Dynamic compression effects on intervertebral disc mechanics and biology

Abstract

Study design: A bovine intervertebral disc organ culture model was used to study the effect of dynamic compression magnitude on mechanical behavior and measurement of biosynthesis rate, cell viability, and mRNA expression.

Objective: The objective of this study was to examine the effect of loading magnitude on intervertebral disc mechanics and biology in an organ culture model.

Summary of background data: The in vivo and cell culture response of intervertebral disc cells to dynamic mechanical loading provides evidence the disc responds in a magnitude dependent manner. However, the ability to link mechanical behavior of the disc with biologic phenomena has been limited. A large animal organ culture system facilitates measurements of tissue mechanics and biologic response parameters on the same sample allowing a broader understanding of disc mechanobiology.

Methods: Bovine caudal intervertebral discs were placed in organ culture for 6 days and assigned to a static control or 1 of 2 dynamic compression loading protocols (0.2-1 MPa or 0.2-2.5 MPa) at 1 Hz for 1 hour for 5 days. Disc structure was assessed with measurements of dynamic modulus, creep, height loss, water content, and proteoglycan loss to the culture medium. Cellular responses were assessed through changes in cell viability, metabolism, and qRT-PCR analyses.

Results: Increasing magnitudes of compression increased disc modulus and creep; however, all mechanical parameters recovered each day. In the anulus, significant increases in gene expression for collagen I and a trend of increasing sulfate incorporation were observed. In the nucleus, increasing gene expression for collagen I and MMP3 was observed between magnitudes and between static controls and the lowest magnitude of loading.

Conclusion: Results support the hypothesis that biologic remodeling precedes damage to the intervertebral disc structure, that compression is a healthy loading condition for the disc, and further support the link between applied loading and biologic remodeling.

Figure 1

Test protocol schematic detailing loading protocol (top). Illustration showing tissue harvest protocol (bottom). Note: Schematic diagram not to scale.

Figure 2

Average ± SEM sulfate incorporation rates for all testing groups (OA, outer annulus; IA, inner annulus; NP, nucleus pulposus). A trend of increasing sulfate incorporation was seen in the anulus regions, but not in the nucleus.

Figure 3

Representative viability images at 20× magnification. Black = live, white = dead. Scale bar in black = 400 μm. Columns represent tissue regions (OA, outer annulus; IA, inner annulus; NP, nucleus pulposus) and rows represent test groups. No changes in cell viability were observed.

Figure 4

Average ± SEM nominal dynamic loading modulus for preload (top) and postload (bottom) tests.

Figure 5

Average ± SEM height loss for the 1 hour dynamic loading protocol. Significantly more height was lost at all time points in the high force dynamic loading groups than in the low force group.

Figure 6

Gene expression fold change results as average ± SEM for anabolic (aggrecan, collagen types I and II, versican), anticatabolic (TIMP-1) (left column), and catabolic (MMP −2, −3, −13, and ADAMTS-4) (right column) genes for tissue from anulus fibrosus (top row) and nucleus pulposus (bottom row) regions. Significant differences (P < 0.05) between groups are marked with a * whereas significant differences from static controls (using a t test with hypothesized mean of zero) are marked with a § symbol (found in low groups in the NP for collagen type I and MMP3). Note that low and high groups are normalized to tail matched static controls.