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Karami K.J.,Michigan State University | Karami K.J.,Johns Hopkins University | Buckenmeyer L.E.,Engineering Center for Orthopaedic Research Excellence | Buckenmeyer L.E.,University of Toledo | And 8 more authors.
Journal of Spinal Disorders and Techniques | Year: 2015

Study Design: A biomechanical ex vivo study of the human lumbar spine. Objective: To evaluate the effects of transpedicular screw insertion depth on overall screw stability and pullout strength following cyclic loading in the osteoporotic lumbar spine. Summary of Background Data: Although much is known about the clinical outcomes of spinal fusion, questions remain in our understanding of the biomechanical strength of lumbar pedicle screw fixation as it relates to screw sizing and placement. Biomechanical analyses examining ideal pedicle screw depth with current pedicle screw technology are limited. In the osteoporotic spine, optimized pedicle screw insertion depth may improve construct strength, decreasing the risk of loosening or pullout. Methods: A total of 100 pedicles from 10 osteoporotic lumbar spines were randomly instrumented with pedicle screws in mid-body, pericortical, and bicortical depths. Instrumented specimens underwent cyclic loading (5000 cycles of ±2 N m pure flexion moment) and subsequent pullout. Screw loosening, failure loads, and energy absorption were calculated. Results: Cyclic loading significantly (P<0.001) reduced screw-bone angular stiffness between prefatigue and postfatigue conditions by 25.6%±17.9% (mid-body), 20.8%±14.4% (pericortical), and 14.0%±13.0% (bicortical). Increased insertion depth resulted in lower levels of reduction in angular stiffness, which was only significant between mid-body and bicortical screws (P=0.009). Pullout force and energy of 583±306 N and 1.75±1.98 N m (mid-body), 713±321 N and 2.40±1.79 N m (pericortical), and 797±285 N and 2.97±2.33 N m (bicortical) were observed, respectively. Increased insertion depth resulted in higher magnitudes of both pullout force and energy, which was significant only for pullout force between mid-body and bicortical screws (P=0.005). Conclusion: Although increased screw depth led to increased fixation and decreased loosening, additional purchase of the stiff anterior cortex is essential to reach superior screw-bone construct stability and stiffness. © Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved. Source


Agarwal A.,Engineering Center for Orthopaedic Research Excellence | Palepu V.,Engineering Center for Orthopaedic Research Excellence | Agarwal A.K.,Engineering Center for Orthopaedic Research Excellence | Goel V.K.,Engineering Center for Orthopaedic Research Excellence | Yildirim E.D.,Engineering Center for Orthopaedic Research Excellence
Journal of Biomechanical Engineering | Year: 2013

In the thoracolumbar region, between 7% and 30% of spinal fusion failures are at risk for pseudarthrosis. From a biomechanical perspective, the nonconformity of the interver-tebral graft to the endplate surface could contribute to pseudarthrosis, given suboptimal stress distributions. The objective of this study was to quantify the effect of endplate-graft conformation on endplate stress distribution, maximum Von Mises stress development, and stability. The study design used an experimentally validated finite element (FE) model of the L4-L5 functional spinal unit to simulate two types of interbody grafts (cortical bone and polycaprolactone (PCL)-hydroxyapatite (HA) graft), with and without endplate-conformed surfaces. Two case studies were completed. In Case Study I, the endplate-conformed grafts and nonconformed grafts were compared under without posterior instrumentation condition, while in Case Study II, the endplate-conformed and non-conformed grafts were compared with posterior instrumentation. In both case studies, the results suggested that the increased endplate-graft conformity reduced the maximum stress on the endplate, created uniform stress distribution on endplate surfaces, and reduced the range of motion of L4-L5 segments by increasing the contact surface area between the graft and the endplate. The stress distributions in the endplate suggest that the load sharing is greater with the endplate-conformed PCL-HA graft, which might reduce the graft subsidence possibility. Copyright © 2013 by ASME. Source

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