Injury tolerance and response of the ankle joint in dynamic dorsiflexion.

Forced dorsiflexion in frontal vehicle crashes is considered a common cause of injury to the ankle joint. Although a few studies have been published on the dynamic fracture tolerance of the ankle in dorsiflexion, this work reexamines the topic with increased statistical power, adds an evaluation of articular cartilage injury, and utilizes methods to detect the true time of fracture. The objective of this study was to measure the response and injury tolerance of the human ankle in a loading condition similar to that found in a vehicle crash with toepan intrusion. A test fixture was constructed to apply forefoot impacts to twenty cadaveric lower limbs, that were anatomically intact distal to the femur mid-diaphysis. Specimen instrumentation included implanted tibial and fibular load cells, accelerometers, angular rate sensors, and an acoustic sensor. Following the tests, specimens were radiographed and dissected to determine the extent of injury. Eleven of the twenty specimens sustained fracture of the ankle joint. Fractures of the medial malleolus were the most common, while two specimens sustained bimalleolar fractures, and two a talar neck fracture. Other injuries included ligament tears, osteochondral fractures, and cartilage abrasions. Analysis of the acoustic emission indicated that fracture did not always occur at the peak ankle moment. Based on the results of this study, an ankle joint moment of 59 N-m represents a 25% risk of ankle fracture in dorsiflexion for a 50(th) percentile male. When applied to the Thor-Lx dummy, the 25% risk of injury occurs at 36 degrees of dorsiflexion as measured by the ankle potentiometer.