There are no known studies of patients with dementia having total hip or total knee joint replacements. Most physicians agree that dementia is one reason patients should NOT have this surgery. There are great concerns for patient safety after joint replacement.
On the other hand, improvements in the implant design and surgical technique have changed the course of treatment. Patients have fewer restrictions. They can get up and walk on the new joint right away.
Physical activity and exercise are very important in all patients. This is especially true for nursing home residents. They are often at risk for other health conditions made worse by inactivity.
The decision to give your mother a knee replacement must be made by the entire team. The group should include family, physicians, nurses, social workers, and rehab staff. Both her primary care physician and gerontologist (if she has one) along with the orthopedic surgeon should assess all aspects of her health, the operation, and the goals for surgery.
If everyone is in agreement, then the surgeon will make further decisions about the type of implant and type of surgical approach to use. Likewise the nursing staff and physical therapists will develop a plan of care to see her safely through the rehab program.