Spinal maturation affects vertebral compressive mechanics and vBMD with sex dependence

The effects of natural aging on the mechanics of the spine are far better understood for the mature adult spine than for the developing (immature) spine. Throughout its chondrification and ossification, the vertebra, which is the primary structural unit of the spine, undergoes enormous cellular, biochemical, and structural changes that should strongly influence its biomechanical response to external forces. Unfortunately, very little data exist for the mechanics of immature vertebrae. Vertebral maturation was therefore investigated in 22 baboon thoracic specimens to elucidate its relationship with biomechanics and volumetric bone mineral density (vBMD). Cadaveric baboon vertebrae were used due to the limited availability of human tissues in the pediatric age range. The specimen ages ranged between 1 and 30 human-equivalent years based on skeletal maturity. Isolated ninth thoracic vertebrae (T9) were subjected to compressive loading to document their compressive mechanical properties (yield load, stiffness, yield strength, and elastic modulus) and ashed to determine their volumetric bone mineral density. Spinal maturation was discovered to significantly increase vBMD (P< 0.0001) and compressive mechanics (stiffness, bulk elastic modulus, failure load, and bulk strength, P< 0.001) in a sex-dependent manner. Vertebral stiffness increased from 1218 N/mm at 1 year to 3534 N/mm at 30 years with a second order polynomial “maturation” relationship. Volumetric bone mineral density and vertebral cross-sectional area together described the developmental patterns of stiffness and yield load of isolated vertebrae. Sex differences were observed throughout development, demonstrating differing growth patterns to accommodate mechanical loading whereby males develop larger size vertebrae and females achieve their mechanical stiffness and strength through greater bone mineral density.