Joints between the vertebral bodies
An intervertebral disc is a secondary cartilaginous joint. The upper & lower surfaces of the vertebral bodies are covered by a thin layer of hyaline cartilage. These plates are joined by a peripheral ring of fibrous tissue, the annulus fibrosus, which is a concentric laminae of fibres, lying at 20-45º to the vertebral bodies. Each alternate layer lies at 90º to the preceding. This means the annulus can withstand strain in any direction. Inside the nucleus is a semiliquid gelatinous substance, the nucleus pulposus, derived from the embryonic notochord, which accounts for 15% of the disc. In the embryo the pulposus lies at the centre of the disc, but subsequent growth laterally & anteriorly leaves it towards the back in the adult. (thus herniates backwards). The nucleus is 90% water at birth & diminishes to 70% in old age. It is the mucoprotein (proteoglycan) content which imbides & retains the water, accounting for a growth of 1cm overnight in a young adult. (less in old age).
The posterolateral lip or uncus, on C3-7, may appear to form a joint with the adjacent disc, because a cavity may develop in this region. These are not synovial joints, but degenerative changes in the disc.
Ligaments between the bodies
The anterior longitudinal ligament is a flat band extending from the anterior tubercle of the atlas, to the front of the upper part of the sacrum. It is firmly attached to the periosteum of the vertebral bodies, but less so over the discs.
The posterior longitudinal ligament extends from the back of the body of the axis to the sacral canal, narrowing gradually from above down. It is broader over the discs to which it is firmly attached, & narrower over the vertebral bodies to which it is loosely attached ( to allow basivertebral veins to exit). At the top the ligament is continued as the tectorial membrane.
Zygapophyseal joints
These are the facet joints between the articular facets. They are synovial with a simple capsule, which blends laterally with the ligamentum flavum. The articular surfaces allow gliding of one on another. The nerve supply comes from the nerve of their own segmental level, & one from the level above. I.e one nerve supplies two joints. Only a small amount of weight transmission occurs through these joints, & the L5/S1 joint is important in preventing spondylolithesis.
Ligaments between the arches
The ligamentum flava are yellowish from their high content of elastic fibres. They attach to the front of the upper lamina & the back of the lower lamina, with shallow grooves marking their attachment. Their stretch in flexion gives some antigravity support.
The supraspinatous ligaments join the tips of the spinous processes, & are strong bands of white fibrous tissue, drawn taut in full flexion. In the neck it is replaced by the ligamentum nuchae.
The interspinous ligaments are weak sheets of fibrous tissue uniting adjacent boarders of spinous processes, well developed only in the lumbar region.
The intertransverse ligaments are also weak ligaments joining the transverse processes.
Atlanto-occipital joint & membranes
The atlanto-occipital joint is a synovial joint between the convex occipital condyle, & the concave facet on the lateral mass of the atlas. Both surfaces are covered with hyaline cartilage, have a lax but strong capsule, & are inervated by the C1 nerve. The joints allow flexion & extension, as well as lateral flexion between the atlas & skull. The centre of gravity lies in front of the joint, thus the head is maintain in position by the neck extensors. The oval shape & half ball & socket nature of the joint prevent rotation at this joint.
The anterior & posterior atlanto-occipital membranes are attached to the upper arches of the atlas & the margins of the foramen magnum. The anterior membrane encloses the space between the two joints, & the posterior is deficient laterally to allow passage for the vertebral artery & C1 nerve.
Atlantoaxial joints
The median atlantoaxial joint is where the back of the anterior arch of the axis articulates with the dens, with the facets covered by hyaline cartilage, with a capsule attached to their margins. The dens is held in place by the transverse limb of the dens, with a bursa between. The lateral atlantoaxial joint is a synovial joint, between the inferior facet of the atlas & superior facet of the axis. The surfaces are circular & flat, with a lax capsule supplied by C2. These joints are concerned with rotation about a central axis, passing through the dens. The head & atlas rotate together on the axis, with the dens spinning beneath the transverse ligament.
The tectorial membrane (posterior occipitoaxial ligament) is in continuity with the posterior longitudinal ligament, attached to the back of the body of the axis, bypassing the atlas to attach to the anterior half of the foramen magnum. This lies in front of the dura to which it is firmly attached.
The cruciform ligament lies in front of the tectorial membrane, & has a transverse band attached to the atlas, & a longitudinal band from the body of the axis to the foramen magnum. The weak apical ligament lies in front of the longitudinal band of the cruciform ligament, joining the apex of the dens to the anterior margin of the foramen magnum. (fibrous remnant of the notochord). The alar ligaments lie obliquely to the side of the apical, attached to the sloping upper margin of the dens, diverging up to insert into the margins of the foramen magnum. These are strong, & limit rotation of the head.
Movements of the vertebral column
Movements occur around the nucleus pulposus, with the facet joint direction determining the direction of movement. In the lumbar region the facets lie in the AP plane, which limits rotation, but allows flexion & extension & some degree of lateral bending.
In the thoracic region the upper facets face upwards & backwards & laterally, (lower reciprocal), like on a circumference of a circle with the centre being the vertebral body. Thus rotation is possible. Flexion & extension, as well as lateral bending occur. With lateral bending, the neural arches slide downwards & backwards on the concave side, & upwards & forwards on the convex side, thus passively rotating the arch. (and spinous process, as in scoliosis).
The other cervical joints, have their joints sloping like the thoracic, but do not lie on the circumference of a circle. Thus pure rotation does not occur, although the similar passive rotation with laterally bending occurs here, as in the thoracic spine.