A literature review (White & Panjabi, 1978) concluded that the representative angle for flexion–extension for C3–C4 is 13°, 12° for C4–C5, 17° for C5–C6, 16° for C6–C7 and 9° for C7–T1. However, Panjabi et al (2001) reported the smaller average ranges of motion for flexion–extension of 7.7° for C3–C4, 10.1° for C4–C5, 9.9° for C5–C6, 7.1° for C6–C7.

A more recent study (Wu et al, 2007) reported mean flexion and extension of the cervical motion segments. More specifically, they reported a mean of 7.56° of flexion coupled with a mean of 0.98 mm of anterior translation and 9.05° of extension coupled with 1.13 mm of posterior translation at C3/C4 levels. The corresponding results were 9.86° flexion with 1.15 mm of anterior translation and 11.24° of extension with 1.26 mm of posterior translation of the C4/C5 level, 9.24° of flexion with 1.15 mm of anterior translation and 9.88° of extension with 1.19 mm of posterior translation at the C5/C6 level. For the C6/C7level, flexion was measured at 9.73° with 0.93 mm of anterior translation and 7.91° of extension coupled with 0.29 mm of posterior translation.

The fact that the cervical intervertebral joints are saddle joints means that the vertebra is also free to move side to side in the plane of the facets around an oblique axis perpendicular to the plane of the facets, in an angle of 45° to the transverse plane (Bogduk & Mercer, 2000). However, this axis does not pass through the apophyseal joints or the discs but through the cranial vertebral body (Penning, 1988). The geometry of the articular surfaces and the movement around the oblique axis dictate that horizontal rotation and side-flexion are always coupled to the same side (Bogduk & Mercer 2000; Penning, 1988). Not surprisingly all studies reporting on the coupling motion of rotation and side-flexion of the motion segments from C3 to T1 agree that the coupling is always on the same side regardless of which of the two side-bends or rotation is the initiating movement (Cook et al, 2006). The only exception is the contralateral coupling with lateral flexion initiation reported in the study of Ishii et al (2006) at the C7/T1 level.

Further studies have determined the centre of rotational motion to be localized in the ventral contour of the vertebral body for both lateral flexion and rotation (Lysell, 1969; Penning, 1988). The position of the centre of rotation anteriorly means that during coupled rotation and side-flexion, more movement occurred at the posterior part of intervertebral joint and less at the anterior.

This characteristic movement pattern is reflected by the morphology of the intervertebral disc. The cervical intervertebral disc is distinctively different from the lumbar one in that it lacks concentric annulus fibrosus, and its core is fibrocartilaginous and has the consistency of soap (Mercer & Bogduk, 1999). The annulus fibrosus is thicker anteriorly and consists of layers of fibres that arise out of the superior surface of the inferior vertebra and insert on the inferior surface of the inferior vertebra. The outer layer of the annulus is orientated vertically at its central part and obliquely at its lateral parts from inferio-lateraly at its origin from the uncinate processes to anteriorly at the inferior margin of the superior vertebra (Mercer & Bogduk, 1999). The thickness of the annulus is about 2 to 3 mm. The deeper fibres of the annulus are orientated obliquely from inferio-laterally to superio-medially and interwoven with the corresponding fibres of the opposite side, towards the midline. The deeper fibres follow the same pattern but they insert more towards the midline of the superior vertebra (Mercer & Bogduk, 1999). This arrangement is consistent with the vertebrae pivoting about its anterior end as the majority of the fibres are arranged as an inverted ‘V’ pointing its apex at the centre of rotation (Bogduk & Mercer, 2000).

The annulus fibrosus is largely lacking posteriorly with only a few fibres in a lamina of about 1 mm thickness, vertically orientated. However, for the larger part of the posterior disc to the uncinate processes there is no annulus (Bogduk & Mercer, 2000; Mercer & Bogduk, 1999). Another characteristic of the cervical disc is the development of transverse fissures at the posterior part of the cervical discs. The fissures first appear at the age of 9 years as clefts in the unconvertebral region and progressively extend medially to form transverse clefts in the third decade of life (Bogduk & Mercer, 2000). With age, the clefts have a tendency to break through transversely, resulting in the posterior disc being comprised of two parts (Mercer & Jull, 1996). It is not clear if the development of transverse fissures is a result of the development of the uncinate processes, an early pathological change, or the result of the shearing forces of rotation (Mercer & Jull, 1996). Whatever the explanation, their presence allows or facilitates axial rotation (Bogduk & Mercer, 2000).

