Accurate measurement of intervertebral kinematics of the cervical spine can support the diagnosis of widespread diseases related to neck pain, such as chronic whiplash dysfunction, arthritis, and segmental degeneration. The natural inaccessibility of the spine, its complex anatomy, and the small range of motion only permit concise measurement in vivo. Low dose X-ray fluoroscopy allows time-continuous screening of cervical spine during patient’s spontaneous motion. To obtain accurate motion measurements, each vertebra was tracked by means of image processing along a sequence of radiographic images. To obtain a time-continuous representation of motion and to reduce noise in the experimental data, smoothing spline interpolation was used. Estimation of intervertebral motion for cervical segments was obtained by processing patient’s fluoroscopic sequence; intervertebral angle and displacement and the instantaneous centre of rotation were computed. The RMS value of fitting errors resulted in about 0.2 degree for rotation and 0.2？mm for displacements. 1. Introduction Neck pain is a common musculoskeletal problem experienced by the vast majority of the population [1, 2]. Alterations of cervical spine mechanics that compromise the stabilizing mechanisms of the cervical spine (such those originated by chronic whiplash dysfunction [3], arthritis [4, 5], and segmental degeneration [6]) can be possible causes of neck pain. Detection of spinal instability (degenerative or traumatic) is based on accurate measurement of intervertebral kinematic [7]. In particular, forward displacement of the vertebrae greater than 3.5？mm and angle between adjacent endplates greater than 11 degree is regarded as a sign of instability [8] and indication for surgery. Quantitative measurements of segmental kinematics also find use in the evaluation of cervical arthroplasty, assessment of disc prosthesis, postsurgery followup, and so forth [9–11]. In spite of their paramount importance in clinical application, accurate measurement of the intervertebral kinematic are hindered by the natural inaccessibility of the spine, the complexity of its anatomy, and physiology and the extremely small range of motion achieved at segmental level. Although most of the injuries and degenerative pathologies of the cervical spine are associated with reduced mobility and pain, there is no gold standard for the measurement of the kinematics of the cervical spine, not even for the measurement of its range of motion as a whole. Many techniques were proposed to measure spine kinematics [12–15]. These techniques
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