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

Dysfunctional paraspinal muscles in adult spinal deformity patients lead to increased spinal loading

Purpose

Decreased spinal extensor muscle strength in adult spinal deformity (ASD) patients is well-known but poorly understood; thus, this study aimed to investigate the biomechanical and histopathological properties of paraspinal muscles from ASD patients and predict the effect of altered biomechanical properties on spine loading.

Methods

68 muscle biopsies were collected from nine ASD patients at L4–L5 (bilateral multifidus and longissimus sampled). The biopsies were tested for muscle fiber and fiber bundle biomechanical properties and histopathology. The small sample size (due to COVID-19) precluded formal statistical analysis, but the properties were compared to literature data. Changes in spinal loading due to the measured properties were predicted by a lumbar spine musculoskeletal model.

Results

Single fiber passive elastic moduli were similar to literature values, but in contrast, the fiber bundle moduli exhibited a wide range beyond literature values, with 22% of 171 fiber bundles exhibiting very high elastic moduli, up to 20 times greater. Active contractile specific force was consistently less than literature, with notably 24% of samples exhibiting no contractile ability. Histological analysis of 28 biopsies revealed frequent fibro-fatty replacement with a range of muscle fiber abnormalities. Biomechanical modelling predicted that high muscle stiffness could increase the compressive loads in the spine by over 500%, particularly in flexed postures.

Discussion

The histopathological observations suggest diverse mechanisms of potential functional impairment. The large variations observed in muscle biomechanical properties can have a dramatic influence on spinal forces. These early findings highlight the potential key role of the paraspinal muscle in ASD.

The Effect of Posterior Lumbar Spinal Surgery on Biomechanical Properties of Rat Paraspinal Muscles 13 Weeks After Surgery

STUDY DESIGN

Preclinical study in rodents.

OBJECTIVE

To investigate changes in biomechanical properties of paraspinal muscles following a posterior spinal surgery in an animal model.

SUMMARY OF BACKGROUND DATA

Posterior spine surgery damages paraspinal musculature per histological and imaging studies. The biomechanical effects of these changes are unknown.

METHODS

12 Sprague-Dawley rats were divided equally into sham and surgical injury (SI) groups. For sham, the skin and lumbodorsal fascia were incised at midline. For SI, the paraspinal muscles were detached from the vertebrae, per normal procedure. Thirteen weeks postsurgery, multifidus and longissimus biopsies at L1, L3, and L5 levels were harvested on the right. From each biopsy, three fibers and three to six bundles of fibers (∼10-20 fibers ensheathed in their extracellular matrix) were tested mechanically to measure their passive elastic modulus. The collagen content and fatty infiltration of each biopsy were also examined histologically by immunofluorescence staining. Nonparametric statistical methods were used with a 1.25% level of significance.

RESULTS

A total of 220 fibers and 279 bundles of fibers were tested. The elastic moduli of the multifidus and longissimus fibers and longissimus fiber bundles were not significantly different between the SI and sham groups. However, the elastic modulus of multifidus fiber bundles was significantly greater in the SI group compared to sham (SI median 82 kPa, range 23-284; sham median 38 kPa, range 23-50, P = 0.0004). The elastic modulus of multifidus fiber bundles in the SI group was not statistically different between spinal levels (P = 0.023). For histology, only collagen I deposition in multifidus was significantly greater in the SI group (median 20.8% vs. 5.8% for sham, P < 0.0001).

CONCLUSION

The surgical injury increased the passive stiffness of the multifidus fiber bundles. Increased collagen content in the extracellular matrix is the likely reason and these changes may be important in the postoperative compensation of the spine.Level of Evidence: N/A.

Larger muscle fibers and fiber bundles manifest smaller elastic modulus in paraspinal muscles of rats and humans

The passive elastic modulus of muscle fiber appears to be size-dependent. The objectives of this study were to determine whether this size effect was evident in the mechanical testing of muscle fiber bundles and to examine whether the muscle fiber bundle cross-section is circular. Muscle fibers and fiber bundles were extracted from lumbar spine multifidus and longissimus of three cohorts: group one (G1) and two (G2) included 13 (330 ± 14 g) and 6 (452 ± 28 g) rats, while Group 3 (G3) comprised 9 degenerative spine patients. A minimum of six muscle fibers and six muscle fiber bundles from each muscle underwent cumulative stretches, each of 10% strain followed by 4 minutes relaxation. For all specimens, top and side diameters were measured. Elastic modulus was calculated as tangent at 30% strain from the stress–strain curve. Linear correlations between the sample cross sectional area (CSA) and elastic moduli in each group were performed. The correlations showed that increasing specimen CSA resulted in lower elastic modulus for both rats and humans, muscle fibers and fiber bundles. The median ratio of major to minor axis exceeded 1.0 for all groups, ranging between 1.15–1.29 for fibers and 1.27–1.44 for bundles. The lower elastic moduli with increasing size can be explained by relatively less collagenous extracellular matrix in the large fiber bundles. Future studies of passive property measurement should aim for consistent bundle sizes and measuring diameters of two orthogonal axes of the muscle specimens.