Paraspinal muscle morphology and composition in adolescent idiopathic scoliosis: A histological analysis

Abstract

Background: Adolescent idiopathic scoliosis (AIS) is a condition resulting in spinal deformity and tissue adaptation of the paraspinal muscles. Although prior studies have demonstrated asymmetries in fiber type and other energetic features of muscle on the concave side of the curve, muscle morphology, architecture, and composition have not been evaluated. Therefore, the purpose of this study was to compare differences in paraspinal muscle microarchitecture and composition between concave and convex sides of a scoliotic curve in individuals with AIS.

Methods: Paraspinal muscle biopsies were obtained at the apex of the scoliotic curve in 29 individuals with AIS undergoing surgical deformity correction. Histological assays were performed to quantify fiber size, evidence of muscle degeneration and regeneration, and tissue composition (proportion of muscle, collagen, and fat). Differences between contralateral muscle samples were compared using two-tailed paired Student's t tests, and relationships between clinical characteristics (age and curve severity) and muscle characteristics were investigated using Pearson correlations.

Results: Muscle fibers were significantly larger on the convex side of the curve apex (P = .001), but were lower than literature-based norms for healthy paraspinal muscle. There were no differences in amount of degeneration/regeneration (P = .490) or the proportion of muscle and collagen (P > .350) between the concave and convex sides, but high levels of collagen were observed. There was a trend toward higher fat content on the concave side (P = .074). Larger fiber size asymmetries were associated with greater age (r = .43, P = .046), and trended toward an association with greater curve severity (r = .40, P = .069).

Conclusions: This study demonstrates that although muscle fibers are larger on the convex side of the scoliotic curve in AIS, muscles on both sides are atrophic compared to non-scoliotic individuals, and demonstrate levels of collagen similar to severe degenerative spinal pathologies. These findings provide insight into biological maladaptations occurring in paraspinal muscle in the presence of AIS.

Keywords: adolescent idiopathic scoliosis; deformity; muscle; spine.

FIGURE 1

Representative hematoxylin and eosin (H&E) image (×20) of muscle biopsies from the concave (A) and convex (B) sides of the curve in a single patient. Morphological features including collagen (Co), fat (*), and muscle fibers with centralized nuclei (black arrows) are highlighted. Black line is 100 μm

FIGURE 2

Mean (SD) fiber areas (A) and percentages of centralized nuclei (B) on the concave (black) and convex (gray) sides of the apical scoliotic curve. The dotted lines indicate literature‐based normative values for healthy skeletal muscle, and * indicates a significant difference between concave and convex sides (P < .05)

FIGURE 3

Mean (SD) of tissue composition proportions between concave (black) and convex (gray) sides of the scoliotic curve for muscle (A), collagen (B), and fat (C). + indicates a trend toward a significant difference between sides (P < .10)

FIGURE 4

Correlation plots between fiber size asymmetries between concave and convex sides (x‐axis) and both age; r = .43 (A) and Cobb angle; r = .40 (B). A solid line indicates a significant relationship (P < .05), whereas a dotted line indicates a trend (P < .10)