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In vitro biomechanical study of cervical intervertebral distraction.To investigate the forces required for distraction to different heights in an in vitro C5–C6 anterior cervical distraction model, focusing on the influence of the intervertebral disk, posterior longitudinal ligament (PLL), and ligamentum flavum (LF).No previous studies have reported on the forces required for distraction to various heights or the factors resisting distraction in anterior cervical discectomy and fusion.Anterior cervical distraction at C5–C6 was performed in 6 cadaveric specimens using a biomechanical testing machine, under 4 conditions: A, before disk removal; B, after disk removal; C, after disk and PLL removal; and D, after disk and PLL removal and cutting of the LF. Distraction was performed from 0 to 10 mm at a constant velocity (5 mm/min). Force and distraction height were recorded automatically.The force required increased with distraction height under all 4 conditions. There was a sudden increase in force required at 6–7 mm under conditions B and C, but not D. Under condition A, distraction to 5 mm required a force of 268.3±38.87 N. Under conditions B and C, distraction to 6 mm required <15 N, and further distraction required dramatically increased force, with distraction to 10 mm requiring 115.4±10.67 and 68.4±9.67 N, respectively. Under condition D, no marked increase in force was recorded.Distraction of the intervertebral space was much easier after disk 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 stress to the endplate spacer.