Intervertebral discs

The bodies of adjacent spinal vertebrae are separated from each other by an intervertebral disc. The disc functions to absorb compressive forces (acts like a shock absorber), and to allow movement of adjacent vertebrae.

Each disc comprises a central, gel like, nucleus pulposus, comprising primarily of water, with collagen (structural protein) fibres, as well as hydrophillic (water attracting) proteoglycans. The proteoglycans, acting to draw water into the disc via osmosis, internally pressurise the disc. The nucleus pulposus is surrounded by the annulus fibrosus, consisting of layers of fibres arranged in alternating directions.

To put it simply, each intervertebral disc consists of a softer gel like region, surrounded by a tough, but flexible, fibrous region. 

The outer fibres of the disc are the only region to have a nerve supply.

The disc is avascular (does not contain blood vessels), this contributing to the disc's poor healing properties. Blood vessels which supply the disc terminate in the cartilaginous vertebral end plate located between the disc and the vertebral body.

Daily change in disc height occurs, with reduction in height due to sustained compressive loading (greater in heavy, compared to light, manual work), and increase in height following sleep/sustained recumbence. Reduction in disc height results in extra pressure on structures such as facet joints, and reduction in size of the intervertebral foramina (spaces through which nerve roots exit the spine).

The amount of proteoglycans in the nucleus pulposus tend to reduce with age, thus reducing water content of the nucleus, and reducing disc height. This causing similar effects to daily changes, as well as more permanent osteoarthritic changes, for example, to facet joints.

While intervertebral disc injuries can occur anywhere they are present, the most common area is that of the lower lumbar spine, therefore being a common cause of low back pain, and, if a nerve root is affected, giving symptoms in the leg, i.e. sciatica.  

Many herniated discs are asymptomatic (without symptoms), this being evidenced by MRI scans showing herniated discs in pain free patients. Just because an MRI scan shows a herniated disc does not mean that it is the cause/source of pain felt. 

The four stages of disc herniation

Degeneration-degeneration weakens the outer annulus fibrosus of the disc

Prolapse-there may be some impingement on surrounding structures

Extrusion-part of the nucleus pulposus breaks through a tear in the annulus fibrosus

Sequestration-part of the nucleus pulposus which has passed through the tear in the annulus fibrosus separates from the disc

The Sacroiliac (SI) joints

The sacroiliac joints are a pair of joints located below the lumbar (small of back) region of the spine, between the sacrum of the spine, and the ilium of the pelvis. These are large joints, strongly reinforced by various ligaments, which allow a small amount (a few millimeters) of movement. The functions of the sacroiliac joints include shock absorption/force transference from the lower limbs (legs) to the body, and vice versa, stability during the push off stage of the gait cycle (walking), as well as allowing widening of the pelvic girdle while giving birth.

Dysfunction of the sacroiliac joints results in their irritation, and may occur from too little, or too much, joint movement. This may be due to repetitive small movements, or a larger traumatic force, e.g. stepping off a high pavement unexpectedly. Sacroiliac joint dysfunction is common in pregnancy due to the actions of hormones on the ligaments which surround the joint, combined with the extra strain of increased weight during pregnancy. While, generally, patients do not suffer repeated sacroiliac joint dysfunction, there are, for example, biomechanical reasons which may cause this. 

Sacroiliac joint dysfunction is one of the most common causes of low back/buttock pain, and may refer pain to the back of the thigh and groin. Very occasionally pain may be felt below the knee.

Facet (apophyseal) joints

Facet joint injury can cause back and neck pain.

While there are major differences in size and shape, the majority of vertebrae in the cervical (neck), thoracic (where the ribs attach), and lumbar (small of back) regions of the spine exhibit broadly the same structure, including facet joints.

While the intervertebral disc functions as part of the weight bearing joints of the spine, facet joints, which are found at the back of each vertebra, articulate directly with the adjacent vertebrae, and act to guide, and limit, range of movement at the various levels of the spine. 

While facet joint injury can occur anywhere the joints are present, the most common area is that of the cervical spine. There may also be reduced range of movement at the injured joint level. The causes of injury are varied, and may be influenced by postural/biomechanical adaptations of the body, e.g. increased lordosis (concavity) in the lumbar, or cervical spine, and/or changes in the spine due to age, e.g. due to reduction in intervertebral disc height, or osteoarthritic changes. Classic injury presentation may, for example, follow sustained looking up while painting a ceiling, or, more often, this is the neck pain patients wake up with following going to sleep without pain.