Continuous Passive Motion after a Total Knee Replacement

Total knee replacement rehabilitation.
Your physical therapist can help guide you through rehabilitation after total knee replacement surgery. Francesco Ruggeri/Getty Images

If you have had a total knee replacement surgery, your doctor may order a continuous passive motion, or CPM, machine.

If you have knee pain and stiffness due to osteoarthritis, your doctor may recommend that you have a total knee replacement (TKR) surgery to correct the problem and to improve your overall functional mobility.

Physical therapy after a TKR usually begins soon after the surgery. At this time, you can expect a physical therapist to visit you in the hospital, who will work with you to improve your knee range of motion (ROM) and strength, and to help you begin walking again.

While in the hospital, your doctor may require that you use a continuous passive motion (CPM) machine as you lie in bed: a mechanical apparatus that attaches to your leg and slowly bends and straightens your knee.

What Does CPM Feel Like?

If your doctor requires that you use CPM after your total knee replacement, you may wake up in the hospital after surgery with the machine attached to your leg. The machine has metal bars and a hinge that bends at your knee joint, and straps are used to hold your leg in place while the unit is plugged into the wall. A small motor slowly flexes and extends the bars at the hinge as your knee bends and straightens.

While using the CPM, you may feel some mild pain or discomfort when your knee is pushed into the bent position; this is normal. If the pain is too much to tolerate while using the CPM, be sure to tell your doctor, nurse, or physical therapist. The amount of bending that the CPM requires of your knee can be adjusted.

Increasing the Range of Motion of the CPM Machine

As your knee ROM improves while using the CPM in the hospital, you may need to increase the amount of bending that the machine causes at your knee joint, which is done by a handheld digital control unit attached to the CPM machine. You should ask your doctor, nurse, or physical therapist to increase the amount of knee flexion if allowed: some surgeons have strict TKR protocols to follow after surgery and do not want for your knee to bend too far in the days following surgery.

Typical settings for the CPM machine start at about 60 to 70 degrees of knee flexion and 0 degrees of knee extension (fully straightened) on the first day. The amount of knee flexion is increased by about 5 degrees each day. Once you increase the amount of bending caused by the continuous passive motion machine, you can expect to again feel a slight increase in knee pain and discomfort; this is also normal. If your pain is excessive, tell your doctor, nurse, or physical therapist, and adjustments can be made.

Is the CPM Really Necessary?

Recently, the use of CPM during the post-operative period after TKR surgery has been called into question. Many well-designed studies that examined the effects of CPM after TKR have failed to show an added benefit from CPM when compared to active and passive physical therapy knee ROM exercises.

Some people theorize that the use of CPM after total knee surgery may help decrease the incidence of deep vein thrombosis (DVT). Again, a review of the available literature failed to show a decrease in DVT with the use of continuous passive motion machines following TKR surgery.

While there is lack of evidence showing that CPM is superior in gaining ROM and function after TKR, there's also very little evidence that CPM is dangerous after surgery.

Most often, the use of CPM after TKR is based on the preference of your surgeon. If you have any questions or concerns about CPM after your total knee surgery, you must speak with your surgeon.

Continuous passive motion is a common treatment that is used after TKR surgery. Your physical therapist can work with you to ensure that you are using the CPM properly, and that the ROM settings on your CPM machine are correct. Remember that the CPM machine does not take the place of active knee exercises, and your physical therapist will most likely prescribe exercises that also help improve the ROM and strength of your knee after your TKR surgery to help you return to normal function quickly and safely.


Bruun-Olsen, V. etal. Continuous passive motion as an adjunct to active exercises in early rehabilitation following total knee arthroplasty – a randomized controlled trial, Disability and Rehabilitation, 31:4, 2009, 277-283.

Hertling, Darlene, and R. Kessler. Management of common musculoskeletal disorders:physical therapy principles and methods. 4. Baltimore: Lippincott Williams and Wilkins, 2005

Herbold,J. etal. Effectiveness of Continuous Passive Motion in an Inpatient Rehabilitation Hospital After Total Knee Replacement: A Matched Cohort Study, PM&R, 4(10) October 2012, Pages 719-725

Continue Reading