Summary of Key Points
The nucleus pulposus (NP) is capable of attracting water molecules and expands its volume through swelling. In a healthy normal state, the NP is, thus, pressurized.
The pressurized disc provides the normal intervertebral spacing and smooth motion of the spine and allows balanced load transfer among the adjacent vertebral bodies.
Disc degeneration alters the intradiscal pressure and thus segmental motion characteristics.
Newer technologies, which will make the measurement of the intradiscal pressure easier, safer, and more reliable, might be useful in delineating symptomatic degenerated discs.
The intervertebral disc is composed of a central gel-like nucleus pulposus (NP) and surrounding annulus fibrosus (AF) with a unique lamellar structure. Each layer of the AF is composed of mainly type I collagen, in which the collagen fibers are organized in an oblique direction that alters at sequential layers. In contrast, the NP has a gel-like structure mainly made by proteoglycans embedded in a type II collagen matrix.
The NP is capable of attracting water molecules and expands its volume through swelling. In a healthy normal state, the NP is, thus, pressurized because the expansion of the volume is restricted by the surrounding AF lamellae and end plates. It has been shown that the disc is a highly viscoelastic fluid-like material with limited compressibility. The water content and, therefore, the intradiscal pressure are critical for maintaining normal motion of the spinal segments and for avoiding neurologic complications. The pressurized disc provides the normal intervertebral spacing and smooth motion of the spine and allows balanced load transfer among the adjacent vertebral bodies.
Intradiscal Pressure in the Normal and Degenerated Discs
Early efforts of measurement of disc pressure began in the 1960s. Nachemson and Morris were the first to assess intradiscal pressure. In 1965, Nachemson observed that the pressure in the AF and NP were different and that disc degeneration could alter the pressure. They measured in vivo loading of the spine during different postures via assessing disc pressure. As with their subsequent studies, they used the disc as a “load transducer” and did not characterize the pressure properties of the disc itself. Wilke and colleagues again performed a similar study on a volunteer in 1999. They demonstrated similar pressure levels to those shown by Nachemson.
The first comprehensive characterization of disc pressure was accomplished by McNally and Adams. They used a small needle with a pressure transducer mounted at the tip to measure pressure changes within a cadaveric lumbar disc loaded under a variety of conditions. They also collected pressure measurements in both axial and transverse directions by simply rotating the needle during placement in the disc. They demonstrated that a healthy normal disc had a uniform pressure distribution due to hydrostatic pressure that was confirmed in both vertical and horizontal measurements. In contrast, degenerated discs demonstrated an irregular stress distribution across the disc. In the degenerated disc, they observed slight differences in horizontal and vertical directional measurements. They also demonstrated that the pressure (stress) characteristics of the disc depended on the loading conditions.
Sato and colleagues also performed in vivo experiments using a similar technology. Confirming the findings of McNally and Adams, they reported that normal healthy discs yielded an isotropic pressure profile evidenced by similar pressures in axial and transverse directions. This was in contrast to degenerated discs, in which axial and transverse pressures were found to differ. They also showed that disc with loss of water content, detected by magnetic resonance imaging (MRI), was associated with less pressure under physiologic loading. McNally and coworkers noted an association between pain and pressure. They showed that painful discs (after provocation) determined by provocative discography were associated with less pressure in the NP in both vertical and horizontal directions.
The loss of water content in the disc due to degeneration translates into the loss of disc height of the intervertebral space, because the NP becomes more compressible and the anular layers lose their ability to withstand axial compressive loading. The loss of disc height results in the decrease of the gap in the facet joint and subsequent changes in the facet loads. This gradually changes the load transfer patterns in the dorsal and ventral spine and end plates.
Pollintine and colleagues measured the intradiscal pressure profiles in normal and degenerated cadaveric spines loaded axially or in flexion. They demonstrated that the percentage of load transferred via the dorsal spine increased with disc degeneration. Similarly, the loading profile of the end plates lost its homogeneity in healthy normal specimens and shifted toward the perimeter. They also confirmed that the degeneration of the disc was associated with anisotropy of pressure within the disc.
As shown in the aforementioned stress profilometry experiments, the AF in the degenerated discs is associated with increased stress levels and localized stress concentrations. When the anular layers are compressed under axial loading, they bulge inward and outward. This stress profile and deformation pattern, in contrast to healthy normal disc where anular layers are compacted and able to withstand axial loading, sets the stage for anular tear and clefts. These, in turn, may be a cause of pain.
The effect of the disc degeneration on the segmented spinal motion, especially the neutral zone, was discussed in an earlier chapter. Cannella and associates investigated the relationship of intradiscal pressure coupled with the degeneration and motion. They looked at the pressure changes in the intervertebral disc under axial compression as they gradually removed the nucleus via a vacuum tissue removal system. They showed that denucleation of the ventral spinal column unit caused increased instability, confirmed by an increased range of motion and a larger neutral zone. Parallel to this phenomenon, they also measured decreasing intradiscal pressure by means of a pressure transducer placed on a needle. The researchers studied various increasing levels of compression. They did not, however, assess pressure changes in the other planes of motion (e.g., sagittal, lateral).
Disc pressure is also a mechanism by which a painful disc is delineated from other normal discs. Discography, a procedure in which a contrast agent is delivered into the disc through a needle to increase the pressure within the disc, is a widely used method to manually provoke pain in suspect discs. Recently, a group of investigators tested the hypothesis that discography-related pressure could be an indication of disc degeneration and a quantitative marker for the painful disc. Unlike previous studies seeking the minimum discography pressure that provokes pain, they collected the opening pressure, the pressure required to inject contrast agent into the disc, and compared it with the MRI signal—which is an indicator of degeneration. They showed that the opening pressure decreased with increasing levels of disc degeneration and that painful discs had a lower opening pressure than those with no pain. The disc pressure was also shown, in another study, to be transferred to the adjacent levels to some extent during provocative pressurization of a lumbar disc.
As a biomechanical indicator, the disc pressure also has a diagnostic value. Gradual changes in the pressure might provide biomechanical evidence of biochemical changes in the disc during degeneration or regeneration. As the technology develops and measurement of the intradiscal pressure becomes easier, safer, and more reliable, it might be an adjunct evaluation technique to MRI in delineating symptomatic degenerated discs. Similarly, Buser and colleagues used the disc pressure as an evaluation method for the regeneration of pig intervertebral discs. They studied the effect of fibrin injection in the regeneration of the nucleus in pigs who had undergone complete nucleotomy. In their study, they showed that fibrin inhibited the encroachment of the anulus and facilitated proteoglycan redevelopment. They confirmed histologic findings of new nucleus formation with recovery in the disc pressure.