Knees. They’re a pain, as my nonna used to say.
Personally, I had no issues with my knees despite being an avid hiker for years, but then a nasty fall skiing back in the winter of 2016 changed all that. I tore my meniscus and parted ways with my ACL. A Denver surgeon made me a replacement one using part of my hamstring tendon, then after two years of hobbling around I had a cleanup surgery to remove scar tissue, some of my meniscus and hardware.
