Facet joints in the spine allow movement in flexion, extension, and rotation. Degeneration of these joints can (but doesn’t always) cause pain. In fact doctors are still puzzled as to why normal-appearing joints can be painful for some people, while degenerating joints don’t cause any problems for others.
In this article, doctors review the results of recent studies on the source of facet joint pain. Animal studies show special receptors called nociceptors located at the joints and in the muscles and tendons near the joints. The nociceptors transmit messages of pain.
Nociceptors in the joints are different from nociceptors in the muscles. Messages from the joints aren’t even sent if the problem isn’t significant. And when the message is sent, the pathway is very slow.
In one study of rabbits, researchers injected carrageenan (causes joints to stiffen) into the facet joints. They found inflammatory changes in the joint capsule and space around the muscles. There was an increase in firing or discharge of the facet-joint neurons as a result proving that facet joints can be the source of the pain.
Other studies focused on whiplash injuries also show facet joints as the pain generator. Facet joints in the cervical spine (neck) appear to be more sensitive and painful than similar joints in the lumbar spine (low back). This may help explain why there are so many cases of post-whiplash syndrome.
It’s not clear yet if the joint capsule is being pinched, compressed, stretched or strained. A study of goats applied various strains and stretches to joint capsules. Some nociceptors responded early when a low level of strain was applied. Others fired only with high strains more typical of a whiplash injury.
The common consensus from all the studies reviewed is that injury or overstretching of the joint capsule may be the most likely source of chronic pain after a whiplash injury. High strains cause nociceptors to fire with after shocks. Damaged nerve endings in the capsule may lead to pain that doesn’t go away. Finding the source of pain linked with whiplash injuries may help researchers find better ways to treat it.