Poster Presentation 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

INTRODUCTION

Computed tomography (CT) and magnetic resonance imaging (MRI) provide insight into paraspinal muscle structural changes associated with spine degeneration and deformity, enabling a non-invasive assessment of muscle quality. Previous literature has linked poor paraspinal muscle health to low back disorders in humans, but lengthy and variable progression of these disorders makes muscle contractile data difficult to obtain. Chondrodystrophic canines present a naturally occurring model of spine degeneration and deformity, providing an alternative option to study. The purpose of the current work was to explore the interrelationship between muscle quality and contractile function in canines experiencing intervertebral disc extrusion. It was hypothesized that significant relationships would exist between imaging-based measures of muscle quality and single muscle fibre contractile function.

METHODS

20 paraspinal muscle biopsies were collected from 20 chondrodystrophic canines (erector spinae on surgical side) during a hemilaminectomy surgery to treat intervertebral disc extrusion. Muscle biopsies were chemically permeabilized and single fibres were extracted (an average of 20 fibres per biopsy were tested). Single fibres were maximally activated in a calcium bath and specific force (i.e. normalized force generating capability) was determined. Prior to surgery, canines either underwent a thoracolumbar CT (n=11) or a thoracolumbar MRI (n=9). Erector spinae quality at the surgical level was assessed by calculating a muscle:fat index on T1-weighted MRI and muscle density on CT. Pearson correlations were calculated between the average erector spinae specific force for each canine and the corresponding muscle:fat index (MRI) or muscle density (CT).

RESULTS

A trending moderate positive correlation (r=0.577, p=0.08; Figure 1A) was observed between muscle fibre specific force and the CT measure of muscle density. Similarly, a trending moderate positive correlation (r=0.652, p=0.08; Figure 1B) was observed between specific force and muscle:fat index on T1 weighted MRI.  

DISCUSSION

Surprisingly, muscle quality measures from CT and MRI provided opposing relationships with single fibre contractile function. Higher muscle density, as measured by CT, is generally associated with higher muscle quality (i.e. less fatty infiltration). However, a higher muscle:fat index, as calculated here, indicates greater fatty infiltration. Thus, higher muscle quality measured via CT was significantly associated with stronger single muscle fibres, while lower muscle quality measured via MRI was significantly associated with stronger single muscle fibres. Both relationships have physiological plausibility: higher quality muscle possesses stronger single muscle fibres, or lower quality muscle possesses stronger single muscle fibres to compensate for the lost strength of neighbouring degenerated fibres. Continued work will probe this relationship deeper.