Biomechanical Properties of Paraspinal Muscles and Their Influence on Spinal Loading: An Experimental and Computational Investigation

Adult spinal deformity (ASD) is prevalent among ~30% of the population above 65 years old. While decreased back extensor strength in the form of muscle weakness/dysfunction is welldocumented for these patients, it is unclear what muscle properties lead to such decreased strength. The overall goal of this research project was to investigate if biomechanical properties of the paraspinal muscles are different in ASD patients; and whether those differences could influence spinal loading and be associated with the initiation/progression of ASD. To achieve this goal, four studies were conducted. The first study examined the effect of spinal level on elastic modulus, slack sarcomere length, and collagen deposition in paraspinal muscles of 13 rats revealing independence of those properties on the spinal level. In the second study, the influence of the size of single muscle fibers and fiber bundles on elastic modulus was investigated for both rodents and humans. Smaller fibers and fiber bundles manifested larger elastic moduli, highlighting the necessity of aiming for consistent bundle sizes for passive property measurement. For the third study, ethical and technical requirements for intraoperative human muscle biopsy acquisition were addressed and paraspinal muscle biopsies were collected from nine ASD patients. Although patient-recruitment halted because of COVID-19 and thus the small number of patients did not allow performing a statistical comparison between the patients, thought-provoking observations were made. In situ- and slack sarcomere lengths had large variations; several fiber bundles exhibited substantially high stiffnesses, and histopathological analysis unveiled a variety of extracellular and intracellular case-specific abnormalities. The variations observed for the biomechanical properties were input to an enhanced musculoskeletal model of the thoracolumbar spine, which predicted increases in the intradiscal pressures by several orders of magnitudes in some cases. This fourth study highlighted the importance of biomechanical properties along with the muscle force-length curve to the spinal forces. The entire thesis demonstrated that biomechanical properties of paraspinal muscles do vary among ASD patients and this can dramatically influence the spinal loads. Therefore, future experimental and computational studies should be conducted to provide further insight on the potential role of these properties in the initiation/progression of ASD.

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Year

Entry

2001

Anderson FC, Pandy MG. Static and dynamic optimization solutions for gait are practically equivalent. J Biomech. February 2001;34(2):153-161.

1990

Pandy MG, Zajac FE, Sim E, Levine WS. An optimal control model for maximum-height human jumping. J Biomech. 1990;23(12):1185-1198.

2006

Damsgaard M, Rasmussen J, Christensen ST, Surma E, de Zee M. Analysis of musculoskeletal systems in the AnyBody modeling system. Simul Model Pract Theory. November 2006;14(8):1100-1111.

1984

Yang KH, King AI. Mechanism of facet load transmission as a hypothesis for low-back pain. Spine. September 1984;9(6):557-565.

1998

Vasavada AN, Li S, Delp SL. Influence of muscle morphometry and moment arms on the moment-generating capacity of human neck muscles. Spine. February 15, 1998;23(4):412-422.