Biomechanical properties were assessed from the tibias of 17 adult males 17-46 years of age. Tissue-level mechanical properties varied with bone size. Narrower tibias were comprised of tissue that was more brittle and more prone to accumulating damage compared with tissue from wider tibias.
Introduction: A better understanding of the factors contributing to stress fractures is needed to identify new prevention strategies that will reduce fracture incidence. Having a narrow (i.e., more slender) tibia relative to body mass has been shown to be a major predictor of stress fracture risk and fragility in male military recruits and male athletes. The intriguing possibility that slender bones, like those shown in animal models, may be composed of more damageable material has not been considered in the human skeleton.
Materials and methods: Polar moment of inertia, section modulus, and antero-posterior (AP) and medial-lateral (ML) widths were determined for tibial diaphyses from 17 male donors 17-46 years of age. A slenderness index was defined as the inverse ratio of the section modulus to tibia length and body weight. Eight prismatic cortical bone samples were generated from each tibia, and tissue-level mechanical properties including modulus, strength, total energy, postyield strain, and tissue damageability were measured by four-point bending from monotonic (n = 4/tibia) and damage accumulation (n = 4/tibia) test methods. Partial correlation coefficients were determined between each geometrical parameter and each tissue-level mechanical property while taking age into consideration.
Results: Significant correlations were observed between tibial morphology and the mechanical properties that characterized tissue brittleness and damageability. Positive correlations were observed between measures of bone size (AP width) and measures of tissue ductility (postyield strain, total energy), and negative correlations were observed between bone size (moment of inertia, section modulus) and tissue modulus.
Conclusions: The correlation analysis suggested that bone morphology could be used as a predictor of tissue fragility and stress fracture risk. The average mechanical properties of cortical tissue varied as a function of the overall size of the bone. Therefore, under extreme loading conditions (e.g., military training), variation in bone quality parameters related to damageability may be a contributing factor to the increased risk of stress fracture for individuals with more slender bones.