CPM Machine After Knee Replacement

CPM also called continuous passive motion, is a device that is used to gently flex and extend the knee joint. The CPM machine can be used after surgery to allow the knee joint to slowly bend.

The initial thought was that CPM would improve motion following knee replacement surgery, as well as other knee procedures, and eliminate the problem of stiffness. By placing the knee in this device soon after surgery, scar tissue would not develop, and the problem of stiffness would not be a concern.​

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

Use of CPM

For many years, CPM machines were seen as a significant medical advancement that could help prevent postoperative complications from knee surgery. By getting the knee joint bending immediately following surgery, the goal was to improve restoration of mobility, and ultimately to speed recovery.

CPM machines were used routinely after a number of different surgical procedures, particularly knee replacement surgery. Knee surgery has come a long way in the past 50 years, and orthopedic surgeons are always looking into ways to improve their results.

One persistent problem following joint surgery is stiffness of the joint. Stiffness after knee replacement can be a serious complication and is one of the more common reasons why people are dissatisfied with knee replacement surgery.

While at least 80% of people are satisfied with the results of knee replacement, there are complications that can occur, and reasons why people are not happy with the results of their surgical procedure. Stiff knees are especially problematic because, in order to resume our normal activities, we depend on excellent knee motion.

The continuous passive motion, or CPM, was developed in an effort to begin motion as soon as possible following surgery, and, hopefully, alleviate the problem of postoperative stiffness.


Click Play to Learn About CPM Therapies

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

Latest Developments

Several recent studies have investigated the use of the CPM following knee replacement surgery and ACL reconstruction surgery. In several studies the results are essentially the same—there is some benefit in the first days and weeks following surgery, but there is no difference in the overall outcome of the surgery.

It does not seem to matter if the CPM is used, ultimately, the results are the same.


The argument for a CPM is that patients do have an initial increase in motion following surgery that is more rapid than patients who do not use a CPM.

Also, patients often feel a strong desire to be "doing something" to help their recovery. While CPM may not actually alter the result of the surgery, it can give patients a sense that they are doing something to help their recovery, even when resting in bed.

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. 


Overall, 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, and not getting more effective active therapy.

  • Faster initial increase in motion

  • Psychological benefit of an action being taken

  • Makes no difference in recovery and range of motion for longer term

  • Keeps patient in bed rather than doing active therapy

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 is a CPM machine?

    Continuous passive motion machines are motorized devices that move your joint through a set range of motion. One version is used on knees after surgery, but other machines can be used on joints like the hip or shoulder.

  • How much does a CPM machine cost?

    The 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.

Was this page helpful?
6 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.
  1. Trochimczuk R, Kuźmierowski T, Anchimiuk P. Study of CPM Device used for Rehabilitation and Effective Pain Management Following Knee AlloplastyInternational Journal of Applied Mechanics and Engineering. 2017;22(1):241-251. doi:10.1515/ijame-2017-0014.

  2. Rex C. Continuous passive motion therapy after total knee arthroplasty. Nursing. 2018;48(5):55-57. doi:10.1097/01.NURSE.0000531010.25095.80

  3. Scott CE, Howie CR, MacDonald D, Biant LC. Predicting dissatisfaction following total knee replacement: a prospective study of 1217 patients. J Bone Joint Surg Br. 2010 Sep;92(9):1253-8. doi:10.1302/0301-620X.92B9.24394

  4. Howard JS, Mattacola CG, Romine SE, Lattermann C. Continuous passive motion, early weight bearing, and active motion following knee articular cartilage repair: Evidence for clinical practice. Cartilage. 2010;1(4):276-86. doi:10.1177/1947603510368055

  5. Mcgrory BJ, Weber KL, Jevsevar DS, Sevarino K. Surgical management of osteoarthritis of the knee: Evidence-based guideline. J Am Acad Orthop Surg. 2016;24(8):e87-93. doi:10.5435/JAAOS-D-16-00159

  6. Medicare.gov. Continuous passive motion devices.

Additional Reading