The patella or kneecap is the largest sesamoid bone in the body and one of 4 in the stifle, the other three being the 2 of
the gastrocnemius and the popliteal muscle. The patella lies within the tendon of insertion of the quadriceps muscle. The
part of the tendon between the apex or distal aspect of the patella and its insertion on the tibial tuberosity is called the
patellar ligament. The caudal surface of the ovate shaped sesamoid should articulate with the trochlea or trochlear sulcus
of the distal femur. In animals with patella luxations, the patella displaces from the normal positioning within the trochlea.
Luxations are typically congenital or developmental but can also be traumatic. True traumatic luxation occurs in light of
normal bony anatomy and is the result of disruption of soft tissues and forceful luxation of the patella. Luxation can be
lateral or medial and animals will be very lame and painful initially. If addressed early, surgery may only need to address
the disrupted fascia and joint capsule and further corrective procedures may not be needed if the conformation is normal.
Congenital or developmental patella luxations in small animals occur in both the dog and cat. In dogs, luxations occur in
both large and small breeds although luxations in small breeds are more common. The most common luxation is medial regardless
of the size of the animal. Lateral luxations, although less common, are seen more often in large breed dogs. Since the patella
plays an important role in the entire extensor mechanism, abnormalities along the system can influence the function and position
of the patella and vice versa. Patellar luxations may also be associated with cranial cruciate ligament deficiency, abnormal
coxofemoral joint confirmation such as increased or decreased femoral neck angle (coxo valga/vara) and hip dysplasia, angular
deformity of the femur or tibia (congenital or traumatic), abnormalities of the quadriceps muscle or a patellar ligament that
is too long or short. At times it can be difficult to determine cause and effect. The entire system should be evaluated prior
to delving into surgical correction. Greater than 50% of patients have bilateral luxations.
Most owners will report that the dog uses the leg normally but will periodically skip, hop, kick out the leg or stretch the
leg when the patella luxates. As the disease progresses, the degree or frequency of clinical signs may increase. Each time
the patella luxates there is potential cartilage damage on both the underside of the patella and the trochlea. Dogs with continuous
medially luxated patellas, especially young growing dogs, may have a 'bow-legged' appearance or genu varum. Dogs with continuous
laterally luxated patellas, especially young large breed dogs, may have a 'knock-kneed' appearance or genu valgum. Some pets
and owners have developed an almost symbiotic relationship in regards to patella luxations. When the pet luxates a patella
they may approach the owner or 'signal' to the owner. The owner then massages, stretches or rubs the leg ultimately reducing
the patella. Concurrent rupture of the cranial cruciate ligament can be seen in 15-20% of dogs. These dogs may show significantly
more lameness due to the cruciate tear compared to the luxating patella along with progressive development of osteoarthritis.