• Takuhei Kozaki, Hannah J Lundberg, Steven P Mell, Dino Samartzis, Mamoru Kawakami, Hiroshi Yamada, Nozomu Inoue, and Howard S An.
• Department of Orthopedic Surgery, Rush University Medical Center, Orthopedic Building, Chicago, IL.
• Spine. 2023 Oct 15; 48 (20): E355E361E355-E361.

Study DesignThis study compared hip stress among different types of lumbopelvic fusion based on finite element (FE) analysis.ObjectiveWe believe that the number and placement of S2 alar iliac (AI) screws and whether the screws loosen likely influence hip joint stress in the FE model.Summary Of Background DataSpinopelvic fixation has been shown to increase the risk of progression for hip joint osteoarthritis. The biomechanical mechanism is not well understood. We hypothesize that the rigid pelvic fixation may induce stress at adjacent joints.Materials And MethodsA three-dimensional nonlinear FE model was constructed from the L4 vertebra to the femoral bone. From the intact model, we made four fusion models, each with different lower vertebrae instrumentation: (1) intact, (2) L4-S1 fusion, (3) L4-S2 AI screw fixation, (4) L4-S2 AI screw fixation with S2 AI screw loosening, and (5) L4-S1 and dual sacral AI screw fixation. A compressive load of 400 N was applied vertically to the L4 vertebra, followed by an additional 10 Nm bending moment about different axes to simulate either flexion, extension, left lateral bending, or right axial rotation. The distal femoral bone was completely restrained. The von Mises stress and angular motion were analyzed across the hip joints within each fusion construct model.ResultsHip joint cartilage stress and range of motion increased for all postures as pelvic fixation became more rigid. The dual sacral AI screw fixation model increased stress and angular motion at the hip joint more than intact model. Our results suggest that more rigid fixation of the pelvis induces additional stress on the hip joint, which may precipitate or accelerate adjacent joint disease.ConclusionsDual sacral AI fixation led to the highest stress while loosening of S2 AI decreased stress on the hip joint. This study illustrates that more rigid fixation among lumbosacral fusion constructs increases biomechanical stress on the hip joints.Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.

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