Well, after consultations with three vets, weeks and weeks of rest, and x-rays, it has been decided; Penny is to have surgery on her left knee. (One thing I've found out over the course of these stressful weeks is that dogs only have knees on their back legs, which seems logical, as we are so closely related and only have two knees.)
We took a video of Penny with us when we went to the specialist vet clinic today - Advanced Vet Care - but didn't need to show it to the vet. I thought I'd post it here as it might be interesting to other people who see their dog has a limp and would like to see the symptoms.
What I see in the video is that when Penny gets up, she slips on the floor, and I think that might be because she is stiff. When she gets up speed - which we've been doing our best to prevent, I might say - she runs with her two back legs together, like 'bunny hops'. She goes about her normal wake-up routine and, just as we do every morning, we get our hopes up and say, "Oh, it's a miracle, she's recovered during the night". But it never lasts...
By evening, after a day spent mooching around, with no walks except toileting outside, she is stiff and sore - and horrifies us by stumbling and falling over as she leaves the kitchen.
The vet surgeon was great, and spent over an hour examining Penny and talking to us, and then he explained the operation. It's a version of the TPLO, which should return her to full mobility, but not for some months.
Veterinary Orthopedic Sports Medicine Group has clear explanations of the operation and the recovery schedule, so we've printed it out and we'll prepare for the long haul.
Our surgeon told us we can expect Penny to return to full use of her leg - well, not completely full, but it should be so good that we won't notice the difference, and she won't be in pain. It should also prevent the onset of arthritis. Here's hoping!
I'll include a few links that I've found helpful in this difficult time:
Joint Pain in Dogs
Pet Surgery Topics
Dog Knee Injury
Practical Tips for Diagnosing and Treating Meniscal Tears - a discussion paper for vets