Luxation means that a joint is completely dislocated, while subluxation means that the joint is only partially dislocated. Shoulder instability subluxations in dogs happen to the forelegs where the leg meets the scapula, which is above the rib cage.
The dog's shoulder partially dislocates usually because the dog has fallen or received a terrible injury, such as getting hit by a car. Genetics also plays a factor, making some breeds more susceptible than others, according to "Dog Owner's Home Veterinary Handbook."
Veterinary Surgical Associates notes that small and toy-sized breeds like Chihuahuas, Yorkshire terriers or Shetland sheepdogs are most prone to shoulder luxation and shoulder subluxation.
Dogs with shoulder subluxation have sudden lameness, swelling and the affected leg will often be coked in a bizarre angle. The dog may sit in a manner peculiar for that individual dog, according to Veterinary Surgical Associates.
The dog's shoulder needs to have struck some object with great force in order to knock the shoulder partially out of the socket. This may also lead to damage of tendons, cartilage or bone fractures, which can complicate treatment.
Treatment includes pain medication, surgery, wearing a sling and rest. The dog also should have physiotherapy, according to "Dog Owner's Home Veterinary Handbook."