The Continuous Passive Motion machine (CPM) does not increase a patient’s range of motion or strength. It does not or shorten recovery. It doesn’t help, and can even hurt a patient’s knee.

The CPM machine is a story of failed early research and technology. If you are interested in the enormous body of research about the Continuous Passive Motion machine click here (as compiled by orthopedic surgeon, Dr. David Halley).

Robert Salter (MD) and John Saringer (an engineer) commercialized the CPM in 1978. The original idea was to stimulate cartilage production in knees, at which the machine succeeded extremely well in rabbits, but not for humans. Human knee cartilage has no blood supply and consequently cannot grow back.

Soon patients began using the Continuous Passive Motion machine to increase knee range of motion. Most research studies, before 2005, compared the range of motion of patients who used the CPM to patients who had only bed-rest. When that comparison was made it looked as though the CPM had a tremendous effect. However, after 2005, the experiments on the CPM evolved. These studies compared people who used the CPM to those who did not use the CPM, but who also were not confined to their beds. The non-CPM control group could move about.  None of these studies showed that the CPM could increase range of motion better than those who simply moved about. The results were remarkably clear: the Continuous Passive Motion machine failed to increase knee range of motion.

In the early 1970’s most civilian knee surgery patients had their knees in a cast for six weeks, and were largely confined to bed. The military not only did not cast injured knees, they immediately got patients up and moving. At that point in time the research showed that some movement was superior to no movement. Dr. Salter reasoned that if some movement was better, continuous movement would be better still, and hence the invention of the CPM.

Basically, the CPM flexed a patient’s leg through a prescribed arc. In order to work properly, the patient was to lay absolutely still for up to 20 hours a day. The purpose behind laying still was to preserve the alignment between the patient and machine. However, simply moving a patient’s head disrupted the alignment, and to make matters worse, the machine itself moved over the surface of the bed. Even bolting the machine down did not solve the alignment problem. Because of the alignment problem patient’s legs only experienced 68% of the prescribed arc. And the CPM was painful for most patients. Lastly they were not very compliant.

Surgeons who have kept abreast of the research, as a rule do not prescribe the CPM, though some will admit that they offer it if patients ask for it.

How the CPM failed (the research) – Read More

Click the Play button above to listen to a full analysis of the CPM including recent research by hospital, surgeons, and therapists from the U.S. to The Netherlands to China. We welcome your feedback in the comments below.

We call it a “Meta-Blog.” In these articles we step back and give you a broad perspective on all aspects of knee health. We explore surgery and recovery and such subjects as ‘How the CPM Machine Failed.’

This is a one-of-a-kind blog. We gather together great thinkers, doers, and writers. And it is all related to Knee Surgery, Recovery, Preparation, Care, Success and Failure. Meet physical therapists, coaches, surgeons, and patients. And as many smart people as we can gather to create useful articles for you. You may have a surgery upcoming. Or in the rear-view mirror. Maybe you just want to take care of your knees to avoid surgery. In all cases you should find some value here. Executive Editor: PJ Ewing ([email protected])