TPLO (tibial plateau leveling osteotomy) is a surgery performed when a dog’s cranial cruciate ligament, the knee stabilizer equivalent to the human ACL, fails. The real question behind “what causes TPLO” is: what caused the ligament to break down in the first place? Unlike human ACL tears, which usually happen from a single bad twist during sports, most dogs develop cruciate ligament failure through slow, progressive degeneration. The ligament weakens over months or years until it partially or fully tears, and TPLO becomes the recommended fix.
Degeneration, Not Trauma, Is the Primary Cause
This is the single most important thing to understand: your dog’s ligament probably didn’t snap because of one unlucky jump off the couch. Most canine cruciate ruptures happen secondary to chronic degenerative changes in the ligament itself. The fibers gradually weaken, fray, and lose structural integrity long before the moment you notice your dog limping.
What drives this degeneration? Enzymes inside the ligament break down collagen faster than the body can rebuild it. Dogs with cruciate problems show significantly higher levels of specific proteins that dissolve collagen fibers, far more than what’s found in humans with the same injury. This is why the canine version of cruciate disease looks so different from the human version. It’s less of an accident and more of a slow structural failure.
Joint inflammation plays a major role in accelerating this breakdown. A study examining dogs with still-intact cruciate ligaments found that nearly 45% already had both joint inflammation and early degenerative changes in the ligament. The severity of the inflammation directly correlated with how damaged the ligament was. In other words, a low-grade inflammatory process inside the knee joint is likely chipping away at the ligament well before any symptoms appear. Immune cells, including certain types of white blood cells and both pro- and anti-inflammatory immune cells called macrophages, infiltrate the joint lining and contribute to this progressive damage.
Knee Anatomy That Sets Dogs Up for Failure
A dog’s knee is built differently than a human’s in ways that put constant stress on the cruciate ligament. The top of the shinbone (the tibial plateau) is sloped, so every time a dog bears weight, there’s a forward-sliding force on the joint that the cruciate ligament has to resist. The steeper that slope, the harder the ligament works.
Dogs with a tibial plateau angle above 30 degrees are at higher risk. Some dogs with cruciate disease have angles ranging from 26 to 37 degrees. A steeper angle means more mechanical load on the ligament with every step, which accelerates the degenerative cycle. This is actually the exact problem TPLO surgery corrects: it cuts and rotates the top of the shinbone to flatten that slope, eliminating the forward thrust so the knee stays stable even without a functioning ligament.
The shape of the groove where the ligament sits also matters. A narrower groove between the thigh bone’s lower knobs increases contact and friction on the ligament. That repeated rubbing accelerates collagen breakdown and speeds up degeneration.
Breeds at Higher Risk
Genetics load the gun. Among breeds with large study populations, Rottweilers, Newfoundlands, and Staffordshire Terriers show the highest prevalence of cruciate rupture. Larger, heavier breeds in general face greater risk because their ligaments bear more force with every stride. Interestingly, some large breeds like Basset Hounds and Old English Sheepdogs have relatively low rates, which points to specific skeletal geometry and genetics rather than size alone being the deciding factor.
Dachshunds also rank among the lowest-risk breeds despite their well-known orthopedic vulnerabilities elsewhere. This reinforces that cruciate disease isn’t just about “wear and tear” in a generic sense. It’s about the specific combination of tibial slope, intercondylar notch width, body composition, and inherited inflammatory tendencies that certain breeds carry.
How Body Weight and Obesity Contribute
Excess weight amplifies every mechanical risk factor. A heavier dog generates more forward thrust on an already-sloped tibial plateau, meaning the cruciate ligament absorbs more force per step. Over thousands of steps per day, over years, that extra load compounds the degenerative process. Weight management is one of the few modifiable risk factors owners can directly control.
Early Spaying and Neutering
The timing of spaying or neutering has a measurable impact on joint disease risk, particularly for medium and large dogs. In dogs weighing 20 kg (about 44 lbs) or more, neutering before one year of age is associated with joint disorder rates roughly three times higher than in dogs left intact. One study found that neutering was linked to a threefold increase in excessive tibial plateau angle, the exact structural risk factor for cruciate disease.
The numbers get more dramatic as dogs get larger. For large mixed-breed dogs (30 to 39 kg), intact females had a 0% rate of joint disorders in one study, while females spayed before six months had a 10% rate and those spayed between six and eleven months reached 23%. For giant breeds over 40 kg, males neutered before six months showed joint disorder rates of 28%, compared to 9% in intact males.
For dogs under 20 kg, early neutering did not significantly increase joint disease risk. The hormonal influence appears to matter most in larger dogs, where sex hormones play a bigger role in guiding proper bone growth and joint development during the first year or two of life. Removing those hormones early may result in subtle changes to leg length, joint angles, and tibial slope that increase cruciate stress over time.
Why the Other Knee Often Follows
If your dog has already torn one cruciate ligament, the other knee is at risk. About 19% of dogs who rupture one side go on to tear the opposite ligament, with a median time of roughly 13 months between injuries. The underlying reasons are straightforward: whatever genetic, anatomical, and degenerative factors caused the first knee to fail exist in the second knee too. On top of that, a dog favoring one leg after surgery shifts extra weight to the other, accelerating an already-vulnerable ligament’s decline.
The risk does decrease with age. Dogs who experience their first rupture at eight years or older have a lower rate of contralateral injury than younger dogs, likely because the degenerative process in younger dogs is more aggressive and has more years to progress on the opposite side.
Putting It All Together
TPLO surgery becomes necessary when multiple factors converge. A genetically predisposed dog with a steep tibial plateau, low-grade joint inflammation brewing for months or years, possibly neutered early, and carrying extra weight gradually loses ligament integrity until it partially or fully tears. The dog becomes lame, the joint destabilizes, and arthritis begins setting in quickly. TPLO addresses the mechanical problem by changing the geometry of the knee so it no longer depends on the cruciate ligament for stability. But the root cause is almost always that slow, invisible degeneration rather than a single traumatic event.