The results usually are good. Prognosis depends on what's wrong. With removal of the bone chips, some horses can perform very successfully. Spend a Buck won the Kentucky Derby after surgery to remove a bone chip. With direct trauma, successful recovery depends on which structures are damaged.
Prognosis ranges from limited to full recovery.
The prognosis for popped knees due to infection depends on the invading organisms, how long they've been there, and the cause of injury. The horse might make a complete recovery, or might need to be euthanized. With developmental abnormalities in foals, the prognosis is guarded to favorable. Some horses, after they reach maturity, never have problems. The risk of your horse getting popped knees can be reduced.