Not sure if this has been covered.
Is it genetic? Can it happen twice to the same leg?
Does anyone cover this as part of your gaurantee?

Yes it can happen on a daily or weekly basis. It kind of "pops in and out". They run and in mid stride hold up offending lef, yelp in pain, limp a while and then go back to normal. It is considered genetic. I do cover against this.

I personally have never heard of luxating patellas in Labrador Retrievers being a hereditary condition. Injury-induced I would believe! How common is it?

Not very common at all but it does happen ask a vet and they'll tell you hereditary. Mostly see in small dogs.

There is info on the OFA site about patella luxation, re grading.
A Grade 4 could be from trauma... I think some of what the specialist would be looking at is if the trochlear groove is shallow (if it's shallow, it's been going on for awhile), if it was an acute onset conformation of the other leg.

"if it was an acute onset conformation of the other leg."

.... if it was an acute onset and also look at the conformation of the other leg.

My girl was diagnosed with MPL - medial luxating patellas - in both knees. First knee started luxating at 9 mos. She became lame because her patella luxated so frequently and the groove was so shallow that eventually the ligament holding the patella tore. The patella was to the side of her knee all the time. Surgery at 1 year. Several months later her other knee started luxating. We did conservative management for a while hoping to avoid another surgery, but again the groove was so shallow that the patella luxated frequently and she was unable to manipulate it back into place without assistance. She would stop and wait for one of us to help. I could tell she was enduring pain. My vet recommended surgery again at this point.

Patella surgery has a very high success rate and with labs it's important to find an ortho vet who has experience doing this kind of surgery on larger breeds. I got a great referral from my regular vet to an excellent ortho vet - 3 hour drive from where I live, but well worth it. He did the surgery on both her knees, wedge recession trochleoplasty.

Both the ortho vet and my regular vet said this was congenital. My girl is the only one in the litter affected. I'm not educated in genetics but it's my understanding and my feeling that things like this can happen. Both the parents had all the clearances recommended - there was no history of patella problems in either.

I can't tell you how helpful and supportive my breeder has been. She is not only a wonderful breeder, she's a wonderful person. She seems so proud of this pet quality lab with the scared up knees. We keep in touch regularly and I see her occassionally. I got an email from her when I forgot to send the most recent b'day picture - I felt so bad. I would not hesitate to get another pup from her again.

The surgery wasn't cheap but it was worth every penny. She has 2 good knees now.

what are the odds of the same knee going twice? First time in Aug 2006 and again in April. I would think that it couldn't happen again if repaired correctly. Dog is 4 yrs old when it first happened. Active hunting dog that is in and out of the bush all the time. Only one side has had a problem and was fixed by an ortho vet. Also only one in the litter with a problem and dam has had 4 litters total with no other pups being affected.
Thoughts??

Are you sure it's not a torn cruciate at this point? It has been stated that this injury can happen as a result of having/having had luxating patellas.
From my understanding, once it's fixed, it's fixed because it is the actual bone being repaired, unlike muscle or ligament surgeries that the injury can occur again.