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

Muscle tissue is known to remodel in response to changes to its mechanical environment. Alterations in passive mechanical properties of muscles can influence spine stiffness and stability. This study aimed to determine whether passive muscle elastic moduli and passive muscle stresses increased 28 days following mechanical disruption of the thoracolumbar fascia and erector spinae aponeurosis, and injury induced by facet joint compression. Male Sprague Dawley rats were randomly assigned to three groups (Incision n=8; Injury n=8; and Control n=6). The thoracolumbar fascia and erector spinae aponeurosis were incised in the Incision and Injury groups to expose the left L5–L6 facet joint. In the Injury group, this facet was additionally compressed for three minutes to induce facet injury and cartilage degeneration. Twenty-eight days after surgery, rats were sacrificed and muscle samples were harvested from lumbar and thoracic erector spinae and multifidus for mechanical testing. Histologic staining revealed mild cartilage degeneration and boney remodeling in the Injury group. However, the hypotheses that either (1) disruption of the thoracolumbar fascia and erector spinae aponeurosis (Incision group) or (2) the addition of facet compression (Injury group) would increase the passive elastic modulus and stress of surrounding muscles were rejected. There was no effect of surgery (Incision or Injury) on the passive elastic modulus (p=.6597). Passive muscle stresses were also not different at any sarcomere length between surgical groups (p>.7043). Disruption of the thoracolumbar fascia and erector spinae aponeurosis and mild facet damage do not lead to measurable changes in passive muscle mechanical properties within 28 days. These findings contribute to our understanding of how spine muscles are affected by injury and fundamental aspects of the initial stages of spine surgery.