Biomechanics of Upper cervical Spine
February 10, 2010 by Dr Arun Pal Singh
Filed under Cervical Spine, Spine Injuries
The three bony components of the upper cervical spine are
- Skull base
- Atlas
- Axis
these three components form a functional unit. There are five joints in the upper cervical spine which are stabilized by ligamentous check rein and muscular control.
Together these contribute to the movements in the neck substantially. This enables us for a rapid response and large-scale head excursion.
Upper cervical spine is thought to contribute approximately 60% of rotation, 40% of flexion-extension, and 45% of overall neck motion.
The normal axial plane C1–2 rotational excursion amounts to 80 to 88 degrees from left to right.
The Atlanto-occipital joint and C1–2 flexion/extension excursion is similar for both joints at 20 to 30 degrees at each level.
Total left to right lateral bending at the C1–2 segment amounts to 20 degrees.
The alar ligaments play a key role in protecting normal craniocervical motion. At midposition of the head these ligaments are slack.
By turning the head in one direction, the alar ligament contralateral to the direction of turning tightens, while the ipsilateral ligament slackens. Together with the tectorial membrane the alar ligaments limit flexion but they play no role in limiting extension.
The contralateral alar ligament limits lateral bending.
Other ligamentous stabilizers of the craniocervical junction are the cranial portions of the anterior longitudinal ligament and posterior longitudinal ligament of the spine and joint capsules of the respective articulations.
Anteriorly, the well-developed atlanto occipital membrane limits extension, with the thinner anterior atlantoaxial membrane contributing to a less significant degree.
A number of smaller ligaments, such as the apical and cruciate ligaments, the
obliquely aligned accessory atlantoaxial ligaments, the anterior atlantodental ligament, and the facet joint capsules also provide support.
The specific arrangement of ligaments at the craniocervical junction utilizes the atlas as a washer or base for a coupled, multiplanar motion.
The combination of a high degree of motility and relatively delicate ligamentous and bony structures makes the upper cervical spine susceptible to injury from indirect high-energy trauma.
Fracture-dislocations of the craniocervical junction is the leading cause of death of motor vehicle accidents.
The atlas is the most fragile vertebral segment in humans. It will fracture with as little as 1 to 2 mm of deformation and is very susceptible to bursting-type fractures with relatively low axial loads.
The two most vulnerable bony structures of the axis are the pars interarticularis and the odontoid waist. Forced hyperextension can lead to failure of either structure.
Flexion is believed to be causative in 80% of odontoid fractures by forcing the transverse ligament against the odontoid.
Atlantoaxial rotation of more than 50 degrees in either direction as measured by CT scan is suspicious for alar ligament insufficiency.
If it is more than 56 degrees, it is diagnostic of disruption.
An intact transverse ligament limits anterior subluxation of the atlas relative to the axis to 3 mm in adults and 5 mm in children.
Similarly, more than 5 degrees of atlantoaxial flexion is indicates transverse
ligament insufficiency.
If atlantoaxial translation exceeds 9 mm in adults, comprehensive failure of all key craniocervical ligaments has to be assumed.
Related posts:
- Basic Anatomy of Upper Cervical Spine
- Upper Cervical Spine Injury – An Overview
- Radiography and Other Imaging Studies In Upper Cervical Spine Injury
- Surgery In Upper Cervical Spine Injury
- Non Operative Treatment Of Upper Cervical Spine Injury
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