What is it?
Patellar luxation (dislocating knee cap) is a common cause of lameness in dogs. It tends to be a developmental problem of limb misalignment but can occur secondary to trauma. Toy and small breeds (e.g. terriers, poodles) are most commonly affected but it also occurs in larger breeds as well as cats. The patella most commonly gets displaced towards the inside (medial patellar luxation), but can also become displaced towards the outside (lateral patellar luxation). The displacement can be partial, complete, continuous or intermittent. It often affects both knee joints.
The patella (knee cap) is a small ovoid sesamoid bone that is an important structural part of the stifle (knee joint). It sits in the patellar tendon which attaches it to the strong thigh muscles and glides within a groove in the femur (thigh bone). If this groove is too shallow or there is misalignment of the bones of the back leg (common in smaller breeds) then this can allow the patella to dislocate.
Clinically dogs may skip when running, or a pop may be felt or heard in the stifle or the dog may hold its leg up with the knee bent (patella dislocated) then stretch it behind them (patella relocates); this may be accompanied by vocalisation. Patellar luxations are graded in severity from grade 1 to 4 with the latter being most severe.
Grade 1 luxations can often be managed without surgery. Grade 2 luxations can sometimes be managed without surgery. Grade 3 and 4 usually require surgery to make the dog comfortable. In general, surgery is recommended for dogs that are lame regardless of grading. Surgery usually consists of three procedures:
Possible post operative complications include implant failure, implant loosening, and reluxation of the patella.
After the surgery the patient needs to be strictly rested for six weeks to allow the soft tissues and bones to heal. Physiotherapy and hydrotherapy is recommended to speed recovery. Most patients regain near normal function of their knee joint though this may take up to 3-6 months.