PROM is an abbreviation for 'Passive Range-of-Motion'.

Normal range of motion of the knee.
Using a helper
The passive range of motion needs a helper to take the knee through the available range of motion without the patient trying to help, so that the muscles are relaxed.
Synonyms:
passive range of motion
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FORUM DISCUSSIONS
When to start worrying about arthrofibrosis?
CLINICAL PUBLICATIONS
Patient Self-Assessed Passive Range of Motion of the Knee Cannot Replace Health Professional Measurements. Borgbjerg J, Madsen F and Odgaard A. J Knee Surg. 2017 Oct;30(8):829-834. doi: 10.1055/s-0037-1598174. Epub 2017 Mar 1.