It's a smooth glistening surface. The surgeon needs to know about it preoperatively to know whether to leave it alone, or to fuse it. A mistake in this area can doom a surgical procedure to failure. Also, this fuses late in a person's body. It's hard to call an osacromiale on anyone under the age of 25, since it's one of the last ossification centers in the body to fuse.
Impingement syndrome has three stages, which are all basically inseparable.
Stage I is edema and hemorrhage. Stage II is fibrosis and tendonitis. Stage III is a full thickness tear. With impingement there shouldn't be fluid in the subacromial bursa until it gets to be severe tendonitis or a partial tear. Then there will be small amounts of fluid in the subacromial bursa from the irritation.