I have never bought a puppy with a health guarantee or on a contract will never do so. I find it difficult to guarantee something that I have no control over. So, if you gave a guarantee to the buyer - you should honor it. If you did not, you have to make the decision of whether to pay for the surgery or not. The article linked above stated that the following may cause OCD "trauma to the joint, genetics, rapid growth, hormone imbalances, and nutrition" Using an unscientific method where the causes are not weighted, we can say there is a 20% chance that it is genetics related. Let’s look at the factors that are probably more prevalent. Rapid growth, Hormone imbalances, nutrition, and trauma to the joints - in my opinion are all associated with having a large breed active molosser. They play rough and trauma is probable. Nutrition - don't know what they fed, rapid growth - duhhhh --- hormone imbalances - whatever!. So, the pup is now 11 months old and for the past 9 months or so the owners were in charge of 60% of the possible causes (not counting hormone imbalances and genetics). I would not pay for anything in this case. You may offer to give them their money and take the dog back. Anything else would be very generous. Dogs are not perfect and there are usually problems regardless of how carefully the breeding was constructed. Wikipedia had this to say. Despite much research, the causes remain unclear but include repetitive physical trauma, ischemia (restriction of blood flow), hereditary and endocrine factors, avascular necrosis (loss of blood flow), rapid growth, deficiencies and imbalances in the ratio of calcium to phosphorus, and anomalies of bone formation. Although the name "osteochondritis" implies inflammation, the lack of inflammatory cells in histological examination suggests a non-inflammatory cause. It is thought that repetitive microtrauma, which leads to microfractures and sometimes an interruption of blood supply to the subchondral bone, may cause subsequent localized loss of blood supply or alteration of growth. Trauma, rather than avascular necrosis, is thought to cause osteochondritis dissecans in juveniles. In adults, trauma is thought to be the main or perhaps the sole cause, and may be endogenous, exogenous or both. The incidence of repetitive strain injury in young athletes is on the rise and accounts for a significant number of visits to primary care; this reinforces the theory that OCD may be associated with increased participation in sports and subsequent trauma. Good luck.