EVALUATION OF QUANTITATIVE MRI, BIOCHEMICAL AND MECHANICAL PROPERTIES OF TRYPSIN-TREATED INTERVERTEBRAL DISCS UNDER PHYSIOLOGICAL COMPRESSION LOADING

Abstract

Despite a relentless search for adequate and effective treatment, low back pain is one of the most prevalent and costly illness in today’s society. While disc degeneration has been implicated as a major etiologic component of low back pain, there has been relatively little study in developing an objective, accurate, non-invasive diagnostic tool in the detection and quantification of matrix changes in early disc degeneration. The aim of the present study was to establish the correlations between magnetic resonance (MR) parameters and the biochemical and mechanical properties of the nucleus pulposus (NP) undergoing targeted trypsin digestion and axial compression.

Three-disc segments from bovine tails were either unloaded or loaded (cyclic compression: 50N-300N-50N at 1 Hz for 16h) to evaluate the effect of compression loading and the interactive effects of trypsin treatment and mechanical loading. The MR examinations were carried out in a 1.5-Tesla whole-body Siemens Avanto System (Siemens AG, Germany). The frozen NP and annulus fibrosus (AF) tissue sections reserved for mechanical analysis were tested under confined compression; swelling pressure was calculated based on the increase in measured force throughout the initial dwell period. Total water, proteoglycan, collagen, and denatured collagen contents were also measured.

Results showed that loading had a significant effect on the MR properties (T1, T2, T1ñ, MTR, ADC) of both disc tissues. Loading had a greater effect on the MR parameters and biochemical composition of the NP than trypsin. In contrast, trypsin had a larger effect on the mechanical properties. Results also indicated that localised trypsin injection predominantly affected the NP. T1ñ was sensitive to loading and correlated with the water content of the NP and AF but not with their proteoglycan content.

Results support the concept that physiologic loading is an important confounder and that T1ñ is an essential parameter in efforts to develop quantitative MRI as a non-invasive diagnostic tool to detect and quantify matrix and material changes in early disc degeneration. Further studies are required to determine the potential of the T1ñ technique to be used as a non-invasive diagnostic tool of the biochemical and mechanical changes occurring in disc degeneration.