Q: Have you ever heard of someone having a knee dislocate out from underneath them? I saw it happen at a party last night. The guy was plenty overweight but still -- can that happen to just anyone?

A: Complete knee dislocations occur most often in athletes or as a result of a traumatic injury (e.g., car or bike accident, fall). But for some people, knee dislocation can occur during daily activities. Knee dislocations have been reported when stepping off a curb, going down a stair, walking, or even while just standing still. This type of knee dislocation is referred to as a spontaneous dislocation, ultra-low energy trauma, or low-velocity injury.

Fortunately, low-velocity knee dislocations of this type are rare. The most common risk factor for low-velocity knee dislocations (just as you described) is severe obesity. In fact, according to a recent study from the University of Tennessee, the higher the body mass index (BMI), the greater the risk of knee dislocation.

The extreme load from their massive, shifting body weight put more pressure on the soft tissue structures than they could bear. The result can be rupture of the ligaments, shift of the bones, stretching of the nerves, and tearing of the blood vessels. Normal body-mass index (BMI) is less than 25, while severe obesity is anything 40 or higher. The full range of BMIs in the adults in the University of Tennessee study was from 30 to 68.

Analysis of the data collected from patient records in the Tennessee study showed that nerve injuries occurred when the BMI was 42 or higher. Vascular injuries occurred at a BMI of 48 or higher. Patients with a BMI of 51 and higher were at increased risk of both nerve and blood vessel trauma. There is also a potential for serious complications such as loss of limb (amputation) and even death associated with these injuries.

Your friend may very well fall into one of the body-mass index categories reportedly linked with spontaneous knee dislocations such as you witnessed last night. Besides obesity, there could be other factors such as a previous knee (ligamentous) injury that could have contributed to what seemed like a sudden and unexpected injury.

Reference: Frederick M. Azar, MD, et al. Ultra-Low-Velocity Knee Dislocations. In The American Journal of Sports Medicine. October 2011. Vol. 39. No. 10. Pp. 2170-2174.