If you're using an STL as-is with whatever pose you found it in and don't intend on changing it, then no. Sculptris isn't necessary.
If your're needing the anatomy to be positioned in a mesh, then YES, you'll need Blender or Sculptris to target (mask) portions for bending or twisting. I mention the free Sculptris app since it'll provide easy-to-grasp tools to smooth away unsightly creases that may occur from bending.
Rhino allows taking a mesh (STL or OBJ) and performing Transform Bending/Shearing or even Cage Editing, but I don't think the comparative results will be the same.