Abstract
Ankle instantaneous axis of rotation (IAR) measurements represent a more complete parameter for characterizing joint motion. However, few studies have implemented this measurement to study normal, injured, or pathological foot ankle biomechanics. A novel testing protocol was developed to simulate aspects of in vivo foot ankle mechanics during mid-stance gait in a human cadaveric specimen. A lower leg was mounted in a robotic testing platform with the tibia upright and foot flat on the baseplate. Axial tibia loads (ATLs) were controlled as a function of a vertical ground reaction force (vGRF) set at half body weight (356 N) and a 50% vGRF (178 N) Achilles tendon load. Two specimens were repetitively loaded over 10 degrees of dorsiflexion and 20 degrees of plantar flexion. Platform axes were controlled within 2 microns and 0.008 degrees resulting in ATL measurements within ±2 N of target conditions. Mean ATLs and IAR values were not significantly different between cycles of motion, but IAR values were significantly different between dorsiflexion and plantar flexion. A linear regression analysis showed no significant differences between slopes of plantar flexion paths. The customized robotic platform and advanced testing protocol produced repeatable and accurate measurements of the IAR, useful for assessing foot ankle biomechanics under different loading scenarios and foot conditions.
Highlights
The kinematic and structural properties of human joints may be affected by diseases, injuries, or surgical alterations
The results of the study are one of a handful to address the instantaneous axis of rotation of the ankle joint and provide the most accurate measurement to date
The instantaneous axis of rotation (IAR) measurement was repeatable within one millimeter, smaller than what is feasibly measurable in a clinical setting
Summary
The kinematic and structural properties of human joints may be affected by diseases, injuries, or surgical alterations. In the case of the ankle joint, any disease, injury, or elected surgery, like osteoarthritis or ankle arthroplasty, will impact both the motion behavior of the joint (how it moves) and its structural stiffness properties (how it rotates under a muscle load) [1,2,3,4,5,6]. Joint motion can be described by rolling and sliding of articular surfaces during motion representative of a moving axis of rotation with dependency on loading scenarios [12]. The two-dimensional instantaneous axis of rotation (IAR) represents a more advanced parameter for characterizing joint kinematics, yet it has been void from most ankle biomechanics research. The ability to detect shifts in IAR may help in defining injury type and/or the impact of injury on foot ankle mechanics, as well as the effects of surgical procedures and implant and orthotic design
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