Why Zach Mettenberger Can't Play Through Latest Shoulder Injury

Zach Mettenberger is no stranger to injury.

The Tennessee Titans rookie quarterback tore his ACL late last year, ending his college career, and reports of a back issue surfaced heading into the 2014 NFL draft, according to NFL Network's Ian Rapoport (h/t College Football 24/7's Chase Goodbread). Now, he is recovering from an acromioclavicular (AC) joint injury.

Mettenberger played through a Grade-1 AC sprain on Sunday, but according to The Tennessean's Jim Wyatt, he suffered a more serious injury in the process. It comes as no surprise that Rapoport adds an "AC joint separation" will end his season, as the next levels of severity up from Grade-1 involve complete ligament tearing.

At minimum.

The AC joint marks the location where the clavicle—the collarbone—meets the acromion, a bony extension of the shoulder blade. The AC ligament connects the end of the clavicle to the acromion, stabilizing the two bones during the many complex movements of the shoulder. The coracoclavicular (CC) ligaments attach the clavicle to the coracoid process, a second extension of the shoulder blade.

Other joints, such as the glenohumeral (GH) and sternoclavicular (SC) joints, work in concert with the AC joint to allow optimal shoulder function.

AC injuries, sometimes called "shoulder separations," disrupt that coordination.

Wikimedia Commons with edits by the author.

Mettenberger's first injury, a Grade-1 sprain, mildly overstretched the AC ligament, but the ligament remained grossly intact. The clavicle also stayed in its proper position. With good pain control, the quarterback could play through his injury, as he did for a period of time on Sunday.

However, Grade-2 and worse injuries imply complete tearing of one or more ligaments.

For instance, Grade-2 injuries completely disrupt the AC ligament, while Grade-3s also tear the CC ligaments. The resulting lack of support may allow the clavicle to displace upward from its normal anatomical location.

Fortunately, even Grade-3 separations usually do not require surgery. That said, the altered anatomy within the joint always necessitates shoulder immobilization to allow the ligaments to reattach, scar down and stabilize the joint once more.

In other words, throwing a 40-yard bomb downfield is out of the question.

Mettenberger's recovery time will depend on the extent of tearing and clavicle separation within his shoulder. At the very least, a complete tear of his AC ligament—a Grade-2 separation—will certainly need more than a week or two to heal and will almost certainly end his season. A Grade-3 injury undoubtedly will.

After all, not only would Mettenberger be unable to play well through a higher-grade injury, but another plant onto his injured shoulder by a defender also could prove disastrous with one or more ligaments already out of commission.

Nevertheless, head coach Ken Whisenhunt told ESPN's Paul Kuharsky that he is "hopeful (Mettenberger) will be back this year."

The Titans, sitting at 2-11, certainly need to allow Mettenberger to gain as much as experience as possible. However, the team and medical staff will need to look long and hard at the risk-versus-reward balance of putting their young quarterback out there again this year—if, somehow, he can heal in time in the first place.

Dr. Dave Siebert is a second-year resident physician at the University of Washington and a member of the Professional Football Writers of America. He plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine.

Follow @DaveMSiebert