Investigation of the passive mechanical properties of spine muscles following disruption of the thoracolumbar fascia and erector spinae aponeurosis, as well as facet injury in a rat

The effect of fatty infiltration, revision surgery, and sex on lumbar multifidus passive mechanical properties

Purpose

Previous research has demonstrated increased stiffness in the multifidus muscle compared to other paraspinal muscles at the fiber bundle level. We aimed to compare single fiber and fiber bundle passive mechanical properties of multifidus muscle: (1) in 40 patients undergoing primary versus revision surgery and (2) in muscle with mild versus severe fatty infiltration.

Methods

The degree of muscle fatty infiltration was graded using the patients' spine magnetic resonance images. Average single fiber and fiber bundle passive mechanical properties across three tests were compared between primary (N = 30) and revision (N = 10) surgery status, between mild and severe fatty infiltration levels, between sexes, and with age from passive stress-strain tests of excised multifidus muscle intraoperative biopsies.

Results

At the single fiber level, elastic modulus was unaffected by degree of fatty infiltration or surgery status. Female sex (p = 0.001) and younger age (p = 0.04) were associated with lower multifidus fiber elastic modulus. At the fiber bundle level, which includes connective tissue around fibers, severe fatty infiltration (p = 0.01) and younger age (p = 0.06) were associated with lower elastic modulus. Primary surgery also demonstrated a moderate, but non-significant effect for lower elastic modulus (p = 0.10).

Conclusions

Our results demonstrate that female sex is the primary driver for reduced single fiber elastic modulus of the multifidus, while severity of fatty infiltration is the primary driver for reduced elastic modulus at the level of the fiber bundle in individuals with lumbar spine pathology.

Sex-Related Pain Behavioral Differences following Unilateral NGF Injections in a Rat Model of Low Back Pain

Low back pain (LBP) is a globally prevalent and costly societal problem with multifactorial etiologies and incompletely understood pathophysiological mechanisms. To address such shortcomings regarding the role of neurotrophins in the underlying mechanisms of pain, an LBP model was developed in rats involving two unilateral intramuscular injections of nerve growth factor (NGF) into deep trunk muscles. To date, behavioral investigations of this NGF-LBP model have been limited, especially as it pertains to female pain behaviors. This study compared mechanical sensitivity to noxious (hyperalgesia) and non-noxious (hypersensitivity) stimuli in control and NGF-injected male and female rats through pain resolution. Although the baseline testing revealed no differences between males and females, NGF-injected females demonstrated prolonged ipsilateral deep trunk mechanical hyperalgesia that resolved seven days later than males. Moreover, females showed bilateral trunk mechanical sensitivity to noxious and non-noxious stimuli compared to only ipsilateral behaviors in males. Sex differences were also observed in the severity of behavioral responses, with females displaying greater mean differences from baseline at several timepoints. Overall, these NGF-LBP behavioral findings mirror some of the sex differences reported in the clinical presentation of LBP and accentuate the translatability of this NGF-LBP model. Future studies using this LBP-NGF model could help to elucidate the neurobiological mechanisms responsible for the development, severity, and/or resolution of muscular LBP as well as to provide insights into the processes governing the transition from acute to chronic LBP.

Somatosensory behavioral alterations in a NGF-induced persistent low back pain model

Low back pain (LBP) is a major global burden in part due to the underlying pathophysiological mechanisms being poorly understood. A LBP rat model involving two injections of nerve growth factor (NGF, an endogenous pain-related neurotrophin) into trunk musculature was recently developed. Additional behavioral work in this NGF-LBP rat model is required to better characterize local and remote somatosensory alterations related to NGF-induced peripheral and central sensitization. This work characterizes the time-dependent development of hypersensitivity to trunk and hindpaw cutaneous mechanical stimulation and deep muscle mechanical hyperalgesia in adult male Sprague-Dawley rats (n = 6/group). Behavioral assays were performed at baseline (Day 0, D0), D2, D5 (pre- and 4 h post-2nd NGF or control injection), D7, D10, and D14 in NGF and control groups. Trunk and hindpaw cutaneous mechanical hypersensitivity were tested using von Frey filaments. Deep trunk mechanical hyperalgesia was determined using a small animal algometer. NGF rats demonstrated increased cutaneous sensitivity to ipsilateral trunk mechanical stimuli at D7, D10, and D14. NGF rats also demonstrated ipsilateral deep mechanical hyperalgesia on D2, D5 + 4 h, D7, D10, and D14. Cutaneous hypersensitivity was delayed compared to deep hyperalgesia in NGF rats. No additional sensory changes were noted. Together, these results indicate that male mechanical somatosensory changes develop primarily locally in the ipsilateral trunk following unilateral NGF injections. These findings contrast with a previous report in female rats using this NGF-LBP model showing more widespread (bilateral) hyperalgesia and remote mechanical hypersensitivity. Future studies will examine potential sex-related pain behavioral differences in the NGF model.

