Low back pain (LBP) is a commonly observed condition and is one of the most serious physiological concerns worldwide [1]. Trunk muscles are important for normal spinal function and are etiologically significant in LBP [2]. In particular, the multifidus muscle (Mm)—the most medially located back muscle and the largest muscle that spans the lumbosacral junction—maintains the erector posture of the trunk as well as abducts and rotates the trunk [3]. Furthermore, intervertebral disc degeneration (IVDD) is considered the principal tissue responsible for LBP [4].

Recently, several studies have performed quantitative evaluation of the Mm fatty degeneration with magnetic resonance spectroscopy (MRS) [5-10] and IVDD with magnetic resonance imaging (MRI) T2 mapping [11-13]. First, MRS analysis of muscle physiology has facilitated detailed analyses of the muscular fat masses by recording the concentration of intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL) [8-10,14-17]. IMCL cannot be visually detected using conventional MRI because they are stored in the form of spheroid droplets in the cytoplasm of muscle cells in close contact with skeletal mitochondria [9,10] and are directly used as an energy source by the mitochondria. Physical inactivity adversely affects skeletal muscle metabolism, manifesting as poor oxidative capacity [18,19], lower glycogen storage, and decreased capillary density [20]. In contrast, EMCL are defined as more or less compact areas of adipose tissue in the subcutaneous layers or along fasciae and are considered metabolically inactive lipid deposits involved in the reduced functionality associated with obesity and a sedentary lifestyle [8,14,15]. Lost muscle strength is associated with EMCL accumulation, which can interfere with sufficient muscle nutrition [8,15,16]. In our previous study [5-7], IMCL using the MRS analyses in the Mm of patients with chronic low back pain (CLBP) was significantly higher than that in the control subjects. Next, MRI T2 mapping utilizes the T2 values for the quantification of moisture content and the collagen sequence breakdown of intervertebral disc (IVD). In a previous work [12], we used MRI T2 mapping to quantify the extent of IVDD, which showed a decrease in the T2 values with increase in the Pfirrmann classification grade [21]. Visual evaluation of the annulus fibrosus (AF) is difficult with ordinary MRI. However, this strategy using T2 values enables us to quantify the degeneration of the AF.

Although fatty degeneration of the Mm as well as IVDD should be a key structural feature of spinal degeneration, few reports exist regarding the MRI analysis of the relation between the Mm and IVDD. The purpose of this cross-sectional quantitative MRI study was to determine the association between the IMCL and EMCL of the Mm and IVDD.

The institutional review board of the Sapporo Medical University approved this study (IRB approval no., 262-1074). All the subjects were provided written and verbal explanations of the study and their consent was obtained before study participation.

The mean body mass index was 24.6±0.5 kg/m². The mean T2 values in IVD were 63.4±2.7 ms for the anterior AF, 73.5±3.3 ms for NP, and 58.0±1.9 ms for the posterior AF. The mean IMCL of the Mm was 10.6±1.62 (×10²) mmol/L, and the mean EMCL was 3.72±0.41 (×10³) mmol/L. There was a significant negative correlation between the IMCL and T2 values of anterior AF (r=−0.65, p<0.01) (Fig. 4A). There were no significant correlations between IMCL and T2 values of NP (r=−0.16, p=0.30) (Fig. 4B) and posterior AF (r=0.07, p=0.62) (Fig. 4C). There were no significant correlations between the EMCL and T2 values of anterior AF (r=−0.11, p=0.46) (Fig. 5A), NP (r=0.15, p=0.32) (Fig. 5B), and posterior AF (r=0.07, p=0.66) (Fig. 5C). Table 1 shows multiple linear regression analysis adjusted for age and sex. There was a significant negative correlation between the IMCL and T2 values of anterior AF (standardized partial regression coefficient=−0.65, p<0.01).

The Mm and IVD are important for the proper functioning of the lumbar spinal column and degeneration in either impairs function [22,23]. Few MRI studies have examined the association between paraspinal muscle fatty degeneration and IVDD. Kader et al. [22] showed a tendency of Mm fatty degeneration being associated with the number of degenerated discs. Teichtahl et al. [23] revealed that severe disc degeneration at all intervertebral levels was associated with high fat content of the paraspinal muscles. Sun et al. [24] reported that Mm atrophy and IVDD were positively correlated at the L3/L4 disc level. However, these studies used visual evaluation to evaluate muscular fatty degeneration and IVDD.

In this study, we used quantitative MRI evaluation methods, such as MRS and MRI T2 mapping, and demonstrated a correlation between IMCL of the Mm and T2 values of anterior AF. Previously, we have identified the associations of spino-pelvic alignment (lumbar lordosis and sagittal vertical axis) with the IMCL of the Mm [5]. The Mm provides two-thirds of the spinal segmental stability [25] and is important for maintaining spinal alignment [26]. In addition, we evaluated the extent of IVDD and compared this with the T2 values in degenerative spondylolisthesis (DS) and no-spondylolisthesis groups; we found that the T2 values decreased in the anterior AF of IVD in the DS group [13]. It is speculated that dysfunction of the Mm leads to anterior gliding because of reduced resistance to anterior shear forces. More mechanical stress loaded at the anterior part of the IVD might lead to pressure on the anterior AF. Thus, the Mm as well as anterior AF of IVD are important for proper functioning of the lumbar spinal column and are cross-correlated to each other.

IVDD [27] and collapsed vertebral bodies [28] irreversibly develop with advancing age; however, an increase in the IMCL of the Mm can probably improve if conservative treatments, such as therapeutic exercise, are initiated. There is a correlation between IMCL of the Mm and the T2 values of anterior AF; therefore, it is expected that therapeutic exercises might have the potential to identify novel targets for the treatment and prevention of IVDD.

This study has certain limitations. First, this study has a cross-sectional design; thus, it cannot be determined whether associations between IMCL increase of the Mm and IVDD are a cause or the result of one another. Longitudinal studies will help address such concerns. Second, we did not perform any evaluation about other factors involved in IVDD, such as smoking, diabetes mellitus, hypertension, and physical activity.

The results indicated that the IMCL of the Mm might be accompanied with posterior AF degeneration. Therapeutic exercises using IMCL of the Mm as evaluation index might have the potential to identify novel targets for the treatment and prevention of IVDD.