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Summary: An in vivo murine experiment was conducted to measure the capacities of viable intervertebral disc cells to recruit inflammatory cells. The objective was to determine whether compounds secreted from viable cells induce inflammation or whether inflammation in disc herniation simply requires exposure to structural cell or matrix components. Three tissue preparations were inserted into the right lower peritoneal cavity of male mice: tissue with viable annulus fibrosus and nucleus pulposus cells, tissue with viable annulus fibrosus cells, or devitalized annulus fibrosus and nucleus pulposus tissue. Controls included sham-operated and nonoperated groups. Mice were killed 1, 2, or 7 days after surgery. Macrophage recruitment occurred after exposure to viable disc tissue but not after exposure to devitalized disc components; recruitment increased over time. Viable disc cells play a role in the etiology of inflammation in disc herniation.

In vivo creep-recovery and disc pressure measurements were performed on the lumbar spine of immature and mature swine. The creep-recovery measurements were performed using a custom materials testing apparatus designed to apply static or dynamic loads to the spine of anesthetized animals. A series of three separate experiments were performed to assess the effects of: (I) animal death, (II) graded injury to the disc anulus, and (III) respiratory mechanics on the biomechanical response of the porcine L1-L3 vertebral unit (VU). In Experiments I and II, creep rate, modulus, and viscosity parameters were computed using a three-parameter solid rheological analysis of the displacement-time response recorded during the application of a 300-N load. In Experiment III, the effects of respiratory volume and frequency changes on disc pressure were assessed in the unloaded, statically loaded, and immobilized porcine VU. Our results indicated that the adult VU tended to be stiffer, deform or creep more slowly, and had a significantly higher viscosity than the VU of immature pigs. The results of Experiment I demonstrated that the biomechanical response for the VU was significantly altered by the death of the animal; the VU of the living animal (adolescent or mature) was more compliant and deformed at a faster rate than the VU of the same animal after death. Disc injury produced changes in stiffness, viscosity, and creep rate analogous to that of aging, and on the basis of the graded injuries created in this study, it appears that a small defect in the annulus is just as deleterious as removing a large section of anular material. The results of Experiment III indicated that respiration plays an important role in the normal, in vivo mechanical and nutritional behavior of the porcine VU. Altogether, these results demonstrate that, in the absence of normal physiologic conditions, one may not be able to reliably predict the mechanical response of the lumbar spine, and suggest that standards for the testing, handling, and storage of biologic tissue should be established.

The in vitro viscoelastic "creep" behavior was examined in 18 cadaveric human lumbar motion segments subjected to static axial compressive loads. Axial deformation was followed for 30 min under constant applied load. Compressive material constants (moduli and viscosity coefficients) were then determined for each intervertebral disc using a linearization method based on a Taylor series expansion of experimental data for the "three parameter" viscoelastic creep model. The degree of disc degeneration and bone mineral content (BMC) were also assessed. Good correlation between the experimentally determined and model predicted strain values were found, with the average error less than 1%. We found that motion segments from older and more degenerated lumbar discs were less stable and had lower material constants than segments from younger and less degenerated discs. Material constants and BMC correlated closely, suggesting that an interdependency of disc and vertebral body properties exists. No correlation between the creep characteristics and disc height, disc area, segment level, or sex were noted.

The viscoelastic properties of normal and surgically altered lumbar intervertebral disks from eight immature swine were examined in vivo. Rheological models were used to mathematically characterize the compressive creep-relation behavior of the disks before and after alteration by either chemonucleolysis [chymopapain (Chymodyactin) injection] or denucleation. In the normal disks, a significant modulating effect of respiration was observed that tended to increase the creep deformation response in comparison to that observed during similar in vitro and in situ studies. These results suggest that, in terms of assessing the absolute magnitude of the viscoelastic properties of lumbar disks, the influence of normal physiological function on adjacent vertebrae and surrounding tissues cannot be neglected. Preliminary results obtained from the experimentally altered disks indicated that partial denucleation primarily affected the initial stiffness behavior of the disk (24% decrease in elastic modulus), whereas chemonucleolysis caused changes in both the time-dependent (15% increase in creep rate) and instantaneous (23% decrease in elastic modulus) properties of the disk. Both partial denucleation and acute chemonucleolysis produced biomechanical changes that were comparable to grade II or slightly degenerative, age-related changes found in human lumbar intervertebral disks.

Seventeen cadaveric human lumbar motion segments from eight spines were cyclically loaded in vitro under axial compression. Loading frequency and magnitude were chosen to simulate rigorous activity within an in vivo physiological level. The load magnitude was determined as a percentage of the ultimate compressive load, the latter estimated from the bone mineral content (BMC) of lumbar vertebrae determined by dual-photon absorptiometry. Following testing, the degree of macroscopic disc degeneration was assessed and the type of fracture in each specimen was determined from serial sagittal sections. Fractures were found in all but one specimen. Three types of fractures were formed: the node of Schmorl and Junghanns (type I), central endplate fracture (type II), and a crush or burst fracture (type III). The results suggested that type I fractures were predominantly associated with segments with normal discs, type II fractures were found primarily in segments with moderately degenerated discs, and type III fractures were associated with segments that failed on the first cycle. Segment stiffness and fatigue strength (cycles to failure) were correlated with disc degeneration, age, and segment BMC, the latter an in vivo measure of bone density. Fatigue strength also decreased in proportion to a power coefficient with increasing relative stress (cyclic stress range/ultimate stress).