Material properties of the degenerative intervertebral disk components

Degenerative changes in structural properties of the motion segment

The function of the motion segment is to provide the spine with axial stability while allowing mobility. The intervertebral disk is responsible for carrying enormous amounts of compressive loading while maintaining flexibility. The load on the disk is mainly compressive, but it is also subjected to other types of loads such as tensile and shear stresses. As the compressive load is subjected to the disk, hydrostatic pressure develops within the inner core of gelatinous nucleus pulposus, which pushes outward causing the outer ring of fibrous anulus fibrosus to bulge and experience tensile stress in the fibers. Loads on the lumbar disk (L3/4) of volunteers performing different body postures as well as disk pressures have been measured in vivo. These studies revealed that the load on the L3/4 level disk in a sitting position and in a standing position with 20° of flexion was 250% of the total body weight, although the portion of the body above the L3/4 level represented only 60%. Such large loads have been validated with mathematical models. This finding suggests that the load on the lumbar disk is composed of external and internal inputs. The external load is the weight of the body above the lumbar disk, and the internal load is the muscle force required to stabilize the spine in different postures. Increases in disk pressure should also be expected when a fluid is injected, as Andersson and Schultz showed when they inquired about the effects of injecting saline in a disk, and found varied responses in cases in which the injected fluid was retained, notably the large increases in pressure (up to 83%). In contrast, a decrease in pressure was observed in the degenerated disk.

Several animal models have been established to investigate degenerative changes in structural properties of the lumbar motion segment. A commonly used mechanical damage method to cause disk degeneration is the needle puncture or stab wound. Several researchers have recently arrived at the same conclusions when reporting that the diameter of the wound has to be large enough to create degeneration. Korecki and colleagues showed that, in an in vitro cyclic testing setting, bovine disks showed immediate and progressive differences in the dynamic modulus and stiffness of the anulus fibrosus tissue after puncture. Aside from the lamellar disturbances, cell viability and matrix remodeling were observed. Another animal model (ovine) of disk degeneration from induced lesions has also shown regeneration in the midwall of the anulus fibrosus. In a different loading condition, a murine tail model has also shown differences in the anulus fibrosus tissue as a consequence of dynamic compression, but did not achieve degenerate disk quality after long cycles of compression. These reports suggest that puncture injuries lead to degenerative remodeling including granulation tissue, which current image-based diagnostics methods might not be able to distinguish.

MacLean and colleagues investigated static viscoelastic behaviors of rat caudal motion segments, vertebrae, and isolated disk explants in different permeability conditions and showed that differences in endplate permeability conditions had a significant effect on the viscoelastic behaviors. Johannessen and colleagues showed a decrease in stress relaxation after 10,000 cycles of compressive loading in adult sheep lumbar motion segments and recovery of the stress relaxation after 18 hours of unloading in phosphate-buffered saline solution, suggesting intervertebral disk fluid transport during loading and unloading.

Boxberger and colleagues used a degenerative disk model in rat by injection of chondroitinase-ABC to the disks. In this model, nucleus pulposus degeneration has been successfully induced with GAG loss as a consequence of chondroitinase-ABC injection to the disks, which were tested in a linear viscoelastic tension/compression regime afterward. Results showed that the dynamic stiffness was decreased at low loads. Nucleus pulposus GAG content was shown to be related to the neutral zone properties in the tension-compression cyclic tests. However, the tension and compression extremes of the load displacement curve were not shown to be related. This finding shows that a degenerate nucleus produces hypermobility in addition to low pressures. Such distortion in load sharing leads to the development of hoop stresses in the anulus that resist compressive loads.

