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.³ In 1965, Nachemson observed that the pressure in the AF and NP were different and that disc degeneration could alter the pressure.⁴ They measured in vivo loading of the spine during different postures via assessing disc pressure. Similar to their subsequent studies, they used the disc as a “load transducer” and did not characterize the pressure properties of the disc itself. A similar study was again performed on a volunteer by Wilke et al. in 1999. They demonstrated similar pressure levels to those shown by Nachemson.⁵

The first comprehensive characterization of disc pressure was accomplished by McNally and Adams.⁶ 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 et al. also performed in vivo experiments using a similar technology.⁷ 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 MRI, was associated with less pressure under physiologic loading. McNally et al. noted an association between pain and pressure.⁸ 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 et al. measured the intradiscal pressure profiles in normal and degenerated cadaveric spines loaded axially or in flexion.^9,10 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 et al. investigated the relationship of intradiscal pressure coupled with the degeneration and motion.¹¹ 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.¹² 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.¹³

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 et al. used the disc pressure as an evaluation method for the regeneration of pig intervertebral discs.¹⁴ 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.

Limitations and New Technologies in Intradiscal Pressure Measurement

In vivo and in vitro disc pressure measurements have gained popularity since they were first tried by Nachemson. Many researchers have used intradiscal pressure assessment as a tool for understanding load transfer and load sharing in the spine, evaluating disc degeneration, or assessing the spinal kinetics after interventions performed on the spine. Although the intradiscal pressure can be a source of various types of information, all of these studies have a common limitation—the methodology by which the pressure measurements are acquired.

The majority of the studies in the literature used a miniature pressure transducer that is attached at the tip of a needle or rigid substrate vehicle, which is designed to deliver the transducer to the NP or AF. The introduction of the needle to the disc causes the destruction of the anular layers to a degree depending on the size of the needle used. The effect of this disruption may be minimal in terms of the overall kinetics and kinematics of the spine in cadaveric human lumbar spine studies. However, it increases the risk of NP herniation under axial loading.^15–17 As in cadaveric cervical studies,¹⁸ this technique becomes more unreliable due to the decreased ratio of disc height to needle diameter and the orientation of the end plates in animal studies, especially small animal models. In addition, because of the dramatic difference between the stiffness of the needle and the surrounding anulus, the natural stress distribution within the disc may be altered during the experimental procedure.

In vivo animal studies have shown that with a sufficiently thick needle, the normal physiology of the disc can be interrupted by a single anular puncture, which in turn causes a rapid and complete degeneration of the disc.¹⁹ Therefore, traditional methods of intradiscal pressure measurements have an intrinsic risk of influencing the results of an experiment.

Moreover, the measurement of disc pressure is difficult and not of high fidelity because the majority of the pressure transducers are contact based and directional. Therefore, the form of the anular layers and consistency of the nucleus and their interaction with the transducer influence the measurements. Due to the lack of visibility while the transducer is within the disc, control of these variables is usually not possible and their effect on the results remains unknown. Similarly, calibration of the transducer for the unique material properties of an individual disc is another limitation in contact-based transducers.

Researchers have vigorously sought improved devices and techniques for disc pressure measurement because of these limitations. Nesson et al. have developed a fiberoptic pressure sensor for intradiscal pressure measurement in rat spines.²⁰ The device has an outer diameter of 366 μm and can measure up to 70 kPa with the resolution of 0.17 kPa. Dennison et al. used a custom-made pressure sensor using a new fibre-Bragg gratings.²¹ Their device was 0.5 mm in diameter and able to measure up to 50 MPa. They placed the device, through a 25-G hypodermic needle, in human cadaveric lumbar discs and measured the pressure under 2000-N axial compression incrementally with a sensitivity of −0.7 to 1.07 kPa/N. Glos et al. utilized microelectromechanical systems (MEMS) technology to miniaturize the pressure sensor.²² The device, with an overall size of 3.2 × 15 × 1 mm, was composed of a stainless steel carrier and a silicon-based piezoresistive sensor (2 × 2 × 0.9 mm) mounted at the tip.

In conclusion, disc pressure is an indicator of the state of disc composition and a tool for assessing the kinetics and kinematics of the segmented spinal motion. With improving technology, the limitations intrinsic to the technique may be overcome and intradiscal pressure data may be used for improving surgical outcomes. Benzel et al., for instance, have studied wireless technology that can be implanted into a patient and used to monitor disc pressure telemetrically.²³ However, it appears that more in vitro studies need to be performed in order to establish the theoretical foundations for such technologies.

References

The complete reference list is available online at expertconsult.com.