To preface my post, keep in mind that I am using the limited information we are given about Cushing's wrist surgery and applying logic to reconstruct a scenario that in my mind is likely.
Sounds like Cushing may have been dealing with an unrecognized scaphoid (also sometimes referred to as "navicular") bone fracture that never quite healed (arrow in figure below points to fracture of the scaphoid bone).
Scaphoid fractures account for 2-7% of all orthopedic fractures, and they are the most common of all carpal (wrist)bone fractures. Symptoms of a "nonunion" of the scaphoid bone can be quite subtle. You may have pain when you use your wrist. However, the pain may be very minimal. It is isn't uncommon for "real world" doctors to see a nonunion of the scaphoid bone on X-rays, but the patient can't remember an injury. These patients usually suffered a wrist injury months or even years ago that they were told was a simple sprain. If x-rays are not taken at the time and then repeated 10-14 days later to account for resobtion of bone to identify a fracture, it can easily be missed. The MRI is the "gold standard" for picking up scaphoid fractures that are not defined on plain x-rays of the wrist. Still, the most common symptom of a nonunion is a gradual increase in pain. Over several years the nonunion can lead to degenerative arthritis in the wrist joint, accompanied by loss of grip strength. Scaphoid fractures are the most commonly undiagnosed fracture.......at least in the "real world." However, with MRI's so readily accessible to NFL teams and their players, overlooking this diagnosis may be considered less defensible.
As to why Cushing would have been initially told that his wrist would be in a cast for 4 weeks, then the time period extended to 8 weeks? Well, certainly not simply because they just wanted to keep him from pushing things too early. Keep in mind that the longer a joint is immobilized, the more difficult the rehab, the more atrophy of muscles, and the more possibility of loss of range of motion. No, this change of events could be more readily explained by the fact that going into the surgery, his surgeon believed that the fracture could be repaired with a simple screw......which would require only 3-4 weeks cast immobilization. Once being able to thoroughly assess the injury DURING surgery, a change of approach may have been indicated.
If there was no evidence that wrist arthritis was developing, a decision could likely have been made to get the scaphoid to best stabilize the fracture site and affect more reliable healing by using a bone graft in addition to a metal screw. Traditionally, the bone graft will be taken from the iliac crest (the rim of the hip bone) or the radius (one of the forearm bones, depicted in the figure below). Approximately 85% of scaphoid non union cases will heal with this treatment.
After this type of surgery, a plaster cast is worn for............. ~8 weeks.