In 2007, a study 67 was conducted at the Veterinary School of Colorado State University whose objective was to identify risk factors for development of excessive tibial plateau angle (TPA) in large-breed dogs with cranial cruciate ligament disease. Excessive TPA is a known risk factor for the development of cranial cruciate ligament tears or rupture. This study documented a threefold increase in excessive TPA for dogs neutered (or spayed) early.
The explanation is fairly simple. The long bones (e.g. the tibia in the knee joint) have a growth plate which closes (and terminates growth) when the bone has grown to its genetically determined proper length. The closure of bony growth plates generally occurs when the dog is between the ages of 4 and 18 months and is regulated by sex hormones. Spay/neuter drastically alters the sex hormones in female and male dogs at least for a time after the surgery. It is well documented that if a dog is spayed or neutered before the growth plates in the long bones at the elbow, knee or hip joint have closed, the closure of the growth plates will be delayed.61 The variability with respect to dogs developing excessive TPA is dependent upon the time frame chosen for the spay/neuter and the actual genetically determined time frame for closure of growth plates in the individual dog. Additionally we have the variable ability of each dog to compensate for the loss of the sex hormones. Depending upon the length of delay, the long bones continue to grow longer – beyond their genetically determined length. In the knee, the longer tibia creates an excessive tibial plateau angle (TPA).
UC Davis has produced a series of studies wherein they combined the incidence of all three joint disorders that have shown evidence of being increased by spay/neuter (i.e., hip dysplasia, cranial cruciate ligament tear, and elbow dysplasia) for one data-point representing the incidence of dogs diagnosed with at least one of the joint disorders.
A 2014 UC Davis study revealed that in Labrador Retrievers, spay/neuter at less than 6 months doubled the incidence of one of the three joint disorders, (including CCL tear) in both sexes. In the same study Golden Retrievers spayed/neutered at less than 6 months had an increased incidence of joint disorders, including CCL tear, to 4-5 times that of intact dogs.61
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, with CCL tear being the most likely to occur.108
As a practical matter, young, active dogs will present with a CCL tear, but repair of the ligament will be short-lived without a modification of the TPA. Enter the tibial plateau leveling osteotomy (TPLO). For those of you who have gone through a TPLO procedure with your dog you know the outcome is generally excellent, however the initial injury and recovery are quite painful and your dog will have 8 weeks of downtime per knee. It follows the second knee will have the same problem at some future date because the second knee would have an excessive TPA as well. Unfortunately, the TPLO surgery does not prevent arthritis in the repaired joint, and in fact it has been shown arthritic changes start almost immediately after surgery.64 Further, the expense is significant, coming in at about $5,000.00/knee.
It is troubling that the root cause of our dog’s CCL tear was never revealed to us by our primary care vet or the orthopedic surgeon. It concerns us that failure to acknowledge the cause of CCL tear and other orthopedic problems is what allows the practice of early spay/neuter to persist.