Hooks versus Pedicle Screws at the Upper Instrumented Level: An In Vitro Biomechanical Comparison
- Department of Orthopaedics, Walter Reed National Military Medical Center, Bethesda, MD, USA; Department of Surgery, Uniformed Services University of the Health Sciences, Bethesda, MD, USA
- DoD-VA Extremity Trauma and Amputation Center of Excellence, Walter Reed National Military Medical Center, Bethesda, MD, USA
- School of Medicine, Georgetown University, Washington, DC, USA
- School of Medicine, Uniformed Services University of the Health Sciences, Bethesda, MD, USA
- Department of Surgery, Uniformed Services University of the Health Sciences, Bethesda, MD, USA; DoD-VA Extremity Trauma and Amputation Center of Excellence, Womack Army Medical Center, Fort Bragg, NC
- DoD-VA Extremity Trauma and Amputation Center of Excellence, Walter Reed National Military Medical Center, Bethesda, MD, USA; Department of Rehabilitation Medicine, Uniformed Services University of the Health Science, Bethesda, MD, USA
Study Design. Controlled laboratory study Objective. To compare motions at the upper instrumented vertebra (UIV) and supra-adjacent level (UIV+1) between two fixation techniques in thoracic posterior spinal fusion (PSF) constructs. We hypothesized there would be greater motion at UIV+1 after cyclic loading across all constructs and bilateral pedicle screws (BPS) with posterior ligamentous compromise would demonstrate the greatest UIV+1 range of motion (ROM). Summary of Background Data. Proximal junctional kyphosis (PJK) is a well-recognized complication following long thoraco-lumbar PSF, however its mechanism is poorly understood. Methods. Twenty-seven thoracic functional spine units (FSU) were randomly divided into three UIV fixation groups (n=9): (1) BPS, (2) bilateral transverse process hooks (TPH), and (3) BPS with compromise of the posterior elements between UIV and UIV+1 (BPS-C). Specimens were tested on a servohydraulic materials testing system in native state, following instrumentation, and after cyclic loading. FSUs were loaded in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Results. After cyclic testing, the TPH group had a mean 29.4% increase in FE ROM at UIV+1 versus 76.6% in the BPS group (P<0.05). The BPS-C group showed an increased FE of 49.9% and 62.19% with sectioning of the facet joints and interspinous ligament respectively prior to cyclic testing. Conclusion. Bilateral pedicle screws at the UIV led to greater motion at UIV+1 compared to bilateral TPH after cyclic loading. This is likely due to the increased rigidity of BPS compared to TPH leading to a “softer” transition between the TPH construct and native anatomy at the supra-adjacent level. Facet capsule compromise led to a 49.9% increase in UIV+1 motion, underscoring the importance of preserving the posterior ligamentous complex. Clinical studies that account for fusion rates are warranted to determine if constructs with a “soft transition” result in less PJK in vivo.
- Research Organization:
- Oak Ridge Institute for Science and Education (ORISE), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0014664
- OSTI ID:
- 2425697
- Journal Information:
- Spine, Journal Name: Spine Journal Issue: 7 Vol. 48; ISSN 0362-2436
- Publisher:
- Wolters Kluwer
- Country of Publication:
- United States
- Language:
- English
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