Background: Early onset scoliosis is a three-dimensional deformity of the spine and trunk – diagnosed before the age of 10 – and occurs in 3 to 5 out of 100,000 infant and juvenile populations. Surgical intervention is often necessary for pediatric patients with severe curvatures. During thesis procedures, spinal rods – known as pediatric non-fusion devices (i.e. growing rods) – are placed in the patients to allow for spinal growth. ASTM standards exist for fusion constructs. Growing-rod constructs place greater demands on the rods as no fusion is performed and the rods are continually loaded. Also, the constructs are serially lengthened at 6-month intervals placing additional demands on the construct. Currently, there are no standards on how to develop and utilize a finite element model – to predict durability of commonly used constructs within the patient. Methods: Finite element models representing static compression bending were implemented using Abaqus CAE (Dassault Systemés Simulia Corporation, Providence, Rhode Island). The model geometry and loading was based on ASTM standard 1537 and 136. The models were created with C3D8R type elements. The model applied a controlled displacement until failure or contact occurred between the superior and inferior UHMWPE blocks. Failure was defined as the point at which permanent deformation of the construct occurs, due to fracture, plastic deformation, or slip. Eight constructs were analyzed based on the rod material (titanium and cobalt chrome), active length (76-mm and 376-mm), and construct type (F1717 and growing-rod). Results: The force-displacement curves and maximum principle stress for each whole model were evaluated. The longer active lengths required a smaller force for failure. Failure at the pedicle screw head was found on each model. Conclusions: A new ASTM standard for growing-rod constructs should be created based off of a longer active length, the overlap of the rod connectors, and location of the rod connectors.
University of Minnesota M.S. thesis. January 2016. Major: Biomedical Engineering. Advisors: Victor Barocas, Joan Bechtold. 1 computer file (PDF); viii, 36 pages.
Method to Determine Compressive Bending Performance in Static and Fatigue Loading for Pediatric Non-Fusion Devices.
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