Segmental malalignment with the Bryan cervical disc prosthesis--contributing factors.

Part 1 of the current study found that use of the Bryan Cervical Disc prosthesis resulted in a median loss of 2 degrees in functional spinal unit (FSU) lordosis when compared with preoperative imaging (P<0.0001, range: 8-degree loss to 5-degree gain). The observed changes were generally small but varied among both the patients and the surgeons, suggesting that variables may exist which affect postoperative sagittal alignment. The aim of the current study was to identify which, if any, of a range of patient and surgical variables may contribute significantly to postoperative FSU malalignment. The change in FSU angulation between the preoperative and postoperative neutral, erect x-rays of 67 consecutive patients (88 disc levels) were correlated with 35 demographic and radiographic variables. Postoperative change in disc space height, angle of prosthesis insertion, and the amount of bone removed from the anterior aspect of the cephalad vertebra varied significantly among the 3 surgeons and correlated with change in FSU alignment. Intraoperative disc space distraction correlated with subsequent loss of disc space height. Multiple linear regression analysis confirmed that loss of disc space height and angle of prosthesis insertion contributed independently to a model with a coefficient of determination of 0.39 (P<0.0001). Attempts to identify factors contributing to change in alignment have not shown any single factor to be wholly responsible. Although the prescribed surgical technique is relatively standardized, it seems likely that a number of surgical variables, particularly those leading to loss of disc space height and affecting annular tension are important.