Three-dimensional finite element stress analysis of a polymeric orthotic device: Biomechanics of the ankle-foot orthosis

Pathological motion of the ankle-foot complex presents a major problem in the rehabilitation of stroke and head injury patients. For example, stroke patients often develop "drop-foot", a problem involving excessive and uncontrolled plantar flexion. Frequently, Ankle-Foot Orthoses (AFOs) are prescribed to restore normal motion or to constrain and inhibit abnormal motion. However, the application/use of these orthotic devices is based on "trial and error", and may cause a great deal of discomfort. Most important, there is a lack of a fundamental understanding in the area of biomechanics of the AFO, and there is also a lack of good engineering design of the orthotic device.; The objectives of the present study were (1) to develop a 3-Dimensional Finite Element Model of the AFO system; (2) to analyze stress distribution in the AFO under various conditions using the 3-D FEM; and (3) to conduct parametric studies.; The investigation began with the development of the geometry of the 3-D FEM of the AFO complex in the UNIX system with the pre-processor of PATRAN. A total of 323 elements and 596 nodes were generated. AFO was represented by 128 3-D solid elements; soft tissue was represented by 146 3-D solid elements; and 39 3-D solid elements represented bones of the foot. Ligaments of the foot were represented by 10 truss elements.; Results from the present investigation clearly indicated that the maximum compressive stress in AFO occurred in the heel region and the maximum tensile stress was generated in the neck region of AFO. During the swing phase, the inversion generated the highest stresses in the AFO. Plantar flexion produced high compressive stresses in the heel region.; Heel strike produced a high compressive stress in the AFO. The toe-off produced a high tensile stress. The toe strike produced a high tensile stress in the AFO. Moreover, stress distribution varied in the AFO with a change of the ground contact point during the heel strike, but was not significantly altered during the toe-off.; The magnitude of the peak stress increased with increasing Achilles tendon force, and also increased with increasing stiffness of the orthotic device. However, the magnitude and location of the peak stresses were insensitive to ligament stiffness.; The 3-D FEM of the AFO system has been developed. The model results confirmed the hypothesis that the peak stress occurred in the neck and the heel region of the AFO.