The morphological and geometrical characteristics discussed above support a pattern of movement of the typical cervical intervertebral movement of coupled side-flexion and rotation of the same side, in which the superior vertebra pivots about its anterior part and slides with its posterior. Few studies have reported on the ranges of motion of the intervertebral segments. At the C3/C4 level, White and Panjabi (1978) reported representative angles of 11° of side-bending and 11° of rotation. Mimura et al (1989) reported a 5.8° rotation from right to left at this level coupled with a side-flexion of 6.2° and a 2.9° of extension at this level. Penning and Wilmink (1987) reported a mean of 3° of rotation from right to left. Ishii et al (2004a) reported 4.5° of axial rotation coupled with lateral-bending at the same side, extension, inferior translation, opposite lateral translation and posterior translation. Ishii et al (2006) found 3.5° of lateral-bending coupled with axial rotation of the same side, flexion, superior translation lateral translation to the opposite side and anterior translation of the vertebra. Similarly, Panjabi et al (2001) reported 5.1° of axial rotation from right to left and 9.1° of lateral-bending from right to left.

For the C4/C5 level, White and Panjabi (1978) reported a mean 11° of right to left side-flexion and a mean 12° of rotation from left to right and Penning and Wilmink (1987) reported a mean 6.8° of rotation from right to left while Mimura et al (1989) reported a mean of 4.2° rotation from right to left with a mean of 6.2° of lateral-bending to the same side and a mean of 2.1° of extension. For the same level, Panjabi et al (2001) reported a mean of 6.8° right to left rotation and 9.3° mean right to left side-flexion. Ishii et al (2004a) found that the C4/C5 level rotates to a mean of 4.6° and it is coupled with ipsilateral side-flexion, extension and contralateral lateral translation while Ishii et al (2006) reported a mean of 3.3° of lateral-bending coupled with ipsilateral rotation, flexion, superior translation, contralateral coupled lateral and posterior translation.

At the C5/C6 level, White and Panjabi (1978) reported a representative angle of 8° of lateral-bending and an angle of 10° of rotation, while Penning and Wilmink (1987) reported a mean of 6.9° of rotation at this level from right to left. Mimura et al (1989) state an average of 5.4° of total rotation coupled with a mean of 4° of ipsilateral lateral-bending and 2.1° of flexion, while Panjabi et al (1988) reported a mean of 5.1° of rotation and a 6.5° of lateral-flexion respectively. Ishii et al (2004b) reported a mean of 4° of rotation coupled with ipsilateral lateral-bending, flexion, superior, opposite lateral and anterior translation while side-flexion, according to Ishii et al (2006) was measured to an average 4.3° coupled with ipsilateral rotation, flexion superior, opposite lateral translations. Similarly, for the C6/C7 level the average range of side-flexion reported by White and Panjabi (1978) was 7° and 9° of rotation. An average range of 5.4° of rotation has been reported by Penning and Wilmink (1987) and a slightly higher average range of 6.4° coupled with ipsilateral lateral-bending of 2.7° and 2.5° of flexion has been reported by Mimura et al (1989). An even lower rotation range of motion has been reported by Panjabi et al (2001) at 2.9° and an average range of lateral-bending of 5.4°. Similarly, Ishii et al (2006) reported an average of 5.7° of lateral-bending coupled with ipsilateral rotation, flexion, superior and contralateral lateral translation; there was also an average range of rotation of 1.6° coupled with ipsilateral lateral-bending, flexion, superior opposite lateral and anterior translation of the vertebra.

At the C7–T1 level the average ranges, according to White and Panjabi (1978), are 4° for lateral-bending and 8° for rotation, while Penning and Wilmink (1987) reported 2.1° of rotation. Ishii et al (2004a) reported an average of 1.5° of rotation coupled with ipsilateral lateral-bending, flexion, superior, opposite lateral and anterior translation. However, Ishii and colleagues’ results (Ishii et al, 2006) disagree with the results of more studies at this level as they report a mean of 4.1° of lateral-bending coupled with contralateral rotation to a mean of 0.4° also coupled with flexion, superior ipsilateral lateral translation. The results of the studies of Ishii et al (2006) and Ishii et al (2004a) reveal that regardless of which movement initiates the coupling, the range of the lateral flexion component of the movement is greater in range than the rotation component. Furthermore, there is no progressive reduction of the available range, as is expected from the top to the bottom of the typical cervical spine column.