Actual knee motion during continuous passive motion protocols is less than expected.

Investigations of the usefulness of continuous passive motion (CPM) after TKA have yielded mixed results, with evidence suggesting its efficacy is contingent on the presence of larger motion arcs. Surprisingly, the range of motion (ROM) the knee actually experiences while in a CPM machine has not been elucidated. In this study, the ability of a CPM apparatus to bring about a desired knee ROM was assessed with an electrogoniometer. The knee experienced only 68% to 76% of the programmed CPM arc, with the higher percentages generated by elevating the head of the patient's bed. This disparity between true knee motion and CPM should be accounted for when designing CPM protocols for patients or investigations evaluating efficacy of CPM.