The following license files are associated with this item:

Attribution 3.0 United States

This is an open access article, originally published at https://doi.org/10.1186/s12891-018-2290-5
Except where otherwise noted, this item's license is described as Attribution 3.0 United States This is an open access article, originally published at https://doi.org/10.1186/s12891-018-2290-5