A 73-year-old man with a history of a left shoulder hemiarthroplasty 10 years ago sustains a fall and presents the the emergency room with acute pain and weakness in his left shoulder. Injury radiographs are shown in Figure A. He undergoes successful closed reduction of the left shoulder. On routine follow up 3 months later, he is still unable to elevate his arm beyond 30 degrees despite rigorous physical therapy. New radiographs depict anterosuperior escape. What is the structure likely injured and what is the treatment that would result in the most predictable outcome?