The knee can be divided into three compartments: patellofemoral, medial and lateral compartments. The patellofemoral compartment is the compartment in the front of the knee between the patella and femur. 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. This malalignment can damage the underlying cartilage.
Patellofemoral instability can be caused by variations in the shape of the patella or its trochlear groove as the knee bends and straightens. 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- Flat feet or fallen arches and congenital abnormalities in the shape of the patella can cause misalignment of the knee joint.
Abnormal Q angle-The high Q angle (angle between the hips and knees) often results in abnormal tracking of the patella such as in patients with knock knees.
Patellofemoral arthritis- Patellofemoral arthritis occurs when there is a loss of the articular cartilage on the back of the kneecap. This can eventually lead to abnormal tracking of the patella.
Improper muscle balance- Weak quadriceps (anterior thigh muscles) can lead to abnormal tracking of the patella, causing it to subluxate or dislocate.
Young active individuals involved in sports activities are more prone to patellofemoral instability.
Patellofemoral instability may cause 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 will evaluate 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 pain medications, rest, ice, physical therapy, knee-bracing, and orthotics. If the patella has been completely dislocated, the patella 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.