The effect of orthotics on in vivo axial tibial and second metatarsal strains

Background: In a previous military study a custom-made tri-layer polyurethane shoe orthotic was shown to be effective in lowering the incidence of stress fractures in infantry recruits. Stress fractures are caused by repetitive high strains. Objective: To compare the effect of tri-layer polyurethane orthotics with other types of orthotics on in vivo strains at sites where stress fractures are common. Design: Subjects underwent surgical implantation of strain gauged staples to the dorsal surface of the midshaft 2nd metatarsal and to the medial surface of the midshaft tibia. Setting: University Hospital. Patients: Two male members of the research staff aged 40 and 54. Measurements: In vivo simultaneous peak-to-peak axial compression-tension strains during treadmill walking at 5 km/h while wearing standard army boots and Nike Air Max running shoes with and without either a non custom pre-fabricated orthotic with a three-quarter length thermocork module, a semirigid custom-made orthotic, with a three-quarter length polypropylene module made from nonweightbearing neutral subtalar position casts, a soft full length custom-made orthotic fabricated from 60 durometer plastazote, and a soft full length custom-made orthotic with a neutral heel post molded from three layers of polyurethane of different density (grade 80 upper layer, grade 60 middle layer and grade 80 lower layer). Results: Only the custom polyurethane composite orthotic was affective in lowering both peak-to-peak compression-tension 2nd metatarsal and tibia strains for both subjects when worn with army boots. When worn with Nike Air Max shoes the polyurethane composite orthotic lowered the peak-to-peak compression-tension 2nd metatarsal but not tibia strains. Limitations: Small subject number, each with a different foot type.Conclusions: The lower peak-to-peak compression-tension tibial and 2nd metatarsal strains while wearing army boots with custom polyurethane composite orthotics can explain the observed lower incidence of stress fractures among military recruits who used these orthotics.