Refinement of Elastic, Poroelastic, and Osmotic Tissue Properties of Intervertebral Disks to Analyze Behavior in Compression

Abstract

Intervertebral disks support compressive forces because of their elastic stiffness as well as the fluid pressures resulting from poroelasticity and the osmotic (swelling) effects. Analytical methods can quantify the relative contributions, but only if correct material properties are used. To identify appropriate tissue properties, an experimental study and finite element analytical simulation of poroelastic and osmotic behavior of intervertebral disks were combined to refine published values of disk and endplate properties to optimize model fit to experimental data. Experimentally, nine human intervertebral disks with adjacent hemi-vertebrae were immersed sequentially in saline baths having concentrations of 0.015, 0.15, and 1.5 M and the loss of compressive force at constant height (force relaxation) was recorded over several hours after equilibration to a 300-N compressive force. Amplitude and time constant terms in exponential force–time curve-fits for experimental and finite element analytical simulations were compared. These experiments and finite element analyses provided data dependent on poroelastic and osmotic properties of the disk tissues. The sensitivities of the model to alterations in tissue material properties were used to obtain refined values of five key material parameters. The relaxation of the force in the three bath concentrations was exponential in form, expressed as mean compressive force loss of 48.7, 55.0, and 140 N, respectively, with time constants of 1.73, 2.78, and 3.40 h. This behavior was analytically well represented by a model having poroelastic and osmotic tissue properties with published tissue properties adjusted by multiplying factors between 0.55 and 2.6. Force relaxation and time constants from the analytical simulations were most sensitive to values of fixed charge density and endplate porosity.

Osmotic Pressure Alters Time-dependent Recovery Behavior of the Intervertebral Disc

“…The increase in water uptake and disc height recovery with hypoosmotic pressure agrees well with data in the literature. 13,27 Mechanical loading 13 and loading from osmotic pressure during recovery hinder fluid-flow into the disc and disc height recovery (Figure 2A). For example, disc recovery in the 1.5 M PBS group exhibited features of loading after 4 hours of recovery, suggesting a change in fluid-flow direction from inflow to outflow.…”

“…3,23,25 -27 Stokes and coworkers observed a decrease in joint stiffness with an increase in water uptake during stress-relaxation (constant applied displacement). 27 Vergroesen et al 24 demonstrated that the effect of osmotic pressure on water absorption was highly dependent on magnitude of the applied load. However, it is not clear how osmotic pressure affects disc recovery behavior under low loading conditions that simulate bed-rest conditions.…”

An Anisotropic Multiphysics Model for Intervertebral Disk

“…Varying the external osmolarity is an important approach to investigate the contribution of fixed charges to the overall mechanical function of the tissue. Stokes et al [57] studied the response of human lumbar disks under constant compressive strain, in bath with varying saline concentrations. They observed that varying of external osmolarity from low to high causes a stress relaxation phenomenon.…”

The feasibility of using compression bioimpedance measurements to quantify peripheral edema

“…Recently, interest has grown in attempts at quantitative assessment of tissue hydration using BIS in many disease states [1,2,3]. In addition, the tissue response to stress in the setting of edema has recently been the subject of study as a quantitative tool for edema assessment [4,5,6]. The examination for early stage/pitting peripheral edema has been performed for centuries by grasping and pressing on the patient's extremity.…”