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A discussion about Continuous Passive Motion (CPM)

During periods of little to no movement the collagen fibres in the connective tissue in muscles, tendons and ligaments contracts and the structure of the fibres changes, making them less flexible. This even occurs overnight [1], and it occurs in all people to some extent. CPM is helpful to ensure sufficient movement especially when active movement is painful or difficult. CPM can be useful for increasing range of motion (ROM) [2] and reducing inflammation.

CPMs should never be used to forcefully increase bend. Like any tool, CPM can be dangerous if it’s not used properly.

The IAA has been getting many questions on why and how to use the CPM in the treatment of arthrofibrosis. We'll start with our general advice and then dive into the concerns and questions below in our discussion.

How to be used by Arthrofibrosis patients

Care must be taken not to force the joint beyond its ability to stretch – CPMs should never be used to forcefully increase bend. Each session should begin in the safe zone. When the joint is comfortable moving through the range of motion the limits of bend and extension are increased by one degree, never more than that. This process is repeated until the maximum safe limits for that session are reached. Legs should not be strapped in. It is our view that CPMs should be used often, at least twice a day in the early stages of arthrofibrosis. Some people sleep with the affected limb in a CPM, and this may be necessary in the early healing stages if there is aggressive arthrofibrosis. The amount of inflammation and aggressive healing influences the treatment time needed for each session, and this varies between individuals.

Isn't active movement better than passive movement?

In a healthy joint active movement is beneficial. However, in arthrofibrosis active movement can do harm by placing stress and pressure on important structures in the joint that are inflamed and swollen. In knees the Hoffa’s fat pad (IFP) is typically swollen after surgery, and active movement will pull the bones of the joint together and pinch the swollen tissues. This causes more inflammation and swelling, and if this happens often enough the Hoffa’s fat pad will be permanently trapped between the bones, making standing and walking painful. So, while the joint is inflamed, CPM is useful for keeping the joint moving, preventing adhesions and helping to keep the tissue healthy. The more inflammation there is, the more quickly and strongly adhesions form, sticking tissues together. The tearing of adhesions increases inflammation and may be experienced as pain after the affected limb is moved after a period of no movement, as occurs while in bed.

Doesn't advice like this allow for blaming of the patient for non-compliance and self-
blame: 'You/I didn't do sufficient CPM'

The general recommendation of post-op CPM therapy is for guidance only, and every individual needs to decide which physical therapies and medications they’re comfortable with. Blame of any type has no place in arthrofibrosis treatment.

Might this advice be misinterpreted to support the harmful idea that you have to do work more, harder and longer?

CPM should always be gentle and not stressful. It’s a passive therapy, meaning that

muscles are not activated, so it’s not actually physical “work” in that sense, you just go along for the ride. If CPM becomes stressful then it is better to stop. For example, if you’ve been told that it’s helpful to sleep with your leg in the CPM, but you find that

you’re not able to sleep, then you shouldn’t keep trying to do that. It can help to be

trained in CPM use before a planned surgery to become familiar and comfortable with it.

Will it (unnecessarily?) scare and upset people if they can't get a CPM post OP?

Gentle CPM can help to prevent adhesions and contractions, especially in the immediate post-operative period when there are high levels of inflammation driving the activation of myofibroblasts. However, people can, and do, recover without CPM, it all depends on the individual, their predisposition and history. And, on the other side of that equation, there are never guarantees of better post-op outcomes even with CPM use.

There is no scientific evidence as far as I know, right?

This is true, there are no studies of CPM use in people with arthrofibrosis that I’m aware of. A Cochrane Review of the post-op use of CPM in the general population found a small reduction in the need for an MUA, which presumably means that fewer people were given a diagnosis of arthrofibrosis after CPM use. A properly conducted study that applies frequent and prolonged CPM therapy post-op is badly needed. CPM once or twice a week for a short time is often not enough to prevent adhesions in the early post-op phase. There is scientific evidence that CPM reduces inflammation (see main article), and some evidence that it reduces contractions.

In cartilage cell transplant (MACI) surgery, CPM is sometimes prescribed not for ROM but to stimulate cartilage cell growth in the graft due to the frictional movement. Are we sure it won't stimulate growth of other cell types as well?

CPM doesn’t induce much friction in a joint - the surfaces of cartilage won’t rub together much with CPM. This is because muscles are not activated during passive movement, so the bones are not being pulled together to create friction and pressure as happens during active exercise (which can also pinch the Hoffa’s fat pad). However, the movement of the synovial fluid during CPM results in an increase of nutrients and oxygen into the cartilage, permitting growth, and the removal of cell fragments and waste. So CPM mimics natural healthy movement that feeds cartilage without loading and stressing the joint.

If one very gently rubs their skin for hours and hours, it's surely gets irritated sooner or later. Why wouldn't that happen in a knee?

Skin is not designed to cope with constant friction like the surfaces of a joint are. Joints are very well lubricated and cartilage has a special, complex design. If the skin was very well lubricated and there was almost no pressure applied then it would be quite tolerant of this gentle form of touch.

People are likely to push / eager improve or to keep ROM. Unknowing physical therapists might even advise to strap down the knee to force ROM.

It’s true that there has been a lack of education of PTs and other clinicians about how to properly and safely use CPM, and how it is designed to work. ROM should never be forced, either manually or with CPM, since this will likely cause small tears and internal bleeding. Even a small amount of bleeding can activate myofibroblasts. People using a CPM should carefully read the instructions on the IAA, and only ever increase the maximum ROM setting by one degree at a time, and make sure that they are comfortable at this setting before increasing it again. Like any tool, CPM can be dangerous if it’s not used properly.

Some surgeons moved away from CPM use post arthrolysis so it's at least controversial.

Some people think that the mechanical stress caused by movement will activate the myofibroblasts. However, the mechanical stress that activates the myofibroblasts is caused by the stiff scar tissue that surrounds the myofibroblasts – that is, by their immediate environment. This stiffness creates tension when the myofibroblasts contract (in response to inflammation etc) and further activates them [3], which is how feedback effects occur in organ fibrosis without active movement. We also know that no movement (using a brace) is one of the most powerful ways to create arthrofibrosis, even in healthy young subjects without trauma.

Mental aspect: wouldn't it help me more to do some nice things that make me feel better or get a good night sleep, instead of being scared to leave the CPM?

Yes, post-op CPM therapy is a general recommendation based on our scientific

understanding, as well as the extensive experience of several of our board members. However, every individual needs to decide which physical therapies and medications they’re comfortable with, everybody is in charge of their own body and what is done to it. Mediation and mindfulness are other suggestions that can help to reduce inflammation and recovery.

We hope this helps to make the CPM your friend during this struggling time. Read more about the use and benefits of CPM for arthrofibrosis here. Feel free to tell us about your experience with CPM or ask questions in the comments below.

  1. Corcoran, P. Use it or lose it –the hazards of bed rest and inactivity- adding life to years. Western J. Med. 154, 536-538 (1991).

  2. Aspinall, S. K. et al. Medical stretching devices are effective in the treatment of knee arthrofibrosis: A systematic review. J Orthop Translat27, 119-131, doi:10.1016/ (2021).

  3. Putra, V. D. L., Kilian, K. A. & Knothe Tate, M. L. Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment. Commun Biol 6, 75, doi:10.1038/s42003-022-04320-w (2023).

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