The Geometrical Arrangement of Joint Constraints that Makes Natural Motion Possible: Experimental Verification on the Ankle

The passive motion of a joint, namely the articular motion under no external loads, provides insights into the joint physiology. It represents the baseline motion of an articulation before passive structures are loaded by external loads. Moreover, during natural motion, the strain energy density stored within ligaments and cartilage is minimized, reducing the risk of microdamage and the corresponding metabolic cost for tissue repairing. In a recent paper, we showed that the line of action of resultant forces of all the constraints provided by the passive structures in a joint must intersect the instantaneous helical axis to make the natural motion possible. In other words, the lines of action of all these constraints must cross the same line at each flexion angle to guarantee the natural motion of the joint. This geometrical property was proven theoretically and verified experimentally for the knee. To prove its generality, in this work we will verify the same property on nine ankle specimens.