What is facet joint arthrosis? That's what I have at the L4/L5 level.

At each level in the spine there are a pair of small joints called facet joints (25 pairs in total). The facet joints allow movement between two (or more) vertebrae. The shape and direction of these joints changes in different parts of the spine making different types of local movement possible (bending, extending, twisting).

Arthrosis means joint disease. Inflamed osteoarthritic joints can cause low back pain (LBP). This occurs most commonly at L4/5 and L5/S1 in the lumbar spine. Scientists have shown that facet joints are usually not the single or primary cause of LBP. Disc disease and degeneration can also cause low back pain. Sometimes people have both.

Reference: 

Jean-Charles Le Huec, MD, PhD, et al. Influence of Facet and Posterior Muscle
Degeneration on Clinical Results of Lumbar Total Disc Replacement: Two-Year Follow-Up. In Journal of Spinal Disorders. June 2005. Vol. 18. No. 3. Pp. 219-223.

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