Confusion after total knee replacement is common, but it's not normal. It's definitely more common in older patients (over 85 years old), compared to younger patients. Doctors aren't sure what causes this in some people. It may be linked to blood clots or blocked arteries from therosclerosis.
More research is needed to find out risk factors other than age. Perhaps it's the type of anesthesia used or the drugs needed for pain control afterward. Sometimes confusion is the first sign of a problem like pneumonia or bladder infection. Confusion is a major factor in falls and injury. Make sure your father-in-law's doctor is aware of this new symptom.