Hypothesis 3 - Developmental

Congenital Luxation of the Patella

Congenital luxation of the patella may result from either malformation of the femoral trochlea or poor alignment between the distal femur and the proximal tibia. A shallow trochlear ridge will allow medial patellar luxation to occur.   Luxation may also be caused by the rotation of the proximal extremity of the tibia, which displaces the tibial tuberosity medially.  Any one of these or combination can be responsible for the congenital luxation of the patella.

Abnormal angulation of the distal femur may be seen because the patellar straight ligament has been out of line with the trochlear groove.  This abnormal angulation is more common in small breed dogs such as Pomeranians, in which patellar luxation is a common finding.  The luxation is almost always medial.  Clinically affected animals carry the limb intermittently. The shallow trochlear ridge will allow the patella to luxate and resolve spontaneously, resulting in an intermittent lameness.  Palpation reveals the luxation when the limb is held in extension.  The patella can be equally displaced manually on examination.  Dogs may even resent this maneuver.

The reason the clinical signs have appeared so late in life is due to the traumatic rupture of the cranial cruciate ligament, which precipitates the signs of lameness.  In fact, the presence of the medially luxating patella combined with the fact that Titan is an overweight dog, has made him more vulnerable to stifle injury.  This is witnessed in the pronounced injury suffered when the dog jumped off the bed.