like birdgirl says ,
could be numerous thingz . very high strung energetic dogs have probs. w/ a.c.l's and knees also . the top of a dogs tibula is on a 65deg. angle so the femur kinda slides down the front of the tibula stressing the hell out of the a.c ligiments . humans are about 15 deg. so it's easier for our tibula to carry our body weight . the lil brown -n- white pit in my avatar had to have a t.p.l.o this year cause of it . tibular plane leveling ostemy . 4 mos. in a cage doggy p/t . and we now give her glocosamine everyday . good as new now but a cpl. thousand $ later . lol . when you go get the dog it's shots ask the vet to check it out .
good luck
crispi