Lameness, or limping, is one reason a pet owner would seek a veterinary consultation. The causes are myriad, though some conditions are certainly over-represented.
A partial or complete tear of the cranial cruciate ligament (CCL), called the anterior cruciate ligament (or ACL) in humans, is one of the commonest clinical conditions seen in dogs with lameness involving a hindlimb.
The cranial cruciate ligament (from the Latin crux, meaning to cross) and the caudal cruciate ligament cross each other in the knee or stifle joint (see image below). Basically, they provide major stability to the joint. In humans, the ACL is often torn acutely from a injury. It is now thought the vast majority of CCL ruptures in dogs occur over time, from chronic instability, arthritis, injuries, or a combination of all of these. Partial tears are extremely common and essentially require the same surgical treatment, though long-term studies looking at orthotics/braces to prevent full CCL rupture in dogs with partial CCL ruptures have not been done (as far as I know).
Lameness in young, large-breed dogs, where the lameness "travels" from leg to leg, can often be attributed to panosteitis, a painful though relatively benign pathology of the bones - these dogs eventually grow out of it, the vast majority by a year of age.
Hip dysplasia, a genetically inherited disease of large breeds, is often diagnosed on the basis of pain attributed to the coxo-femoral joints (or hips), and radiographic evidence of this disease. It is treated surgically in younger dogs with severe disease and is managed with medication, mild-moderate exercise restriction, and rehabilitation in both surgical and non-surgical cases.
It must not be forgotten that neuropathies are often the source of limping in cats and dogs. This means that the cause is neurological, stemming from pathology of the central nervous system such as the spinal cord (e.g., intervertebral disc disease), or a peripheral neuropathy. See below for what a blown disc looks like - ouch.
Interverterbral disc disease is invariably associated with spinal pain, while acute unilateral lameness without spinal pain is commonly attributed to fibrocartilaginous embolism, a syndrome where a tiny fragment believed to be from a disc is caught in the tiny blood vessels of the spinal cord, and often causes severe lameness, though the prognosis of this syndrome in the vast majority of cases is actually very good!
Broken nails, splinters, abrasions, diseases of the nails and nailbeds, joint and muscle disorders, Lyme disease, and other infections, are other causes of lameness in dogs and cats.
Whatever the cause of lameness, in virtually all cases, the vet should be trotting the dog down a hallway or outside to evaluate the dog's gait. With a cat, the vet can omit the trotting.
Dog carts can be used for patients with severe hindlimb disabilities, though not this type of cart. Love, love, this photo.