Systematic review of skeletal muscle passive mechanics experimental methodology

Understanding passive skeletal muscle mechanics is critical in defining structure-function relationships in skeletal muscle and ultimately understanding pathologically impaired muscle. In this systematic review, we performed an exhaustive literature search using PRISMA guidelines to quantify passive muscle mechanical properties, summarized the methods used to create these data, and make recommendations to standardize future studies. We screened over 7500 papers and found 80 papers that met the inclusion criteria. These papers reported passive muscle mechanics from single muscle fiber to whole muscle across 16 species and 54 distinct muscles. We found a wide range of methodological differences in sample selection, preparation, testing, and analysis. The systematic review revealed that passive muscle mechanics is species and scale dependent-specifically within mammals, the passive mechanics increases non-linearly with scale. However, a detailed understanding of passive mechanics is still unclear because the varied methodologies impede comparisons across studies, scales, species, and muscles. Therefore, we recommend the following: smaller scales may be maintained within storage solution prior to testing, when samples are tested statically use 2-3 min of relaxation time, stress normalization at the whole muscle level be to physiologic cross-sectional area, strain normalization be to sarcomere length when possible, and an exponential equation be used to fit the data. Additional studies using these recommendations will allow exploration of the multiscale relationship of passive force within and across species to provide the fundamental knowledge needed to improve our understanding of passive muscle mechanics.

Paraspinal muscle pathophysiology associated with low back pain and spine degenerative disorders

Low back pain disorders affect more than 80% of adults in their lifetime and are the leading cause of global disability. The muscles attaching to the spine (ie, paraspinal muscles) are critical for proper spine health and play a crucial role in the functioning of the spine and whole body; however, reports of muscle dysfunction and insufficiency in chronic LBP (CLBP) patients are common. This article presents a review of the current understanding of the relationship between paraspinal muscle pathophysiology and spine-related disorders. Human literature demonstrates a clear association between altered muscle structure/function, most notably fatty infiltration and fibrosis, and low back pain disorders; other associations, including muscle cell atrophy and fiber type changes, are less clear. Animal literature then provides some mechanistic insight into the complex relationships, including initiating factors and time courses, between the spine and spine muscles under pathological conditions. It is apparent that spine pathology can directly lead to changes in the paraspinal muscle structure, function, and biology. It also appears that changes to the muscle structure and function can directly lead to changes in the spine (eg, deformity); however, this relationship is less well studied. Future work must focus on providing insight into possible mechanisms that regulate spine and paraspinal muscle health, as well as probing how muscle degeneration/dysfunction might be an initiating factor in the progression of spine pathology.

Investigating how combined multifidus injury and facet joint compression influence changes in surrounding muscles and facet degeneration in the rat

PURPOSE

To examine whether unilateral multifidus damage could promote degeneration at the L5-6 facet joint (FJ) and compensatory changes in lumbo-pelvic muscles in rats.

METHODS

12 facet clamp, 12 facet sham and 7 control rats were studied. Facet clamp and sham animals had the left L5-6 FJ exposed, and the clamp group had a mild compressive clamp applied using hemostatic forceps to model post-traumatic arthritis. Both groups then had the left multifidus detached from the L1-L6 spinous processes. Animals were euthanized 28 days post-surgery. Muscle mass and fascicle length were evaluated bilaterally for the paraspinal muscles, gluteal muscles and biceps femoris. Intra-muscular collagen of the paraspinal muscles was measured histologically. FJ transverse plane angles were measured from micro-computed tomography scans. L5-6 FJ degeneration was evaluated through the 24-point OARSI scale.

