Frozen shoulder is a condition where your shoulder becomes painful and stiff. No one is really sure why this happens. Treatment, including painkillers and exercises, can help ease your symptoms. Your shoulder is what’s called a ‘ball and socket’ joint. The ball at the top of your upper arm bone (humerus) moves in the shallow socket on the edge of your shoulder blade (scapula). The whole joint is enclosed within a strong, fibrous capsule. Frozen shoulder happens when this capsule becomes thickened, contracted and tighter than it should be.