Biomechanics of the unstable ankle joint and clinical implications

Purpose: The aim of this paper is to provide fundamental information about the biomechanics of the unstable ankle joint and to establish a rational fo r the daily clinic when dealing with patients in both, the acute and chroni c unstable condition of the ankle joint complex. Methods: The problem of th e unstable ankle joint is worked up by analyses of the basic anatomy and bi omechanics followed by an overview of its clinical manifestation including a differential diagnosis. Results: The ankle joint and its surrounding liga ments represent a complex mechanical structure whose mechanical properties highly depend on ligament integrity. Recent in vitro studies have supported the hypothesis that, besides maintaining lateral ankle stability, the late ral ankle ligaments play a significant role in maintaining rotational ankle stability and in transferring movement between leg and foot. Instability o f the ankle results from acute ligament injuries and may become chronic whe n complete ligament healing does not occur. Chronic instability syndrome ma y manifest with recurrent injuries with chronic lateral pain, tenderness, s welling, or induration with great difficulties in sports and daily activiti es. Symptomatic instability can be caused by mechanical instability with de monstrable instability, but it can be also present with no demonstrable ins tability. Impairment of ankle proprioception has been shown to be a major c ause of symptomatic ankle instability. Other conditions may mimic ankle ins tability. Conclusions: The cause of chronic functional instability is often not mechanical instability but impairment of ankle proprioception. A histo ry of insecurity, instability, and giving way is far more important in diag nosis than the physical and radiographic examination. If surgical treatment is advised, anatomical reconstruction of the ankle ligaments is mandatory for fear of altering the biomechanics.