The elbow has some unusual and very individual anatomy that can make a difference after injuries. Fractures and dislocations can alter the normal bumps and grooves that give the elbow joint its alignment. Elbow dislocation with even one bone fracture increases the risk of problems.
The elbow is a hinge joint with its major point of axis for movement and rotation where the bones of the forearm insert into the humerus (upper arm bone). The bottom of the humerus called the trochlea is spool-shaped. The top of the ulna wraps around this spool to form the hinge. The joint gets its stability from this alignment.
Besides the formation of the bones and joints, there's also the strength and stability given by the surrounding ligaments. Many times when an elbow is dislocated, the ligaments are torn or damaged, too.
It's the combination of dislocation, fracture, and soft tissue damage that creates an unstable joint. Until all three of these structures are repaired, restored, and strengthened, there is a risk of recurrent dislocation and instability.