Kinematics of the Axially Loaded Ankle

Abstract

An apparatus that allowed the application of a 900 N axial load and the simultaneous measurement of rotation in the sagittal, coronal, and axial planes was used to study the normal kinematics of the ankle in 13 below-knee amputation specimens. Two testing routines were done on all specimens. In the first sequence, specimens were moved through a dorsiflexion (DF) and plantarflexion (PF) arc of 60° (25° DF and 35° PF). DF was associated with an average of 2.5° of external rotation, and PF was associated with an average of <1° of internal rotation. In the coronal plane, PF and DF were both associated with <1° of varus. In the second part of the testing, the ankle position in the sagittal plane (DF/PF) was fixed and the axial load was increased from 50 N to 750 N in 100-N intervals. Increasing the axial load caused an increase in external rotation and valgus of 1° to 2°. For axial rotation, external rotation was more pronounced in PF than DF. The effect of load on the increase on valgus was not affected by sagittal ankle position. The effect of increasing axial load on sagittal rotation was to increase DF or PF <2° over the entire range of loads and sagittal positions. The understanding of ankle biomechanics is essential to the formulation of rational guidelines for the treatment of ankle pathology and the prediction of the long-term consequences of ankle injuries. The incomplete understanding of this subject is evident when the disparate recommendations for a number of common conditions are considered. By examining the three-dimensional motion of the stable ankle, a more precise understanding of the abnormal three-dimensional motions associated with instability can be achieved. This knowledge will permit a logical approach to treatment of ankle fractures.