Development of a Remodeled Caspar Retractor and Its Application in the Measurement of Distractive Resistance in an In Vitro Anterior Cervical Distraction Model.

STUDY DESIGN: In vitro biomechanical study of cervical intervertebral distraction using a remodeled Caspar retractor.

OBJECTIVE: To investigate the torques required for distraction to different heights in an in vitro C3-4 anterior cervical distraction model using a remodeled Caspar retractor, focusing on the influence of the intervertebral disc, posterior longitudinal ligament (PLL), and ligamentum flavum (LF).

SUMMARY OF BACKGROUND DATA: No previous studies have reported on the torques required for distraction to various heights or the factors resisting distraction in anterior cervical discectomy and fusion (ACDF).

METHODS: Anterior cervical distractions at C3-4 was performed in six cadaveric specimens using a remodeled Caspar retractor, under four conditions: A, before disc removal; B, after disc removal; C, after disc and PLL removal; and D, after disc and PLL removal and cutting of the LF. Distraction was performed for 5 teeth, and distractive torque of each tooth was recorded.

RESULTS: The torque increased with distraction height under all conditions. There was a sudden increase in torque at the 4th tooth under Conditions B and C, but not D. Under Condition A, distraction to the 3rd tooth required 84.8±13.3 cN·m. Under Conditions B and C, distraction to the 3rd tooth required <13 cN·m, and further distraction required dramatically increased torque. Under Condition D, no marked increase in torque was recorded.

CONCLUSIONS: Distraction of the intervertebral space was much easier after disc removal. An intact LF caused a sudden marked increase in the force required for distraction, possibly indicating the point at which the LF was fully stretched. This increase in resistance may help to determine the optimal distraction height to avoid excessive stress to the endplate spacer. The remodeled Caspar retractor in present study may provide a feasible and convenient method for intraoperative measurement of distractive resistance.