Knee Patellofemoral Instability
The knee can be divided into three compartments: patellofemoral, medial and lateral compartment. The patellofemoral compartment is the compartment in the front of the knee between the knee cap and thigh bone. The medial compartment is the area on the inside portion of the knee, and the lateral compartment is the area on the outside portion of the knee joint. Patellofemoral instability means that the patella (kneecap) moves out of its normal pattern of alignment in its groove. This malalignment can damage the underlying cartilage.
Patellofemoral instability can be caused because of variations in the shape of the patella or its trochlear groove. Normally, the patella moves up and down within the trochlear groove when the knee is bent or straightened. Patellofemoral instability occurs when the patella moves either partially (subluxation) or completely (dislocation) out of the trochlear groove.
A combination of factors can cause this abnormal tracking and include the following:
- Anatomical defect: congenital abnormalities in the alignment and shape of the femur and tibia can cause misalignment of the knee joint, and a shallower trochlear groove can cause maltracking.
- Improper muscle balance: Weak quadriceps (anterior thigh muscles) and hip abductors (hip side muscles) can lead to abnormal tracking of the patella, causing it sublux or dislocate.
- Ligament tear or hyperlaxity: Hyperlaxity or tears of the medial patellofemoral ligament can lead to patella subluxation or dislocation
Patellofemoral instability causes pain when standing up from a sitting position and a feeling that the knee may buckle or give way. When the kneecap slips partially or completely you may have severe pain, swelling, bruising, visible deformity and loss of function of the knee.
Your doctor evaluates the source of patellofemoral instability based on your medical history and physical examination. Other diagnostic tests such as X-rays, MRI and CT scan may be done to determine the cause of your knee pain and to rule out other conditions.
If your kneecap is only partially dislocated (subluxation), your physician may recommend non-surgical treatments, such as anti-inflammatory medications, rest, ice, physical therapy, knee-bracing, and orthotics. If the kneecap has been completely dislocated, the kneecap may need to be repositioned back in its proper place in the groove. This process is called closed reduction.
Surgery is sometimes needed to help return the patella to a normal tracking path when other non-surgical treatments have failed. The aim of the surgery is to realign the kneecap in the groove.
Patellar realignment surgery is broadly classified into proximal re-alignment procedures and distal re-alignment procedures.
Proximal re-alignment procedures: During this procedure, structures that limit the movements on the outside of the patella are lengthened and/or ligaments on the inside of the patella are reconstructed.
Distal re-alignment procedures: During this procedure, lateral force is decreased by moving the tibial tubercle towards the inner side of the knee.
The surgery is performed under sterile conditions in the operating room under spinal or general anesthesia. The surgeon will make two or three small cuts around your knee. The arthroscope, a narrow tube with a tiny camera on the end is inserted through one of the incisions to view the knee joint. Specialized instruments are inserted into the joint through other small incisions. The camera attached to the arthroscope displays the image of the joint on the monitor. A sterile solution will be pumped into your knee to stretch the knee and provide a clear view and room for the surgeon to work. With the images from the arthroscope as a guide the surgeon can look for any pathology or anomaly and repair it through the other incisions with various instruments. After the evaluation is completed, a larger incision is made over the front of the knee. Depending on your situation, a lateral retinacular lengthening may be performed. In this procedure, the tight ligaments on the outer side of the knee are lengthened, thus allowing the patella to sit properly in the femoral groove. Your surgeon may also reconstruct the ligament on the inside, or medial side of the knee to realign the quadriceps.
In cases where the malalignment is severe, a procedure called a tibial tubercle transfer (TTT) will be performed. In this procedure, a section of bone where the patellar tendon attaches to the tibia is removed. This bony section is then shifted and properly realigned with the patella and reattached to the tibia using screws. Once the malalignment is repaired and confirmed with arthroscopic evaluation, the incisions are closed with sutures.
After surgery, physical therapy is necessary to restore motion and recondition the muscles. Return to full activities is usually 5-6 months after surgery.
Risks and complications
Possible risks and complications associated with the surgery include:
- Recurrent dislocations or subluxations
- Persistent pain