MEASUREMENT OF DIFFUSION COEFFICIENTS IN THE NUCLEUS AND ANNULUS OF THE INTERVERTEBRAL DISC

Abstract

The aim of this study was to measure diffusion coefficients of solutes through the disc in relation to molecular weight.

The intervertebral disc is avascular thus nutrients and other factors from the blood supply are transported into the intervertebral disc by diffusive and convective flow. For small solutes such as lactate and glucose and oxygen, diffusion appears to predominate however convection may aid transport of larger molecules such as growth factors.

At present there however, there is virtually no information on diffusion of solutes of different molecular weights through the disc; this information is necessary for assessing and modelling transport pathways.

Diffusion coefficients were measured in nucleus and annulus sections of bovine intervertebral discs by a novel method which prevented tissue swelling and proteoglycan loss. Briefly strips of fluorescent or radiolabelled solute-saturated filter-paper were placed adjacent to the disc and the resulting concentration gradients measured at appropriate times. Solute sizes from 0.01 to 70 kDa were investigated. All results are reported as mean + s.e.m (n=6).

Diffusion coefficients (D) fell steeply with increase in molecular weight following a log-log relationship as predicted by theory. For small solutes (lactate) D for the outer annulus was 3.4 ± 1.1.10⁻⁶ cm²/sec while for 70 kDa dextran, D was 1.4 ± 0.6.10⁻⁷. There was no significant difference between values of D for nucleus and outer annulus for any solute.

Diffusion coefficients through the disc follow relationships seen in other cartilages and are dependant on tissue properties and molecular weight. The similarities between values for nucleus and outer annulus demonstrate the conflicting roles of proteoglycan and water contents in governing diffusion through the matrix with D decreasing both with increase in proteoglycan and decrease in water content.