A 45-year-old woman presents to the emergency department with right knee pain and deformity after falling down the stairs. A sunrise view radiograph (Figure 1) shows a lateral patella dislocation.
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The medial patellofemoral ligament (MPFL) attaches the medial patella to the medial femur and is the primary restraint against lateral dislocation of the patella. Cadaveric studies show that the MPFL provides 50% to 60% of the total restraining force against lateral dislocation. Other structures that provide a lesser role in preventing lateral instability include the medial retinaculum and the medial patellotibial ligament. Patella dislocations generally occur with the knee in extension and the leg externally rotated. Risk factors for patella instability include patella alta, trochlea dysplasia, increased Q angle, and ligamentous laxity.1,2
The incidence of MPFL injury after patella dislocation varies widely in the literature from 40% to 90%. MRI is the study of choice to diagnose an MPFL injury. Other injuries that may occur with patella dislocations include osteochondral injuries, meniscus tears, and MCL tears. Osteochondral injuries of the patellofemoral joint and lateral femoral condyle occur in up to 40% of lateral patella dislocations. This correlates closely with studies showing a 35% incidence of patellofemoral arthritis 13 years after patellofemoral dislocation.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Rhee SJ, Pavlou G, Oakley J, Barlow D, Haddad F. Modern management of patellar instability. Int Orthop. 2012;36:2447-2456.
- Sillanpaa PJ, Mattila VM, Maenpaa H, et al. Treatment with and without initial stabilizing surgery for primary traumatic patellar dislocation. J Bone Joint Surg Am. 2009;91:263-273.