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.  

Radiographically we would expect find the patella lying to the medial side on a craniocaudal view of the stifle joint.  On the mediolateral view, the patella is absent from the trochlear groove and lies superimposed on the femoral condyles.  A skyline, or tangential, view of the distal femoral trochlea will show the displaced patella and possibly a shallow trochlear groove.  Associated bone abnormalities are frequently evident, including a shallow trochlear groove, rotation of the proximal tibia, curvature and rotation of the proximal tibia, and abnormal angulation of the femorotibial articulation.

Brief Explanation of C-Map For Hypothesis 3:

Lame in Right Hind Leg:  Clinically affected animals carry the limb (lameness)

Hind Right Leg Patella Easily Luxated to medial Side of trochlea Groove when Leg extended:  Palpation reveals the luxation when the limb is held in extension.  The patella can be equally displaced manually on examination. The luxation is almost always medial

On stifle flexion, patella returns to trochlear groove:  This is a common finding on manual palpation of an affected stifle.

Fell off bed, limped with weight bearing lameness on right hind leg:  The presence of a medially luxation patella more vulnerable to stifle injury

Treated with antinflammatories with no effect:  This is a congenital development abnormality which can be treated with antiinflammatories to reolve 

11.6 pounds:  Overweight dogs are predisposed to joint injuries especially with a congenital luxating patella

Pain response on left stifle when cranial drawer performed:  Animals often resent this movement when there is an injury

Pain response on left stifle when patellar luxation attempted:  Dogs may even resent this maneuver.