The facet joints are the most 'pull-apartable' structures of the spine, with their freedom controlled by the extraordinarily strong facet capsules (also known as 'capsular ligaments'). Although facet joint arthropathy is usually secondary to disc thinning, it is possible to injure a healthy facet joint by a chance ricking of the back. As with any other synovial joint injury, the joint swells and the back muscles clench to protect it.
You can injure a healthy facet joint in a myriad different ways, although the mechanism of injury usually involve a degree of bending twist. The resulting capsular damage is similar to a mini-scale joint sprain, like a twisted ankle in the back. Acute swelling can cause searingly painful sciatic leg pain, that usually develops within the following 24hours.
Sciatica can be felt in different parts of the leg depending which spinal level is affected. Most sciatic leg pain is caused by referred pain from an inflamed facet joint - not as commonly thought, from a bulging or 'slipped' intervertebral disc.
Severe inflammation of the facet causes the joint capsule to weep which, if not dispersed by normal movement, will harden to form stringy junk tissue called 'adhesions'. Advanced cases are known as 'root sleeve fibrosis' where the nerve itself becomes choked where it is bound to the joint capsule. You can read more this in about sciatica and its treatment.
Better still, why not download this video and see Sarah explaining in person the different sorts of sciatica, both acute and chronic caused in some cases by facet joint irritability and others by the disc.
More commonly though, facet joint arthropathy is a secondary to disc thinning at the same level. This causes overriding of the opposing facet joints surfaces at the back of the spine, where load-bearing may be increased by up to 70%. As the upper vertebra settles down on the lower one it creates bony rub between parts of the spine that should only have glancing contact.
Early on, this simply inflames the soft tissues around the facet but eventually it causes arthritic change as the friction erodes the cartilage covering the bony surfaces. Facet joint arthropathy is the second most common degenerative spine disorder. The extra-good blood supply of the facet joints also means that even quite markedly degenerated facets can be rendered pain-free with suitable treatment.
Facet joint stiffness simply caused by not bending properly can also create arthropathy. The facet capsules are extremely strong (they can suspend twice body weight). They readily shrink (adaptively shorten) if not kept stretchy through bending normally with everyday activity. This is common with back sufferers who feel they should not bend. Shortened, inelastic facet capsules are therefore vulnerable to incidental trauma and further inflammation.
A traumatised facet joint inflames easily. Unlike the discs, facet capsules have a rich blood supply, so they exhibit a marked inflammatory response with even minor strains. Their equally robust nerve supply easily picks up the pain messages. Inflamed facet joints are a potent source of back pain. The pain is typically at the side of the spine and craves deep pummelling with the fingers to gain relief.
The left picture above demonstrates the pain distribution from normal healthy facet joints after injecting an amount of normal saline solution. The diagram on the right demonstrates the much deeper and more extensive spread of referred pain when injecting a not-so-healthy set of facet joints.
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