Design enhancement of a solid ankle foot orthosis: Real-time contact pressures evaluation and finite element analysis

Date of Completion

January 1997


Health Sciences, Rehabilitation and Therapy|Engineering, Biomedical|Health Sciences, Medicine and Surgery




Adult neuropathic diabetic patients with acquired foot deformities often develop contact pressure related plantar foot ulcerations, leading to extended hospital procedures. These patients are often fit with a custom made ankle-foot-orthosis (AFO) designed to reduce these pressures. Although effective, AFOs have poor patient compliance due to their high weight.^ Our study performed the first quantification of the entire patient - AFO contact surfaces, and used finite element analysis (FEA) to suggest geometric AFO modifications to reduce weight.^ Four normal males were fitted with custom-molded AFOs of the same design as those designed for clinical purposes.^ Real time contact pressures between the subject and the AFO, including the entire inner surface, plantar exterior surface, and strap were obtained using a FSCAN pressure measurement system. Pressure measurements were obtained during normal gait, rising from and sitting on a chair, climbing stairs, and pivoting on the AFO. Peak and average pressures were collected. These pressures were evaluated for high and low pressure regions, and used as inputs to the FEA model of the AFO.^ A sample AFO was digitized using a 3SPACE digitizer linked to CadKey7 software. The data file was imported to MARC/Mentat FEA software program, and the model created using 3-D hexagonal 8-node elements. The FSCAN contact pressures were used as boundary conditions, as were ground interactions. Following successful completion of the analysis, regions exhibiting low stresses in the AFO were removed and the analyses rerun.^ Contact pressure measurements demonstrated the largest pressures along the inner AFO medial metatarsal region and plantar heel region during the mid-stance phase of gait and stair rise, respectively. Low pressures were noted along the AFO calf region.^ FEA showed that the largest overall stresses in the AFO occurred during gait's toe-off phase and stair climbing. The AFO calf shell showed negligible stresses in all the activities. Calf region material was removed and FEA performed on the resulting modified AFO. Results showed that the increased stresses resulting from material removal remained well under the AFO material tensile strength. It is concluded that the AFO can be enhanced by removing material from the calf shell region. ^