Biomechanics

The bones of the foot are arranged to not only provide stability and absorb impact, but to provide maximal efficiency in movement. Beginning with their axes of motion, which are purposefully arranged to allow motion in a specific sequence as the body's weight transfers over the foot from posterior to anterior. The ankle allows dorsiflexion, the subtlalar and mid tarsal joints allow pronation (shock absorption), which  converts to supination (rigid lever), returning motion back to the ankle as plantar flexion. It is a spring-loaded structure. And stability is not just structural - it's dynamic.  It is crucial to understand that the foot and ankle are merely a system of levers and pulleys. Understanding the anatomic arrangement of the tendons around an axis describes the relative contribution of a muscle (by way of tendons) to the system. The larger the tendon, the more force it can exert. And the farther from the joint axis, the greater the lever arm, also resulting in more force. And when a tendon crosses over a joint axis, we can determine which direction the force will occur.  Tendons on the lateral side of the ankle will pull the foot into eversion, while tendons on the medial side will invert. Posterior tendons plantar flex, while anterior tendons dorsiflex. However the arrangement becomes more complex when we see tendons crossing multiple axes, and becoming maximally loaded and thus effective at specific points during the gait cycle. Pathology occurs when joint axes shift from ideal, or motion is somehow limited either by physical restriction (tight tendons or boney impingement) or by anatomic malalignment. Such is the example with a short or elevated first ray, or a first ray with an axis moving away from the traverse plane. The result of inefficiency in the peroneal complex to sufficiently load the medial column will result in prolonged pronation, the downstream effects being hyper mobility, poor shock absorption, distal deformity such as bunions and hammertoes, and load transfer issues like metatarsalgia, stress fractures, and neuromas.  Thus you can appreciate how biomechanics is the foundation of many of the pathologies we see in foot and ankle medicine. Correcting these deficiencies is essential to surgical practice.    The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.