The intervertebral joint is therefore an articulation with six degrees of freedom (DOF), three DOF in translation, and three DOF in rotation .
The movements occur in the disc and articular facet joints, assimilated into three coaxial joints which therefore have no locking position (Fig. 2). The tensioning of the ligamentous structures, including the intervertebral disc and the osseous constraints are stabilizing structures which can be exceeded in extreme situations, especially in micro- or macro-trauma.
According to Panjabi , when a load is applied to a spinal functional unit (SFU) , the result is a range of motion (ROM), which, before reaching its maximum, passes through a neutral zone and an elastic zone. The neutral zone is the portion of the intervertebral mobility area closest to the rest position, in which the joint has the largest capacity for movement with minimal resistance to intervertebral mobility. The elastic area corresponds to the magnitude of intervertebral mobility located between the end of the neutral zone and the limit of the ROM . It is interesting to note that in the setting of disc degeneration , the neutral zone will increase, as well as translations, which signifies instability (Figs. 3 and 4).
Methods for Measuring Spinal Mobility
This evaluation can be done in vitro and in vivo.
In Vitro Measurements
In vitro measurements are performed on cadaveric subjects, usually of elderly subjects but isolated from any musculoligamentous envelope, which explains why angular values are usually increased compared to those measured on living subjects. The physical measurement means are displacement sensors, ultrasound or X-rays.
In Vivo Measurements
The in vivo measurements are for active movements which seek to assess the overall and intersegmental mobility. Many processes can be used: simple goniometers or inclinometers (liquid or gravity) and especially more accurate electronic (cervical range of motion® or CROM®) , electrogoniometers, magnetic devices (e.g., Fastrack® or Isotrack®) , ultrasound devices (Zebris®), videofluoroscopy, and finally optoelectronic devices (Vicon®).
Medical imaging includes dynamic X-rays, cineradiography, CT, and MRI.
Dynamic radiographs are performed routinely in the clinical setting, mainly for cervical and lumbar evaluation. On these images, the flexion–extension and less often the right and left lateral inclination can be measured. Rotation is perfectly explored only through computed tomography (CT).
The dynamic flexion–extension lateral cervical radiographs are in a sitting position: the subject is asked, for the exploration of flexion, to try to touch the sternum with the chin and to explore extension by bringing the head as far back as possible. The dynamic lumbar images of flexion–extension can be done according to different techniques. The technique of Putto , which is with the patient seated or standing, hyperflexion of the trunk in hyperextension with gluteal support is the most recognized (Fig. 5); the same author showed that he obtained less amplitude by positioning the patient in less flexion and extension. We can also achieve these positions of flexion–extension on a Swedish chair, a method used in the evaluation of spinal fusion  (Fig. 6). Wood  studied patients with spondylolisthesis and shows that images performed in flexion–extension on the supine subject were more sensitive than those practiced in standing.
Dynamic images for lateral inclination were investigated by Weitz  to recognize indirect signs of lumbar disc herniation. Dupuis  did a study of dynamic radiographs in lateral inclination to recognize signs of instability.
More conventionally, the intervertebral instability is likely if it exists between extremities of flexion and extension, an angular intervertebral mobility of greater than 10° , or even 20° , and vertebral translation of more than 3 mm , 4 mm , or even 5 mm .
With the help of image software, one can refine the evaluation of intersegmental mobility and calculate the position of the instantaneous centers of rotation (ICRs) . Gertzbein  demonstrated on cadaveric parts that there was dispersion of these ICRs in degenerate and unstable intervertebral segments (Fig. 7).
Finally, the evaluation of dynamic views of lumbosacral mobility is crucial to recognize the patient’s ability to correct their pelvic retroversion in the setting of anterior truncal imbalance. We can evaluate the amplitude of anteversion in standing with an image in a single leg lunge position as described by Hovorka in the chapter “The Reserve of Hip Extension and its Relationship with the Spine” and by Lazennec  (Fig. 8), in a procubitus position with a cushion positioning the femurs in hyperextension (Fig. 9).
CT is less used in this setting but enhances evaluation of rotation. It was used by Penning  at the cervical level, Morita  to evaluate flexion–extension in the thoracic region, and Fujimori  to evaluate lateral inclination in the thoracic region. Husson  describes signs of lumbar instability in the face of abnormal decoaptation (uncoupling) on rotating scanners.
