Frozen shoulder is caused by trouble in the joint capsule, Gabriella Ode, MD, sports medicine surgeon at HSS tells SELF. The shoulder joint capsule is basically a pocket of strong ligaments and lubricating synovial fluid that anchors and stabilizes the upper arm bone within the shoulder socket. With frozen shoulder, this capsule first becomes inflamed and thickened, which leads to stiffness and pain.

In some cases, said Dr. Ode, frozen shoulder follows an injury or surgery that keeps you from moving the joint (what doctors call “secondary” frozen shoulder). Usually, though, frozen shoulder just happens with no identifiable cause.

Some other higher-risk groups include people with diabetes or thyroid disease, Dr. Ode says. It’s not clear why, but researchers speculate that the inflammation caused by those diseases might be one reason.

Read the full article at SELF.com.