Biomechanical effects of iliopsoas muscle hypertrophy on the lumbar spine: finite element analysis study

In our study; lumbar 1-5, sacral vertebrae, pelvis, femur, iliopsoas, multifidus; were modeled with SolidWorks ® slime program in accordance with the original anatomical structures. As a reference, a computerized tomography image of a 33-year-old, 175 cm tall, 75 kg individual who had no previous trauma or surgery; who did not have any chronic disease was taken as the basis. According to this model; 4 separate models were created in order to monitor the effects of the differences in the balance of iliopsoas and multifidus muscle strengths on the discs and facets. The reference model (model 1) where both muscle strengths were modeled as normal, the multifidus muscle was at normal reference values and the iliopsoas was strengthened by 30% (hypertrophy), and the model 2 where both muscle groups were strengthened by 30% ( hypertrophy ). Hypertrophic model 3, in which it is simulated, and finally the multifidus atrophic iliopsoas hypertrophied model 4 was created. Maximum equivalent stress measurement was selected as lumbar 1 and 5 facets, lumbar 4-5, lumbar 5-sacral 1 for discs. The highest values were accepted as the loading of that level and structure without making any right-left distinction. Sequential axial loads and 5 N/ m rotational force were applied to the models starting with 100 Newton (N), increasing up to 500 N and then decreasing again. In addition, combined loading was applied as flexion - extension - rotation. The stresses observed in all these structures were analyzed. In the analyzes, the reaction forces formed on the intervertebral disc and facet joints and the maximum equivalent stresses (MEG) acting on these regions were evaluated in terms of Newton (N) /mm2, i.e. Megapascal ( MPa ). The reaction forces were obtained vectorially in terms of N. MEGs affecting the discs show that the L5-S1 disc is exposed to more tension than L4-5 and the loading is much higher than flexion-extension. No significant difference was found between these two disc levels in terms of rotational and cyclic loading. In cases where the psoas and multifidus muscles are hypertrophied at the same time, there was a decrease in MEGs at both levels, especially in flexion and extension, more pronounced in the L5-S1 disc compared to the reference model. However, the highest loading was observed in flexion. It was observed that it was in the L5-S1 disc in the model where the multifidus was weak and the iliopsoas was strong (Model 4).