The disc / facet joint complex
Although there are differences between the five regions of the spine (cervical / thoracic / lumbar / sacral / coccygeal) there are also similarities, with the vertebrae of the first three regions moving in relation to the adjacent vertebra across a disc and two joints (facet or zygo-apophyseal joints). The thoracic vertebrae are identified by having a pair of ribs.
A lumbar vertebra, of which there are usually five, has a disc above and below, attached to the larger round part at the front (called the body), which is joined to the posterior portion as a ring surrounding the spinal canal (containing nerves) by two pedicles (left and right) to which are attached the left and right transverse processes (like short ribs). Behind are the laminae, which overlap like roof tiles and join to the spine (spinous process) which projects backwards and can be felt under the skin. Projecting upwards from the pedicles are the superior facet processes which join the inferior processes of the one above to form a joint on each side, and corresponding inferior processes meet the ones below. These joints vary in their orientation, at times being oriented more fore and aft (sagittal) while others are more transverse (coronal). They are similar to other synovial joints and can become arthritic.
Lumbar movement occurs in all three planes allowing flexion (bending forwards) and extending (going backwards), as well as sideways and to a limited extent rotation. These movements occur across the discs and at the facet joints.
Ligaments and the joint capsules, with the disc, restrict excessive motion.
There are three main groups of muscles in relation to the lumbar spine – those at the back of the spine, the erector spinae, quadratus lumborum and latissimus dorsi, those in front, the iliopsoas and those which surround the abdomen, the abdominal muscles, rectus, oblique and transverse. With reduced activity these muscles quickly waste.
The disc is a complex structure, with a central gelatinous portion contained by a ring of fibres (annulus) arranged like a cross-ply tyre which are attached to the cartilaginous end-plates on the bodies of the vertebrae. With aging the gel becomes stiffer and the fibres can separate. The disc may lose height allowing excessive movement, causing degeneration of the facet joints. Extra bone may form on the bony rims of the bodies next to the disc (osteophytes). With progressive aging the situation can restabilize.
At times the central gelatinous portion can separate between the fibres and press on the nerves causing pain. This may cause sciatica – see leg pain. What initiates this process is debated. Is it a tear in the outside of the disc or is it loss of gel pressure? Bulging of the discs may cause pain by stretching nerves present in the peripheral part of the fibres of the annulus or by allowing abnormal movement patterns. Stress transferred to the bone may alter bony architecture with thickening (sclerosis) and changes seen on MRI scan described by Modic (Types I, II and III). See imaging.
Studies done with patients having operation under local anaesthetic indicate that the following structures can be painful to mechanical stimulation – the peripheral portion of the annulus, the synovium (lining) of the facet joints and inflamed nerve roots. Muscles and ligaments were not mechanically sensitive.
IMPORTANT POINT It is our belief that abnormalities of the disc are the primary cause of back pain and our treatments are based on this. Many patients with acute low back pain are said to have strained muscles – this muscle pain is spasm and is usually a secondary response to a problem with the disc.
For an interesting account of how mechanical treatments can effect back pain see the writings of Robin McKenzie, a NZ physiotherapist, e.g., Treat your own Back or his book, The Lumbar Spine (Spinal Publications NZ Limited, P O Box 93 Waikanae, NZ). Not all patients respond to extension exercises but the McKenzie method emphasizes the importance of differentiating which patients will respond to which treatments. A similar discrimination is made by the Canadian Back Institute – see non-surgical care. When patients fail to respond to such treatments further assessment is required.