The patella (kneecap) is a part of the stifle joint (knee). In patellar luxation, the kneecap luxates, or pops out of place, either in a medial or lateral position.
Medial patellar luxation is generally classified by grade, from 1 to 4. Grade 1 luxations usually do not cause lameness or cause very minor signs of lameness but can be found on physical exam because the patella is fairly easy to push out of the groove it normally runs in. Grade 2 luxations are slightly more severe and cause the dog to occasionally carry one hind leg in a motion often described as “skipping”. Grade 3 luxations cause prolonged carrying of the affected leg or obvious lameness and the patella is usually luxated. Grade 4 luxations usually involve visible deformity of the leg and severe lameness and the patella is permanently luxated.
This is one of the main reasons why small dogs should not be allowed to jump on and off of furniture and run the stairs at will. Unless you provide pet steps, If they don’t get up on the sofa by themselves then they should not be allowed to get down by themselves.