Study of load in the sacroiliac joint during dynamic simulation of movements in the lumbar spine on skeletal muscle models after posterior bisegmental fusion

Abstract

Background. Complications are the main concern of patients and surgeons when considering spine surgery. One of the risk factors for complications in the thoracic and lumbar spinal segments, as well as segments adjacent to those with fusion, is changes in sagittal spinal-pelvic balance. Objective: to determine the effect of muscle changes that occur during surgical access for posterior bisegmental LIV-SI fusion on the load of the iliac crest surface in the sacroiliac joint. Materials and methods. Dynamic motion simulation modeling was performed using OpenSim software with the use of additional programs to calculate loading forces. The complete model of the human musculoskeletal system was taken as a basis. To compare the load force, four models were created: 1 — basic, all spinal motion segments are fully functional, 2 — fixation without changing the anatomical curves of the spine, 3 — fixation in the position of hyperlordosis; 4 — fixation with reproduction of hyperlordosis. For models 2–4, changes in the muscles were made that correspond to the effects of surgical posterior access to the lumbar spine for posterior instrumented fusion LIV-SI. The load on the area of interest was measured as the value of the projection of the force vector depending on the angle of torso inclination as a percentage of body weight. Results. Muscle strength and function were the same for all types of instrumental spinal fusion, and trauma during access was not taken into account. In model 1 (normal) with the upright position, the projection of the load force falls on the center of gravity of the vertebra. When tilted, the load force in the sagittal direction acts exclusively on the anterior ilium with a slight shift of 10 % forward. In normosthenic and hyperlordotic fixation, there is a shift in the projection of the load force on the posterior iliac crest in the upright position and its displacement to the center with the tilt. The displacement of the load center with the upright position in normosthenic fixation is associated with the exclusion of some extensor muscles from the calculation of fibers, which reduces their total strength and leads to sagittal imbalance with an increase in lordosis. Hypolordotic fixation (model 4) slightly shifts the projection of the load force in the upright position (by 3 %) and approaches the normal values of model 1 when tilted. Regarding vertical loads, for all models with muscle integrity impairment (models 2, 3, 4), the load in the upright position is greatly increased — on average by 60 % compared to the norm, with a decrease in body weight by 40–45 % when tilted. Conclusions. It has been proved that the load force on the surface of the iliac crest in the sacroiliac joint depends on the angle of instrumental fusion performed. The greatest changes are observed with the displacement of the load center during upright standing in the sagittal direction. In normosthenic and hyperlordotic fixation with decreased back muscle strength, there is a load shift to the posterior part of the iliac crest. In hypolordotic fixation, the center of the load remains close to the central position. The distribution of the load in the vertical direction is primarily influenced by the reduced strength of the back muscles, which increases the load by 60 %.