Do I need a bending machine (CPM) after my knee replacement?

Last reviewed: September 29, 2026

Probably not. A continuous passive motion machine, or CPM, is a device that slowly bends and straightens your knee for you while you lie in bed. For years it was standard after a knee replacement.

The American Academy of Orthopaedic Surgeons now lists it among the things to avoid using routinely after a knee replacement, and physical therapy guidelines say the same. Pooled results from twenty trials found the machine improved knee bending by two or three degrees, too small to matter to anyone actually using the knee, with no gain in function or pain and no shorter hospital stay. One trial found the group without a machine went home sooner.

Used instead of therapy rather than on top of it, it looks a little different. One older trial compared a home machine directly against professional therapy and found no difference at six months in bending, knee scores or stiffness, at lower cost. But those patients had spent days in hospital first. No study has looked at a machine in someone who goes home the same day, which is now common, so nobody can say what it adds there.

The machine is not a shortcut. What it substitutes for is your own effort.

In general:

• Guidelines advise against routine use after an uncomplicated knee replacement.
• Bending the knee yourself does more than a machine bending it for you.
• No study has tested one in patients who go home the same day.
• A knee that stays stiff in the first weeks is worth reporting early.

Drafted with AI assistance from published sources, reviewed and edited by Babak Sheikh, MD.

Sources:

• American Academy of Orthopaedic Surgeons. Avoid Routine Use of Continuous Passive Motion (CPM) After Knee Replacement. Choosing Wisely. 2020. https://www.choosingwisely.org/societies/american-academy-of-orthopaedic-surgeons/
• Harvey LA, Brosseau L, Herbert RD. Continuous Passive Motion Following Total Knee Arthroplasty in People With Arthritis. Cochrane Database Syst Rev. 2014. https://doi.org/10.1002/14651858.CD004260.pub3
• Yang X, et al. Continuous Passive Motion After Total Knee Arthroplasty: A Systematic Review and Meta-analysis of Associated Effects on Clinical Outcomes. Arch Phys Med Rehabil. 2019. https://doi.org/10.1016/j.apmr.2019.02.001
• Yang Y, et al. The Effect of Continuous Passive Motion in Patients Treated With Total Knee Arthroplasty for Osteoarthritis. Am J Phys Med Rehabil. 2021. https://doi.org/10.1097/PHM.0000000000001718
• Jette DU, et al. Physical Therapist Management of Total Knee Arthroplasty. Phys Ther. 2020. https://doi.org/10.1093/ptj/pzaa099
• Worland RL, et al. Home Continuous Passive Motion Machine Versus Professional Physical Therapy Following Total Knee Replacement. J Arthroplasty. 1998. https://doi.org/10.1016/S0883-5403(98)90011-9
• Lenssen AF, et al. Effectiveness of Prolonged Use of Continuous Passive Motion (CPM), as an Adjunct to Physiotherapy, After Total Knee Arthroplasty. BMC Musculoskelet Disord. 2008. https://doi.org/10.1186/1471-2474-9-60

This is general information about what published research shows. It is not medical advice and it is not about your situation. Reading it does not create a doctor-patient relationship with me. I am a retired orthopedic surgeon and I do not see patients. Talk with your own doctor before changing anything about your care. If something feels suddenly or seriously wrong, get medical care promptly rather than waiting.

This page is provided as is. I make no guarantee that it is accurate, complete or current, and medical evidence changes. Use of it is at your own risk.

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