We don't have synonyms for back-bone yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are backbone, back bone and back-bond.
(n) :
Alternative spelling of backbone. [(countable) The series of vertebrae, separated by disks, that encloses and protects the spinal cord, and runs down the middle of the back in vertebrate animals.]
(n) :
Alternative spelling of backbone. [(countable) The series of vertebrae, separated by disks, that encloses and protects the spinal cord, and runs down the middle of the back in vertebrate animals.]