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Study Design: Real time in vivo measurement of forces in the cervical spine of goats following anterior cervical discectomy and fusion (ACDF)., Objective: To measure interbody forces in the cervical spine during the time course of fusion following ACDF with plates of different stiffnesses., Summary of Background Data: Following ACDF, the biomechanics of the arthrodesis is largely dictated by the plate. The properties of the plate prescribe the extent of load-sharing through the disc space versus the extent of stress-shielding. Load-sharing promotes interbody bone formation and stress-shielding can inhibit maturation of bone. However, these principles have never been validated in vivo. Measuring in vivo biomechanics of the cervical spine is critical to understanding the complex relationships between implant design, interbody loading, load-sharing, and the progression of fusion., Methods: Anterior cervical plates of distinct bending stiffnesses were placed surgically following ACDF in goats. A validated custom force-sensing interbody implant was placed in the disc space to measure load-sharing in the spine. Interbody loads were measured in vivo in real time during the course of fusion for each plate., Results: Interbody forces during flexion/extension were highly dynamic. In animals that received high stiffness plates, maximum forces were in extension whereas in animals that received lower stiffness plates, maximum forces were in flexion. As fusion progressed, interbody load magnitude decreased., Conclusion: The magnitude of interbody forces in the cervical spine is dynamic and correlates to activity and posture of the head and neck. The magnitude and consistency of forces in the interbody space correlates to plate stiffness with more compliant plates resulting in more consistent load-sharing. The magnitude of interbody forces decreases as fusion matures suggesting that smart interbody implants may be used as a diagnostic tool to indicate the progression of interbody fusion., Level of Evidence: N/A.

Study Design: In vitro biomechanical testing of human cadaveric cervical and goat cervical motion segments., Objective: The aim of this study was to measure the effects of plate stiffness on load-sharing, instantaneous axis of rotation (IAR), and posterior element loading after anterior cervical discectomy and fusion (ACDF)., Summary of Background Data: ACDF is intended to create an environment, which facilitates sufficient stability and biomechanical conditions to promote bone formation. The relationship between cervical plate stiffness, load-sharing, and the IAR is complex. The ideal cervical plate is sufficiently stiff to limit interbody motion but is compliant enough to facilitate load-sharing rather than stress-shielding., Methods: Anterior cervical plates of distinct bending stiffnesses were applied to human and goat cervical motion segments following ACDF. A validated custom force-sensing interbody implant was placed in the disc space to measure load-sharing in the spine. Interbody loads, posterior element strain, and the IAR were measured during flexion/extension for each plate., Results: Load-sharing in the interbody space, posterior element strain, and the location of the IAR were all significantly affected by plate stiffness. More compliant plates resulted in more load sharing, less posterior element strain, and a more dorsally located IAR relative to stiffer plates., Conclusion: A more compliant plate fosters more consistent load-sharing through the entire range of flexion/extension, which may promote faster bone formation and better fusion. A more compliant plate causes less posterior element strain, which may reduce facet joint loads and in turn reduce facet joint arthrosis. An ideal plate may be one that is stiff enough to minimize interbody motion and yet compliant enough to allow consistent load-sharing and minimal increase in posterior element strain., Level of Evidence: N/A.

Background Context: Achieving a successful spinal fusion requires the proper biological and biomechanical environment. Optimizing load-sharing in the interbody space can enhance bone formation. For anterior cervical discectomy and fusion (ACDF), loading and motion are largely dictated by the stiffness of the plate, which can facilitate a balance between stability and load-sharing. The advantages of load-sharing may be substantial for patients with comorbidities and in multilevel procedures where pseudarthrosis rates are significant., Purpose: We aimed to evaluate the efficacy of a novel elastically deformable, continuously load-sharing anterior cervical spinal plate for promotion of bone formation and interbody fusion relative to a translationally dynamic plate., Study Design/setting: An in vivo animal model was used to evaluate the effects of an elastically deformable spinal plate on bone formation and spine fusion., Methods: Fourteen goats underwent an ACDF and received either a translationally dynamic or elastically deformable plate. Animals were followed up until 18 weeks and were evaluated by plain x-ray, computed tomography scan, and undecalcified histology to evaluate the rate and quality of bone formation and interbody fusion., Results: Animals treated with the elastically deformable plate demonstrated statistically significantly superior early bone formation relative to the translationally dynamic plate. Trends in the data from 8 to 18 weeks postoperatively suggest that the elastically deformable implant enhanced bony bridging and fusion, but these enhancements were not statistically significant., Conclusions: Load-sharing through elastic micro-motion accelerates bone formation in the challenging goat ACDF model. The elastically deformable implant used in this study may promote early bony bridging and increased rates of fusion, but future studies will be necessary to comprehensively characterize the advantages of load-sharing through micro-motion., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

Study Design: After undergoing anulotomy, lumbar intervertebral discs from sheep were treated with small intestinal submucosa (SIS) and assessed functionally at 24 weeks after surgery., Objective: To determine the efficacy of an SIS-based patch and plug scaffold to facilitate anular defect closure and anular functional recovery after anulotomy and partial discectomy., Summary of Background Data: The incidence of reherniation following discectomy remains high and mechanical means of anular closure have met with limited success. SIS is a naturally occurring collagen-based material, which acts as a resorbable scaffold in vivo that promotes soft tissue regeneration., Methods: Twelve sheep underwent retroperitoneal exposure of the lumbar spine. Three levels were assigned to either: no additional procedure, box anulotomy alone, or box anulotomy followed by placement of an SIS "patch and plug" anchored by titanium bone screws. At 26 weeks after surgery, 18 motion segments underwent pressure-volume testing to assess the competency of the anulus. High resolution MRI images were taken of the remaining 18 segments. Undecalcified histology was conducted on all specimens., Results: Radiographs, MRI images, and histology indicate that there was an exuberant tissue response at SIS-treated levels. New tissue formation in SIS-treated specimens was integrated well with the native anulus, but did not resemble the organization of native anulus. The extent of anular closure was substantial enough to allow the disc a functional recovery to a mean 66% of its capacity to develop internal pressure. MRI images indicate that SIS-treated levels did not maintain signal intensity comparable to exposure-only (intact) levels, but SIS-treated discs were statistically significantly higher than anulotomy-only levels., Conclusion: SIS-treated discs were better able to maintain hydration and resulted in a functional recovery relative to anulotomy alone levels. The SIS patch and plug reduced the cascade of functional degeneration that an intervertebral disc undergoes following anulotomy.