Etiology and Occurrence.--This, the most common luxation met with in the equine subject, has been described by writers as existing in many forms. Patellar disarticulation may be more practically considered as _momentary_ and _fixed_, regardless of the position taken by the patella. Described under the title of false luxation are recorded cases wherein the quadriceps (crural) muscles become contracted in such manner that a condition simulating true disarticulation of the patella obtains. Also, some practictioners report cases of patellar luxation and refer to pseudo-luxations, without clearly defining the conditions which constitute pseudo-luxation. This has contributed to the extant cause of misconception as to actual differences between luxation and conditions simulating dislocation. Luxation of the patella is a condition wherein the articular portions of the femur and patella assume abnormal relations whether such displacement of the patella be momentary and capable of spontaneous reduction, or fixed and requiring corrective manipulation. Spasmodic contraction of the crural muscles which sometimes retains the patella in such position that the leg is rigidly extended, does not in itself constitute luxation of the patella; and unless this bone becomes lodged on the upper portion of a femoral condyle or laterally displaced out of its femoral groove, luxation cannot be said to exist in the horse. These are sub-luxations. Occasionally one may observe in suckling colts outward luxation of the patella wherein there is history of navel infection and no marked evidence of rachitis is present. Some of these cases recover. In a unilateral involvement of this kind in a three-month-old mule colt, the author observed a case wherein an unfavorable prognosis was given and destruction of the subject advised, because of the extreme dislocation of the patella. This colt, however, was not destroyed and in three weeks had apparently recovered. No treatment was given in this instance; the colt was allowed the run of a small pasture with its dam and in time it matured, becoming a sound and serviceable animal. Classification.--Two forms of true patellar luxation in the horse may be considered; one which is due to the patella becoming 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 possible 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 influenza and pneumonia.