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Biology of Frozen Shoulder

The cause of stiffness in shoulder motion called frozen shoulder is slowly unfolding. Thanks to improved technology and lab studies, researchers are now able to examine the affected tissues. Identifying tissue changes at the cellular level may help us prevent and treat frozen shoulder.

In this study, tissue from four patients with frozen shoulder was removed and stained with a special dye. Scientists were able to see that a protein called vimentin was the cause of changes in the anterior (front of the shoulder) capsule. The capsule is a fibrous layer of tissue surrounding the entire shoulder like an envelope.

Staining other sections of the capsule showed that fibroplasia occurs throughout the capsule. Fibroplasia is a term used to describe an excess amount of fibrous or scarlike tissue.

The authors conclude that frozen shoulder is caused by increased levels of vimentin. These changes only occur in the anterior portion of the capsule. It's the presence of vimentin that causes contracture or tightness. Fibroplasia is a separate process.

Fibroplasia is not the same as contracture. The decreased range of motion associated with frozen shoulder is not caused by fibroplasia. That's why surgical release of the anterior shoulder capsule is usually enough to restore motion.

References: Hans K. Uhthoff, MD, FRCSC, and Pascal Boileau, MD. Primary Frozen Shoulder. In Clinical Orthopaedics and Related Research. March 2007. No. 456. Pp. 79-84.

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