The pope's doctor had "prescribed a period of greater rest for his leg" because of "acute gonalgia", or knee pain, the Vatican explained in a note.

The note did not specify whether the problem related to Francis's previous difficulties with sciatica, a chronic nerve condition that causes back, hip and leg pain, makes him walk with a limp, and has forced him to cancel or modify high-profile appearances in the past.

That condition, which Francis has called his "troublesome guest," also makes it difficult for him to stand for long periods.

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