kneepan vs rotula

kneepan

noun

  • The kneecap; the patella. 

rotula

noun

  • The patella; the kneecap. 

  • One of the five radial pieces in the dentary apparatus of the sea urchin. 

How often have the words kneepan and rotula occurred in a corpus of books? (source: Google Ngram Viewer )