A biomechanical model to analyze normal, degenerated, and fused cervical spines using IAR’s concept

During voluntary flexion/extension of the head-neck system, the cervical spine undergoes a stepwise motion from upper to lower regions with a specific pattern.  The motion of each vertebra is composed of a translation and rotation with respect to lower vertebrae, which may be considered as an absolute rotation about an axis called instantaneous axis of rotation (IAR). The location of this axis is different between normal and degenerated spines. A biomechanical computer model of the head and cervical vertebrae, including eight separate rigid links and nine spinal muscles as actuating elements was developed to evaluate and compare inter-segmental force-moments and muscle forces in normal, degenerated, and fused situations.  Kinematics data were used as the input to our model and the joint reaction forces and moments and muscle forces were analyzed as model output in solving an inverse dynamic problem.  As a result, significant differences were noticed among these three situations which are thoroughly discussed in this paper.