Why does body movement still affect fractured bone when it is immobilized?

Question

Let's assume we have a type 1 closed fracture in the coronoid process:

The treatment is to immobilize that joint in plaster. However, the patient should also limit their movement.

I wonder why is that, when by definition the joint is immobilized? As long as there is no pressure on the surrounding bones (e.g. carrying objects with that arm) I though movement of irrelevant bones (e.g. walking) will not affect the healing process. Yes, there are swinging if the arm is hung before chest, but it's the movement of the whole arm, and the joint is still fixed.

Is there any factor that affect the healing process, beside immobilizing the fractured location?

^{A minor question: if arm swinging is the problem, then would eliminating that swing by attaching the whole arm to the body be fine?}

Answer 1

By immobilization of the elbow you are just keeping the elbow joint in place, which means the forearm does not move in the relation to the upper arm.

But there are other things that can move the coronoid process:

• The movements in the shoulder joint, wrists and fingers in which the muscles attached to the humerus or ulna are involved
• The movements of the body that shake the bones, such as running and jumping
• Carrying things in the hand and anything else what pulls the ulna down
• Pushing objects or anything else what pushes the ulna up

So, by casting, you can't 100% immobilize the olecranon.