David Dycus, DVM, MS, CCRP, DACVS Dr. David Dycus is director and chief of orthopedic surgery at Nexus Veterinary Bone & Joint Center and medical director of Nexus Veterinary Specialists. He lectures locally, nationally and internationally, has published numerous research articles, has authored or co-authored several book chapters, and is co-editor of Complications in Canine Cranial Cruciate Ligament Surgery. At Nexus, his focus is total joint replacement, complex and minimally invasive fracture repair, angular limb deformity correction, 3D implant printing, and arthroscopy.

As a veterinary orthopedic surgeon, I have done a lot of stifle surgeries. At one point in my career, I started to notice something interesting. My casual observation led to a formal research undertaking that permanently affected my views of spaying and neutering young dogs. Here's how it all came about.

The origin of our study

In tibial plateau leveling osteotomy (TPLO), one of the most common procedures veterinary orthopedists perform, we measure the tibial plateau angle to figure out how steep it is. In human knees, the tibial plateau is relatively flat, so when we stand upright, there's not much load on our anterior cruciate ligament (ACL). But think about a football or baseball player who's crouched down in an athletic position. They're putting a tremendous load on the ACL — and that's how dogs live their lives.

As I measured these tibial plateau angles year after year, I began to recognize that many were higher than what had previously been reported in the literature. Those studies were from the '70s, '80s and early '90s, and some were also from other countries.

To investigate this phenomenon more carefully, we asked a student intern to compile a massive data sheet of dogs that had undergone TPLO. First we looked at whether their tibial plateau angles truly were higher than what had previously been reported, which was around 25 or 26 degrees. Our average tibial plateau angle was 29 degrees — outside the standard deviation of the other studies. A higher tibial plateau angle in almost 4,000 dogs was important as it pertained to us doing surgery. But our next question was, why? Why were our 4,000 dogs different from what had been reported in the literature?

Some might speculate that in Maryland and the Northeast part of the United States, dogs are bred with steeper tibial plateau slopes. But we had cases from almost all 50 states, so we felt they were fairly representative of the total population.

Next we looked at sexual alteration. We pulled the cases involving intact dogs and looked at their tibial plateau angle, then we pulled cases involving dogs that had been sexually altered. We found that the dogs that had been altered had a significantly higher tibial plateau angle than the intact dogs. And the intact dogs' lower tibial plateau angle was in line with the original studies and some studies from Europe. We published our findings in 2020.¹

Implications of our research

So what can we take away from this research? First and foremost, the tibial plateau angle in altered dogs may be higher because spaying and neutering before skeletal maturity causes delayed closure of the growth plates. We know that in earlier decades in the United States, ideas about spaying and neutering were different from what they are now, with alteration likely to happen much later in a dog's life if it happened at all. And there are many parts of Europe where spaying and neutering are rare.

I'm drawing some conclusions that are not definitively proved in our study, but the rough comparisons are still interesting. The biggest limitation those in favor of early sterilization might point out is that we didn't know the age at which our dogs were spayed and neutered. That's true. When they came to us, most of these dogs were adults. However, our philosophy in the United States is early spay-neuter, so I think it's fair to deduce that many, if not most, of our dogs were altered before skeletal maturity.

I also realize there's a difference between a shelter medicine approach to spay-neuter in preventing overpopulation and non-shelter medicine principles focusing on health of the individual animal. I don't think it's realistic to keep shelter animals intact until they're skeletally mature, then sexually alter them and adopt them out. But when we're talking about client-owned puppies, sexual alteration very early in life is not the best decision, at least from an orthopedic standpoint.

Delayed growth plate closure and potential arthritic changes

With delayed growth plate closure, we know animals get a little taller and lankier. If there's an issue during development — let's say the distal ulnar growth plate is damaged — and we still have substantial growth left, that can precipitate severe angular limb deformities that load the joints atypically, which leads to arthritic changes. Premature closure of the distal ulnar growth plate can also lead to subluxation of the elbow or the carpus, resulting in a very painful joint that, unfortunately, we can't do much about.

The other factor to consider is hormonal changes induced by sexual alteration — changes we don't completely understand. What do these hormonal changes do to the homeostasis of the joint while it's developing? Could they cause alterations in joint loading even apart from delayed growth plate closure? In dogs, arthritic changes are always secondary to something; they don't develop just because dogs get old.

