The anatomy of the cervical spine is somewhat different to that of the lumbar spine. This is important to understand when assessing and treating the spine mechanically. This reading is a brief introduction to the cervical spine.
There is some uniqueness to the function and structure of the cervical spine; Firstly, there are no intervertebral discs between the occiput and C1 as well as C1 and C2. The function of this upper vertebra is quite different to that of C2-T1 and thus we look at both these areas with a different lens. During this reading, the reference to the lower cervical spine refers to c5-t1, the mid cervical refers to C2-5, and the upper cervical spine refers to the occiput to c2.
With the cervical spine, we have a coupling of movements. By this we mean, one movement is accompanied with another and they are not able to be isolated. For example, in the upper cervical spine at the levels of c1-2 – where the majority of our cervical rotation comes from, we see with a rotation to the right that we also have a lateral flexion to the left – we call this a contralateral coupling. In the middle and lower cervical spine, we see an ipsilateral coupling – for example, with rotation to the right we also have a lateral flexion of those vertebral bodies to the right. You cannot have one without the other (Figure 2).
The cervical spine is capable of a number of movements. Flexion, Extension, rotation, lateral flexion, protrusion and retraction. We will now cover any specifics to each of these movement.
Rotation – Most rotation, roughly 45 degrees for a normal ROM individual, will come from C1 - C2. There is significantly less rotation at the remaining vertebra, with roughly 3-7 degrees at each other level (Figure 5). As aforementioned it is important to keep in mind that rotation is also coupled with lateral flexion and vice versa.
Flexion and extension is available at all levels of the spine. The movement is instigated in the lower cervical spine. It is also normal for the passive range of motion (ROM) to be greater than active ROM, but with age we tend to see a decrease in range. As the disc component of the cervical spine accounts for roughly 40% of the cervical spine height, degenerative changes over time and diurnal changes may explain the resulting decrease in height and ROM over time.
In reference to the disc model flexion creates an anterior to posterior force upon the disc allowing for a movement of the nucleus pulposus to move in an anterior to posterior direction. Extension has the reverse effect with a movement of the nucleus pulposus posteriorly to anteriorly (Figure 6).
The zygapophyseal joints (facet joints) are a synovial joint with the joint plain angle changing at each level. Figure 1 shows that the joint plain of these facet joint are steeper in the lower cervical and become more shallow in the upper cervical. A general guiding principle is the plain of the join will glide towards the sinus/nose.
The disc model for the lower cervical spine is also often referred to. Although understanding disc mechanics is helpful in understanding the mechanics of the cervical spine, we recommend considering other educational strategies for the patient as this has high potential for negative iatrogenic effects.
The shape of the intervertebral disc is crescent shaped, different to that of the lumbar spine. The annulus fibrosis is also thicker anteriorly and thins out laterally (Figure 3) . The shape of the vertebra is also unique with the inclusion of uncinate processes (also known as joints of luschka (Figure 4). The uncinate processes give the vertebral bodies a saddle shape that result in the coupling of movements of rotation and lateral flexion. These uncovertebral joints (that are formed from the uncinated process’ are most developed from c2-4 and less developed or minimal from c5-c7. These joints facilitate flexion and extension in the sagittal plane and couple rotation and ipsilateral flexion in the plain of the zygapophoseal joints. These uncovertebral joints also allow for reinforcement of the disc posterolaterally.
Retraction is a combination of lower cervical extension and upper cervical flexion, whereas protrusion is the opposite, with an increase in lower and mid cervical flexion and an increase in upper cervical extension.
Mechanical movements have been shown to have an effect on intradiscal pressures and neural foramen diameter – where the nerve root exits the vertebra.
Flexion creates a displacement of the nucleus pulposus posteriorly as we mentioned before but also results in an increase in neural foramen diameter, an increase in spinal canal space, has a tensioning effect on the spinal cord, dura matter and nerve roots and facets of the superior adjacent vertebrae glide upwards and anteriorly which increases the space of the zygapophoseal joints.
Extension has the opposite effect; we see;
An anterior displacement of the nucleus pulposus (Figure.);
A decrease in neural foramen diameter;
Narrowing of the spinal canal;
A decrease in neural tension;
And the zygapophoseal joints move posteriorly and inferiorly on their inferior adjacent vertebrae.
Rotation and or lateral flexion in one direction result in;
A contralateral displacement of the nucleus pulposus (Figure.);
A decrease in neural foramen on the ipsilateral side;
An increase in neural tension in the contralateral nerve root and dura;
Ipsilateral zygapophoseal joints close while the contralateral zygapophoseal open.
Movement of the upper limbs also result in tensioning of the upper limb nerve roots, resulting in a decrease in cervical and thoracic mobility. This neural innervation of the upper limb also gives us a sound reason to assess and consider the involvement of the cervical spine in the presentation of shoulder and other peripheral complaints.
Cervicogenic headaches are headaches resulting from the function of c1 through to c3. Differentiation between various culprits can prove difficult. The following points help differentiating between cervicogenic and other head symptom generators;
Reduced ROM in Cx – specifically flexion/rotation test
Neck movements affect pain
Palpation of C2 may affect pain
Shoulder /scapula / arm symptoms will not generally be present with migraine
The helpfulness in understanding these concepts of anatomy are currently highly debatable in the physical therapy communities. There is minimal evidence supporting the specificity of our special tests for identifying underlying pathology, and in many cases passive and active interventions have limited evidence suggesting they evoke more than non-specific effects. So the question being asked is, what benefit does trying to identify specific pathology provide (with exceptions of red-flags and a short list of specific, identifiable pathology) and does it alter our treatment strategy? Limitations in imaging also exist, with studies suggesting a wide variety of presentations, degeneration and abnormal findings are present in asymptomatic populations. For this reason we teach a process of critical thinking and patient centred reasoning to provide the most appropriate evidenced based care. Despite this, having a general idea of how this complex is moving and supported can be helpful in understanding literature and biomechanical discussion.