"Sacralization of the fifth lumbar vertebra" (or "sacralization") is a congenital anomaly, in which the transverse process of the last lumbar vertebra (L5) fuses to the sacrum on one side or both, or to ilium, or both. These anomalies are observed at about 3.5 percent of people, and it is usually bilateral but can be unilateral or incomplete (ipsilateral or contralateral rudimentary facets) as well. Although sacralization may be a cause of low back pain, it is asymptomatic in many cases (especially bilateral type). Low back pain in these cases most likely occurs due to biomechanics. In sacralization, the L5-S1 intervertebral disc may be thin and narrow. This abnormality is found by X-ray.