochlea (groove) being too shallow which would not allow the patella to engage and stay in the groove.
The common symptoms for a dislocation include pain, tenderness, swelling around the knee joint, restricted movement of the knee, numbness below the knee, and discoloration of the area where the injury has occurred. The patient would actually see the knee cap sitting on the peripheral side of the knee. Symptoms related to subluxation include pain, swelling and recurrent popping of the patella in and out. Very commonly, when the knee cap does not come out, the patient experiences recurrent deep and vague pain at the front of the knee.
Your doctor will examine your knee and suggests diagnostic tests such as X-ray, CT scan, and MRI scan to confirm condition and provide treatment.
- Typically, if a loose chip of cartilage or bone did not dislodge, the treatment for a first time dislocation is conservative. This includes nonsteroidal anti-inflammatory medications, a brace and physical therapy for rehabilitation.
If a loose chip dislodged as a result of a first time dislocation then a knee arthroscopy is performed to remove the piece and smooth down the damaged cartilage.
The treatment for those individuals who have recurrent patella dislocation is usually reconstructive. The goal is to re-align or repair structures so to prevent the knee cap from coming out. Some of the surgical options include:
- Medial patellofemoral ligament reconstruction – In this procedure, the torn MPF ligament is removed and reconstructed using grafting technique. Grafts are usually harvested from the hamstring tendons, located at the back of the knee and are fixed to the patella tendon using screws. A cadaver donor graft can also be used. This procedure can be performed using minimally invasive techniques with assistance of an arthroscopy.
- Tibia tubercle realignment or transfer – This procedure is reserved for the most serious of cases where the patients anatomy is so poorly aligned that the bone needs to be cut and moved and then fixed to help stabilize the patella.
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