If Rusty is limping, one possible cause of his lameness could be a torn knee ligament. A number of things can tear knee ligaments in adult dogs. Consider your dog's breed, age and weight -- and see a vet pronto -- to determine whether his limping could be a result of a ligament injury.
Dogs endure a lot of stress on their knees when they run, particularly when they push to the side to make a sharp turn. If Rusty is an active pooch, stress and overuse can contribute to a torn knee ligament. In his 2011 book "Sports Medicine for Hunting Dogs," Dr. Martin Coffman says that athletic dogs are predisposed to knee injuries much like human athletes are.
Rusty's breed could be at the root of his torn knee ligament. Larger dogs and those whose breed specialty is hunting are at the top of WebMD's list of dogs at risk for torn ligaments: Labrador and golden retrievers, German shepherds and Rottweilers. Boxers are another large breed who tend to tear their knee ligaments. Poodles and bichon frises are smaller breeds who are inclined to sustaining knee injuries.
Mature Bone Structure
You would think that older dogs are more likely to develop knee problems. It's not so much that the ligaments are worn, although overuse is one cause. The issue is that younger dogs don't have as much of a problem because their bones aren't developed and mature enough to supply enough leverage to tear ligaments. Usually knee ligament tears occur to dogs older than 5 years.
Dogs who are overweight put a lot of stress on their knee joints and ligaments. When they get up from lying down, or when they climb stairs, run or jump, their body weight pounds on their knees. This can cause the ligaments to tear slowly, gradually worsening. It starts out as a small tear that degenerates with use. Early on, Rusty may limp. Eventually, the dog might not be able to use his injured leg at all.
The Neutered Angle
In his article "The Pros and Cons of Neutering a Dog" for DogChannel.com, veterinarian Jon Geller lists an unusual negative aspect of neutering: Dogs who were "fixed" before they were 5 months old are more likely to tear their knee ligaments. Dr. Geller says some researchers think neutered dogs have weaker ligaments with a greater tendency to rupture.