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A review of 27 fractures of the acromion process during a 15-year period revealed five distinct types that were classified into three groups. Stress fractures are rare, do not result from acute trauma, and gain little benefit from nonoperative treatment. Type I fractures are minimally displaced. Type IA fractures are avulsion fractures and heal rapidly. Type IB fractures result from direct trauma to the extremity, and are minimally displaced. Most heal with nonoperative treatment. Type II fractures are displaced laterally, superiorly or anteriorly and do not reduce the subacromial space. Most are pain free with full motion after 6 weeks of nonoperative treatment. Type III fractures reduce the subacromial space. This may occur by an inferiorly displaced acromion fracture, or an acromion fracture associated with an ipsilateral, superiorly displaced glenoid neck fracture. Patients in this group sustained significant trauma to the involved extremity. All type III fractures treated nonoperatively develop significant limited shoulder motion with pain, suggesting that early surgical intervention may be indicated.