The dislocation of the patella is the dislocation of the kneecap sideways— either laterally, to the outward facing side of the leg, or medially, to the inward facing side of the leg. While the condition is rare, accounting for around 3% of knee injuries, acute patellar dislocations are painful health emergencies with long-lasting consequences if not properly treated. This is especially true in a wilderness medicine context: a patellar dislocation may create dangerous and even deadly scenarios in the backcountry, where long, complicated evacuation processes will prolong patient discomfort and the precarious wilderness environment may lead to further danger. Thus certified wilderness first responders are permitted to perform straightforward dislocation reductions in a backcountry setting to alleviate pain and assist evacuation.
As the person responsible for the well-being of those under their care, a wilderness first responder may want to better understand the mechanism of injury and factors increasing risk of patellar dislocation. And in a biomechanics context, patellar dislocation provides an interesting case study since it is a highly mechanical event dependent on intricate biomechanical equilibriums surrounding the knee. So how does a patellar dislocation occur? And what factors increase the probability of a dislocation occurring?
Key to keeping the patella from shifting laterally or medially is the patelloformal joint, which allows knees to bend and straighten. This is the connection point where the triangular patella slides in and out of the trochlear groove, a groove within the lower thigh bone. The patellofemoral joint is restricted to a singular direction of motion back and forth by tendons and ligaments as the leg bends and straightens. Ligaments stretch over and around the patella in constant tension to keep it from shifting to either side. As the leg flexes, the tightening of the quadriceps increases the amount of tension in these bands, pinning the patella securely in place.
So where can it go wrong? As the knee starts to flex, between 20-60 degrees the tension in the ligaments is comparatively low, and the patella has not yet settled deeply into the trochlear groove. In addition, the quadriceps tendons and muscles actually exert a force at a slight angle. This pulls the ligaments, then the patella, slightly sideways. When a sudden, very large force is applied, the patella can slip inward out of its normal path and dislocate there, or sling around and come to rest on the outward facing side of the knee.
Patellar dislocation is most likely to occur in sports, affecting primarily young adults. Factors that can increase injury risk include having a flattened trochlear groove, misaligned patella, or any injury or condition affecting the ligaments and tendons surrounding the patella. In a backcountry setting, especially on lengthy, remote trips, it is essential to perform a thorough intake of existing medical history for trip attendees. Being aware and prepared before-hand can keep a fascinating biomechanical failure and treatable injury from becoming a dangerous, even life-threatening, emergency.