abnormal sacral vertebrae morphology vertebral fusion sacral vertebral fusion In humans, vertebrae S1-S5 are normally fused in maturity, with no intervertebral discs. In rodents, these vertebrae remain distinct in adulthood. MP:0004622 MPheno.ontology fused sacral vertebrae the union of one or more sacral vertebrae into a single structure in species where this does not normally occur fusion of sacral vertebrae fused sacral vertebra and sacral vertebra