This is a slightly odd class. What does it mean exactly?
* "backbone" (no spaces) is sometimes used as a synonym for the entire vertebral column - which isn't a bone, it's a collection of bones
* a "bone of the back" would be a grouping class for any bone in the back of the organism. Depending on your organism subdivisions, this may just be a synonym for "vertebra"
The MA class "back bone" includes "vertebral column" as a subclass, but also includes individual vertebrae as subtypes - see attached.
the MA class 'back bone' should probably be merged into MA:0000309 ! vertebra
Similarly lower and upper back bone into lumbar and thoracic vertebra
If the intention is really to have a grouping class that includes the column and bones as subclasses, then the name might be better as "skeletal structure of back". But it's not clear why this would be necessary to have this grouping - having "vertebral column" as the part-parent seems fine.