I’ve been working on some medical illustration for a while now, both 2D and 3D, and have in all this time been wondering about ribcage deformation in real life. I know that in a lot of 3D animation, and even in a lot of drawing, the ribcage is treated as a single rigid shape. But, for what I am doing that is not going to be enough.
Looking at all the illustrations and diagrams I have been able to find, I understand where and how the ribs connect to the spine in the back, and to the sternum in the front via cartilage (talking here about the “true” ribs), or to each other (in the case of the “false” ribs).
What I have NOT been able to find a good explanation of, is exactly how the ribs stay connected when, for example, bending sideways. While the spine bends in between each vertebra, the sternum is a fixed solid, and the ribs have to stay attached to both. In the back, I know there is a certain amount of articulation from the spine, and I imagine there is some give in the cartilage in the front as well. Plus, there is some flexibility in the ribs themselves. But, to what degree each of these things play a part, I haven’t been able to determine from any of my resources.
Also, what determines the angle of the sternum? Does it rotate along with the top pair of ribs, or does it sort of move with a combination of all the ribs together?
I will probably post a similar question in the rigging forum looking for practical advice in terms of 3D animation. But here, I am concerned with the actual anatomy, looking for diagrams, explanations, references of any kind that will help me with my understanding.