The foramen magnum (FM) is located in the occipital bone, which has three parts: – the squamosal portion in the dorsal aspect; – the clival portion located anteriorly; the condylar part, connecting these two portions, that includes the occipital condyle, posterior margin of the jugular foramen and the hypoglossal canal. Occypital condyle receives the C1 lateral mass. The most posterior margin of the foramen magnum is called “opisthion”, while the “basion” represents its most anterior midline. C1 differs from the other cervical vertebra by being a ring shape and lacks a vertebral body and a spinous process. It has two thick lateral masses which are situated at the anterolateral part of the ring. C2 has many attributes of the more caudal cervical vertebrae, but its transitional nature dictates a complicated anatomy configuration. Odontoid process represents its rostral extension: this fundamental structure originates by the developmental fusion process between the caudal part of the C1 somite and the cranial part of the C2 somite. The odontoid process begins to fuse with the body of C2 at 4 years of age and at 7 years of age the fusion is completed. In about one-third of adults, a remnant of cartilaginous tissue will be present between the odointoid and the C2 vertebral body. The pars interarticularis projects from the lamina in a rostral and ventral direction to attach to the lateral masses. C1 allows the odontoid process of C2 between its lateral masses. In other terms, odontoid occupies the usual position of the vertebral body [6]. Odontoid articulates with the dorsal aspect of the ventral portion of the C1 arch by an anterior oval facet and posteriorly with the transverse ligament that is attaches to the tubercles on the medial aspect of the C1 ring. The lateral masses of C1 articulate with the occipital condyles and C2 by kidney-shaped articulations, while C2 is directly connected to the occiput by the alar and apical ligaments and the tectorial membrane. In a sense, C1 functions as an intermediate “fulcrum” that regulates movement between the occiput and C2 [7]. The special arrangements of the occipitoatlantoaxial ligaments are remarkable to allow for complex motion, yet provide stability to this area. Infact, the capsules of the C1–C2 lateral facets surround the articular surfaces and are reinforced by ligaments and lateral fibers that pass in a rostral direction from the tectorial membrane [8]. There are ligaments between the C1 anterior arch and the odontoid and behind it: the cruciate ligament that has a vertical component from the rim of the FM to the midportion of the C2 vertebral body; the apical ligament from the rim of the FM to the tip of the odointoid process; the alar ligaments from the lateral anterior rim of the FM to the dorsal aspect of the odontoid. The cruciate ligament is considered one of the most important ligament of the human body and its rupture, identified by high resolution MRI, can lead to craniocervical instability. As stated by Quercioli [9], pioneer of occipito-atlanto-axial biomechanics, the integrity of the transverse and occipitoaxial ligaments is the essential condition for maintaining a stable odontoid process in the axis.

The occipitoatlantoaxial complex is the most mobile of the axial skeleton [10]. This functions as a single unit, considering C1 as a cradle for the occiput and C2 as a washer between the skull and the cervical spine. This complex is responsible for 40 % of total cervical flexion-extension and 60 % of total cervical rotation.

The atlanto-occipital joint allows flexion-extension and minimal degrees of lateral flexion and rotation, while the atlanto-axial joint works in coupled, rotation plus minimal lateral bending (Table. 2.1). C1 flexion-extension (e.g. nodding movements) are possible because the C1 superior articular surfaces are concave whereas the occipital condyle are convex. Flexion is achieved by the condyles rolling forwards and sliding backwards across the anterior walls of the notches. A converse combination of movements occurs in extension. Axial forces apply by the mass of the head and the muscles prevent upward displacement, maintaining the condyles nestled on the floor of their cavities. The total normal range of flexion-extension at the atlanto-occipital joint has been described as having a mean value between 14 and 35°, a range from 0 to 25° or a mean value of 14° with a standard deviation of 15° [3, 11, 12]. During these movements, a minimal anterior or posterior translation was observed [13]. Moreover, other restraints to flexion are fixed by the impaction against the skull base, tension of the posterior muscles and capsules and contact of the submandibular tissues against the throat. Extension is limited by compression of the suboccipital muscles against the occiput. Rotation and lateral flexion of atlanto-occipital joint are extremely limited, approximately 5°, due to the depth of the atlantal notches in which the occipital condyles rest. In biomechanical terms, during axial rotation to one side, the contralateral occipital condyle contacts the anterior wall of its atlantal notch, while ipsilateral condyle impacts the corresponding atlantal posterior wall. Therefore, joint stability stems largely from the depth of the C1 notches: their side walls prevent lateral translation, the front and back walls prevent anterior and posterior dislocation.

The IAR for the C0–C1 articulation has not been defined, although x-axis is considered to pass through the mastoids and the z-axis to be 2–3 mm above the tip of t he odontoid process [14]

