Objective: On a cervical disk, the bulge may be produced in the poster lateral direction when there is a combination of lateral bending and compression loading. Although attempts have been made to measure the kinematics of intact vertebral spine but has not been compared with a vertebral spine when the dicks are bulged. The study aims to investigate the effect of a bulged disk on the kinematics of a cervical spine during a flexionextension motion then flexion-extension of five subject patients who had history of bulged disk in their cervical spine were studied. All patients had bulged disk at C5/C6 level.Design: This study uses a three dimensional model of cervical vertebras which was created from CT data of the subject patients. Flexion-extension range of motion of each subject patients was recorded using a fluoroscopic imaging method. Then motion of the cervical vertebra next to bulged disk were tracked using a 2D-to-3D registration method. The result of this measurement was compared with motion of intact cervical spines obtained by previous researchers. Results:The range of motion for normal C5/C6 is less than bulged C5/C6. Conclusion:The kinematics of flexion-extension motion for bulged intervertebral disk and the normal intervertebral disks are different.Keywords: Spine cervical vertebra; Kinematics; Fluoroscopy; Bulged disk IntroductionThere are three main reasons for study on spinal kinematics better understanding of the spine function, understanding of the spinal diseases specifications and spinal treatment outcomes.The spine disorders may cause the vertebral instability especially in the spine cervical region. Although the internal causes like biochemical irritants may trigger the inter-vertebral disk degeneration, the external causes like trauma may result the disk balance perturbation and degeneration.The movements of the head are controlled by muscle. However, the type of movements may depend on the form and structure of the cervical vertebrae and interaction between them. The kinematics of the cervical spine are, therefore, depends the structural anatomy of the neck [1,2]. Diagnosis, treatment and prevention ameliorative efforts of cervical spine is required understanding its anatomy and biomechanics [3]. In spite of the advantage of the anterior cervical discectomy and fusion to stabilize the patient's cervical region this procedure may followed by the adjacent level degeneration and there is no gold standard to assess its rate except the static radiologic method that is shown the vacancy of the three-dimensional weight bearing analysis [4].Disk bulging is one of the spinal disease. It is an aging related phenomenon and can be detected by Magnetic Resonance Imaging (MRI) as an effective means of detecting cervical disc deformity. However, traditional MRI has limitation to scan the disk at rest position. The scan normally performed when the patients are at anatomical supine position which is non-weight-bearing position. This position is different with the position which patient being observe...