Distractive Force Relative to Initial Graft Compression in an In Vivo Anterior Cervical Discectomy and Fusion Model

Study Design. An in vivo biomechanical anterior cervical discectomy and instrumented fusion (ACDFI) model employing a calibrated distractor and a subminiature load cell used to intraoperatively measure distractive force across the discectomy site and subsequent compressive force across the interbody load cell following distractor removal. Objective. To determine the relationship between the distractive force and resultant initial graft compression in an in vivo ACDFI model. Summary of Background Data. The relationship between the distractive force and subsequent graft compression following distractor removal has not been studied in an in vivo ACDFI model. The consequences of over or under distraction and its subsequent effect on graft compression with regards to axial neck pain, endplate failure with graft subsidence, and fusion rates is an area of clinical significance for ACDFI. Methods. Intraoperative measurements were obtained from 17 discectomy sites in 12 patients undergoing one and two level ACDFI. Informed consent was obtained from all subjects before surgery. A calibrated Caspar distractor was used to measure the distraction across the discectomy site and a subminiature interbody load cell was placed into the discectomy site and used to measure the resultant initial compressive force following distractor removal. The statistical significance and correlation between the distractive force across the discectomy site and the subsequent compressive force across the load cell were investigated with the Pearson correlation coefficient. Results. The average distractive force across the discectomy site was 33.5 ± 11.6 lbs and the subsequent compressive force across the interbody load cell was 16.9 ± 5.9 lbs following distractor removal. The Pearson correlation coefficient was r = 0.912 (P < 0.0005). The data showed a statistically significant linear correlation between the distractive force and the subsequent compressive force across the range of distraction investigated. Conclusion. This study demonstrated a statistically significant linear correlation between the distractive force applied across the discectomy site and the subsequent compressive force across a load cell placed in the interbody space following distractor removal in an in vivo ACDFI model.