Health & Testing
Patellar Luxation in Toy Breeds: Grading, Genetics, and What Buyers Must Know
Patellar luxation affects an estimated 7% of dogs overall — but is dramatically more common in toy breeds. Here's the grading system and why OFA patella clearance matters.
April 20, 2024 · 5 min read
What Is Patellar Luxation?
The patella (kneecap) sits in a groove (trochlear sulcus) at the end of the femur. In patellar luxation, the kneecap slips out of this groove — either medially (inward, most common in toy breeds) or laterally (outward, more common in large breeds). Medial patellar luxation (MPL) is extremely common in Yorkshire Terriers, Toy Poodles, Pomeranians, Chihuahuas, and other toy breeds.
The Grading Scale
- Grade 0: Normal — patella does not luxate.
- Grade I: Patella can be manually luxated but returns spontaneously. Usually no clinical signs.
- Grade II: Patella luxates spontaneously during movement. Dog may skip or hold leg up briefly. Moderate concern for breeding.
- Grade III: Patella remains luxated most of the time. Can be manually reduced but re-luxates. Significant lameness possible.
- Grade IV: Permanent luxation — patella cannot be manually repositioned. Usually causes severe lameness. Surgery required.
OFA Patella Testing
OFA patella evaluation is a physical examination by a licensed veterinarian. Dogs grades 0 or I are typically considered acceptable for breeding (I should be disclosed). Grades II–IV are considered abnormal. OFA records all submitted results — both normal and abnormal — in their public database when owners consent to disclosure.
The Genetic Component
Patellar luxation has a significant heritable component — it does not behave as a simple Mendelian trait but rather as a complex polygenic condition influenced by multiple genes affecting bone geometry and musculature. Breeding two affected dogs will produce more affected offspring. Selecting breeding dogs with OFA-normal patellas reduces incidence in offspring, though it cannot eliminate it entirely.