CPM Machine After Knee Replacement

A device used to gently work the knee joint to reduce stiffness

A continuous passive motion (CPM) machine is a motorized device that gently moves a joint through a set range of motion. It is sometimes used after knee replacement surgery to prevent scar tissue development, reduce stiffness, and restore normal function.

In theory, placing the knee in this device soon after surgery does just that. But although there is some benefit to using CPM in the first days and weeks of recovery, there is no difference in the overall outcomes of those who use CPM vs. those who don't.

This article delves into why CPM may be recommended after knee replacement surgery (and why it may not). It reviews the pros and cons of using a CPM machine, how to use one, and other surgeries for which CPM may be a part of the recovery period.

Woman with leg in a cpm in a hospital bed
bojan fatur / Getty Images

Benefits of CPM

A 2022 study found that total knee replacement patients who received CPM and did active exercises reported more improvement in pain, stiffness, and function than people who only did active exercises.

Some healthcare providers also argue that patients do have an initial increase in motion following surgery that is more rapid than patients who do not use a CPM.

Additionally, there are some specific procedures, such as a release of contracture or adhesions, where a CPM can be an important part of the recovery from knee surgery.

Limitations of CPM

Overall, though, studies have shown that a CPM does not make any difference in the long run after knee replacement or ACL surgery. Studies have shown that within four to six weeks of knee replacement surgery, patients who use CPM and those that don't have the same range of knee motion.

While there may be the psychological effect stated above, there has been no evidence to show that the use of CPM will ultimately improve the outcome of a knee replacement or anterior cruciate ligament (ACL) reconstruction surgery.

Many surgeons worry that CPM may ultimately slow down recovery by keeping the patient in bed, limiting more effective active therapy.

Pros
  • Faster initial increase in motion

  • Psychological benefit of an action being taken

Cons
  • Makes no difference in recovery and range of motion in long term

  • Keeps patient in bed rather than doing active therapy

When CPM Is Used

For many years, CPM machines were seen as a significant medical advancement that could help prevent postoperative complications from joint surgery. They were used routinely after a number of different surgical procedures, particularly knee replacement.

And the need is there: While at least 80% of people are satisfied with the results of knee replacement, stiffness of the joint remains a persistent problem. This can be a serious complication and is one of the more common reasons why people are dissatisfied with knee replacement surgery.

The machines are no longer widely used because of the multiple studies that found that CPM following knee replacement surgery and ACL reconstruction surgery has minimal benefits.

However, some surgeons still recommend CPM following knee surgery when the limited pros outweigh the cons in a particular case.

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Click Play to Learn About CPM Therapies

This video has been medically reviewed by Anju Goel, MD, MPH.

How to Use a CPM Machine

A CPM machine is typically used while you are lying down. Follow your healthcare provider's instructions, or those provided with the machine, when putting on the device and adjusting it. 

CPM machines are usually controlled with a remote control that starts and stops the machine and lets you adjust the speed and the amount of flex. 

How Long Should You Use a CPM Machine?

You should use a CPM machine only for as long as your healthcare provider recommends. This is usually around four hours a day for three to four weeks after surgery, but it will depend on what procedure you had done.

Other CPM Uses

CPM is most commonly used after knee replacement surgery, but it can also be used after other joint surgeries, such as:

  • Hip surgery: Some studies have shown that CPM after hip impingement surgery can help improve flexibility. Similarly, A 2021 study concluded that people who used CPM after hip arthroscopy had reduced pain and required less pain medication than those who did not.
  • Elbow surgery: CPM is sometimes used following elbow surgery, but studies have found that it may have no real benefit for people who have undergone this procedure.
  • Shoulder surgery: A 2021 review of studies concluded that CPM after rotator cuff surgery helped improve pain and mobility. The study also found that people who underwent surgery to correct a condition called frozen shoulder had faster improvements in pain after using CPM.
  • Wrist fractures: A 2020 study in people recovering from wrist fractures did not find significant differences in pain, function, or range of motion between patients using CPM and those using traditional exercises.

CPM has also been used to help treat lower back pain. Research into this use is limited, but one small study found that people who used the machine for 10 minutes one to three times a day for three weeks had less pain.

A Word From Verywell

As stated previously, overall studies do not demonstrate any long-term benefit to the routine use of CPM following a knee replacement or ACL surgery. As the studies clearly show, patients are likely to be at the same point within several weeks of surgery regardless of the use of a CPM.

More surgeons are recommending against the routine use of CPM and encouraging patients to focus on active therapy efforts of getting up and out of bed.

Frequently Asked Questions

  • What's the price of a CPM machine?

    A CPM machine usually costs more than $2000 to buy, but you can also rent them for about $400 for two to three weeks. Check with your insurance company to see if they will cover part of the rental cost. Medicare Part B covers CPM machines prescribed by a physician for up to 21 days of use in your home.

  • Do they use a CPM machine after every joint surgery?

    CPM machines are not effective for every type of joint surgery. Furthermore, some healthcare providers no longer recommend them and will suggest alternative therapies instead, such as physical therapy. You should use CPM only if your healthcare provider recommends it.

13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.