Abstract & Commentary
Synopsis: The 2 hypothesized causes of chronic ankle instability are mechanical instability and functional instability.
Source: Tropp H. Functional ankle instability revisited: Special communications/commentary. Journal of Athletic Training. 2002;37(4):512-515.
This commentary was provided by Hans Tropp, MD, PhD, for a Journal of Athletic Training special issue on chronic ankle stability. Tropp, a pioneer in ankle instability research, explains that mechanical instability and functional instability are the 2 hypothesized causes of chronic ankle instability. He defines mechanical instability as ankle movement beyond the physiologic limit of ankle range of motion, and functional instability as the subjective feeling of ankle instability or the presence of recurrent, symptomatic ankle sprains.
Tropp presents the functional biomechanics of ankle instability and emphasizes that while the talocrural joint is often considered the "ankle joint," the subtalar joint has greater relevance to this malady. Stability of the foot underneath the center of gravity occurs from postural corrections at the subtalar joint as rotations around the center of pressure. The effect of external loads (torques) on the ankle is dependent on the line of action of the ground-reaction force upon the subtalar joint axis. Tropp explains that the ground-reaction force usually acts lateral to the subtalar joint axis and anterior to the talocrural joint axis, and consequently external loads usually evert and dorsiflex the ankle.
Hyperinversion and "giving way" occurs when the inverted, weight-bearing ankle is subjected to an external load that further forces the foot into inversion. This mechanism is exacerbated if a shear force is added, with the evertor muscles unable to withstand the lever produced by the body-weight load. The potential for hyperinversion is compounded with the wearing of a shoe, since the shoe increases the length of the lever arm, and the friction between the shoe and the ground can add a shear force component.
Comment by David H. Perrin, PhD, ATC
Tropp challenges the theory that damage to the lateral ankle ligaments during hyperinversion disrupts the mechanoreceptor system arising from the sensory receptors in these ligaments. He explains that anesthetizing the lateral ligaments has little effect on ankle joint proprioception and that the wearing of an ankle brace improves ankle joint position sense. He deduces from this information that a variety of receptor sources, including cutaneous, joint, and muscular sources, are likely necessary for motor control.
Tropp also explains the importance of neuromuscular preparation for weight-bearing during the swing phase of gait. He postulates that inappropriate positioning of the lower limb before heal strike likely increases the potential for injury, since the line of action of the reaction force is determined when the foot reaches the ground.
Clinicians typically use a battery of proprioception and strengthening exercises for the rehabilitation of ankle instability. Tropp suggests that the main factor in functional instability is a change in coordination, mainly due to a transition from ankle synergy to hip synergy during postural corrections. It is this transition that might also affect the tendency to invert the foot during the swing phase of gait. Consequently, he recommends including coordination training activities that provoke ankle inversion and eversion in the rehabilitation protocol.
Dr. Perrin, Dean, School of Health and Human Performance, University of North Carolina—Greensboro, is Associate Editor of Sports Medicine Reports.
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