Is knees a common noun

Yes, the noun 'knee' is a common noun, a Word for a knee of anyone or anything.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Peggy Sheahan Knee and Robert A. Knee, The Knee Law Firm LLC, Hackensack, NJ
• Knee Street, Tatura, VIC Australia or Knee Road, Wabash, IN
• Knee Deep Vintage (retail clothing), Chicago, IL
• "Knee High by the Fourth of July", a novel by Jess Lourey