Anatomic and biomechanical considerations

In the upper cervical spine instability is quite well understood and defined both clinically and biomechanically. The advancement of relationships between anatomy and function will be presented.

The anatomic stability of occipito-atlanto-axial joint complex is gained mainly by the ligament, capsule, and structural membrane (Fig. 51.2). The stability of occiput–C1 joint is provided by their capsule, along with the anterior and posterior atlanto-occipital membrane, enhanced by the additional structures such as the alar ligament, apical ligament, and the tectorial membrane. The transverse, dentate, and apical ligaments play an important role in structural stability of the atlantoaxial joint. The injury or dysfunction of the transverse ligament leads to the translation of C1 on C2 and widening of the atlantodens interval (ADI), which can be detected by lateral X-ray of the cervical spine. Functional loss of the alar ligaments indicates a potential for rotatory instability, which, however, must be determined in conjunction with other clinical findings, such as neurological dysfunction, pain, and deformity.⁹

Fig. 51.2 Axial schematic of the major ligaments involved in the clinical stability of the upper cervical spine.

Knowledge of the normal movements of the occipito-atlanto-axial joint complex is important for evaluating clinical cases that may be potentially unstable. The occiput and axis are sometimes described as two rotating squares with the atlas as a bearing between them. The joints between the atlas and axis are almost horizontal and therefore allow for a wide range of motion in rotation, whereas rotation between the occiput and atlas is completely prevented by the geocentric anatomy of the articulation. The investigations of measurement of range of motion of C0–C1–C2 are shown in Tables 51.1 and 51.2. Both joints, C0–C1 and C1–2, participate in flexion and extension. Extension at occiput–C1 is one of the largest single motions in the spine. Lateral bending occurs at occiput–C1, but is almost negligible at C1–2. The greatest intervertebral motion in the spine is axial rotation at the C1–2 joint.^10,11

Analysis of lateral neutral, flexion, and extension radiographs was performed using two measurements: occipitocervical angle (OCA), and occipitocervical distance (OCD). The OCA was defined by the junction of McRae’s line (a line intersecting the basion and the opisthion) and a line drawn parallel to the superior endplate of C3. The superior endplate of C3 was used for the caudal landmark because it was found to parallel the inferior endplate of C2 and was more readily visualized on all radiographs. The OCD was obtained by measuring the shortest distance from the superior-most aspect of the C2 spinous process to the occipital protuberance, found directly behind the mastoid process.

The mean occipitocervical angles were 24.2°, 44.0°, and 57.2° in flexion, neutral, and extension, respectively. The mean occipitocervical distances were 21.5 mm in neutral, 28.0 mm in flexion, and 14.8 mm in extension.¹⁰ These are simple to determine on routine lateral radiographs. The differences in the occipitocervical angle and occipitocervical distance in neutral, flexion, and extension are significant. These measurements should be a valuable intraoperative tool for achieving occipitocervical fusion in appropriate alignment.

The term coupled motion denotes that motion about one axis consistently occurs simultaneously with motion about another axis (rotation or translation).¹² It is most dramatic in the cervical spine. Two kinds of coupled motion are especially well known in the cervical spine: axial rotation in the same direction as applied lateral bending, and lateral bending in the same direction as applied axial rotation. Posture affects motion coupling patterns of the upper cervical spine. The most dramatic change due to modification in posture is found in coupled sagittal plane rotation, which changes from extension at extended posture to flexion at flexed posture at both levels and in response to both load types. For the axial torque, the main axial rotation and coupled lateral bending changes little with posture.

Diagnosis

In daily clinical practice, one often deals with patients with neck pain or stiffness resulting from injury or the ongoing degenerative changes rather than those with no relevant complaint. Active motion in patients with complaints of neck pain or stiffness is usually restricted by the motion-induced pain. Due to the closely related trigeminus nuclei, segmental functional disorders in the suboccipital area cause, in addition to local neck pain, frontoparietal and retro-orbital pain that can also irradiate to the upper and lower jaw. In addition, tendomyoses with trigger points and difficulties in swallowing may occur in the anterior part of the neck.¹³ The historical information collected from the patient is important in making the diagnosis, even given the aid of radiologic technology.

In order to differentiate between hypomobility, hypermobility, or instability, clinical diagnosis requires specific examination of each particular segment in accordance with its biomechanics. Once the nature of the functional disorder is defined, the whole repertoire of therapeutic measures can be applied, with segment-oriented manual techniques being the most efficient. In cases of persistent and recurrent disturbances, rehabilitation must be completed by regular back exercises and optimal ergonomics.

A few recognized criteria of radiology for C0–C1–C2 instability have been established. For instance, when the ADI value, which may describe the range of C1–2 translation, is larger than 3 mm in lateral flexion and extension view, the atlantoaxis is considered unstable.¹⁴ Asymmetry of the odontoid lateral mass interspace on open-mouth view is also an important clue for diagnosis. One should also assess whether vertical migration of the odontoid process exists, especially in rheumatoid patients. For evaluation of the function of the upper cervical spine, especially assessment of the range of motion, functional X-rays films are useful in addition to the clinical examination (Fig. 51.3). For the diagnosis of segmental instability, passive motion should be induced in order to obtain the full range. If anterior instability of the upper cervical spine is suspected, flexion–extension X-rays in the lateral view are appropriate. If a lesion of the alar ligaments is suspected, then lateral flexion X-ray films should be taken. In the normal situation, the atlas glides in the direction of bending, coupled by forced rotation of the axis. In cases with rotatory instability of the upper cervical spine, functional computed tomography should be performed. Atlantoaxial rotation of more than 52° should be considered pathological as a result of a lesion of the alar ligaments. For examination of the relationship between the spinal cord and bony structures or inflammatory tissue in patients with rheumatoid arthritis, functional MRIs are helpful.

