Slipping Kneecaps Or Patellar Luxation (And Tibial Torsion)
Common in toy breeds, particularly the Yorkshire Terrier, this inherited condition varies in severity and need for treatment. Being born with slightly rotated tibias leads to a slackness to the knee ligaments. This allows the kneecap to slip off, inwards (medially). Usually there is no pain associated with the kneecap slipping (patellar luxation) but once it is off, the leg no longer supports weight normally. An affected dog continues walking or running as if nothing happened but hops along, not bearing weight on the affected hind leg. If the kneecap spontaneously slips back, which often happens, the leg again functions normally and bears weight.
Diagnosis and treatment
The disease is diagnosed by physical examination alone. For many individuals, most in my experience, no treatment is necessary. For others, especially those with the most severe luxations or individuals who have torn a lateral knee ligament, surgical correction is beneficial.