When I was a teenager I had a variety of joint problems, including my knees. Walking down off a mountain one day the weather took a turn for the worse and a strong gust of wind knocked me off my feet and I landed knee-first on a rock. Nothing broken, but couldn’t walk on it. We’d barely seen anyone else that day but it just so happened that a couple were just approaching us as I fell. The wife was a nurse, the husband was ex-special forces.