RESULTS

Differences, compared to control, were observed in the detached multifidus from both facet clamp and sham groups; namely decreased mass and fascicle length and increased collagen content. However, no between group differences were found for any other muscle. Further, mild FJ degeneration was more prevalent in the groups that had experienced multifidus injury but was not exacerbated by the mild compressive clamping of the FJ.

CONCLUSION

Unilateral multifidus injury with or without FJ compressive clamping does not have a clear impact on the characteristics of surrounding spinal musculature within 28 days post-surgery in rats. Mild FJ degeneration was present in some animals from all three groups, and the impact of multifidus injury on this degeneration is inconclusive.

Experimentally induced spine osteoarthritis in rats leads to neurogenic inflammation within neurosegmentally linked myotomes

Naturally occurring spine osteoarthritis is clinically associated with the manifestation of chronic inflammatory muscle (myofascial) disease. The purpose of this study was to investigate the causal association between experimentally induced spine osteoarthritis and neurogenic inflammatory responses within neurosegmentally linked myotomes. Wistar Kyoto rats were randomly assigned to spine facet compression surgery (L4-L6) or sham surgery. Animals exposed to facet compression surgery demonstrated radiographic signs of facet-osteoarthritis (L4-L6 spinal levels) and sensory changes (allodynia, thermal hyperalgesia) at 7, 14 and 21 days post-intervention, consistent with the induction of central sensitization; no radiologic or sensory changes were observed after sham surgery. Increased levels of proinflammatory biomarkers including substance P (SP), calcitonin gene related peptide (CGRP), protease-activated receptor-2 (PAR2) and calcium/calmodulin dependent protein kinase II (CaMKII) were observed post-surgery within neurosegmentally-linked rectus femoris (L2-L5) muscle when compared to the non-segmentally linked biceps brachii (C4-C7) muscle; no differences were observed between muscles in the sham surgery group. These findings offer novel insight into the potential role of spine osteoarthritis and neurogenic inflammatory mechanisms in the pathophysiology of chronic inflammatory muscle (myofascial) disease.

The effect of vertebral level on biomechanical properties of the lumbar paraspinal muscles in a rat model

INTRODUCTION

Passive mechanical properties of the paraspinal muscles are important to the biomechanical functioning of the spine. In most computational models, the same biomechanical properties are assumed for each paraspinal muscle group, while cross-sectional area or fatty infiltration in these muscles have been reported to differ between the vertebral levels. Two important properties for musculoskeletal modeling are the slack sarcomere length and the tangent modulus. This study aimed to investigate the effect of vertebral level on these biomechanical properties of paraspinal muscles in a rat model.

METHODS

The left paraspinal muscles of 13 Sprague-Dawley rats were exposed under anesthesia. Six muscle biopsies were collected from each rat: three from multifidus (one per each of the L1, L3, and L5 levels) and similarly three from longissimus. Each biopsy was cut into two halves. From one half, two to three single muscle fibers and two to six muscle fiber bundles (14 ± 7 fibers surrounded in their connective tissue) were extracted and mechanically tested in a passive state. From the resulting stress-strain data, tangent modulus was calculated as the slope of the tangent at 30% strain and slack sarcomere length (beyond which passive force starts to develop) was recorded. The other half of each biopsy, which represented the muscle at the fascicle level, was snap frozen, sectioned, stained for Collagen I and its area fraction was measured. To evaluate the effect of spinal level on these biomechanical properties of multifidus and longissimus, one-way repeated measures ANOVA (p < 0.05) was performed for tangent modulus and slack sarcomere length, while for collagen I content linear mixed-models analysis was adopted.

RESULTS

In total, 192 fibers and 262 fiber bundles were mechanically tested. For both muscle groups, no significant difference in tangent modulus of the single fibers was detected between the three spinal levels (p = 0.9 for multifidus and p = 0.08 for longissimus). Similarly, the tangent modulus values for the fiber bundles were not significantly different between the three spinal levels (p = 0.13 for multifidus and p = 0.49 for longissimus). In both muscle groups, the slack sarcomere lengths were not different among the spinal levels except for multifidus fibers (p = 0.02). Collagen I area fraction in muscle fascicles averaged 6.8% for multifidus and 5.3% for longissimus and was not different between the spinal levels.

DISCUSSION

The results of this study highlighted that the tangent modulus, slack sarcomere length, and collagen I content of the lumbar paraspinal muscles are independent of spinal level. This finding provides the basis for the assumption of similar mechanical properties along a paraspinal muscle group.