each case is 'different' so much depends on the conformation and overall health of each dog afflicted with knee weaknesses/injuries.
I do know of a rottie/gsd who had one stifle go and then the other, no surgery was done and she live to almost 13, but did suffer form arthritis for that last few years.
if the injury is a result of a conformational defect, litle can be done other than surgery to repair the damage, if left alone arthritis will develope as the dog ages, also the opposite leg will suffer because of the extra weight it must bare.
google and check out MSM supplememnts for helping with repair to damage ligaments.
Hugz to you and yours.