One of the most common non-contact knee injuries is a patellar (or kneecap) dislocation. With significant pain and functional limitation, patellar dislocations are a severe injury for athletes and non-athletes alike. The vast majority of dislocations are lateral, due to a variety of factors we will discuss below.
Anatomy: The patella normally sits in the trochlear groove (see above), which gives the patella some bony stability. Laterally the patella receives contributions from the ilio-tibial band, lateral retinaculum, lateral patellotibial ligament, lateral epicondopatellar ligament, and muscularly by the vastus lateralis(1). Tightness of these structures contributes to lateral patellar tracking. On the medial side, the vastus medialus obliquous muscle (VMO) and the medial patella-femoral ligament (MPFL) are the primary contributors to medial motion (and therefore lateral restraint). The patella is situated in the patellar tendon, a floating bone in the tendon connecting the quadriceps muscles to the tibial tubercle.
Contributing Factors to Injury: Patellar dislocation etiology is multifactorial. One anatomic variation that may predispose someone to a dislocation is a shallow trochlear groove (see below left) on the lateral side. This leads to poor anatomic support, and the kneecap is more prone to dislocate laterally.
Another anatomic adaptation predisposing someone to a patellar dislocation is anteversion of the hip, in which the femoral neck is bend forward, causing the shaft of the femur to rotate internally (see above right), which results in lateral displacement of the patella when the foot is fixed. When the femur rotates on a fixed foot, the patella stays relatively stable, as shown below. This shows the lateral translation of the patella during weight bearing and femoral internal rotation.
Left: patella with neutral hip Right: patella on trochlea with hip internal rotation
Another contributing anatomic factor that may contribute to lateral dislocations is a large Q-angle, which is the angle between the ASIS of the pelvis to the center of the patella to the tibial tubercle (See below). A large Q angle has been linked to increased internal rotation of the femur.
An increase in Q angle could occur due to deficiencies above and/or below the patella. Weakness of the hip external rotators (gluteus maximus and posterior gluteus medius(PGM)) leads to increased internal rotation. This will occur generally with an imbalance of abductor strength, as the TFL (a hip internal rotator) is overused in relation to the PGM. Below the patella, internal rotation of the tibia as occurs during pronation of the foot will also increase the Q angle (4).
Women are more common dislocators than men (2). This is due to a number of factors, including wider pelvis’ which contributes to a larger Q angle(12 vs 15-16 degrees for men vs women), altered muscular activation of hip external rotators during jumping and cutting tasks(3,4), increased joint laxity, and a relative predisposition to cut and land on a straighter knee (2). Due to the patella being in it’s loose-packed position on a straight knee, the patella will have a predisposition to dislocate when the knee is straight versus bent.
Lastly, patellar dislocation may also be due to trauma, usually due to a laterally displaced blow to the kneecap causing a lateral dislocation.
In summary, there are numerous factors which may predispose people to dislocation, a number of which can be addressed with physical therapy. The treatment of acute dislocation will be discussed in the next post.
- Fithian et al. Patellar Dislocation: A Natural History. Am J Sports Med. 2004;
- Homaechevarria A. Patellar instability. St Lukes Sports Medicine Conference. March 17, 2012
- McConnell J. Rehabilitation and nonoperative treatment of patellar instability. Sports Med Arthrosc 2007;15(2):95–104.
- Powers CM. The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Ortho Sports Phys Ther. 2003; 33(11): 639-646