Dynamic foraminal dimensions during neck extension and rotation in fusion and artificial disc replacement: an observational study

Publication date: Available online 4 September 2017
Source:The Spine Journal
Author(s): Yener N. Yeni, Timothy Baumer, Daniel Oravec, Azam Basheer, Colin P. McDonald, Michael J. Bey, Stephen W. Bartol, Victor Chang
BackgroundChanges in the dimensions of the cervical neural foramina (CNF) are considered to be a key factor in nerve root compression and development of cervical radiculopathy. However, to what extent foraminal geometry differs between patients who underwent anterior cervical discectomy and fusion (ACDF) and those who underwent total disc arthroplasty with an artificial disc (AD) during physiological motion is largely unknown.PurposeThe objective of this study is to compare compare CNF dimensions during physiological neck motion between ACDF and AD.Study Design/SettingThis is a retrospective comparative analysis of prospectively collected, consecutive, non-randomized series of patients at a single institution.Patient SampleA total of 16 single-level C5-6 ACDF (4M, 12F; 28-71 years) and 7 single-level C5-6 cervical arthroplasty patients (3M, 4F; 38-57 years), at least 12 months after surgery (23.6 ± 6.8 months) were included.Outcome MeasuresPatient demographics, pre-operative MRI based measurements of cervical spine degeneration and 2-year post-operative measurements of dynamic foraminal geometry.MethodsBiplane x-ray images were acquired during axial neck rotation and neck extension. A CT scan was also acquired from C3 to the first thoracic vertebrae. The subaxial cervical vertebrae (C3 to C7) were reconstructed into 3D bone models for use with model-based tracking. Foraminal height (FH) was calculated as the 3D distance between the superior point of the inferior pedicle and the inferior point of the superior pedicle using custom software. Foraminal width (FW) was similarly calculated as the 3D distance between the anterolateral aspect of the superior vertebral body inferior notch, and the posterolateral aspect of the inferior vertebral body superior notch. Dynamic foraminal dimensions were quantified as the minimum (FH.Min, FW.Min), the range (FH.Range, FW.Range) and the median (FH.Med, FW.Med) of each trial and then averaged over trials. Mixed model ANOVA framework was used to examine the differences between ACDF and AD groups. The initial severity of disc degeneration as determined from pre-operative MRI images was introduced as covariates in the models. There were no study-specific biases related to conflicts of interest.ResultsAt the operated level (C5-6), FH.Med and FH.Range were smaller in ACDF than in AD during axial rotation and neck extension (p<0.003 to p<0.05). At the superior adjacent level (C4-5), no significant difference was found. At the inferior adjacent level (C6-7), FW.Range was greater in ACDF than in AD during axial rotation and extension (p<0.05). At the nonadjacent level (C3-4), FW.Range was greater in ACDF than in AD during extension (p<0.008).ConclusionsThis study demonstrated decreases in foraminal dimensions and their range for ACDF compared to AD at the operated level. In contrast, it demonstrated increases in the range of foraminal dimensions during motion for ACDF compared to AD at the nonoperated segments. Together, these data support the notion that increased mobility at the nonoperated segments after ACDF may contribute to a greater risk for adjacent segment degeneration. Due to the significant presence of Range variables in the findings, the current data also indicate that a dynamic evaluation is likely more appropriate for evaluation of the differences in foramina between ACDF and AD than a static evaluation.



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