Dynamic MRI is mainly used to evaluate the neurological content of the spinal canal. Vitzhum  used it to evaluate thoracic movements.
Finally, intraoperative rigidity measurement was described for the first time by Ebara  with intraoperative distraction of the spinous processes surrounding the tested intervertebral segment according to the force (F) and displacement (D); the rigidity is proportional to the ratio: delta F/delta D (Fig. 10). An unstable segment will have low rigidity, characterized by significant displacement for a small applied force. Brown  has developed an automated device that can best adjust the intensity of the applied force. Like Hasegawa , who has a great deal of experience in this field, Brown maintains that the indication of a flexible or rigid arthrodesis is governed by these results for measurement of rigidity.
Amplitude of Spinal Movements
Figure 11 shows the distribution of movement amplitudes between the cervical, thoracic, and lumbar segments in flexion, extension, rotation, and lateral inclination.
White and Panjabi  have shown, in vitro, that these last two movements of inclination and rotation are reflexively or automatically associated (Fig. 15).
Ishii [22 and 23] has shown more recently, in vivo, that lateral inclination and lateral rotation were in the same direction in the lower cervical spine but that there was an opposite rotation in the upper cervical spine (Fig. 16). This coupling was confirmed by Fujimori .
The diagrams of Castaing  show the amplitudes of cervical flexion–extension (Fig. 17), cervical lateral inclination (Fig. 18), cervical rotation (Fig. 19), the different movements of the thoracic spine (Fig. 20), and finally the different movements of the lumbar spine (Fig. 21).
At the cervical level, Ordway  describes on the dynamic views in flexion–extension of the cervical spine the presence of protraction (or protrusion) and retraction movements. In protraction, there is a flexion of the C3C7 segment and an associated extension of the OC1C2 segment. In retraction, there is an extension of the C3C7 segment and an associated flexion of the OC1C2 segment (Fig. 22).
At the thoracic level, with CT, Morita  found a flexion–extension of 31.7° and Fujimori  a lateral inclination of 25°.
Segmental Amplitudes and Motion Analysis
We recall that these movements are mainly in rotation and also in translation which is much smaller and which become pathological if too important.
The Upper Cervical Spine (OC1C2)
At the level of the upper cervical spine (OC1C2) , Table 2 summarizes the amplitudes proposed by the different authors. At OC1, despite the spheroidal shape of the surfaces, there is practically only a flexion–extension motion; the occipital condyles recede with respect to the upper articular surfaces of C1 in flexion and advance in extension. The center of the movement is at the occiput (Fig. 23).
At the C1C2 level in flexion, the neural arc of C1 slightly loses its parallelism with that of C2, without C1’s nosing forward as in certain high cervical instabilities. In extension, the neural arc tilts backwards. The center of the movement is in the middle of the articular mass of C1 (Fig. 23).
The rotational movement is essential at C1C2 since it has an amplitude of more than 25° for each side, i.e., half of the total amplitude of rotation in the cervical region. In this movement, which mainly affects the atlanto-axial joints, there is a shift toward the front of the lower articulation of C1 on the side opposite to the rotation and a sliding toward the rear of this same lower articulation of C1 from the side of the rotation (Fig. 24). This is reflected in the open-mouth radiograph by an asymmetry of the AO distances between the axis (O) and the lateral masses of C1 (A) (Fig. 25) which is not pathological and does not mean in any way a C1C2 rotary subluxation.
The ICR of the movement is located in the middle of the dens process, at mid-distance from the joints involved in the movements, lateral and anterior atlanto-axial. For Castaing , there are two types of rotation: one around the odontoid with symmetrical displacements of the two C1C2 joints and the other around a fixed C1C2 articulation (Fig. 26). It is interesting to note that there are two types of C1C2 rotary subluxation involving these two modes of rotation. Finally, Fig. 27 shows that there is a lateral inclination of 8° at C2C3, 3° at OC1, and virtually no inclination at C1C2.
The Lower (Sub-Axial) Cervical Spine
Tables 3, 4, and 5 show different amplitudes reminiscent of intersegmental mobility noted in vitro and in vivo in the literature. The C5C6 segment is the most mobile, especially in flexion–extension and one will recall that the lesions of degenerative instability are most common at this level. The amplitudes of movement in rotation and lateral inclination are slightly variable from one level to another. The C7T1 segment is the least mobile in all movements.