After 2-3 months, a Grade 3 AC separation may be irreducible, and the conoid and trapezoid (coracoclavicular) ligaments may be retracted and incapable of healing even with downward reduction of the clavicle.
This is the type of case that has, in the past, been addressed with a distal clavicle resection, clavicular reduction and stabilization using the CA ligament--the "Weaver-Dunn arthroplasty"
Mazzocca has developed an arthroscopic alternative using the Arthrex tightrope device for acute stabilization, and a Tibialis anterior tendon allograft for long term healing.
There are no long term studies, but for the patient with a chronic, painful AC separation, this type of reconstruction is feasible, and appears to restore pain-free function in my experience.
