Effects of sagittal plane axis shifts in hinged ankle-foot orthoses on gait mechanics and shaft-to-leg relative motion.

BACKGROUND

The alignment of the axis in ankle-foot orthoses is essential for ensuring optimal biomechanical function and comfort for the user. Correct alignment reduces orthosis displacement, minimizes skin irritation and joint stress, and improves overall performance. This study explores the effects of varying joint axis positions on ankle-foot orthoses functionality and shaft-to-leg movement.

METHODS

Six healthy adult males (mean age: 35 ± 12 years) participated, walking with bilateral ankle foot orthoses configured in five joint axis positions and a shod condition. A 3D motion analysis system captured kinematic and kinetic data, examining the effects of anterior, posterior, proximal, and distal axis shifts on range of motion, joint moments, energy, and shaft-to-leg relative movement (pistoning).

FINDINGS: Anterior-posterior axis shifts significantly affected joint mechanics. Anterior alignment produced the highest dorsiflexion moments and reduced pistoning during dorsiflexion. Range of motion was reduced in all orthotic conditions compared to the shod condition. Proximal-distal shifts had minimal biomechanical impact but increased pistoning. Contrary to expectations, neutral alignment due to the recommendations of the manufacturer did not consistently minimize pistoning or preserve motion.

INTERPRETATION: This study underscores the importance of anterior-posterior alignment in optimizing ankle joint function with orthotics. Specifically, anterior alignment reduced pistoning more effectively than the manufacturer-recommended neutral position, particularly during dorsiflexion. These findings highlight the clinical relevance of aligning the orthotic axis with natural joint mechanics, which can reduce motion restrictions, improve gait efficiency, and prevent harmful pressures that may cause discomfort, bruising, and inflammation around the ankle.