UPWARD LUXATION OF THE PATELLA. 959 



upper portions of the articular surface may lie nearer to the point 

 of insertion of the ligament than the portions below. As a conse- 

 quence 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 con- 

 siderable exertion of strength to draw it down. Solleysel, Lafosse, 

 Meyer, and many others have attributed the fixation of the patella 

 to cramp, spasm, or tonic contraction of the patellar muscles, including 

 the long vastus. Whatever the explanation may be, there is no 

 doubt that 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 interfered with. The condition 

 may be regarded as fixation of the patella in a position of excessive 

 extension. It has also been termed subluxation or arrest of the 

 patella. 



Causes and symptoms. Upward luxation or arrest of the patella 

 may be due to one of two causes : — (1) flattening of the surface of 

 the inner trochlear lip 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 the lameness is often recurrent, but only lasts 

 for a few moments on each occasion (habitual luxation). This may, 

 therefore, be termed momentary upward luxation of the patella; 

 and that produced by fixation of the lower border of the patella 

 on the upper border of the internal lip of the trochlea, stationary 

 upward luxation. Movement of all the joints of the limb, with the 

 exception of the hip- joint, is dependent, however, on movement of 

 the stifle, 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. 529, and " Rupture 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 stride. 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 flexion of the 

 limb which follows the check has produced the erroneous impression 

 that it is due to cramp in the vasti and rectus muscles. It is not 

 uncommon after influenza and other debilitating diseases. 



