The Impact of Hounsfield Units and Cage Position on Subsidence Following Anterior Cervical Discectomy and Fusion

Introduction: Recent studies have investigated the ability of computed tomography (CT) scans to assess bone quality via Hounsfield unit (HU) measurements. Prior work on 1-level ACDF has noted increased subsidence rates in patients with lower HUs. The purpose of this study was to try to validate the proposed relationship between decreased HUs and subsidence, and to evaluate whether this relationship played a role in multi-level ACDF.

Methods: All adult patients (2018-2022) who underwent a 1-3 level ACDF and pre-operative CT scan within the last 4 years(average < 10 months). Radiographic measurements were performed on 6-month postoperative films to determine cage/graft positioning, screw loosening, and subsidence. Bivariate analysis was performed using independent t-tests with p-value statistical significance at < 0.05. Area under the curve(AUC) values were calculated from receiver operating characteristic(ROC) curves to assess the predictive value of segmental HU (average of the vertebrae just caudal and cranial to the instrumented vertebrae), minimum HU, and maximum HU in regards to screw loosening and/or subsidence.

Results: A total of 42 patients met inclusion criteria. Demographics were similar among between groups. Mean segmental HU, segmental HU above and below 343.7HU (subsidence threshold found by Wang et al.), minimum HU, and maximum HU were similar between patients who did versus those who did not have subsidence at any level. Amongst the HU measurements, the maximum AUC value was 0.524(95%CI 0.343-0.705) for screw loosening as predicted by maximum HU with a cutoff of 295HU. Subsidence of cage 1 was found to be associated with middle positioning as compared to anterior positioning of cage 1(38.5%vs6.9%, p=0.021).

Conclusion : This study found no association between segmental, minimum, or maximum HU values and subsidence or screw loosening. There was a lack of predictive ability of these factors with all AUC values less than acceptable(0.7-0.8). Middle positioning of cage 1 was associated with subsidence of cage 1. This finding suggests that central positioning of the cage/graft over cancellous bone is inferior to anterior positioning, which may be related to the anterior aspect having more cortical bone and thus greater integrity and ability to withstand biomechanical burden.