Early Postoperative Kinetic Changes in Cervical Spine after Anterior Cervical Fusion

Park, Seung Won; Kim, Young Baeg; Min, Byung Kook; Hwang, Sung Nam; Choi, Duck Young; Suk, Jong Sik

Abstract

Although anterior cervical fusion(AC) was accepted as an ideal method to treat spondylotic myeloradiculopathy, it was found that the vertebrae adjacent to the fused level tended to undergo an accelerated degeneration. The reasons for this phenomenon thought to be the hypermotility of the intervertebral joints without any pertinent biomechanical explanations. In this study, early kinetic changes in cervical spine after ACF were investigated to find out biomechanical factors related to the accelerated degeneration of adjacent vertebrae. Twelve patients with ccervicalspondylotic myeloradiculopathy who underwent ACF from May 1995 to February 1996 were included. Male to female ratio was 1:2. Mean age was 47.6 years. Simple radiographic flexion and extension views of cervical spine were taken from 40 days to 224 days after the operations. The flexion-extension angle(FEA), top angle, radius of coupling motion at each intervertebral joint were measured. The postoperative changes of FEA were variable. In eleven cases, the top angle decreased at the upper vertebra adjacent to the fused level. Also, eleven cases revealed decreased radius at the adjacent upper vertebra. The decreased top angle and radius indicate the increment of stiffness and tension according to the stiffness coefficient(Stiffness coefficient=orque/Angular displacement in radians, Nm/rad) and torque equation(Torque=istanceXForce, Nm), respectively. As a result, it can be said that the important early kinetic factors acting on the accelerated degeneration after ACF are the increased stiffness of vertebral joints followed by shortening of radius and increased stress.