Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about Missy's rear leg non-weight bearing lameness for the past 5 weeks.
I understand that she is slim and active, which makes a cruciate ligament injury seem unlikely but with her history that injury would make the most sense, especially because on radiographs there is no sign of bone disease. Although she is strong and athletic even professional athletes that train with appropriate professional guidance can suffer cruciate ligament injuries. While it is true we see more cruciate injuries in overweight, poorly conditioned dogs even the best athletes can suffer cruciate ligament damage if they are unlucky and slip or twist the knee just the wrong way during activity. Her short legs and her particular conformation, the angle her knee is set at, can predispose her to a cruciate strain/tear.
Dogs are non-weight bearing lame for a few reasons:
1) It hurts too much to bear weight on the leg
2) neurologic disease such that the pup cannot feel the leg or control the muscles in the leg
3) instability of one of the joints such that the leg either cannot support the pups weight or the dog feels the instability and doesn't feel confident using the leg.
Over time when a dog doesn't bear weight on a leg and learns that she can move quickly on 3 legs it may become a habit to not use the injured leg. So even if healing takes place the loss of muscle while the ligament is healing, and her habit of not needing to use that leg, may interfere with her doing so now.
I am glad to hear that she was weight bearing some yesterday on her walk, that is a positive sign. It may take a while for her to learn to use the leg consistently again. It is important to not push her though to return to full activity before she is ready. Healing can take months. If she resumes full activity too soon she can reinjure herself and lose all of the healing that has taken place.
It may help to have her evaluated by a veterinary physical therapist and learn some exercises that you can do at home with her to help maintain muscle mass and flexibility so she will hopefully be able to get back to normal function eventually.
Some dogs with cruciate ligament tears also suffer meniscal damage. The meniscus is a cartilage cushion in the knee joint. It cannot be seen on radiographs and if it tears it can fold in the joint. This makes bearing weight on the joint painful (like walking on a pebble). This won't heal even with strict rest and anti-inflammatories and will require surgical intervention.
So if she continues to be lame then surgery to remove a damaged meniscus may be needed.
Please let me know if you have any further questions.