To analyze, understand, and correct the various malfunctions of the spine, it is essential that we first fully characterize the normal functions. The three functions of the human spine are to (a) support the appendicular skeleton and the trunk, (b) protect the neural elements, and (c) allow a significant amount of complex motion. Certain clinical problems are known to be associated with irregularities in the amount and possibly the type of motion present in the spine. Some of the diseases associated with irregular motion in the spine are well recognized and documented through clinical observations. Other diseases may be associated with motion irregularities even though the full pathology and associated clinical consequences have not yet been fully characterized.

The functions and anatomic designs of the occipitoatlantoaxial complex (C0-C1-C2) and the lower cervical spine (C2 through C7) are dissimilar; therefore, it is advantageous to study the kinematics of each region separately. The occipitoatlantoaxial complex is grouped and labeled the upper cervical spine. Because of kinematic (2), kinetic (3), and clinical uniqueness (4,5), the cervical region is further divided into the middle cervical (C3 through C5) and the lower cervical (C6 through T1) regions.

Some key kinematic terms are defined herein to facilitate further reading (6).

Translation: Translation is motion of a rigid body in which a straight line in the body always remains parallel to itself.

Rotation: Rotation is motion of a rigid body in which a straight line does not remain parallel to itself (Fig. 4.1). The angle between the two positions of the straight line, that is, A1-B1 and A2-B2, is the angle of rotation.

Center of Rotation (COR): When a rigid body moves in a plane, there is a point, either in the body or at some hypothetical extension of it, that does not move. This is the COR of the body for that step of motion (see Fig. 4.1). The line perpendicular to the motion plane and passing through the COR is called the IAR. The COR or IAR is obtained by the intersection of the perpendicular bisector of the translation vectors A1-A2 and B1-B2 of any two points, for example, points A and B.

Degrees of Freedom (DOF): The number of independent coordinates in a coordinate system required to completely specify the position of an object in space.

Neutral Position: The erect posture of the spine in which the overall internal stresses in the spinal column and the muscular effort to hold the posture are minimal.

Range of Motion: The range of physiologic intervertebral motion measured from the neutral position. It is divided into two parts: neutral zone (NZ) and elastic zone (EZ).

Neutral Zone: That part of the range of physiologic intervertebral motion, measured from the neutral position, within which the spinal motion is produced with a minimal internal resistance. It is the zone of relatively high flexibility or laxity.

Elastic Zone: That part of the range of physiologic intervertebral motion measured from the end of the NZ up to the physiologic limit. In the EZ, spinal motion is produced against a significant internal resistance. It is the zone of high stiffness.

In vitro biomechanical tests include strength, fatigue, or flexibility (7). A strength test consists of applying a load to an implant or a spinal construct (specimen plus instrumentation) until it fails and determining its load carrying capacity. A fatigue test determines the capacity of the spinal construct to carry varying physiologic loads until the fusion matures and unloads the implant. The load can be varied but is always less than the failure load. The cyclic load is applied until the specimen fails. A stability test determines the capability of the spinal construct to minimize motion at the site of fusion to promote initiation of fusion and maturation. Stability testing can be further divided into flexibility and stiffness testing. To conduct a stability test, the specimen is subjected to load (force or moment) or displacement (translation or rotation). Flexibility testing consists of applying loads and measuring resulting displacements, while stiffness testing consists of applying displacements and measuring resulting loads.

The concept of in vivo passive kinematics has been well described in the work of Dvorak et al. (8, 9 and 10). Investigations with functional x-rays have showed that additional motion can be gained by exerting external forces on the “fully” flexed or extended neck, an important finding to consider when evaluating voluntary flexion/extension tests. This is also consistent with the assertion that the stretch test (passive axial traction) is more likely to show existing pathology than is measurement of active (i.e., voluntary) flexion/extension (6).

The differences between active and passive in vivo ROM must be considered in the interpretation of laboratory and clinical studies of normal ranges of cervical motion. Considering that in daily clinical routine we do not deal with healthy volunteers but rather with patients who have either injury or painful conditions resulting from ongoing degenerative changes, we must be aware that the patient’s active motion may be restricted by motion-induced pain rather than pathology (11, 12 and 13). However, we should be concerned, especially when we suspect a segmental instability, in motion (both rotation and translation) beyond the limit of active motion. In an injured patient population, examination of passive flexion/extension was shown to result in the identification of more hypermobile segments (9,14). Similar observations as related to passive motion of the cervical spine have been made with lateral bending (15) and axial rotation (10). Examining the total motion of the cervical spine with a spine motion analyzer (16), it was found that passive tests resulted in larger ROM with smaller associated standard deviation (SD). Active tests showed significantly less motion than passive tests in measurements of lateral bending and axial rotation of the cervical spine. A meta-analysis of average cervical spine motion (17) of healthy individuals between 20 and 50 years of age found increased passive motion, as compared to active (140 vs. 126 degrees total flexion/extension; 109 vs. 86 degrees total lateral bending; 174 vs. 151 degrees total axial rotation).

