To understand a pathological process, one must first comprehend relevant normal physiology and microanatomy. There are two contrasting types of bone in the adult human skeleton. Cortical bone is compact and dense. It is found encasing all parts of the skeleton but is most prominent in the diaphyses of long bones such as the femur. The femoral cortex is thick, forming an elliptical tube that surrounds a medullary canal containing sparse trabecular bone. In this example, the mechanical function of cortical
bone can be best understood. The femur, a major weight-bearing bone, sustains large bending and torsional forces arising during movement. Imagine the forces while ascending a staircase. With extension of the hip and femur, vector forces in opposite directions place huge bending moments along the longitudinal axis of the femoral shaft.
The other type of bone, more abundant in the spine, is trabecular bone. Also known as cancellous bone, it can be considered as a porous interlocking scaffold of vertical and horizontal columns of bone (Fig. 1). Thus, trabecular bone is best at resisting compressive loads. The vertebral body is made up of mostly trabecular bone. In terms of the biomechanics of the spine, this is well suited to the demands of the anterior spinal column. The vertebral body and intervertebral disc sustain approximately 80% of the load during axial compression, with the remaining 20% sustained by the facet joints .
The structural differences between cancellous and cortical bone also have metabolic significance. In the densely packed cortical bone, nutrition is supplied by low-pressure vessels within the haversian canalicular system. Considering the amount of bone in relation to the amount of vascularity, the ratio is relatively low. In contrast, cancel-lous bone is much more richly vascularized by osseous vascular complexes that pass between the less densely packed trabeculae. This arrangement produces a much higher surface-to-volume ratio of bone to extracellular fluids. Therefore, cancellous bone responds more quickly to metabolic alterations and, for this reason, the vertebral bodies are more susceptible to processes that increase bone resorption, such as osteoporosis .
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