Effects of disc height and distractive forces on graft compression in an anterior cervical discectomy model

Study Design. An in vitro biomechanical study using a calibrated distractor and a subminiature load cell in a cadaver anterior cervical discectomy construct was conducted. Objective. To study the interrelations of preoperative disc height, graft height, and compressive, and distractive forces in an anterior cervical discectomy model. Summary of Background Data. the effects of graft size on compressive and distractive forces in a discectomy model remain unknown. Larger grafts afford neutral decompression through anterior column distraction. This distraction may subject the graft and vertebral bodies to excessive loads, increasing graft fracture, and subsidence risk. Methods. Disc height was measured radiographically in 18 specimens. A Smith-Robinson discectomy was performed, and the superior and inferior ends of the specimens were embedded in polyester resin. Distraction was applied through a calibrated Caspar distractor to measure the distractive force applied while steel spacers rigidly fixed to a subminiature load cell were introduced. After distraction was removed, immediate compressive force was measured. Results. Distractive forces of 112.4 N and 189.9 N were required to insert the 6-mm and 8-mm grafts, respectively. When this distractive force was removed, immediate compressive loads of 8.8 N and 21.5 N on the graft were noted. When a compressive load of 45 N was applied in a loading frame, measured graft loads of 16.2 N and 29.2 N also increased. No statistically significant relation was observed between preoperative disc height and distractive force or compressive of the graft. Significantly lower distractive and compressive forces were associated with insertion of the 6-mm rather than 8-mm graft. Conclusions. Significantly higher distractive and compressive forces were recorded with larger grafts. Preoperative disc height was not an accurate predictor of graft loads.