Research Papers

Effect of Hydration on Healthy Intervertebral Disk Mechanical Stiffness

[+] Author and Article Information
Semih E. Bezci

Department of Mechanical Engineering,
University of California, Berkeley,
2166 Etcheverry Hall,
Berkeley, CA 94720
e-mail: bezsem11@berkeley.edu

Aditya Nandy

Department of Chemical and
Biomolecular Engineering,
University of California, Berkeley,
2166 Etcheverry Hall,
Berkeley, CA 94720
e-mail: aditya.nandy@berkeley.edu

Grace D. O'Connell

Department of Mechanical Engineering,
University of California, Berkeley
2166 Etcheverry Hall,
Berkeley, CA 94720
e-mail: g.oconnell@berkeley.edu

Manuscript received December 25, 2014; final manuscript received August 19, 2015; published online September 3, 2015. Assoc. Editor: James C. Iatridis.

J Biomech Eng 137(10), 101007 (Sep 03, 2015) (8 pages) Paper No: BIO-14-1647; doi: 10.1115/1.4031416 History: Received December 25, 2014; Revised August 19, 2015

The intervertebral disk has an excellent swelling capacity to absorb water, which is thought to be largely due to the high proteoglycan composition. Injury, aging, degeneration, and diurnal loading are all noted by a significant decrease in water content and tissue hydration. The objective of this study was to evaluate the effect of hydration, through osmotic loading, on tissue swelling and compressive stiffness of healthy intervertebral disks. The wet weight of nucleus pulposus (NP) and annulus fibrosus (AF) explants following swelling was 50% or greater, demonstrating significant ability to absorb water under all osmotic loading conditions (0.015 M–3.0 M phosphate buffered saline (PBS)). Estimated NP residual strains, calculated from the swelling ratio, were approximately 1.5 × greater than AF residual strains. Compressive stiffness increased with hyperosmotic loading, which is thought to be due to material compaction from osmotic-loading and the nonlinear mechanical behavior. Importantly, this study demonstrated that residual strains and material properties are greatly dependent on osmotic loading. The findings of this study support the notion that swelling properties from osmotic loading will be important for accurately describing the effect of degeneration and injury on disk mechanics. Furthermore, the tissue swelling will be an important consideration for developing biological repair strategies aimed at restoring mechanical behavior toward a healthy disk.

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Grahic Jump Location
Fig. 1

(a) Osmolality for each saline group. Osmolality increased linearly with PBS concentration (slope = 1851 mOsm/kg/M). (b) Percent change in saline osmolality after tissue swelling experiment (c) Swelling ratio was negatively correlated with the external osmotic environment (Pearson's: ρ < −0.55, p ≤ 0.001). Inset: Representative disk showing location and size of NP and AF tissue cores. * represents significant differences between the NP and AF swelling ratio at each osmotic condition (t-test, p < 0.01). (d) Swelling ratio normalized by the swelling ratio of the 0.15 M PBS group showed no significant differences between NP and AF explants (t-test: p = 0.4).

Grahic Jump Location
Fig. 2

(a) Residual stress from applied osmotic loading condition with respect to the estimated residual stretch (least-squares curve fit, R2 > 0.999). (b) Hydration correlated with residual stretch (Pearson correlation: NP: r = −0.99, p = 0.0001, AF: r = −0.88, p = 0.1). Values for 0.15 M PBS group are shown on each figure by the respective data point.

Grahic Jump Location
Fig. 3

(a) Representative sample in MTS device. Inset: Motion segments were potted in bone cement to ensure parallel-loading surfaces, and then placed in a saline bath (osmotic concentration range = 0.015 M to 3.0 M PBS) for mechanical testing. (b) Force–displacement curves from a representative motion-segment. Disk joint stiffness increased with an increase in saline osmolality. The dashed and solid red lines represent the toe- and linear-regions, respectively.

Grahic Jump Location
Fig. 4

Stiffness measured from slow ramp compression to 1000 N. (a) Overall displacement measured during compression tests, normalized by the displacement measured in the 0.15 M PBS group. (b) Toe- and (c) linear-region moduli with respect to the saline concentration. The mechanical behavior with respect to saline concentration can be described using the equations provided in the figure. All parameters demonstrated a moderate significant correlation with osmotic loading (Pearson's: p < 0.01).

Grahic Jump Location
Fig. 5

Average creep response under (a) 200 N and (b) 1000 N load. Differences in stiffness and time constant, as determined by a rheological model, are reported in Table 1.

Grahic Jump Location
Fig. 6

Rate-dependent change in apparent modulus measured during slow-ramp compression (0.55 N/s) and during the ramp to apply 1000 N for creep (40 N/s). (a) Toe-region apparent modulus was not rate dependent. (b) The linear-region apparent modulus increased by twofold with an increase in loading rate (* represents t-test p < 0.01). Data is presented as mean ± standard deviation.



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