The AC Joint Makes No Sense

I mentioned in a previous article that there are problems that simply do not make any sense … even if you’re the expert that’s trying to explain it to someone else. Problems with the acromioclavicular (AC) joint fit into that category very (not)nicely.

The AC joint is the space between your collarbone and the hook part of your shoulder blade, called the acromion. The top and bottom of the two bones should be lined up with each other, stabilized by ligaments, which are the strong bands of tissues that connect bones to other bones. There should also be a healthy space between the two bones that is filled with two things that do not show up on xray: 1) cartilage, the coating on the ends of bones that allows them to move, and 2) a disk that is like the meniscus in the knee.

If someone falls directly onto the tip of shoulder, usually with their arm tucked in, there can be either a stretching or a tearing of the ligaments that keep the two bones lined up with each other. So far so good, right? Here is where common sense starts to unravel:

  1. Injuries to most ligaments in the body are usually called a sprain (e.g. ankle, wrist, knee, etc.); however, in the AC joint, we do not call injuries to the ligaments sprains — we put them into their own category called AC separations.
  1. Here is the most confusing thing: even if you completely tear all of the ligaments that are holding the two bones together, and you actually have a visible and complete separation like in this xray, this still does not usally need an operation. Most patients with this injury are just fine with a course of NSAIDs and shoulder blade exercises (below). If you have been told you need surgery for this injury and you have not had a course of conservative management, you need a second opinion because even though it is possible, it is very very unusual for surgery to be a reasonable first option for a patient with this injury.

Why does the AC joint tolerate being in that position? Well, the answer comes from an understanding of what the collarbone does, which is to help to keep the shoulder blade in the proper position. Without getting into the technical details and terminology, you can try this yourself and see that how far forward you can bring your shoulder blade is limited by the collarbone, which functions like a strut, keeping the shoulder blade from coming any further forward. For your shoulder to function optimally, you actually want your shoulder blades back as much as possible (more on this later in the section on exercises). If the muscles around your shoulder blade are able to accomplish that without the help of the collarbone, then you are all set.

Which brings us to the other strange AC joint problem — AC joint arthritis. As you can see in this xray, the space between the two bones is very narrow, which means that the cushion between the two bones has completely worn away. The amazing thing is that you can have an xray that has bone on bone AC joint arthritis but have no symptoms whatsoever. This is actually a very common occurrence for people in their 60’s and 70's.

The best explanation that I have heard is that the pain that comes from AC joint arthritis is because the cushion has worn away, but there is still a little bit of motion in the joint, so when the two bones grind on each other this creates painful inflammation (think of this like the heat you generate when you rub your hands together, but it happens at the microscopic level and it doesn’t go away). If you can treat the inflammation, the pain will go away. Eventually, the two bones stop moving, and if they do not grind on each other anymore then the inflammation does not come back.

How do we treat the inflammation? Usually I recommend a long acting anti-inflammatory medicine first for 3–4 weeks, and if that does not work then we talk about doing an ultrasound guided AC joint injection in the office. The two benefits of the injection are that it confirms the diagnosis (when your pain goes away with the numbing medicine in the injection), and it will hopefully have the pain go away for at least some time (because of the powerful anti-inflammatory effect of the cortisone component of the injection).

If the pain does go away completely with the ultrasound guided injection, and it does come back, some patients do choose to have the end of the collarbone removed. This is an xray of a patient who had the end of the collarbone removed arthroscopically, which means the surgery was done using a camera and incisions that are about 1cm in length. As you can see, there is now a flat end to the end of the collarbone and a wide space between the collarbone and the acromion on the other side of the now open AC joint. This surgery is uncommon, and is a last resort for patients that are no longer getting lasting relief from the oral medications or even the ultrasound guided cortisone injections. Just like with any other operation that is not time sensitive, if the patient can live with it, I can live with it, and it is always the patient’s decision when to go ahead and do the operation.

So now you’re an expert on the AC joint. You can explain to someone why you can completely tear ligaments in your shoulder, have two bones not touching each other at all, but still have pretty decent shoulder function. You can also explain to someone why you can have bone on bone arthritis and no pain. Or you can be just as confused as when you first started reading this article … in which case you and I are in the same boat. Listen, not everything makes sense. Some things stay confusing, and that’s ok. If you do have an AC joint problem that seems to have settled down, here are the two exercises for the AC joint that may be helpful, particularly for an AC separation:

  1. Rowing exercise for the middle shoulder blade muscles — you can do these with a rubber band at home if you have one, or use a cable machine at the gym with light resistance setting.

If you do the first exercise you’ll notice that it helps to strengthen the muscles that bring your shoulder blades together. If you do the second exercise you’ll notice that it helps to tilt your shoulder blade back. If you think about it, those two motions are going to compensate for the loss of the collarbone function in the case of AC separation, and even potentially take some of the pressure off your AC joint if you have arthritis there. In any case, see if these exercises help you, but don’t do them if they seem to be making your shoulder worse.

Shoulder specialist orthopedic surgeon, happily married father of 3 awesome kids, always looking to learn and find new ways to make a difference