P33. Risk factors for allograft subsidence following ACDF

Abstract

<h3>BACKGROUND CONTEXT</h3> Studies investigating the impact of interbody subsidence in ACDF suggest a correlation between subsidence and worse radiographic and patient-reported outcomes. Though some interbody subsidence is anticipated, a normal range of subsidence has yet to be defined. Furthermore, no study has investigated the variables influencing allograft subsidence in ACDF. <h3>PURPOSE</h3> The primary purpose of this study was to utilize computed tomography to characterize subsidence of allograft interbodies in ACDF. The secondary aim of this study was to determine the impact of patient- and graft-related variables on subsidence in ACDF. <h3>STUDY DESIGN/SETTING</h3> This was a retrospective study conducted at a single academic medical center. <h3>PATIENT SAMPLE</h3> Adult patients who underwent a 1 to 3 level ACDF with an allograft interbody and anterior plating between 2011-2017 were included. A total of 98 patients were included in the study. <h3>OUTCOME MEASURES</h3> Outcome measurements included superior endplate subsidence, inferior endplate subsidence, VAS scores, pseudoarthrosis, and reoperation rates. <h3>METHODS</h3> We performed a retrospective review of a prospective cohort of patients undergoing 1 to 3 level ACDF with an allograft interbody and anterior plating at a single institution between the years of 2011-2017. We collected demographic information, radiographic variables, and graft-related parameters. Graft subsidence was assessed on CT scan performed at least 6 months postoperatively. We classified subsidence as none if <2mm, mild if 2-3mm, moderate if 3-4mm, and severe if >4mm. Student's t-test and ANOVA were used to compare all means between groups. <h3>RESULTS</h3> We identified 98 patients (152 levels) for inclusion. On sagittal CT for the entire cohort, the mean superior endplate subsidence was 1.62mm+/-0.90 and the mean inferior endplate subsidence was 1.62mm+/-0.91. The number of levels that underwent <2mm, >2mm, and >4mm of subsidence was 73 (48.0%), 79 (51.9%), and 18 (11.8%), respectively. Of the 18 levels with severe subsidence, 14 occurred in multi-level constructs, 2 of which occurred in both levels of a two-level ACDF in one patient. Of the remaining 12 levels of severe subsidence, 11 (92%) occurred at the caudal level and to a significantly higher degree at the inferior end plate (p<0.001). On univariate analysis of levels with severe subsidence, both a decrease in the distance from the screw tip to the inferior endplate and in the screw tip height to vertebral body height ratio were associated with an increase in subsidence (p<0.05). Additionally, both mild and severe subsidence were significantly influenced by loss of vertebral body height on immediate postoperative X-rays (p<0.005). There was no significant difference in preoperative or postoperative VAS between groups (p>0.05). 16 patients (94.1%) with severe subsidence experienced pseudarthrosis compared to only 11 patients (13.6%) with <4mm subsidence (p<0.001). However, reoperation rate was not significantly different between the severe subsidence and non-severe subsidence groups (11.8% vs 17.3%, p=0.44). <h3>CONCLUSIONS</h3> The present study is the first to utilize advanced imaging to quantify and characterize allograft subsidence in ACDF. Univariate analysis of patients with severe subsidence demonstrated that inferior screw placement in the caudal instrumented vertebra was associated with increased subsidence, likely due to decreased buttress support and an increased working length with an associated decrease in construct rigidity. Furthermore, excessive endplate resection was associated with an increase in both mild and severe subsidence. Based upon our study, some allograft subsidence in ACDF is expected, but too inferior screw placement at the caudal vertebral level and excessive endplate resection should be avoided to prevent severe subsidence. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.