Ruptured anterior cruciate ligaments are the bane of hard-charging retrievers. I've been there with a lab and two flat-coats. I had to put the lab down after the other knee went. This was in 1979 when knee surgeries were generally a failure.
In the early 90's Dr.Barclay Slocum (now deceased) of Eugene created and patented the plateau leveling osteotomy procedure that basicly reconstructed the knee joint so that it didn't need an anterior cruciate ligament. He performed the procedure on my 6 year old flat-coat and she did fine after an extended recovery period of about 6 months. Barclay's surgical protege, Richard Howard of the Raleigh Hills Vet Clinic performed the same operation on my 5 year old flatcoat with excellent results.

Dr. Howard is a SUPERB surgeon and gives you the straight scoop on the prognosis. I can recommend him very highly.
Several vets are certified to perform the procedure in the Portland area these days. The procedure is less invasive than it was 10 years ago and most dogs 7-8 years old that are in good health should do fine if the post op regimen is followed to a "t". :smile:
Good luck - "flatcoat"