Never heard of such a thing. Can you get some more specific information from this vet? Or...another opinion?
I'm assuming you didn't get your pup from a decent breeder who can give you some information?
I have Rottweilers; so know a bit about joint issues. Some ideas for maintenance (bear in mind I have no clue what your vet is referring to.)
*Glucosamine/Chrondriotin supplements are an EXCELLENT idea for dogs with joint/connective tissue problems. Daily. For life. Absolutely no side effects. Can make an amazing difference, speaking from experience. Do this!
*Keep her very lean. Not an ounce opf extra fat. This will go a huge way in helping her grow stroing joints, and will help delay offset of early arthritis.
*Good diet. I feed raw, but there are many excellent diets available & it's unlikely you will find any decent kibble in your grocery store or at your vets. There's been a couple of recent discussions on dog food, check them out.
*Lots of low impact exercise, minimize jumping & such for at least another 6 months as bones are still growing. Swimming when possible is excellent. Strong muscles will compensate for joint/connective weaknesses.
Just some ideas; bear in mind I'm not a vet & really don't know what this "slack ligament" thing is. If your vet is unable or unwilling to be more specific or give you any ideas (like I just did) I'd find another vet. All vets are not created equal.