Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

Introduction

Paraspinal muscles are vital to the functionality of the spine and recent work has identified the importance of passive stiffness (i.e. elastic modulus) of these muscles on spinal loading [1]. Therefore, it is important to determine the factors that contribute most to the passive elastic modulus of these muscles. Since fibers (Fs) and fiber bundles (FBs) are basic constituents of muscle, it is valuable to explore their mechanical properties as a model system, which will eventually be used to inform the whole muscle passive mechanical properties.  This study aimed to measure passive mechanical properties of individual Fs and FBs from longissimus and multifidus human muscles to explore potential effects of cross-sectional area (CSA) and number of fibers (N_f) in their passive elastic moduli.

Method

This study examined the fiber and fiber bundle elastic moduli of multifidus and longissimus from the L4–L5 level of four adult spinal deformity patients (two males and two females, aged 69 ± 10 years). Biopsies were collected during surgical exposure and preserved before conducting a total of 144 tests, with each specimen providing six Fs and twelve FBs.

Tensile tests were conducted on specimens within 2 weeks of collection [2]. The oval cross-sectional area was determined through measurements of top and side diameters of specimens. Subsequently, the number of fibers in bundles was counted after completing the tests. The stress-strain data were fitted to a quadratic curve to calculate elastic modulus (E) of fiber bundles at 30% strain (E₃₀). Also, simple and multiple linear regressions were used to assess the effect of CSA and N_f on elastic modulus.

Results

The average of collected stress-strain data for individual fibers showed a linear correlation between stress and strain, while fiber bundles indicated a polynomial relation for both muscles (Fig. 1). The linear regression analysis between E₃₀ and CSA for both Fs and FBs revealed a negative correlation (p-value_{Multifidus Fs/FBs}>0.08, p-value_{Longissimus Fs/FBs}<0.006) (Fig. 2). Additionally, multiple linear regression considering both N_f and CSA showed a high p-value for the N_f effect on E₃₀ in both muscles (p-value>0.18).

Discussion

Quadratic relation between stress and strain in FBs compared to linear relation in Fs highlights the role of extracellular matrix (ECM) in muscle's passive properties. Linear regression for both Fs and FBs indicates that increasing the area led to decreased E; however, it shows no significant effect on Multifidus Fs/FBs, whereas it exhibits a significant effect on Longissimus muscle. Previous studies predicted that a higher elastic modulus in the basement membrane relative to the fiber's contractile area increased the elastic modulus for smaller fibers [3], and the lower E of larger bundles may result from their ECM content [2]. Moreover, the multiple regression for FBs indicates that N_f has no significant effect on E₃₀ in addition to CSA in both muscles. In general, the factors influencing the passive properties of muscles are variable and considering other factors such as fiber size, ECM content, and fiber’s mechanical quality are crucial for a comprehensive understanding of the muscles' passive mechanical properties.