MCL Hypertrophy:

I found "hypertrophy" (expansion and thickening) of the 'Medial Collateral Ligament' in the knee.
The rest of the leg was "pretty normal" (Science term for: Pretty normal)
When you find hypertrophy of the MCL - it means that AT SOME POINT and maybe recently but not yesterday: The ACL ligaments were luxated, torn, partially torn.
Or there's been some other semi-recent or old injury to the knee.
What's an MCL?
An MCL is that ligament runs up the inside of the knee. It's a big ligament that takes over when the ACL's cannot do their job very well anymore.
You can feel it. You'll feel a 'walnut' sized lump of hard plastic growing inside the knee. Compare how that feels to the other knee, or to your other dog's knees.
What do you do?
Well there's a surgery to fix a knee - - but YOU DO NOT have to have surgery on an MCL. It's doing an important job.
Why is my dog lame? A knee that's only being held together by a lonesome MCL is easier to re-injure, not-necessarily-severely.
It's "just my bum-knee" and it aches sometimes", or "I twist it just the right way."
Every once in a while a knee won't ever get right. And those need surgery.
But a big MCL is "pretty okay" at supporting a knee in a dog that's not fat. And that's another story.
Keep 'em skinny and you might just get away without surgery.
Those are only $6,000 to $21,000 depending on where you go.
Doc Johnson