I recently suffered an acromioclavicular (AC) joint sprain and thought I would share what I have learned from rehabbing myself.

The AC joint attaches the end of the clavicle to the top of the shoulder (acromion). It is covered by a joint capsule and re-inforced in all planes by ligaments. The joing contains a piece of cartilage similar to a meniscus in the knee.

Unlike tendons and muscles, ligaments are avascular (not much blood flow) but they are highly innervated (hurt a lot). They have a lot of nerve endings in order to supply feedback concerning the joints location and the various stresses: tension/stretch/hot/cold/pain etc.

This injury will result in tenderness and swelling on top of the joint. You may even feel some popping if you move your fingers back and forth across the joint. Reaching across your body will hurt and reaching behind your back may hurt as well.

AC injury is a recipe for something painful and pretty slow to heal. There are 6 types of AC joint injury:
Type I - basic sprain, ligaments intact, capsule intact
Type 2 - superficial ligaments are ruptured, the deeper coracoclavicular
ligament is sprained but intact, minimal trap/deltoid involvement
Type 3 - rupture of superfiicial ligaments and the deeper coracoclavicular
ligament, detachment of deltoid and trapezius, upto 100%
displacement
Type 4 - As type 3 but the displacement is posteriorly.
Type 5 - More severe form of type 3 where the clavicle punches a hole
in the fascia (covering of muscles) of the deltoid and trap,
distinguished by the shrug test which does not reduce the
deformity.
Type 6 - (rare) as above except clavicle is displaced behind the biceps
tendon

Treatment: My injury seems to be a pretty classic Type 2 but perhaps a little closer to the type 1 than the type 3. Type 4,5 and 6 require surgery. Type 3 injuries are given a trial of conservative treatment (physical therapy and pain meds and anti-inflammatory agents) but may require surgery in severe and persistent cases.

Week 1: Immobilization with a sling (hard for anyone to do that has to
work), rest, ice, meds, in season athlete may be candidate for
injections (case by case basis)
Week 2: active assisted ROM in pain free range especially internal rotation
behind back, isometrics for deltoids and traps, horizontal
abduction, scapular stabilization, early functional activities
Week 3: Progression of ROM in pain-free range, progress exercises for
deltoids, traps, scapular retractors
Week 4: Progress functional actvities e.g. bench press the bar.
Week 5-6: More of the same, slowly progress back to activities, let
pain be your guide. Ice is your friend after a workout. Use
moist heat before workout if you are tight and need to stretch

Of course, I have done nothing like this...I went straight back to bench press after 2 days of rest...this was too soon. So I did some light shoulder work the next day and by day 5 I did get back to benching, however, I kept it light. I just did the bar plus about 20 pounds and did 10 reps with three different grips then did inclines and declines as well. About 300 reps total, mostly painfree. I've still been doing heavy farmers walks with only a little pain about mid-range on the pickup and no pain supporting the weight. Just remember...be careful, take it slow and use strict technique/posture. Do high rep and low weight to focus on getting blood flow. You have to get blood and nutrients to the area to heal and stress the ligament the right way to get the fibers to line back up.