Regional Variations in Microstructure and Biomechanical Properties of the Human Vertebral Endplate and Trabecular Bone.

Abstract

Vertebral compression fractures represent one of the most common injuries resulting from osteoporosis with an incidence rate of 700,000 per year in the United States. Spinal osteoporosis affects the anterior part of the vertebrae, leading to its collapse and kyphosis. However, there is limited data on the regional differences in the microstructure of the vertebral endplate and cancellous bone and their relationships with degenerative disc diseases and vertebral fractures. Does bone resorption occur at a higher rate in the anterior than in the lateral region of the vertebra? We conducted shear testing experiments on isolated cylindrical bone cores of thoracic vertebrae. Our data showed a 5-fold increase in mechanical strength from the anterior to the lateral regions of both endplate and cancellous bone (p=0.04). Material and ash density measurements correlated positively with shear strength. We further studied the regional variation in biomechanical properties inside human and bovine thoracic vertebrae using surgical anterior self-tapping cancellous screws. Our results showed a trend for increased insertion torque, pullout strength, and stiffness from divergent (laterally oriented) to convergent (centrally oriented) screws. Radiological image analyses from Faxitron X-ray, CT scan, and microCT confirmed that density was higher in the central than in the lateral regions of the cancellous bone. However, we observed the opposite relationship at the cervical and lumbar levels where the divergent screw configuration was mechanically stronger than the convergent one. According to our histological analyses, the density of microvessels at the vertebral endplate decreased from central to lateral regions by 15%. The ratio bone volume over total volume (BV/TV) increased by 31% from superior to inferior bony endplates. There was a decrease in the ratio osteoid matrix surface/bone surface (OS/BS) by 20% from superior to inferior bony endplates. We also observed that older individuals with increased degenerative disc diseases (DDD) had lower cancellous bone density, but higher bone remodeling activity than younger ones. The biomechanical properties of both endplate and cancellous bone vary considerably within the human spinal column. The lateral region of the vertebral body (EP and cancellous) is denser and stronger at the cervical and lumbar levels. The opposite relationship is observed in the thoracic segment. Newer implants could prioritize a lateral or central loading accordingly; thus, reducing the likelihood of screw loosening and the subsidence of disc replacement devices. Moreover, the superior endplate is weaker than the inferior, but more porous, and probably more important for the diffusion of nutrients to the nucleus pulposus. Therefore, the likelihood of disc degeneration might increase with superior endplate injuries.