214 LAMENESS OF THE HORSE 



horse may be considered; one wliicli is due to the patella becom- 

 ing fixed upon the internal trochlear rim of the femur and the 

 other when the patella slips over the outer rim of the trochlea. 



The first form is known as upward luxation and is made pos- 

 sible by rupture of the mesial (internal) femeropatellar ligament. 

 According to Cadiot and Almy, it is only by the rupture of 

 this ligament — the femeropatellar — that upward luxation may 

 occur. This type of luxation is rarely observed and is usually 

 due to violent strain and abnormal extension of the stifle joint. 



The second class, outward luxation, occurs in colts and is, 

 in many instances, congenital. This form of luxation is also 

 the one usually seen following debilitating diseases such as influ- 

 enza and pneumonia. 



Vpivard luxation of the patella is characterized by the stiff- 

 extended position of the leg. "When the patella is situated 

 upon the inner trochlear rim, the tibia must be extended because 

 of the traction exerted by the straight ligaments. Since the 

 stifle and hock joints extend and flex in unison, there is presented 

 also an extension of the tarsus. Extension of the stifle joint 

 would increase the distance between the femoral origin of the 

 gastrocnemius and its insertion to the summit of fibular tarsal 

 bone (calcis) were it not for the gastrocnemius and superficial 

 flexor (perforatus). Extension of the hock in upward luxation 

 of the patella, permits of flexion of the phalanges. In upward 

 luxation, then, the leg is extended as if too long, but the pha- 

 langes may be in a state of moderate flexion. If the foot rests on 

 the ground when the extremity is not flexed, it is almost impossi- 

 ble for the subject to step backward. Because of immobilization 

 of the stifle and hock joints in upward luxation, the subject can 

 walk only by hopping on the sound leg and then the extremity 

 is flexed, allowing the anterior portion of the fetlock to drag on 

 the ground. 



In some cases practitioners are called to attend young animals 

 that are reported to be "stifled" (often in young mules that 

 have made a rapid growth) and upon arrival the only noticeable 

 symptom of preexisting luxation is the soiled condition of the 

 anterior fetlock region — evidence of its having been dragged. 

 Such cases may be styled momentary luxation, whether they are 



