Continuous passive motion (CPM) was first used in the 1980s when a well-known orthopedic surgeon did some studies with it to help joints heal after fractures. It's use after total knee replacement has been debated for years.

Some studies show CPM helps patients get more motion back faster. Others say it doesn't make a difference. Some studies show a shorter hospital stay for patients using CPM. Others don't show any difference at all.

Researchers suggest different study designs and methods of research may explain these varied results. For example, in one study the CPM was only used for 24 hours after the operation. Another study used it for three hours twice a day for five days.

Sample size (the number of patients in the study) can make a difference in results, too. Smaller studies may not be as valid as larger studies. Finally surgery has improved over the years making the use of CPM less necessary. Patients are already able to move freely after the operation with fewer muscles cut and less swelling. They don't need a machine to passively bend and straighten the knee for them.