67-year-old woman, sona has a 3 month history of increasing right shoulder pain. She is now unable to lift her arm to brush her hair or to take vessels off a shelf in the kitchen. She denies any numbness or tingling or radiation of the pain down her arm. On physical examination, she exhibits weakness in abduction and external rotation on her right arm. Her shoulder shows a normal passive range of motion. When asked to hold her arms abducted to 90 degrees, she is unable to do so on the right. There is no motor weakness in her forearm or hand. Her right upper extremity has normal pulse and normal sensation. Which of the following is the most likely diagnosis?

The above history describes a rotator cuff tear. Patients classically have difficulty doing overhead activities. On physical examination, she has weakness with abduction and external rotation of her shoulder. The supraspinatus is responsible for abduction and the infraspinatus is responsible for external rotation of the shoulder. These muscles, along with the teres minor and subscapularis, make up the rotator cuff. Brachial plexus injuries commonly present with some type of numbness or tingling in the upper extremity. Glenohumeral ahritis is unlikely because she has a full passive range of motion, and shoulder ahritis is commonly associated with a decreased range of motion. Also this history is relatively sho term. Proximal humerus fracture and shoulder dislocation are unlikely because this process has been present for 3 months. These injuries need more acute treatment. Also she has a full passive range of motion, which is not common with fractures or dislocations.