31.2.1 Physiologic Loads on the Vertebral Body

When placed under dynamic loading, the spine allows multidirectional movements that can be analyzed with a six-degree-of-freedom biomechanical model. To analyze the spine and its biomechanical characteristics, this structure is often subdivided in smaller physiological motion units called FSUs. The FSU is composed of two adjacent vertebrae, the intervertebral disk and the ligaments providing support and stability. According to a purist’s definition, the FSU does not include muscles or other connecting tissues. Each vertebra of the FSU can move in space according to orthogonal X, Y, Z axes in addition to rotations along each axis. Because of the musculoligamentous apparatus and the orientation of the facet joints, physical limitations are exerted on FSUs. Furthermore, these limitations fluctuate depending on the region of the spine studied (i.e., cervical, thoracic, or lumbar). At the thoracolumbar level (where most of the VCFs happen), the forces applied on the FSU can be simplified in four main categories: compression forces (axial forces), shear forces, bending moment, and axial torque (▶Fig. 31.1).

In the thoracolumbar spine, the majority of the axial load is borne by the vertebral bodies. Each vertebral body is composed of two structural constituents that sustain that load: the cancellous core and the cortical shell. First, the cancellous core is a dense network of inner trabeculae oriented vertically and horizontally. The vertical trabeculae support the vertebral body thin cortex (measuring approximately 0.4 mm) and resist to the axial loading by transmitting forces from the upper to the lower end plate. On the other hand, the horizontal trabeculae reinforce and provide support to the vertical trabeculae by preventing sideway displacement under compressive loading with and without shear forces. Although the thickness of the cortical bone is not substantial (approximately 0.4 mm), the bone mass attributable to the cortical shell is surprisingly large when compared to the rest of the vertebral body, estimated to be up to ~40% of the total bone mass and providing approximately 45% of the structural support. ⁶ Also, the cortical shell allows less plastic deformity than the cancellous core before fracture (~2 vs. ~10%). Thus, it is unlikely to have a fracture of the core of the vertebral body without a fracture of the shell.

The physiologic range of motion (ROM) of an FSU consists of a neutral zone (NZ) and an elastic zone (EZ) (▶Fig. 31.2). These characteristics (NZ and EZ) are intrinsic to any two consecutive vertebra ⁷ and have very different biomechanical behaviors. In the NZ, the spine undergoes relatively large motions with minimal force loading due to the laxity of ligaments and capsule. In the EZ, there is resistance to movement leading to increase load per unit of displacement ⁸ until failure. Several studies have demonstrated that the NZ increases with injury, muscle weakness, and degeneration. This increase in the NZ tends to lead to FSU instability. The NZ may be brought within a physiological limit with osteophyte formation, surgical fixation/fusion, and muscle strengthening. ⁸

When a person stands upright, the mass of the trunk, head, and arms presses vertically on the lower lumbar spine with a force of approximately 55% of the body weight, corresponding to 385 N for a 70 kg man. ⁹ The measurements of the various components of spinal loads are complex and vary depending on the region of the spine evaluated, the posture (flexion vs. extension), and the angulation of the motion segment. Sato et al ¹⁰ measured in vivo spinal loads at the L4–L5 level with intradiskal pressure measurement. Those pressures were of 91 kPa in the prone position, 151 kPa in the lateral position, 539 kPa in the upright standing position, and 623 kPa in the upright sitting position. The maximum intradiskal pressure in the flexed standing position was more than twice the upright standing position with a value of 1,324 kPa. The spinal load calculated for healthy 73 kg subjects were then: 144 N prone, 240 N in lateral decubitus, 800 N upright standing, and 996 N upright sitting. ¹⁰ These results demonstrate that the vertebral body loads fluctuate significantly according to the position of the patient. This concept explains well why patients with severe osteoporosis are at risk of developing VCFs when performing simple daily tasks, such as walking, getting down the stairs, or holding groceries.

31.2.2 Impact of Disk Degeneration on the FSU

The role of the disk is to sustain compressive forces in addition to supporting shear and tensile forces. ¹¹ The disk is an important component of the FSU as it redistributes loads from the inferior end plate of the superior vertebra to the superior end plate of the inferior vertebra. As disk degeneration increases, the intradiskal pressure measurements decrease ¹⁰ and higher loads are transmitted from one vertebral body to the other (creating peak loads) including to a posterior load redistribution that results in a greater load placed on the facet joints and vertebral arch (▶Fig. 31.3). The loss of normal nucleus pulposus microarchitecture can first be appreciated on MRI by a T2-weighted signal intensity drop of the nucleus pulposus of the disk which can be quantified with the modified Pfirrmann grading system (▶Table 31.1).

As compressive load within a normal disk increases, the hydrostatic pressure within the nucleus pulposus also increases, and some of the inner pressure (horizontal pressure) is transferred to the annulus fibrosus by circumferential stress (hoop stress). The end-plate forces are normally evenly distributed on the anterior column with less forces transmitted to the posterior elements and facet joints. With increased disk degeneration, there is less load bearing on the ventral end plates (decreasing from 44 to 19%) and more stress on the neural arch, specifically on the facet joints (increasing from 8 to 40%) (▶Fig. 31.3). ¹² Load redistribution can be further exaggerated with extension maneuvers (in that context, up to 90% of the compressive forces can occur on the facets).

This anterior-to-posterior redistribution due to disk degeneration gives rise to Stress Shielding. As the load switches posteriorly, as per Wolff’s law, there is less trabecular turnover and remodeling in the anterior vertebral body, resulting in decrease of up to 20% in the trabecular volume and increase of up to 28% in intertrabecular spacing. ¹⁴ As discussed earlier, upright flexion results in higher stress on the disk. In patients with disk degeneration, the load delta on the anterior portion of the FSU is even further increased. For example, in flexion versus neutral position, the loads applied to the FSU can increase by up to 300%, ¹² further predisposing osteoporotic patients to VCFs.