Most developmental changes that cause arthritic issues come from a genetic basis of disease. We also know that issues like hip dysplasia and developmental elbow disease are multifactorial in origin. Genetics play a role, but there are also environmental cues. One avenue to be explored is whether early spaying or neutering can push a dog that's genetically susceptible to hip laxity or hip dysplasia into clinically relevant disease.

Something else to consider is the effect of hormonal changes on basal metabolic rate and calorie expenditure. Do those changes play a role in altering dogs' body condition? We know that in the U.S., about 60% of adult dogs are obese or overweight. That itself is a massive risk factor for the development of arthritic changes, because, again, we're putting more load onto the joints.

One can also theorize that delaying growth plate closure may lead to conformational changes, resulting in a different conformation from what that dog would have had if left sexually intact and able to develop normally. Could those conformational differences ultimately result in arthritic changes? Conformation plays an important role in joint loading. A Labrador's conformation is very different from a corgi's, and there are variations within breeds. We don't talk about conformation very much, but we should.

Complexities and questions

Of course, an immense number of questions come up when we talk about spaying and neutering. For example, if we decide it's best to avoid early spaying and neutering, what's the appropriate timeline? Should males be the same as females? Should small breed dogs be the same as large breed dogs? A number of questions need to be answered before we can make definitive recommendations.

When it comes to early spaying and neutering, there are valid arguments on both sides. I believe common ground exists; we just need to figure out what it is. Of course, change is difficult, especially a change of mindset or philosophy. Anytime we try to bring that about, it's an uphill battle.

We also have to take into account behavioral issues, especially with male dogs. We would be giving dog owners a lot of leeway by assuming that they won't let their dog won't run around the neighborhood and that they'll actually follow through with spaying and neutering when we know from a developmental standpoint that it's safe to do so.

"When it comes to early spaying and neutering, there are valid arguments on both sides. I believe common ground exists; we just need to figure out what it is."

My current recommendations

Some of my recommendations to dog owners are backed in science. And some are backed in experience; I call these "Davidisms." Across the board, with males and females of all breeds, I would like to see skeletal maturity before spaying or neutering. Here's how to tell: One of the last growth plates to close is that of the tibial tuberosity, and snapping a lateral radiograph of the stifles will tell you whether it's closed. In almost every population of dogs, I feel confident spaying or neutering if that plate is closed.

If it's not closed and the dog is female, I'm OK to spay if the first two heat cycles have occurred. I know the argument that mammary neoplasia risk is higher in dogs that experience one or more heat cycles. But, honestly, that study is outdated.² I'd love to have oncologists look into it more closely. Regardless, I don't lose sleep if a dog has gone through a heat cycle or two and is still intact. For male dogs, it's a matter of controlling behavior. If owners are comfortable with that and the dog's not marking up their walls, then let's for sure wait until skeletal maturity.

I'll be honest — and some will completely disagree with me on this — but realistically we have no medical reason to castrate a male dog. If it develops a testicular tumor, OK, you remove the testicle and it's essentially cured. The argument of increased prostatic cancer incidence has been debunked. If and when prostatitis and enlarged prostate occur, they can be managed as well with castration. So if a dog can be controlled from a behavioral standpoint, and the owner's compliant, one could argue there's no need to remove a dog's testicles. There is an argument to be made for spaying a female dog — obviously, malignant mammary neoplasia is problematic. But I still support waiting until she's skeletally mature.

"[Some] will completely disagree with me on this — but realistically we have no medical reason to castrate a male dog."

Again, this is a mindset change, and the question is whether we're willing to undergo that change. Ultimately, it may be clients who end up turning the tide. Dog owners are learning about this issue and doing research as best they can. I see many younger dogs with developmental orthopedic issues and their owners ask, "If we had kept our dog intact, would this have been an issue?" And I don't know. Maybe so. When I see dogs that are intact, it's because owners have done their research. And they're waiting for skeletal maturity before they spay or neuter.

1. Fox EA, Dycus DL, Leasure CS, et al. Average tibial plateau angle of 3,922 stifles undergoing surgical stabilization for cranial cruciate ligament rupture. Vet Comp Orthop Traumatol. 2020;33(3):167-173.
2. Schneider R, Dorn CR, Taylor DON. Factors influencing canine mammary cancer development and postsurgical survival. J Natl Cancer Inst. 1969;43:1249-1261.