Sometimes the sacrum is fused to the lowest lumbar vertebra: a lumbosacral transition vertebra. “LSTV is the most common congenital anomaly of the lumbosacral spine.” In about a thousand patients studied by Sekharappa et al, it was about twice as common in patients who had sought spinal surgery as it was in patients with no spinal complaint (about 14-16% of patients, instead of 8%). The study also identified a “definite causal relationship” with degeneration of the disc above the LSTV.
Fusion of the sacroiliac joint is closely related and even more common, as high as half the population by the age of 80, and freakishly more common in men than women for some reason, in 287 subjects studied by Dar et al.
These joints — the lumbosacral, and the sacroiliac — are so sturdy and immobile that, in some people, they cross the line and develop into a solid block. Talk about a stiff back! Literally!