Kim and colleagues used a rabbit degenerative disk model with 18-gauge needle puncture of the disk to investigate changes in dynamic viscoelastic properties of the whole disk associated with the disk degeneration. In this model, the proteoglycan content decreased and collagen content increased 4 weeks after puncture. The dynamic viscoelastic test showed a decrease in elastic and viscous properties in the punctured disk ( Fig. 1 ). The correlation study showed that the proteoglycan content positively correlated with the elastic and viscous mechanical properties and height of the disk; however, there was no correlation with the collagen content. These results suggest that the proteoglycan is a governing factor for viscoelastic properties and structural properties of the disk.

Experimental test chamber for an unconfined dynamic compression experiment to record viscoelastic properties of a rabbit disk. The bone-disc-bone complex was secured between 2 porous pucks that prevented friction of the endplates with respect to these structures. Disks can be altered chemically to promote and recover from degeneration. Their dynamic viscoelastic properties can be assessed in this way. IVD, intervertebral disk.

Using the same rabbit degenerative disk model and the dynamic viscoelastic testing method, Miyamoto and colleagues investigated the effects of OP-1 injection in the lumbar disk on the biomechanical and biochemical restoration of the disk. In this study, a significant increase in wet weight and proteoglycan content was observed in both nucleus pulposus and anulus fibrosus tissues of the OP-1-injected disks, compared with the lactose-injected control disks, whereas an increase in collagen content was observed only in the nucleus pulposus. These results suggested that increased proteoglycan content, induced by the injection of OP-1, resulted in tissue hydration in both the nucleus pulposus and anulus fibrosus. The results of the dynamic viscoelastic test showed that the elastic modulus has a significant positive correlation with the proteoglycan content in the nucleus pulposus and the proteoglycan and collagen content in the anulus fibrosus. Similarly, the viscous modulus was shown to have a significant positive correlation with the proteoglycan content in the nucleus pulposus and the proteoglycan and collagen content in the anulus fibrosus.

Degenerative changes in structural properties of the motion segment

The function of the motion segment is to provide the spine with axial stability while allowing mobility. The intervertebral disk is responsible for carrying enormous amounts of compressive loading while maintaining flexibility. The load on the disk is mainly compressive, but it is also subjected to other types of loads such as tensile and shear stresses. As the compressive load is subjected to the disk, hydrostatic pressure develops within the inner core of gelatinous nucleus pulposus, which pushes outward causing the outer ring of fibrous anulus fibrosus to bulge and experience tensile stress in the fibers. Loads on the lumbar disk (L3/4) of volunteers performing different body postures as well as disk pressures have been measured in vivo. These studies revealed that the load on the L3/4 level disk in a sitting position and in a standing position with 20° of flexion was 250% of the total body weight, although the portion of the body above the L3/4 level represented only 60%. Such large loads have been validated with mathematical models. This finding suggests that the load on the lumbar disk is composed of external and internal inputs. The external load is the weight of the body above the lumbar disk, and the internal load is the muscle force required to stabilize the spine in different postures. Increases in disk pressure should also be expected when a fluid is injected, as Andersson and Schultz showed when they inquired about the effects of injecting saline in a disk, and found varied responses in cases in which the injected fluid was retained, notably the large increases in pressure (up to 83%). In contrast, a decrease in pressure was observed in the degenerated disk.

Several animal models have been established to investigate degenerative changes in structural properties of the lumbar motion segment. A commonly used mechanical damage method to cause disk degeneration is the needle puncture or stab wound. Several researchers have recently arrived at the same conclusions when reporting that the diameter of the wound has to be large enough to create degeneration. Korecki and colleagues showed that, in an in vitro cyclic testing setting, bovine disks showed immediate and progressive differences in the dynamic modulus and stiffness of the anulus fibrosus tissue after puncture. Aside from the lamellar disturbances, cell viability and matrix remodeling were observed. Another animal model (ovine) of disk degeneration from induced lesions has also shown regeneration in the midwall of the anulus fibrosus. In a different loading condition, a murine tail model has also shown differences in the anulus fibrosus tissue as a consequence of dynamic compression, but did not achieve degenerate disk quality after long cycles of compression. These reports suggest that puncture injuries lead to degenerative remodeling including granulation tissue, which current image-based diagnostics methods might not be able to distinguish.