Much of the research presented herein requires further confirmation and suggests new avenues for investigation. Attempts have been made to distill fact from postulate and to integrate biomechanical research with clinical applications. As in any research field, there are numerous problems to overcome. The kinematics of interest is that of the in vivo cervical spine. The experimental techniques for precise, no-risk, in vivo measurements continue to evolve. The characteristics of the muscle forces that result in in vivo physiologic motions have recently been emulated during experimental motion testing (18). In vitro studies have been performed to simulate vertebral motion, but whether the motions produced experimentally are the same as those produced in vivo is not known. Obviously, these problems must be addressed. The purpose of this chapter is to present the state of the art in regard to cervical spine kinematics; useful areas of clinical relevance are indicated.

Most of the axial rotation and some of the flexion-extension and lateral bending of the head occur in the upper
cervical spine. The highly specialized anatomy of this region is designed to provide seemingly paradoxical attributes: flexible enough to allow nearly 50% of the cervical spine axial rotation, sufficiently stiff to protect delicate structures of the spinal cord and vertebral arteries, and strong enough to carry the weight of the head and resist muscle forces. It is therefore a very complicated structure, with motion determined by the orientation of the articular processes and limited by the ligaments (Fig 4.3).

The atlantooccipital (C0-C1) joints are spheroid articulations; that is, they allow motion in the shape of a sphere. The joint capsule connecting them is very tight, serving to limit the possible movements and prevent dangerous hypermotion that could pinch the spinal cord or the vertebral arteries. The dominant movement in the atlantooccipital joint is flexion and extension of approximately 24 degrees (Table 4.3). The lateral bending is 5 to 10 degrees. The idea of axial rotation in this joint has long been rejected but, more recently, investigators have shown axial rotation in both in vitro and in vivo studies (33, 34, 35 and 36). The possible movements in the atlantooccipital joint are shown in Table 4.3. The disparity in the results is
mostly due to the differences in the methods used in the mixed group of in vitro and in vivo studies.

The atlantoaxial (C1-C2) joint consists of four joint spaces: the two atlantoaxial lateral joints, the atlantoaxial median joint (between the ventral arch of the atlas and the dens axis), and a joint between the dorsal surface of the dens and the transverse ligament, which is connected to the ventral joint space. From the medial part, there is a large synovial fold in the lateral atlantoaxial joint. This joint capsule, in contrast to that of the atlantooccipital joint, is loose, allowing a great deal of motion. It is here that most of the axial rotation occurs, a fact reflected in the anatomy as the vertical dens of the axis (C2) acts as a pivot about which the atlas (C1) rotates. The possible movements in the atlantoaxial joint are summarized in Table 4.4. As in Table 4.3, the differences in the results of the studies are due to the different methods used.

The motion in the upper cervical spine, especially in the atlantoaxial joint, is mainly limited by the nonstretchable collagen fibers (37) of the alar ligaments, which connect the dens to the medial aspects of the occipital condyles and ventral arch and lateral masses of the atlas (34,38). According to Werne (39), alar ligaments are of great importance in limiting axial rotation—a belief that has been confirmed by newer investigations (Fig. 4.4) (34,40). In conjunction with the tectorial membrane (the only elastic ligament in the upper cervical spine), the alar ligaments also limit the flexion of the occiput. During lateral bending (Fig. 4.5), the alar ligament is responsible for forced axial rotation of the second vertebra in the same direction as the lateral bending (10,39). The spinous process of the axis moves contralaterally.

The cruciate ligament (Fig. 4.3A and B) has cleverly evolved to restrict potentially dangerous ventral motion of the atlas during flexion movement of the head while still allowing the atlas to turn freely around the dens during axial rotation. It consists of two main parts: the horizontally oriented transverse ligament and the vertically oriented longitudinal fibers. At the level of the dens, there is a thin layer of cartilage covering the transverse ligament (8), which allows the ligament to move more freely during rotation and protects it from friction damage. The transverse ligament consists of collagen fibers with an interesting fiber orientation similar to a folding lattice. This allows extensive stretching of the ligament during axial rotation without damage to the fibers. In vitro experiments indicate failure of the transverse ligament at a wide range of loads, between 118 and 1,765 N (8,41,42). The apical ligament has no functional meaning.