The atlanto-axial complex is composed by two lateral facet joints, the unique atlantodental articulation and the joint between the posterior surface of the odontoid and the transverse ligament. Stability at this highly mobile articulation is primarily dependent on ligamentous structures, because the lateral joint capsules, in contrast to that of the atlantooccipital joint, are loose. Its foremost rule is to bear the axial load of the head and atlas and to transmit this load into the remainder of the cervical spine. For this function, C2 laterally presents wide superior articular facets that support the lateral masses of C1 and form the lateral atlanto-axial joints. The centrally-placed odontoid process acts as the “pivot” and forms the atlanto-axial median joint. In order to achieve axial rotation, the anterior arch of the atlas spins and glides around the pivot. Therefore, this movement is anteriorly restrained by the median atlanto-axial joint and inferiorly by the lateral atlanto-axial joints, that also subluxate. In particular, the ipsilateral lateral mass of C1 slides backwards and medially, while the contralateral lateral mass slides forwards and medially (Fig. 2.3). During axial rotation, the lateral atlanto-axial joints glide across their osseous flat surfaces. But the articular cartilages both of the atlantial and the axial facets are convex in the sagittal plane, rendering the joint biconvex in structure. In addition to these anatomical features, the spaces formed anteriorly and posteriorly by the detachment of the articular surfaces, are filled by large intra-articular meniscoids: these serve to keep a film of synovial fluid on articular surfaces. In the neutral position, the summit of the atlantial convexity rests on the convexity of the axial facet, i.e. the apex of the C1 inferior facets is balanced on the apex of the C2 superior facets. As the C1 rotates, the ipsilateral atlantial facet slides down the posterior slope of the respective axial facet, while the contralateral one slides down the anterior slope of axial facet. Upon reversing the rotation, C1 rises back onto the summit of the facets. In conclusion, C1 axial rotation requires anterior displacement of one lateral mass and a reciprocal posterior displacement of the opposite lateral mass. If the articular cartilages are asymmetrical, a small amplitude of lateral bending may accompany axial rotation: the side of coupling depends on the bias of asymmetry [15], but however this movement is considered negligible [16]¹. Principal structures that restrain axial rotation are the alar ligaments and the joint capsules: at the limits of rotation, the lateral atlanto-axial joints are almost subluxated. The normal ranges of rotation of C1 on C2 are varied (see Table 2.1): 32° e 56.7 in cadaveric studies [17a, 18], over 75° using x-ray [19] and 43° using CT [17b] in healthy adults. Recently, some authors [20], measuring in vivo by MRI normal kinematics of the upper cervical spine in neutral position and during Dvorak’s flexio-rotation test, reported respectively 77.6° and 65° of C1–C2 segmental rotation. Sagittal plane motion (flexion-extension) in C1–C2 has been reported by several authors to be on average of 11° and may be facilitated by the rounded tip of the odontoid process [21–23]. More recently, this value has been confirmed by a descriptive study based on computer-aided measurements from lateral flexion-extension radiographs [24]. The biconvex nature of the atlanto-axial articulation means that cervical spine flexion and extension often create motion in the direction opposite that being experienced in the atlas [25]. In other terms, when the entire cervical spine is flexing, C1 extends and when the cervical spine extends, C1 flexes. This paradoxical coupling motion is possible because C1 is sandwiched between the head and C2, undergoing a passive movement. Infact, in neutral condition, C1 is balanced precariously on the convexities of its articular cartilages, but when an axial compression load is applied, C1 starts to move. If the line of compression is anterior to the balance point, C1 moves into flexion. On the contrary, when the line is posterior, C1 will extend. This paradox is governed essentially by the muscles acting on the head and it can be observed even if the rest of the cervical spine is flexed. The restraint to C1 flexion/extension have never been formally established. No ligaments are disposed to limit this motion: essentially C1 is free to flex or extend until the posterior arch hits the occiput or the neural arch of C2, respectively. C1 backward sliding is limited by the impaction of its anterior arch against the odontoid process, while forward slipping is prevented by the transverse and the alar ligaments. Subluxation or dislocation implies destruction of both ligaments. Up to 3 mm of anterior translation of C1 on C2, as measured by anterior atlantodental interval (AADI), is considered normal. As the AADI increases to 5 mm or greater, the transverse and accessory ligaments are disrupted. When the transverse ligament is damaged, also rotary dislocation can occur at 45° of rotation, rather than 65° in normal condition.

The IAR for the C1–C2 sagittal plane motion is located in the region of the middle third of the odontoid. During axial rotation, it is located in the centre of the odontoid.

The middle and lower cervical spine segments have essential similar anatomic and functional features and can be effectively represented by the FSU: two vertebral bodies, the disc, the facet joints with associated ligamentous and capsular structures. Each vertebra consists of 3 pillars that forms 3 parallel columns for the load-bearing functions of the cervical spine. The anterior pillar is the vertebral bodies, which are united by interposed discs to form the anterior column. The two posterior columns are formed by the articular pillars: the superior and inferior facets are opposed to one another and united by a joint capsules. Their specific orientations allow to bear the weight of the segments above and prevent dislocation. The facet joints are the principal restraints against forward translation. End-plates of the vertebral bodies, that are stacked on one another, separated by the intervertebral disc, are not flat as in the lumbar spine. In the sagittal plane , they appear gently curved, tilting greatly downwards and forwards. The anterior inferior border of each vertebral body forms a lip that hangs, like an hook, towards the anterior superior edge of the vertebra below. As a result, the plane of intervertebral disc is not perpendicular but somewhat oblique and supports flexion-extension motion as cardinal movement of these typical cervical segments. The body supplies the strength and support for two-thirds of the vertebral compression load. The upper surface is typically concave from side to side and convex in the antero-posterior direction. On its lower surface, it is convex from side to side and concave in the antero-posterior direction. Also, the upper projection on the lateral superior surface of the vertebra below is called “the uncus” . These bilateral uncinate processes are related intimately with the convex lateral inferior surfaces of the upper vertebral body and form the uncovertebral joints or joints of Luschka. The exact rule of these joints is not known: they would seem to prevent posterior dislocation and limit lateral bending.