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Passive Stretching and Continuous Passive Motion (CPM)

CPMs should never be used to forcefully increase bend

Care must be taken not to force the joint beyond its ability to stretch – CPMs should never be used to forcefully increase bend. CPM can be useful for increasing range of motion (ROM)[1] and reducing inflammation[2]. CPM has been controversial with studies citing little evidence of efficacy [3]. However, these studies were not about treating arthrofibrosis, they were conducted on the general surgical population undergoing their first joint replacement without complications [3]. The great majority of people who have surgery do not develop arthrofibrosis. Therefore, any positive response to CPM from the small number of people who would have developed post-surgical arthrofibrosis was diluted in the pooled result [4]. More research is needed into the efficacy of CPM for treating people with arthrofibrosis [3,4].


Care must be taken not to force the joint beyond its ability to stretch. The joint should only be bent or straightened to the point at which it is mildly uncomfortable – think of the discomfit of pushing your finger backwards and stretching it. Pushing further than discomfit risks tearing tissue, and that is a major setback.

Tissue contraction

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 [5], and it occurs in all people to some extent. This is the reason that stretching first thing in the morning feels pleasurable. When tissues are inflamed the contraction process is greatly increased due to the presence of the special cells that make scar tissue, called myofibroblasts. This contraction can create a severe lack of ROM, particularly in the morning and after periods of immobilisation. For more information on tissue contraction, see [5,6]. Passive stretching is therefore likely to be most beneficial early in the morning. It gently stretches collagen fibres out, permitting greater ROM and thereby avoiding tearing during daily use. Mobilisation and stretching also flushes out toxins (inflammatory cytokines, dead cells etc) and rehydrates connective tissue and membranes so that they slide against each other properly and do not form adhesions.

How to be used by Arthrofibrosis patients

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 1 degree. This process is repeated until the maximum safe limits for that session are reached. ROM settings should only ever be increased by 1 degree at a time, never more than that. In our opinion, 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.

Knee CPM device ,Rehabilitation after knee surgery.jpg

Computer controlled CPM

Computer controlled CPMs have an emergency button to quickly lower the amount of bend, and for safety the person being treated should always hold the controller and know how to operate it. The joint should be flexed through the comfortable range of motion and held the maximum position for 2 to 10 seconds to permit stretching of tissues. Optiflex describes their program for knee CPMs “When the carriage reaches the Programmed (maximum) Flexion Angle, it will oscillate between the Programmed Flexion Angle and 10° less than the flexion angle, three times. On the third oscillation cycle, the carriage will hold at the flexion angle for the Programmed Extension/Flexion Delay Time”.

In the US there are several hand-operated stretching devices available for the treatment of knees, but in our opinion motorised, computer controlled CPMs do a better job since they have precise 1o incremental control of ROM and there is less likelihood of accidental tears. In addition, a CPM used correctly takes a joint through the full range of motion many times during a session, so there are greater benefits from mobilisation. Good reconditioned second-hand CPMs can sometimes be purchased on Ebay (ex-hospital) and these can be a good option. Always look for computer controlled CPMs.



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. 


Should I work more, harder and longer on my flexion?

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.


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.


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, 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.


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.


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.


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.


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.


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.


  1. 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).

  2. Ferretti, M. et al. Anti-inflammatory effects of continuous passive motion on meniscal fibrocartilage. J. Orthop. Res. 23, 1165–1171 (2005).

  3. Chaudhry, H. & Bhandari, M. Cochrane in CORR ((R)): Continuous passive motion following total knee arthroplasty in people with arthritis (Review). Clin Orthop Relat Res 473, 3348-3354, doi:10.1007/s11999-015-4528-y (2015).

  4. Usher, K.M, S. Zhu, G. Mavropalias, J.A. Carrino, J. Zhao, and J. Xu (2019) Pathological mechanisms and therapeutic outlooks for arthrofibrosis. Bone Research 7:9

  5. 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).



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