A CPM (continuous passive motion) machine is a motorized device that slowly moves a joint through a preset arc of motion without any effort from your muscles. It’s most commonly used after knee replacement surgery to help prevent stiffness and maintain range of motion during early recovery. The machine does all the work, gently bending and straightening the joint while you rest.
How a CPM Machine Works
The concept behind CPM dates back to the 1960s, when researchers found that early, consistent movement after joint surgery reduced stiffness and improved range of motion. Animal studies confirmed that passive movement, meaning motion generated by something other than the patient’s own muscles, helped joints heal with better mobility than keeping them immobilized.
A typical CPM machine looks like a padded cradle or trough that supports your leg (or other limb). A motor inside the device slowly moves the joint back and forth through a range you or your care team set ahead of time. The speed is gentle and continuous, cycling through the motion over and over for the duration of your session. You don’t push, pull, or resist. You simply let the machine guide your joint through its arc.
What Joints CPM Machines Are Used For
Knee CPM machines are by far the most common. They’re primarily prescribed after total knee replacement (also called total knee arthroplasty), but they’re also used after surgical repair of fractures around the knee joint. Beyond the knee, CPM devices exist for the hip, and some specialized versions are designed for the shoulder, elbow, ankle, or wrist. The design changes depending on the joint, but the principle is the same: controlled, motorized movement through a safe range.
Typical Settings and Duration
When you first start using a CPM machine, the range of motion is usually set conservatively. Some protocols begin with a small arc of just 30 to 40 degrees of bending, increasing by 10 to 20 degrees each day as your joint tolerates more movement. Research has found that protocols using larger motion arcs (70 to 90 degrees) continued for longer periods tend to produce better results than more limited starting ranges.
How long you use the machine each day varies. For hip recovery, a common protocol is about 4 hours per day over a period of 4 weeks. Knee protocols differ depending on the surgeon’s preference and the type of surgery, but sessions can range from a few hours to much of the day in the early postoperative period. Your surgeon or physical therapist will adjust the settings and schedule based on your progress.
One detail worth knowing: a study measuring actual joint movement during CPM found that the knee doesn’t always move through the full arc the machine is set to. The real motion can be somewhat less than what the dial reads, which is why your care team may monitor your progress with direct measurements rather than relying solely on the machine’s settings.
Benefits and What the Evidence Shows
CPM is primarily used to increase knee range of motion after surgery. The idea is straightforward: if you keep a joint moving early, it’s less likely to stiffen up from scar tissue formation. A 2024 meta-analysis looking at 557 patients after total knee replacement found beneficial outcomes when CPM was combined with physical therapy.
A prospective study on patients recovering from tibial head fractures (a type of knee-area break) compared CPM users to non-users at 6 weeks and 6 months after surgery. The CPM group had significantly better range of motion at both checkpoints. They also scored better on knee function assessments, pain scores, and quality-of-life measures, with differences in some outcomes reaching statistical significance by the 6-month mark. Both groups continued improving between 6 weeks and 6 months, but neither group fully matched the range of motion of their uninjured knee even after half a year.
That said, the evidence isn’t universally strong. Some studies show only modest short-term gains in range of motion that level out over time as physical therapy catches up. The benefits appear most consistent when CPM is used alongside active rehabilitation rather than as a replacement for it.
Who Should Not Use One
CPM machines aren’t appropriate for everyone. Conditions that typically rule out CPM use include active joint infection or a history of previous joint infection, progressive nerve damage in the affected limb, and recurring joint tightness caused by inflammatory disease. If you’ve had complications that make passive movement risky, such as unstable fractures or certain wound healing problems, your surgeon will choose a different rehabilitation approach.
Getting a CPM Machine at Home
CPM machines are classified as durable medical equipment, meaning they’re rented rather than purchased for home use. Your surgeon writes a prescription, and a medical equipment supplier delivers the device to your home, typically with instructions on setup and use.
Medicare covers knee CPM machines for up to 21 days of home use after knee replacement surgery, provided your doctor prescribes it and the supplier is enrolled in Medicare. After meeting the Part B deductible, you pay 20% of the Medicare-approved amount if your supplier accepts assignment. It’s worth confirming with the supplier before delivery that they’ll accept Medicare assignment for all rental months. If they don’t, you may need to pay the full cost upfront and seek reimbursement yourself.
Private insurance coverage varies. Some insurers have moved away from covering CPM devices, with at least one major insurer (Cigna) classifying them as not medically necessary for any indication. If your surgeon recommends a CPM machine, check with your insurance provider before renting to understand your out-of-pocket costs.
CPM vs. Physical Therapy Alone
The practical question most patients face is whether they actually need a CPM machine or whether physical therapy alone will produce the same results. The honest answer is that it depends on the surgery and the individual. For straightforward knee replacements, many surgeons now favor early active mobilization (getting you moving on your own with a therapist’s guidance) over prolonged CPM use, since long-term outcomes often end up similar. For more complex procedures, like repair of fractures around the knee, CPM may offer a measurable advantage in both pain and function that persists at 6 months.
Your surgeon’s recommendation will factor in the complexity of your procedure, your baseline mobility, and how aggressively rehabilitation can proceed. CPM works best as one piece of a larger recovery plan that includes hands-on physical therapy and, eventually, your own active exercises.