When the knee encounters a twisting force or direct injury while mildly flexed, the kneecap can dislocate laterally and then jump back into place, often with an audible pop.
Pathophysiology
When the patella dislocates, it impacts the lateral femur, causing a typical bone bruise that may extend to the lateral articular surface. The medial patellar retinaculum and medial patellofemoral ligament which help to hold the kneecap in midline position are stretched and may be partially or completely torn. Occasionally, a bone fragment at these ligament attachments to the kneecap (i.e. bone avulsion) can be be pulled away.
Causes
Patellar dislocations are seen in athletes who make fast pivot type movements with the foot planted. They are also seen non athletes who inadvertently twist the knee or fall, dancers, children with ligamentous laxity, and people with developmental anomalies of the patellofemoral mechanism (i.e. kneecap gliding along the anterior femur).
Symptoms
Patient with patellar dislocations may experience knee pain, swelling, difficulty walking, and knee instability.
Diagnosis
A physical exam by a doctor is performed to evaluate the kneecap and the integrity of the supporting ligaments.
An MRI is helpful to determine if there is loose or displaced cartilaginous or bony fragment, to evaluate the degree of injury of the medial knee ligaments, and to determine if there are morphologic changes to the anterior knee compartment that may predispose to dislocations.
Patellofemoral instability associations
- Shallow trochlear sulcus < 3mm. The groove the kneecap glides along is too shallow which allows the kneecap to easily slip out.
- Patella alta (i.e. high riding patella) where the patellar height is greater than 1.3 times the length of the patellar tendon.
- Lateralization of the tibial tubercle, onto which the patellar tendon attaches. The tibial tubercle to trochlear groove (TT-TG) distance is greater than 15 mm.
- Lateralization patellar subluxation and tilt which may be developmental or related to ligamentous laxity from prior dislocation.
Conservative Treatment
Low grade injuries: If there is no osteochondral fragment or severe ligament damage, patellar dislocations are initially treated with rest, icing, elevation, ace bandaging, and anti-inflammatory medicines. The knee may be immobilized for 3 to 6 weeks and physical therapy will then be performed to strengthen the muscles during the healing process. Often, a brace is recommended for the first several months when returning to sports.
Operative Treatment
High grade injuries: If there is an underlying developmental abnormality or ligamentous damage that will predispose to patellofemoral instability and allow recurrent dislocations or if there is significant osteochondral injury or displace fragment that may cause mechanical problems, surgical reconstruction may be warranted. Surgeries for patellar dislocations include removing loose osseous or chondral fragments, treating osteochondral injuries, tibial tubercle shift procedure to better alignment and stability, and medial patellofemoral ligament reconstruction. The surgical procedure performed will be depend on multiple factors including the MRI findings and clinical examination.