MacLean and colleagues investigated static viscoelastic behaviors of rat caudal motion segments, vertebrae, and isolated disk explants in different permeability conditions and showed that differences in endplate permeability conditions had a significant effect on the viscoelastic behaviors. Johannessen and colleagues showed a decrease in stress relaxation after 10,000 cycles of compressive loading in adult sheep lumbar motion segments and recovery of the stress relaxation after 18 hours of unloading in phosphate-buffered saline solution, suggesting intervertebral disk fluid transport during loading and unloading.

Boxberger and colleagues used a degenerative disk model in rat by injection of chondroitinase-ABC to the disks. In this model, nucleus pulposus degeneration has been successfully induced with GAG loss as a consequence of chondroitinase-ABC injection to the disks, which were tested in a linear viscoelastic tension/compression regime afterward. Results showed that the dynamic stiffness was decreased at low loads. Nucleus pulposus GAG content was shown to be related to the neutral zone properties in the tension-compression cyclic tests. However, the tension and compression extremes of the load displacement curve were not shown to be related. This finding shows that a degenerate nucleus produces hypermobility in addition to low pressures. Such distortion in load sharing leads to the development of hoop stresses in the anulus that resist compressive loads.

Kim and colleagues used a rabbit degenerative disk model with 18-gauge needle puncture of the disk to investigate changes in dynamic viscoelastic properties of the whole disk associated with the disk degeneration. In this model, the proteoglycan content decreased and collagen content increased 4 weeks after puncture. The dynamic viscoelastic test showed a decrease in elastic and viscous properties in the punctured disk ( Fig. 1 ). The correlation study showed that the proteoglycan content positively correlated with the elastic and viscous mechanical properties and height of the disk; however, there was no correlation with the collagen content. These results suggest that the proteoglycan is a governing factor for viscoelastic properties and structural properties of the disk.

Experimental test chamber for an unconfined dynamic compression experiment to record viscoelastic properties of a rabbit disk. The bone-disc-bone complex was secured between 2 porous pucks that prevented friction of the endplates with respect to these structures. Disks can be altered chemically to promote and recover from degeneration. Their dynamic viscoelastic properties can be assessed in this way. IVD, intervertebral disk.

Using the same rabbit degenerative disk model and the dynamic viscoelastic testing method, Miyamoto and colleagues investigated the effects of OP-1 injection in the lumbar disk on the biomechanical and biochemical restoration of the disk. In this study, a significant increase in wet weight and proteoglycan content was observed in both nucleus pulposus and anulus fibrosus tissues of the OP-1-injected disks, compared with the lactose-injected control disks, whereas an increase in collagen content was observed only in the nucleus pulposus. These results suggested that increased proteoglycan content, induced by the injection of OP-1, resulted in tissue hydration in both the nucleus pulposus and anulus fibrosus. The results of the dynamic viscoelastic test showed that the elastic modulus has a significant positive correlation with the proteoglycan content in the nucleus pulposus and the proteoglycan and collagen content in the anulus fibrosus. Similarly, the viscous modulus was shown to have a significant positive correlation with the proteoglycan content in the nucleus pulposus and the proteoglycan and collagen content in the anulus fibrosus.

Instability of the motion segment associated with intervertebral disk degeneration

As disk degeneration progresses, structural failure of the disk is manifested by tears and clefts in the anulus fibrosus. These material disruptions occur in different directions and are the result of a variety of influencing factors, including altered loading of the disk. Potential relationships between osteophytes and peripheral tears were first reported by Schmorl and Junghanns, and also highlighted that, because of the tears, segmental instability would be affected. Farfan and Kirkaldy-Willis concluded that tears were by-products of torsional stresses, implicating them as initiators of the failure of other disk components in the disk degeneration cascade.

