The four grades of developmental dislocation of the canine patella, also known as patellar luxation:

Grade I: The patella can be manually dislocated but returns to a normal position when released. Affected dogs aren't lame.

Grade II: The patella dislocates spontaneously when the knee is flexed or extended. Reduction is possible by extending the knee or applying lateral pressure to the patella. A torsional deformity of the tibia is typical. Some dogs with grade II patellar luxation are lame and some aren't.

Grade III: The patella is permanently dislocated but can be manually reduced. It spontaneously dislocates if lateral pressure is released. Severe bony deformities are typical.

Grade IV: The patella is permanently dislocated and cannot be reduced.

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