es 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.

_Upward 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).