The issue of abnormal development in the absence of gonadal (sex) hormones is the common thread among the studies implicating spay/neuter as a causal factor in the development of hip dysplasia. In the presence of sex hormones, the closure of bony growth plates generally occurs when the dog is between the ages of 4 and 18 months. Spaying or neutering before 6 months, which is common practice in the US, interrupts the production of sex hormones and delays the closure of at least some of the bony plates. Some or all of the bones will grow beyond their genetically determined length, leaving the bones comprising the hip joint mismatched in length. Early spay/neuter significantly increases the occurrence of hip dysplasia.
Beyond the 2007 systematic review 83, a 2008 review 89 of 1.2 million records of dogs at the College of Veterinary Medicine, University of Missouri, concluded castrated male dogs were significantly more likely than other dogs to have hip dysplasia.
The 2014 UC Davis study 61 evaluated the incidence of hip dysplasia in Golden retrievers and Labrador retrievers. In both breeds neutering (or spaying) before 6 months of age significantly increased the occurrence of joint disorders, including hip dysplasia, especially in the golden retrievers.
In 2016, a UC Davis produced study of German Shepherds reveals that in males and females, neutering or spaying within the first year of life is associated with a highly significant, threefold risk of acquiring at least one joint disorder,(i.e., hip dysplasia, cranial cruciate ligament tear, and elbow dysplasia).108