Table 1. . Clinical stages of ACS and histological findings.

Stage		Symptoms	Duration of symptoms (months)	Histology
I	Painful stage	Moderate pain and reduction of movement	<3	Hypertrophic synovitis with hypervascularity Normal capsular tissue
II	Freezing stage	Severe pain and reduction of movement	3–9	Perivascular synovitis and disorganized collagen deposition and scarring
III	Frozen stage	Pain may be present but stiffness predominates	10–14	Dense and hypercellular collagenous tissue of the capsule
IV	Thawing stage	Minimal pain and gradual improvement in movement	14–24	Fully developed scar tissue Pathophysiology remains unclear