Spinal Disorders: Fundamentals of Diagnosis and Treatment Part 9. Spinal disorders are among the most common medical conditions with significant impact on health related quality of life, use of health care resources and socio-economic costs. Spinal surgery is still one of the fastest growing areas in clinical medicine. | BiomechanicsoftheSpine Chapter 2 55 posterior elements vertebral body intervertebral disc Figure 7. Motion characteristics of the spinal segment a The subaxial motion segments exhibit six degrees of freedom 3 translations 3 rotations . Spinal motion is often a complex combination of translations and rotations. b The instantaneous helical axis of motion can be regarded as a screw motion. Range of Motion Spinal kinematics and spinal range of motion can be determined in vivo using . surface markers goniometers pantographs or computerized digitizers. While these methods are adequate for postural measurements they lack the accuracy required for intersegmental motion measurement 51 76 . More reliable in vivo radiographic and in vitro cadaveric measurements have been performed to determine the average range of motion for various levels of the spine 43 72 73 . Intersegmental range of motion is site specific determined by local anatomical geometry and functional demands Fig. 8 . Intersegmental motion is site specific Mechanical Response of the Spinal Motion Segment A common method for measuring and expressing the complex structural properties and motion of the spinal segment is through three-dimensional flexibility testing. Flexibility is the ability of a structure to deform under the application of a load. The mechanical response of the spine is typically determined by applying pure bending moments with or without the addition of an axial compressive preload in each of the three physiological directions of flexion-extension lateral bending and axial rotation and recording the overall principal and coupled motion of the specimen. Measuring the flexibility of individual functional spinal units or multisegment spine segments . the total motion achieved for a given load is somewhat analogous to the clinical concepts of range of motion and spinal instability. The load-displacement curve of the spine is generally non-linear. For small loads displacements are relatively .
