The "sternoclavicular (SC) joint" is the connecting point of the sternum (breastbone) and the collarbone. It is rare, according to the Washington Orthropaedics and Sports Medicine web site, but isn't something that typically requires surgery.
This is likely the injury reported to be associated with his ribs.
SB Nation's Joel Thorman spoke with SB Nation's medical contributor, Dr. Ali Mohamadi, who noted the rareness of the injury:
When they said it was an "unusual" injury, I was wondering if it might be an SC joint injury because this is an uncommon joint to be sprained/dislocated. As opposed to the most common site of a shoulder injury (where the humerus inserts into the shoulder), this is actually the joint that connects the sternum to the clavicle, and the cartilage holding the bones in place is very strong. There are basically two ways it can be sprained, which by definition is tearing of the cartilage holding the joint in place: the clavicle can shift in front of the sternum -- anterior sprain --- or behind it -- posterior sprain. Anterior sprains are more common and cause pain/swelling in the front of the chest as well as a "bump" in the middle of the chest. Posterior sprains cause pain and swelling as well, but because the clavicle is dislocated inward, it can theoretically affect internal structures (trachea, esophagus, blood vessels). In either case, movement of the SC joint will be limited.
The good news is it's not commonly an injury that requires surgery. The bad news is it doesn't appear to be something from which athletes can return in six days - which would put Roethlisberger out for the Steelers Week 11 game against Baltimore.
There are two types of SC joint sprains - partial dislocations of the joint and full dislocations. It was reported Roethlisberger left Heinz Field Monday to get a full MRI done, and other reports suggested he's getting a second one done Tuesday.
As Thorman wrote, the main question is the length of time Roethlisberger could be out. Mohamadi said:
The biggest issue here, even if it is a mild sprain, is that this is his throwing shoulder. A moderate sprain may require placing the joint back into position and allowing it to heal with devices such as a sling/strap for 4-6 weeks. Worst case scenario, for a dislocation, is surgery, which is six-plus weeks for full recovery. Even with a mild sprain, with the injury affecting his throwing shoulder I'd think it's unlikely he will play this week, but then again he hasn't skipped a beat with other injuries that many thought would have kept him out of action.