Session IX - Tibia

Ultra-Low Velocity Knee Dislocations

Frederick M. Azar, MD; Jason C. Brandt, MD; Barry B. Phillips, MD; Robert H. Miller III, MD, University of Tennessee ­ Campbell Clinic Dept. of Orthopaedic Surgery, Memphis, TN

Purpose: To report the incidence of neurovascular injury, associated injury, complications, outcome, and common characteristics of patients with this uncommon injury.

Methods: Seventeen patients had complete knee dislocations sustained during everyday activities. Their average body mass index (BMI) was 48.06 (range 30.8 to 66.7). A BMI of 30 is considered obesity, 35 severe obesity, and 40 or more very severe obesity. Thirteen dislocations were anterior, 2 were posterior, and 2 were lateral. Seven of the 17 patients had vascular (popliteal) injuries, all of which were surgically repaired. Seven had injuries to either the peroneal nerve alone (5) or to both the peroneal and tibial nerves (2). Closed reduction was successful in all. Eight of the 14 knees had surgical repair of ligaments; the other nine were immobilized with crossed pins (6), external fixation (1), or splints (2). Outcome was evaluated at an average follow-up of 28.5 months (minimum follow-up of 1 year).

Results: Two patients with nerve and vascular injuries required above-knee amputations, one patient died of cardiac arrest 7 days after knee dislocation, and 3 were lost to followup. Of the 11 remaining patients, 4 had nerve palsies, which resolved in 2 and persisted in 2. Results were poor in all patients, but those with surgical repair of ligaments had better results than those without.

Discussion: Complete knee dislocation is a relatively rare but potentially devastating injury. Most reports focus on dislocations caused by high-energy trauma. The few reports of low-velocity knee dislocation have concerned primarily dislocations occuring during athletic competition. A subset of patients with low-velocity knee dislocations sustained during normal daily activity has been identified. Although the patient population is small, in general these ultra-low-velocity injuries were more severe, produced more nerve and vascular damage, and had poorer results than those reported after most high-velocity knee dislocations and low-velocity dislocations in athletes. That all of these patients were obese and most were severely obese points to a correlation between obesity and a risk of knee dislocation with ADL. The frequency of neurovascular injury also increased as the BMI increased.

Conclusion: Ultra-low-velocity knee dislocations in obese patients generally are more severe injuries than high-velocity dislocations or low-velocity dislocations in athletes, with more neurovascular damage and poorer results.