Modified techniques to prevent sagittal imbalance after cervical arthroplasty

Study Design. Retrospective study of radiographic outcomes in patients undergoing single level cervical arthroplasty with the Bryan cervical disc (Medtronic Sofamor Danek, Memphis, TN). Objective. This study was designed to determine whether modification of disc insertion angle and insertion depth are effective in preventing segmental or whole cervical kyphosis after arthroplasty. Summary of Background Data. Preservation of segmental motion and aggravation of kyphosis are known challenges after arthroplasty. However, there are currently no proven preventative factors for kyphosis. Change in disc insertion angle was only reported effective for avoiding end-plate kyphosis. Additionally, it was difficult to predict the effect of insertion angle on overall sagittal alignment. There have been no studies regarding the correlation between insertion depth and sagittal alignment. Methods. A total of 41 patients with single-level arthroplasty were evaluated. Radiologic assessment using neutral cervical radiographs at the long-term was performed. Linear regression analysis between insertion angle, insertion depth, postoperative sagittal alignment, functional spinal unit angle, and shell angle were performed. Results. Disc insertion angle and insertion depth demonstrated significant negative correlation with the postoperative shell angle. Lordotic insertion angle and an anteriorly located disc led to lordosis in the shell angle. Overall sagittal alignment showed a tendency to correlate with insertion angle. By the result of effect of insertion angle ranging from 3.5 to 7.5 on the sagittal alignment and shell angle, we can hypothesize paradoxical biomechanical stress on the other segments. Conclusion. Arthroplasty using the Bryan disc provided a favorable clinical and radiologic outcome thus far; however, we should not underestimate emergent adverse outcomes. To prevent postoperative sagittal imbalance after cervical arthroplasty, intentional modification in disc insertion angle and depth would be helpful. Unexpected compensatory biomechanical loads should be elucidated in future studies.