UPWARD LUXATION OF THE PATELLA. 
88 
to such injuries. In cows, Sand noticed spontaneous dislocation of the 
tibia following partial destruction of the interarticular cartilages, in 
consequence of chronic gonitis. 
II.—DISPLACEMENT OF THE PATELLA. LUXATIO 
PATELLAE. 
The patella is retained in position by a capsular ligament strengthened 
by two lateral ligaments, and by its straight ligaments, of which the 
horse and ox possess three; most other animals, however, only one. 
Finally, the pull of the triceps extensor cruris and tensor vaginal femoris 
muscles sustains the patella above. Horses, oxen, and dogs are the 
commonest sufferers, and the luxation is either— 
(a) Upwards, over the internal lip of the trochlea of the femur; or 
(b) To the side, and always to the outer side. 
The two conditions, however, differ not only in their symptoms, but also 
in their prognosis and treatment, and therefore will receive separate 
consideration. 
(a) Luxation of the patella upwards, which occurs in oxen and horses, 
and has been seen by Olivers in a mule, is, strictly speaking, onty a 
displacement of the patella within its articular capsule, the bone passing 
upwards over the internal lip of the trochlea and failing to return. It 
is fixed in this position by its lower border lying against the upper 
margin of the inner lip of the trochlea. This luxation can, therefore, 
only occur after excessive extension of the stifle-joint. Violet has lately 
put forth the theory that the patella is retained on the trochlea of the 
femur by its lateral ligaments, and particularly by its internal lateral 
ligament. The surface of the internal lip of the trochlea describes 
the segment of a circle around the point of insertion of the internal 
lateral ligment into the femur. It can, therefore, easily be imagined 
that under certain circumstances the upper portions of the articular 
surface may lie nearer to the point of insertion of the ligament 
than the portions below. As a consequence the patella might easily 
become fixed in position on that surface of the inner trochlear lip, 
which Violet describes as being concave in the horse, and which in 
oxen is level, and require a very considerable exertion of strength to 
draw it down again. In consequence of this form of the articular 
surface of the internal lip of the trochlea, and of the tension in the 
lateral ligaments, the patella is sometimes arrested for a few moments, 
and, under certain circumstances, for a longer time at the highest point 
in its course, and the action of the limb thus interfered with. The 
condition might, therefore, be looked upon as fixation of the patella in 
a position of excessive extension. It has also been termed subluxation 
or arrest of the patella. 
