sacralization

noun

sa·​cral·​iza·​tion ˌsā-krə-lə-ˈzā-shən 

variants also British sacralisation

-ˌlī-ˈzā- 

: incorporation (as of the last lumbar vertebra or any of its parts) into the sacrum

specifically : a congenital anomaly in which the fifth lumbar vertebra is fused to the sacrum in varying degrees