Facet Arthritis (also called arthrosis, or degeneration)
The facet joints are in the back of the spine, about a half inch from the bump that you can feel in the middle of your back (the spinous process). These joints are indicated in the picture on the right by the asterisk on the photograph of the spine model (*).
These joints have a sliding motion.
When a patient bends forward and backward, the disc remains the center of motion, but the back part of the spine has to get either longer (to bend forward) or shorter (to bend backward). These motions are accommodated by the facet joints which have smooth cartilage surfaces, like the cartilage on the end of a chicken bone.
These joints, just like a knee or hip, can become arthritic. The cartilage can wear away, and the result can be bone rubbing on bone. When this process occurs, the body reacts by forming more bone which on x-ray would look like a cloud or a cotton ball forming over the involved joints, which would look more white on x-ray.
Compare the images above to a what a normal front view looks like, without the calcification or cloud appearance of the facet joints.
However, a better way to look at these joints is with a three dimensional study, like an MRI or CT scan, that can make images in different planes/directions.
In this cross sectional view below which is of normal facet joints, notice that it appears that the two bones that form the joint fit nicely together. There is a thin line of normal appearing joint fluid between the bones of the facet joints.
However, below, are examples of abnormal facet joints.
In this view of a level with facet degeneration, it can be seen that extra bone is seen forming on one side (labeled "bone spurs"), and extra fluid is seen on the other since the two sides no longer fit well together. It is not hard to imagine that these joints don't have smooth motion as the spine flexes and extends.
On this view, the facets also have no fluid signal in the joints, and they have overgrown to a point where they are compressing the nerves in the spinal canal. This condition is called spinal stenosis.
Here is another view of the facet joints using a CT scan (which can give better detail of bones), which is a special x-ray that can take cross sectional images.
The facet joints pictured here are normal, with the two bony sides appearing to fit together. The space between the bones is even and without irregularity.
In this image, however, the facet joints can be seen as irregular and rougher appearing, and on the right side, there is extra bone growth, referred to as osteophytes, which are a sign of arthritis of these joints.
The facet joints can also wear out in the neck, where these joints have a different shape, overlapping like shingles on a roof, and can also develop arthritis with the cartilage worn away.
Since this cartilage is invisible to the xray beam, when cartilage is present, a joint space will be visible. When the cartilage is worn away, there will be a thinner or missing space between the bones. With further progression, there can be bone on bone contact.
Here is another example where the cartilage that remains in the upper part of the cervical spine is noted by the bones appearing separated. However, lower in the cervical spine, the cartilage is missing, and there is bone rubbing on bone.
In the images below, there is a front to back (AP) view that shows the enlargement of the facet joints compared to a normal appearing AP view of a cervical spine.