Testing Pullout Strength of Pedicle Screw Using Synthetic Bone Models: Is a Bilayer Foam Model a Better Representation of Vertebra?

Study Design A biomechanical study. Purpose A new biomechanical model of the vertebra has been developed that accounts for the inhomogeneity of bone and the contribution of the pedicle toward the holding strength of a pedicle screw. Overview of Literature Pullout strength studies are typically carried out on rigid polyurethane foams that represent the homogeneous vertebral framework of the spine. However, the contribution of the pedicle region, which contributes to the inhomogeneity in this framework, has not been considered in previous investigations. Therefore, we propose a new biomechanical model that can account for the vertebral inhomogeneity, especially the contribution of the pedicles toward the pullout strength of the pedicle screw. Methods A bilayer foam model was developed by joining two foams representing the pedicle and the vertebra. The results of the pullout strength tests performed on the foam models were compared with those from the tests performed on the cadaver lumbar vertebra. Results Significant differences (p 0.05) in the pullout strength of pedicle screws between osteoporotic (0.85±0.08 kN) and extremely osteoporotic bone models (0.94±0.08 kN), but there was a significant difference (p 0.05) in pullout strength between cadaver and bilayer foam model in normal bones. Conclusions The new synthetic bone model that reflects the contribution of the pedicles to the pullout strength of the pedicle screws could provide a more efficacious means of testing pedicle-screw pullout strength. The bilayer model can match the pullout strength value of normal lumbar vertebra bone whereas the monolayer foam model was able to match that of the extremely osteoporotic lumbar vertebra

Comportamiento biomecánico de la columna vertebral lumbopélvica deformada postquirúrgica

ilustracionesBased on literature, one of the common lumbar spine disorders reported is the isthmic high-grade spondylolisthesis (HGS), and there is no consensus on its surgical treatment selection. Thus, the present thesis aims to evaluate from an engineering point of view the influence of the fixation configuration for deformed or fractured spine surgery on the stabilization, biomechanical behavior, and stress state of the post-surgical lumbo-pelvic spine, providing a useful source of information for surgical planning and decision making. To evaluate the pathology as HGS (as literature-based selected case of study of deformed spine), a patient specific lumbosacral spine model was obtained with Scalismo and CAD modeling and used as a base to recreate a HGS condition. The diagnosis was made based on clinical literature and consisted of a lumbosacral spine with grade 3 isthmic spondylolisthesis, low dysplasia (L5 rectangular), an unbalanced spine (C7PL in front of FH), and a retroverted pelvis (low SS/high PT, vertical sacrum). Fusion In situ (FIS) with laminotomy and Lumbar interbody fusion (LIF) with reduction and laminotomy techniques were identified as suggested treatments, based on the Mac-Thiong classification scheme and clinical reports. Six variations of each fixation technique, involving adding or removing screws by spine level, were defined as possible instrument configurations, and compared. Based on the case of study and geometrical model, biomechanical Finite element models were developed to evaluate the mechanical response of HGS lumbosacral spine treated with FIS and LIF techniques, along with the proposed configurations. Thirteen models, divided into two groups (FIS and LIF models), were developed as variations of FIS and LIF base models. The spine mesh was built up in Abaqus from the vertebrae, supported by BCPD morphing process. To simulate the mechanical conditions of the surgical procedure in the two groups of FIS and LIF models, Swelling, Reduction/ Displacement, and Fixation standing steps were defined. A comparison between variations by level in the FIS and LIF instrumentation configurations for HGS was developed using FEM. The results obtained can be used to establish which levels are required to fix the system while ensuring the safety of both the biological systems and the instrumental. For model validation, a comparison of FIS and LIF models with experimental, numerical, and clinical outcomes reported in the literature is suggested as an alternative.Con base en la literatura, uno de los trastornos comunes de la columna lumbar reportados es la espondilolistesis ístmica de alto grado (HGS), y no existe un consenso sobre su selección de tratamiento quirúrgico. Por lo tanto, la presente tesis tiene como objetivo evaluar, desde una visión ingenieril, la incidencia de la configuración de fijación para cirugía de columna vertebral deformada o fracturada sobre la estabilización, comportamiento biomecánico y estado de esfuerzos de la columna vertebral lumbo-pélvica postquirúrgica, proporcionando una fuente útil de información en la planificación y toma de decisiones quirúrgicas. Para evaluar una patología como HGS (como un caso de estudio de columna deformada seleccionado basado en la literatura), se obtuvo un modelo de columna lumbosacra de paciente específico utilizando el software Scalismo y modelado CAD, y se utilizó como base para recrear una condición de HGS. El diagnóstico se basó en la literatura clínica y consistió en una columna lumbosacra con espondilolistesis ístmica de grado 3, displasia baja (L5 rectangular), una columna desbalanceada (Línea de gravedad delante de la cabeza del fémur) y una pelvis retroversa (Inclinación sacra baja, inclinación pélvica alta, sacro vertical). Las técnicas de fusión in situ (FIS) con laminotomía y fusión intervertebral lumbar (LIF) con reducción y laminotomía se identificaron como los tratamientos sugeridos, basados en el esquema de clasificación de Mac-Thiong y reportes clínicos. Se definieron seis variaciones de cada técnica de fijación, que implicaban agregar o quitar los tornillos de columna por nivel, como posibles configuraciones de instrumentación y se compararon entre sí. Basándose en el caso de estudio y el modelo geométrico, se desarrollaron modelos biomecánicos de elementos finitos para evaluar la respuesta mecánica de la columna lumbosacra HGS tratada con las técnicas FIS y LIF, junto con las configuraciones propuestas. Se desarrollaron trece modelos divididos en dos grupos (modelos FIS y LIF) como variaciones de los modelos FIS y LIF base. La malla de la columna se construyó en Abaqus a partir de las vértebras, apoyado por el proceso de transformación de malla BCPD. Para simular las condiciones mecánicas del procedimiento quirúrgico en los dos grupos de modelos FIS y LIF, se definieron las etapas de estabilización (estado de hinchamiento de discos intervertebrales), reducción/ desplazamiento y fijación. Se desarrolló un comparativo entre las variaciones por nivel en las configuraciones de instrumentación FIS y LIF para HGS mediante el uso del método de elementos finitos (MEF). Los resultados obtenidos pueden ser utilizados para establecer qué niveles son necesarios para fijar el sistema y, al mismo tiempo, asegurar la seguridad tanto de los sistemas biológicos como de la instrumentación. Como alternativa para la validación del modelo, se propone una comparación de los modelos FIS y LIF con resultados experimentales, numéricos y clínicos reportados en la literatura. (texto tomado de la fuente)MaestríaMagíster en Ingeniería MecánicaBiomecánicaÁrea Curricular de Ingeniería Mecánic