As stated in your patient education materials, studies show that the rate of humeral (upper arm bone) nonunion is higher when the patient has a prosthesis (implant). A fracture of the upper arm (humeral shaft) by itself is a lot simpler than one with an implant down inside the bone.

Anytime the patient tries to move the shoulder or elbow, the force that's transmitted goes right through the fracture site. That disrupts the end of the bones and the blood flow to both sides. The result is a delay in fracture healing.

At the same time, the tip of the implant down inside the humeral shaft may cause the two ends of the bone to shift apart even further whenever the arm is moved. This distractive force must be limited in order to facilitate healing. If every time the body tries to lay down new bone material to cross the gap the two ends of the bone move, then healing is disrupted and the fracture site won't heal.

Anyone who is in poor health, has poor nutrition, or has diabetes or other serious health concerns is at risk for these kinds of complications. Talk to the surgeon if your mother-in-law has any of the risk factors discussed.

There are specific steps the surgeon can take during the procedure to ensure safety of the bone and prevent fracture. Even so, a fracture can occur despite all precautions. That's why the surgical team warns patients ahead of time to prepare you for any and all possibilities.