Complete knee dislocations don't just occur in athletes or as a result of a traumatic injury. For some people, knee dislocation occurs 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. Even so, it would be helpful to identify risk factors for these injuries and prevent them altogether. According to a study from the University of Tennessee, one major risk factor is severe obesity.
In a study of 17 patients with low-velocity knee dislocations that occurred during daily activities, the average body-mass index (BMI) was 48. Normal BMI is less than 25, while severe obesity is anything 40 or higher. The full range of BMIs in these 17 adults was from 30 to 68.
The extreme load from their massive, shifting body weight put more pressure on the soft tissue structures than they could bear. The result was rupture of the ligaments, shift of the bones, stretching of the nerves, and tearing of the blood vessels.
When a knee dislocates, there is usually ligamentous damage and there may be nerve and blood vessel injuries as well. Multiple ligaments can be ruptured including the anterior cruciate ligament, posterior cruciate ligament, and the medial and lateral collateral ligaments.
In some cases, the dislocated knee can be relocated without surgery. But if there's been damage to the nerves or blood vessels then surgery is required. Surgery to repair or reconstruct the ligamentous support was needed in half the group in this study. Two-thirds of the group was stabilized with splints, braces, or external fixation of some type.
The most serious complication of knee dislocation in this group was leg amputation due to loss of blood supply (called ischemia) from vascular injury. Other problems included infection and postoperative arthrofibrosis (stiff knee from adhesions). One patient died of a heart attack, which was considered a complication of the surgery.
Taking a closer look, analysis of the data 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. Patients who had surgery to repair ligamentous and other soft tissue damage had higher function and better overall results.
In summary, the authors point out that severe obesity is a major risk factor for spontaneous, atraumatic (without trauma) complete knee dislocation. There is a potential for serious complications such as loss of limb (amputation) and even death associated with these injuries. The higher the body mass index, the greater the risk of vascular and nerve injuries.
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.