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What is a Continuous Passive Motion Machine?

By Henry Gaudet
Updated: Feb 29, 2024

A continuous passive motion machine is a device designed to gently flex and extend a joint such as a knee. This device is used as a physical therapy tool after surgery to keep the joint from becoming stiff. The limb is strapped to the continuous passive motion machine and cycled through a safe range of motion at a controlled speed.

Dr. Robert B. Salter is credited with developing the idea of the continuous passive motion machine, beginning in 1970. By keeping the joint moving, he hoped to give the patient greater use of the limb with fewer complaints. With the assistance of the engineer John Saringer, he was able to develop a prototype in 1978.

After surgery, patients are likely to experience pain when attempting to actively flex an affected joint. As the name implies, the use of a continuous passive motion machine does not require the patient to actively flex muscles and move the limb. The machine does all of the work, often sparing the patient from experiencing considerable pain.

Frequent or constant motion of the joint after surgery helps the patient to heal more quickly and retain a greater range of motion. Limbs left stationary are prone to stiffness. As the joint heals, scar tissue forms, and without sufficient movement, the scar tissue can adhere to adjacent tissue, restricting the joint’s range of motion.

One advantage of the continuous passive motion machine is portability. The machine can be used in a hospital, but it can also be set up in the patient’s home after discharge. Machines are available for rental and might be covered by health insurance plans.

The effectiveness of a continuous passive motion machine compared with professional physical therapy has been widely debated. A number of studies have been carried out, and it appears that the outcomes are similar. Some studies suggest that patients using the machine might be at an advantage during the first few weeks, but within two months, the results are nearly identical. Either option provides results far better than healing with no form of physical therapy at all.

The primary differences between the use of a continuous passive motion machine and the efforts of a professional physical therapist are price and availability. Machine rental costs are considerably lower than a typical therapist’s fees. These machines remain in the home while the limb heals, as opposed to visits to physical therapy facilities, allowing the patient greater access to the therapy.

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Discussion Comments
By anon118683 — On Oct 14, 2010

I had knee replacement surgery in Aug 2010. Went home with a rented CPM and used it every night as I slept. When I didn't use it, I woke up with horrible knee and leg pain - just awful. The machine made a big positive difference. Insurance paid for one month of renting it.

I'd like to have it about another month for occasional use when my knee wants to seize up. That seems to be the benefit of it - keeps the knee slowly, gradually moving all night. It didn't bother me to have this running as I slept. The only negative is that you have to lie on your back all night. That was a small concession for having none of the severe knee pain. This is a great invention!

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