How is patellar luxation diagnosed?
Examination:
Affected animals exhibit lameness which may vary from a sudden and marked lameness which resolves in a matter of moments (as the patella pops back into its normal position) through to a more persistent lameness with stiffness after rest as a result of secondary arthritis.
Sometimes the patella can be felt or even seen to “flick” back into position when flexing and extending the stifle joint. When a veterinarian is examining a dog with patellar luxation he/she will want to know whether the patella is luxating medially or laterally and how readily it luxates. Various grading schemes exist for patellar luxation. The system we find most useful lists four grades.
Grade 1 In these cases the patella can be pushed out of the groove but immediately pops back into the sulcus when released. This type of luxation doesn’t occur with normal use of the limb.
Grade 2 This seems to be the most common grade of patella luxation. The patella is situated within the groove most of the time but will periodically move out of the groove (luxate). This usually occurs during exercise and may not be associated with obvious clinical signs. It may also become apparent when grooming or lifting the leg up to wash/dry a hind paw for example.
Grade 3 In these cases the patella is displaced out of the groove for the majority of time but it can be manipulated back into the groove although it does not to stay there as the joint is flexed/extended.
Grade 4 These are the most severe cases and fortunately are the rarest. The patella is permanently luxated and cannot be manipulated back into the groove. These are the most challenging cases to correct and may be associated with quite marked bony deformities of the affected limb.
Manipulation will also allow appreciation of any “grinding” (or “crepitus”) that may be associated with the luxation. This indicates cartilage loss on the back of the patella or over the ridge of the groove. This may have an affect on prognosis as it is associated with arthritis.
Some dogs will suffer from concurrent cranial cruciate ligament failure and this will be assessed at the time of consultation and later when the dog is asleep. This may only be confirmed at surgery. Corrective surgery for these cases is a little more complicated than for patellar luxation alone.
Radiography
Radiography is commonly performed following clinical examination in order to evaluate the bony conformation of the hindlimbs and to assess the stifle joints in more detail.