Q: I am having a weird shoulder problem. I don't have any pain but every time I reach back to put my seatbelt on, my left shoulder clunks. What could be causing this?

A: You may have some shoulder instability meaning the head of the humerus (upper arm bone) isn't being held exactly in the middle of the shoulder socket where it belongs. If the humeral head is just slightly forward of the center of the socket, when you reach back, the head shifts back into place.

The clunk you feel may be the round head of the humerus making the change in position from being slightly forward of the center of the shoulder socket back to the middle of the joint. Some experts refer to this as a relocation of the humeral head.

Shoulder instability of this type is usually accompanied by tendon tears, muscle weakness, or some other type of biomechanical change in how the shoulder joint moves and works.

The best thing to do is get an accurate diagnosis. You will probably have to make an appointment with an orthopedic surgeon or physiotherapist (possibly someone who specializes in shoulders).

This specialist will be able to test the structures of the shoulder and determine what's going on and how to treat it. Early intervention can help keep the problem from getting worse, so don't put it off. Call and make an appointment today.

Reference: Oke A. Anakwenze, MD, et al. Recurrent Anterior Shoulder Instability Associated with Bony Defects. In Orthopedics. July 2011. Vol. 34. No. 7. Pp. 538-544.