Effects of enzymatic digestion on compressive properties of rat intervertebral discs

Abstract

Enzymatic treatments were applied to rat motion segments to establish structure-function relationships and determine mechanical parameters most sensitive to simulated remodeling and degeneration. Rat caudal and lumbar disc biomechanical behaviors were evaluated to improve knowledge of their similarities and differences due to their frequent use during in vivo models. Caudal motion segments were assigned to four groups: soaked (control), genipin treated, elastase treated, and collagenase treated. Fresh lumbar and caudal discs were also compared. The mechanical protocol involved five force-controlled loading stages: equilibration, cyclic compression-tension, quasi-static compression, frequency sweep, and creep. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastin's role was greatest in tension and at higher force conditions, where GAG content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions. Equilibration creep and cyclic compression-tension tests were the mechanical tests most sensitive to alterations in specific matrix constituents. Caudal and lumbar motion segments had many similarities but biomechanical differences suggested some distinctions in collagenous structure and water transport characteristics in addition to the geometric differences. Results provide a basis for interpreting biomechanical changes observed in animal model studies of degeneration and remodeling, and underscore the need to maintain and/or repair collagen integrity in IVD health and disease.

Fig. 1

(a) Example motion segment potted and pinned to prevent swelling prior to chemical treatment, (b) Motion segment attached to the loading apparatus with custom grips, (c) Experimental loading apparatus with a fluid chamber, a 3-axis positioning stage and custom grips.

Fig. 2

Mechanical protocol where stage A is equilibration, B is cyclic compression-tension, C is quasi-static compression, D is dynamic compression at 1 Hz, and E is creep.

Fig. 3

Sample trilinear fits to data from the compression-tension conditioning stage, illustrating representative differences between (a) Soaked (Control), (b) elastase treated, (c) Collagenase treated and (d) genipin-treated IVD.

Fig. 4

Transient deformation patters of (a) equilibration (stage A) at 0.15 MPa and (b) creep (stage E) at 1 MPa for simulated stretched exponential fit using mean parameters for each experimental group.