Fig. 51.3 Lateral view of the cervical spine at flexion, neutral position, and extension.

Functional magnetic resonance tomography is an important diagnostic method for assessing the cervical spine in patients with rheumatoid arthritis. In particular, fusion and instabilities as well as compressions of the spinal cord or medulla often can only be detected with the help of functional MRI. Compared with static MRI examinations in patients with rheumatoid arthritis, functional magnetic resonance imaging identifies the extension of pannus tissue cranial, ventral, and dorsal to the dens with possible displacing and impinging effects on the spinal cord during flexion and extension. In addition, it is suitable for demonstration of the degree of instability in the atlanto-occipital and atlantoaxial planes. In contrast to conventional X-rays, CT, and static MRI, basilary impression as well as compressions and angulations of the spinal cord are better visualized by cinematic magnetic resonance tomography.¹⁵ Cineradiography is a valuable continuous recording tool and has been used in motion analysis of the musculoskeletal system (Fig. 51.4).¹⁶ Cineradiography has advantages over other techniques for analyzing the details of cervical spinal motion and diagnosis of spinal instability.^17–20 The outcome from the cineradiographic motion analysis of the cervical spine can be taken into account to determine the level of spinal fusion for cervical spondylosis. An instantaneous center of rotation deduced from cineradiographic films has also been used to clarify the quality of motion and to justify therapy for the neck region.

Fig. 51.4 Radiographic examples and equipment setup. Cineradiographic images were projected onto a digitization table at 3.5-fold magnification. The template was superimposed onto every four frames (time interval, 0.13 seconds), and four vertebral landmarks were digitized.

Hino et al. investigated the kinematics of the normal and pathologic atlantoaxial joints by cineradiography to determine the in vivo kinematic parameters for the quantification of atlantoaxial instability.²¹ The kinematics of the atlantoaxial joints were evaluated by cineradiography in healthy volunteers and in patients with atlantoaxial subluxation. The results revealed the different onset points of a rapid increase in motion between flexion-from-E and extension-from-F in atlantoaxial motion. They were defined, respectively, as points A and B (Fig. 51.5). The discrepancy between these points (i.e. zone A–B) was significantly more remarkable in patients with atlantoaxial subluxation than in the volunteers. Furthermore, in most of the cases with atlantoaxial instability, subluxation occurred when the cervical spine was in a more extended position, and it was reduced in a more flexed position.

Fig. 51.5 The format of cineradiography outcome. Regarding the angulation of sagittal rotation, 0° means lines A1–P1 and A2–P2 (i.e. the x-axis is horizontal). Point A exists in a more extended position and point B in a more flexed position in the atlantoaxial range of motion.

Surgical indication and principle of internal fixation

Although occipito-atlanto-axial region instability can occur suddenly from violent traumatic forces, it can also develop in a slow, progressive fashion as occurs with congenital abnormalities, infections, neoplasms, or rheumatoid arthritis. In certain instances, spinal elements may fail along a continuum; anatomic changes can appear long before pathologic motion develops.

Neurological dysfunction due to spinal instability is the most urgent indication for arthrodesis. Mechanical compression of neural structures can present with occipital radicular pain, myelopathy, cranial nerve deficits, nystagmus, and bulbar dysfunction. Sudden death has been reported with occiput, C1, and C2 subluxations. Subluxations of the craniovertebral junction may also present with vertebral artery compression.

In the absence of neurologic deficits, specific pathological and clinical features become crucial to consider in determining the need for internal fixation. Patients who are at high risk for nonunion of fractures, those at risk of developing neurological deficits from compression or progressive subluxations, or those with predominantly ligamentous injuries may be considered for arthrodesis. However, the decision for internal fixation must be individualized, based upon the patient’s neurological status, age, and medical condition and upon the extent of subluxations, the type of pathology, and the levels of instability.

The conclusion of the analysis of the in vitro biomechanical properties of one-, two-, and three-point fixations at the atlantoaxial segment is that one-point fixation results in high stiffness in flexion–extension, whereas the stiffness in axial rotation and lateral bending is lower, using new nonbone graft-dependent fixation devices. Two-point fixation results in high stiffness in all motion directions except in flexion–extension. The reasons probably being that the screws are located close to the axis of rotation. Three-point fixation results in a high stiffness level in all degrees of freedom. The combination of transarticular screws with the C1-claw device results in stiffness equivalent to the traditional 3-point fixation technique, but without the need of structural bone graft and the use of cerclage wire in the spinal canal. From a biomechanical viewpoint the 3-point fixation technique is the method of choice for C1–2 fusion.