• Shun Yamamoto, Masoud Malakoutian, Marine Theret, John Street, Fabio Rossi, BrownStephen H MSHMDepartment of Human Health and Nutritional Sciences, University of Guelph, Guelph, Canada., Mitsuru Saito, and Thomas R Oxland.
• Department of Orthopaedics, The University of British Columbia, Vancouver, Canada.
• Spine. 2021 Nov 1; 46 (21): E1125E1135E1125-E1135.

Study DesignPreclinical study in rodents.ObjectiveTo investigate changes in biomechanical properties of paraspinal muscles following a posterior spinal surgery in an animal model.Summary Of Background DataPosterior spine surgery damages paraspinal musculature per histological and imaging studies. The biomechanical effects of these changes are unknown.Methods12 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.ResultsA 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).ConclusionThe 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.Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.

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