Disk fissures have been classified in 3 categories, depending on their morphology and anatomic position in the disk: (1) peripheral tears or rim lesions, parallel to the endplates and exhibiting normal separation of the disk from the subchondral bone of the vertebral body, which, with time, developed (2) circumferential tears that present evidence of delamination as a failure mode; and (3) radial tears that, as the name implies, propagate in a direction perpendicular to an imaginary axis of the disk (if it is considered as a flat cylinder), which usually lead to disk herniations and expulsion of nucleus material. The literature shows few reports that address the crack propagation phenomena involved in disk tears, because it is common to report only the resulting condition (ruptured anulus, herniated disk). Many of the models consider the anulus as a bulk material but, recently, more advanced models have incorporated annular layers and implemented permeability and porosity, as well as the disk’s osmoviscoelastic properties. The interlamellar structures have been deemed especially sensitive to shear stresses, and the literature is lacking reports of their allegedly weaker mechanical properties. They are believed to play a predominant role when destructive processes such as delamination occur as part of herniation, as has been attempted in analytical models of the disk, of anulus fibrosus tissue, and of individual lamellae. Schollum and colleagues recently analyzed in detail the interface between annular lamellae in an ovine model by subjecting thin slices of immature and mature anulus fibrosus tissue to microtensile tests. Although their studies mostly described the architecture of the interlamellar interface, important differences in the response to tensile forces were shown between young and old tissue, with the older tissue exhibiting a more ordered and uniform lamellar separation than the young tissue; however, the investigators did not report elastic properties.

Fujiwara and colleagues studied the effect of disk degeneration graded by magnetic resonance imaging on the segmental motion of the lumbar spine using a total of 106 motion segments obtained from 44 cadaveric lumbar spines taken from 18 women and 25 men with a mean age of 69 years. The investigators found that segmental motion increased with increasing severity of disk degeneration to grade 4, but decreased when the disk degeneration advanced to grade 5. Such segmental motion changes were greater in axial rotation compared with those in lateral bending, flexion, and extension, showing the importance of torsional instability in diagnosing spinal instability. The results of these studies are important for understanding the kinematic property changes in relation to the types or grades of disk degeneration. The results were consistent with the previous reports and the concept of 3 stages of spinal degeneration: dysfunction, instability, and restabilization proposed by Kirkaldy-Willis and Farfan.

Instability of the lumbar spine associated with intervertebral disk degeneration

Segmental instability of the lumbar spine is frequently considered a cause of LBP, but instability of the spine is poorly defined and understood. The basic concept of spinal instability is that excessive motion beyond normal constraints causes either compression or stretching of the neural elements or causes abnormal deformations of ligaments, joint capsules, annular fibers, or endplates, which are known to have a significant number of nociceptors. Even though several studies have indicated that excessive motion on flexion/extension radiographs is associated with LBP or degenerative disk disease, other studies cite decreased motion in patients with degenerative changes and such pain.

Lumbar segmental instability may be associated with a spectrum of clinical manifestations of degenerative changes in the intervertebral disk. Intervertebral disk degeneration has been studied using magnetic resonance (MR) imaging, and grades of degeneration have been reported. The relationship between the types (or grades) of disk degeneration and kinematic characteristics of the motion segment has been studied using cadaveric spinal motion segments. Despite some variation in results, likely because different loading conditions and methods of grading degenerative disk changes were used, the overall results of these studies indicate that the biomechanical characteristics of the motion segment can become altered significantly when degenerative changes develop in the intervertebral disk.

