 Extrinsic factors include bone spurs from the acromion or AC joint, shoulder instability and neurologic problems arising outside of the shoulder.It is often difficult to distinguish between pain caused by bursitis or that caused by a rotator cuff injury as both exhibit similar pain patterns in the front or side of the shoulder.Subacromial bursitis can be painful with resisted abduction due to the pinching of the bursa as the deltoid contracts.