It is a contraction of the joint capsule that causes decreased range of shoulder motion. It may be secondary to an idiopathic process (4th through 6th decades of life) or an underlying disorder that leads to disuse (tendinitis, bursitis, fractures) . Treatment of the underlying pathology, if any, along with physical therapy is generally effective and the treatment of choice.
Source
Anderson, B. MD. "Frozen shoulder." Up to Date. Oct 2008.