In vivo Measurement of Lumbar Segmental Movement

There have been numerous in vivo studies on segmental instability of the lumbar spine in which dynamic flexion/extension radiographs were used. However, these dynamic radiographic techniques have been found to be inaccurate. The errors associated with sagittal plane translational motion measurement reported in the literature range from 1 to 4 mm, or 3% to 15% of the vertebral depth. Schaffer and colleagues reported high false-positive and false-negative rates (ie, normal translations are categorized outside of the normal range and vice versa) with significant differences between measurement methods despite high reliability across radiographic quality, raters, and measurement. More sophisticated techniques, such as biplanar stereoradiography, centrode pattern analysis, and traction-compression radiography, have been introduced but have not been widely accepted. More accurate methods involve invasive techniques by inserting metal beads or spinous process wire to determine three-dimension (3-D) motion. However, because these methods are invasive, they are not appropriate for routine use in clinical practice or for in vivo human studies. Studies on segmental instability have also been limited by other factors in addition to these problems associated with accurate measurement of segmental motion in vivo. For example, the range of motion measured in most of these studies is affected by the variability in voluntary efforts that the subject applies at the time of examination and can also be limited because of pain.

Other two-dimensional (2-D) imaging methods for measuring axial rotation, as opposed to flexion/extension, have involved MR imaging of subjects in various rotated positions. Although these studies were noninvasive and controlled for voluntary motions, they could only determine changes in segmental motion around 1 axis. It has been suggested that coupled motions could play an important role in determining spinal instability. To measure these coupled motions, studies have been conducted to measure 3-D motions in vivo. More invasive techniques involve inserting wires into the spinous process of subjects to determine 3-D motion. Although this method has proved more accurate than radiographs, its invasive nature limits its widespread clinical use. Other studies have used biplanar radiography, in which the radiograms of the spine were taken from 2 directions simultaneously and 3-D motions are calculated by the positions of anatomic landmarks in corresponding images. There has been some concern about the accuracy in determining anatomic landmarks for biplanar radiography, as well as a lack of equipment for this method in typical clinical settings.

To overcome some of these limitations to 3-D motion measurement, Lim and colleagues developed a 3-D imaging technique using dynamic computed tomography (CT) to determine 6-degree-of-freedom (3 rotations and 3 translations) transformation of individual cadaveric cervical vertebrae during motion by tracking eigenvectors of the individual vertebrae. The investigators showed that accurate measurements (±1 mm and ±1°) can be made using CT in vitro. The research group expanded on this technique to measure vertebral segmental movements in human lumbar spines in vivo ( Fig. 2 ). Although this method was able to determine the rotations and translations of the lumbar vertebrae during motion in vivo, it was limited in determining transformation of the sacrum because an entire 3-D CT model of a sacrum is difficult to obtain clinically and the eigenvector analysis using a partial-sacrum 3-D model caused an error of the measurement of the segmental motion at L5 to S1. The same research group developed another method to determine transformation of individual vertebrae including the sacrum during motion using the 3-D CT model and a volume merge method ( Fig. 3 ). This method can determine the rotations and translations during motion even if the 3-D geometry of the bone is incomplete, as in the case of the sacrum (an assumption of the bone as a rigid body still holds) at each position. Thus, it is able to determine the transformation of the incomplete sacrum 3-D CT model with an accuracy of less than 0.1 mm in translation and 0.2° in rotation.

A subject’s positioning inside a CT gantry to study torso rotation. Straps hold subject onto a torso rotation apparatus and CT records evidence of coupled motion during torsion. Segmental movements are level dependent and the pattern of segmental movement is different between healthy subjects and subjects with LBP.

Description of the volume merge method for analysis of segmental movement. A vertebral body in the neutral position (a) was virtually rotated (b) and translated (c) towards the rotated position. The position was refined with 0.05° and 0.05 mm increments, respectively, until the maximized volume merging was determined (d). A voxel with a dimension of 1.0 × 1.0 × 1.0 mm was created for each point of the stationary target. The number of points of the moving vertebra ( white dots ) that fell within the voxel of the stationary target ( yellow dots ) was determined and the percentage of volume merge was defined: (e) no volume merge, (f) volume merge achieved within the voxel region of interest.