Derotation hard. We are bettter at at straightening curve s rotating the spine with the tools we currently have is limit correcting the rotational deformity of scoliosis which cause the rib hump is technically difficult to accomplish the ribs not enough leverage, ribs themselves can have ther own deformity that can't be corrected by correcting the spine deformity s.
Ribs deformed. Sometimes, the ribs may deform from the scoliosis or have a deformity of their own along with the scoliosis. Even with the scoliosis corrected, this rib deformity may persist & is a cosmetic issue that can be addressed by another surgical procedure if severe enough that can be done later if not enough of the rib deformity corrected with the scoliosis surgery or at the same time if it is severe.