Adhesions are frond-like filaments which form in the knee in response to inflammation.
They stretch across the walls of the knee joint, binding the surfaces and limiting movement.
Initially they are easily broken with knee movement, but later the adhesion tissue matures into scar-bands and these thicken and make the knee stiff. When the knee is stiffened by adhesions the condition is called arthrofibrosis.
The reason knee surgeons order CPM (continuous passive motion) and early mobilisation is to prevent adhesions forming in the first place. Once formed, if caught early enough they can still be broken with manipulation under anaesthesia (MUA) but if this is unsuccessful they may need to be removed surgically.
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