A continuous description of intervertebral motion by means of spline interpolation of kinematic data extracted by videofluoroscopy

In vivo analysis of intervertebral kinematics provides useful information about spinal disorders and performance of disk prostheses. Diagnosis of intervertebral instability is based on measurement of abnormal range of segmental motion in sagittal plane through functional flexion–extension radiography; however, this concise measure does not take into account the progression of segmental motion in between flexion and extension extremes. Fluoroscopy can support analysis of intervertebral kinematics during patient's motion with an acceptable X-ray dose. A spline-based method designed for a continuous-time description of intervertebral motion extracted by videofluoroscopy is proposed. Fluoroscopic sagittal sequences of lumbar spine were processed by an automated method based on template matching to track vertebrae. A smoothing spline interpolation of the estimated intervertebral kinematic data was performed and a continuous-time description of segmental rotation and translation was obtained; the smoothing parameter was chosen both to preserve motion and to reduce noise. Concise measurements were extracted by the continuous-time kinematics and compared with standard clinical measurements of intervertebral sagittal rotation and translation. The trajectory of instantaneous center of rotation, never presented before for in vivo spinal segments, was provided and compared with standard measurements of the finite center of rotation. Results showed a good agreement with standard clinical measurements: on average, absolute differences resulted 0.74 degree for sagittal rotation, 0.59 mm for translation and 1.02 mm for the x- and y-position of center of rotation. The proposed method offers an effective technique for the continuous-time description of intervertebral motion, maintaining standard clinical measurements for diagnosis of lumbar instability.