Years ago, I came back to Yoga with a knee injury from poor form in my regular leg work out. Yoga more than rehabilitated my knee, it strengthened the muscles around it and the tracking. I learned to be conscientious of my knee tracking between the large and second toes in poses (such as Warrior) and that I'm not hyper-extending in poses, such as Downward-Facing Dog or even Tadasana. Awareness of proper stacking of bones and activating strong feet, strong calves, strong thighs draws attention to proper knee placement in these poses and I'm now injury free, pain-free in my knees! YAY!
Randale Sehcrest's Animated Tutorial of the Knee gives a comprehensive look at the knee joint and surrounding infrastructure.
David Keil's video Working with the Knee in Lotus offers practical solution for easing into one of yoga's most sought after poses.
Chaz Rough's Yoga for the Knees offers a great sequence (about 30 minutes or so) for strong knees, strong legs.
There is a wealth of information available but for starters check out the rest of my Yoga-The Knee Playlist.
I don't know about you, but as a writer and avid computer junkie I'm always battling wrist issues. Up-dog, down dog, plank, and even simple forward bends can having me cringing. Here are several articles & videos that can help with pain in the wrists.
In addition to these videos, I have found that active fingertips, fists (with thumbs OUT!), blocks, and using forearms can alleviate wrist pain! Maybe your Down-dog can be modified into a Dolphin pose or your plank is done on forearms. Modifications are not disabling, they are empowering! Remember, that the goal of the asana is to reap the fullest energetic benefits of the pose, not to be bendy or look perfect. (Yoga is never perfect, that is why it is called a PRACTICE!)
I'll be updating this page with more relevant information over time, so please be sure to check back often!