UPWaUD LUXATION OF THE PATELLA. 
089 
Causes and symptoms. It will be seen that dislocation may be due 
to one of two causes, viz.:— 
(1) Flattening of the articular surface of the internal lip of the trochlea 
in conjunction with abnormal tension in the lateral ligaments; or, 
(2) Fixation of the lower surface of the patella on the upper border 
of the inner trochlear lip. In the first case, it is natural that the 
lameness should often be remittent, but only last for a few moments 
on each occasion (habitual luxation). This form may, therefore, be 
termed momentary upward luxation of the patella; and that produced 
by fixation of the under border of the patella on the upper border of 
the internal lip of the trochlea, stationary upward luxation. Movement 
of ail the joints of the limb, with the exception of the liip-joint, is 
dependent, however, on flexion of the stifle, on account of the insertion 
into or around it of the flexor metatarsi, gastrocnemii, extensor pedis 
and flexor pedis perforans muscles; and therefore when the stifle is 
fixed the other joints are at once immobilised in an extended position, 
and flexion and free movement are lost (see fig. 260 in chapter on 
“ Bupture of the Flexor Metatarsi Muscle”)* In fact, any interference 
with the movement of the patella produces lameness: in the case of 
momentary luxation there is a slight check, similar to that seen in 
stringhalt, just as the foot leaves the ground and should begin its 
striding movement. Bassi, indeed, regards stringhalt as due to 
momentary luxation. The peculiar check known as ‘ ‘ straw cramp” is 
often seen in the stable when the animal turns round, sometimes in one, 
but not infrequently in both limbs. As a rule, it gradually disappears 
during work, sometimes only occurring during the first twenty to thirty 
steps. The sudden and excessive flexion of the limb which follows the 
check has produced the .erroneous impression that it is due to cramp in 
the vasti and rectus muscle. It is not uncommon after influenza and 
other debilitating diseases. 
The degree of lameness varies greatly. Sometimes it is necessary to 
look carefully in order to recognise it at all, but in other cases the 
patella is seen to remain fast for a moment and then suddenly descend. 
The symptoms are different when the patella remains continuously 
fixed above the internal lip of the trochlea, i.e., m stationary upward 
luxation. This condition only affects oxen and solipeds, because in 
other animals the internal lip is more rounded off, and fixation of the 
patella above it is impossible. In consequence of violent contraction 
of the vasti and rectus muscles occurring when rising or falling, kicking 
outwards, or extending the limb, the patella passes too far upwards, 
and becomes fixed. Before this can occur, it is probably necessary 
for the straight ligaments of the patella to become elongated, as 
happens in pregnant animals by continual confinement to the stable, 
Y \ r 
v.s. 
