Biomedical Engineering Advances (Dec 2021)

A finite element study of the relationship between upper body weight and the loads experienced by the human lumbosacral spine, and fusion instrumentation, in a standing upright posture

  • Justin M Warren, MS, PhD,
  • Lloyd A Hey, MD, MS,
  • Andre P Mazzoleni, PhD

Journal volume & issue
Vol. 2
p. 100023

Abstract

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Background: A quantitative relationship between increasing upper body weight and both 1) non-instrumented lumbosacral joint loading and 2) fusion instrumentation reactions is established using a three-dimensional (3D) finite element model (FEM) of the human thoracolumbosacral spine. Methods: A 3D FEM of the thoracolumbosacral spine (both non-instrumented and instrumented with fusion instrumentation) is developed and validated against cadaveric spine specimens. Upper body weight loading is applied to simulate an individual with varying levels of body mass index (BMI) including 25 kg/m2, 30 kg/m2, 35 kg/m2, and 40 kg/m2. Maximum force/moment joint reactions from L1–S1 in the non-instrumented model are recorded. Maximum instrumentation reactions in the fusion model are recorded. Findings: With BMI of 25 kg/m2 as a reference, nearly all joints experience amplified percent increases in joint loading compared to percent weight gain for each level of obesity studied. Considering spinal fusion instrumentation, rod stresses and pedicle screw reactions likewise experience amplified loading increases compared to body weight. Compressive force at L5–S1 increases by 47%, 86%, and 132% for total body weight increases of 20%, 40%, and 60%, respectively. Fusion rod stresses increase by 47%, 86%, and 128% for the same body weight increases. Interpretations: This paper quantitatively illustrates the importance of nutrition and weight control as part of a comprehensive approach to low back pain management and treatment. Increases in loading on the joints in the lumbosacral spine and in fusion instrumentation are amplified when compared to the corresponding increases in body weight for various clinical levels of obesity. These results illustrate that moderate weight loss can produce significant lower back and spine benefits.

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