The development of a dynamic,six-axis spine simulator

Background contextAlthough a great deal of research has been completed to characterize the stiffness of spinal specimens, there remains a limited understanding of the spine in 6 df and there is a lack of data from dynamic testing in six axes.PurposeThis study details the development and validation of a dynamic six-axis spine simulator.Study designBiomechanical study.MethodsA synthetic spinal specimen was used for the purpose of tuning the simulator, completing positional accuracy tests, and measuring frequency response under physiological conditions. The spine simulator was used to complete stiffness matrix tests of an L3–L4 lumbar porcine functional spinal unit. Five testing frequencies were used, ranging from quasistatic (0.00575 Hz) to dynamic (0.5 Hz). Tests were performed without an axial preload and with an axial preload of 500 N.ResultsThe validation tests demonstrated that the simulator is capable of producing accurate positioning under loading at frequencies up to 0.5 Hz using both sine and triangle waveforms. The porcine stiffness matrix tests demonstrated that the stiffness matrix is not symmetrical about the principal stiffness diagonal. It was also shown that while an increase in test frequency generally increased the principal stiffness terms, axial preload had a much greater effect.ConclusionsThe spine simulator is capable of characterizing the dynamic biomechanics of the spine in six axes and provides a means to better understand the complex behavior of the spine under physiological conditions.     

Incidence and risk factors for perioperative visual loss after spinal fusion

Background contextPerioperative visual loss (POVL) is a rare but devastating complication that may follow spinal surgeries. The incidence of POVL after spinal fusion is not well characterized during the past decade.PurposeA population-based database was analyzed to characterize the incidence and risk factors for POVL associated with spinal fusion surgery on a national level.Study designThis study consisted of a retrospective database analysis.Patient sampleA total of 541,485 patients from the Nationwide Inpatient Sample (NIS) database were included in the study.Outcome measuresStudy outcome measures included incidence of POVL, length of stay (LOS), in-hospital costs, mortality, and POVL risk factors.MethodsData from the NIS were obtained from 2002 to 2009. Patients undergoing spinal fusion for degenerative pathologies were identified. Patient demographics, comorbidities, LOS, costs, and mortality were assessed. Statistical analyses were conducted using an independent t test for discrete variables and the chi-square test for categorical data. Binomial logistic regression was used to identify independent predictors of POVL. A p value of less than or equal to .001 was used to denote statistical significance. No funds were received by any of the authors for production of this study.ResultsA total of 541,485 spinal fusions were identified in the United States from 2002 to 2009. The overall incidence of POVL was 1.9 events per 10,000 cases. Of patients who had POVL, 56.2% underwent surgery for a diagnosis of spinal deformity. Patients with POVL were significantly younger on average compared with unaffected patients (37.6 years vs. 52.4 years; p<.001). Length of stay and hospital costs doubled for patients with POVL (p<.001). Logistic regression analysis demonstrated that independent predictors of visual loss were deformity surgery (odds ratio [OR]=6.1), diabetes mellitus with end organ damage (OR=13.1), and paralysis (OR=6.0, p<.001).ConclusionsOur findings demonstrated an overall POVL incidence of 1.9 events per 10,000 spinal fusions. Patients undergoing thoracic fusion for deformity correction accounted for the majority of cases of POVL. Despite being a rare complication after spinal fusion, POVL is an adverse event that may not be entirely preventable. Patients undergoing long-segment fusions for deformity and those with certain risk factors should be counseled regarding the risks of POVL.