Capsulitis is inflammation of any anatomical capsule. In the body, an anatomical capsule is a structure, generally made of fibrous tissue, that encases a part or parts. For example, the facet joints at the back of the spinal column are enclosed in capsules known aptly as facet joint capsules. Perhaps the most well-known form of capsulitis is adhesive capsulitis, or frozen shoulder.
Capsulitis is often found at the sacroiliac joints when you have an inflammatory arthritis disease such as spondylitis. It is considered to be an active inflammatory lesion. For people with spondylitis, an MRI using one of several specialized techniques is generally necessary to find evidence of capsulitis and other active inflammatory lesions.