Fractures of this type occur most often during a total shoulder replacement (versus a hemiarthroplasty where only one side of the shoulder joint is removed and replaced). Sometimes the surgeon has trouble getting to the shoulder socket. The angle and force needed may be too much for the brittle bones.
Older women seem to be at increased risk for humeral (upper arm) fractures. They are especially at risk if they also have rheumatoid arthritis or other health issues such as diabetes contributing to delayed or poor healing.
Surgeons must be aware of potential risk factors for fracture. Surgical approach and techniques must be chosen carefully with these risks in mind. Patient position during the operation is important. The elbow should never be used as a lever to get increased shoulder motion when under anesthesia.
Soft tissue release around the shoulder may be needed before moving the arm through its full range of motion. Special care must be taken when reaming out the humeral bone to place the stem of the implant inside. Hand reaming instead of power reaming is advised. The bone should be compressed, rather than removed, in patients who have low bone density.
These are just a few of the many considerations surgeons must include in the surgical process. When many risk factors present at the same time, then the risk of fracture goes up. Even being aware of all the risks doesn't guarantee complications won't occur. The surgeon can't always predict who might develop intraoperative fractures.