Crystal’s (Abbey Lee) calcium deficiency causes the most horrific deterioration scene in the whole movie; her bones crunch and become contorted into hideous and unnatural shapes as they crack and then heal too quickly. It’s a medical condition, sure, but there’s an implicit judgement of Crystal in the background. The beautiful, much-younger wife of Charles is positioned as being overly fond of her looks and as she starts to age and her body lets her down, she hides in a cave in the darkness rather than be with other people.
https://www.denofgeek.com/movies/old-m-night-shyamalans-twist-ending-explained/
