LUXATION OF THE PATELLA. 59 



230ssible that the patella may be caught on the summit of the internal lip 

 of the trochlea, especially if the trochlea happens to present a flattening 

 at that point. 



Outward luxation may be spontaneous or accidental. It is termed 

 spontaneous, when produced by relaxation of the ligaments of attachment 

 or by irregular muscular action ; accidental, when resulting directly from 

 any external mechanical cause.* Violent contraction of the triceps cruralis, 

 by lifting the patella beyond its normal limit of travel, helps, or at least 

 permits, the patella to be displaced outwardly at the moment when the 

 muscle relaxes. 



Pathological relaxation of the ligaments and muscles, by allowing the 

 patella to descend too far on the trochlea, also renders displacement 

 possible, hence spontaneous luxation sometimes occurs even while the 

 animal is at rest in the stable. This luxation is certainly only of a 

 temporary character, or perhaps only of the nature of subluxation, and 

 is often reduced by mere muscular contraction when the animals are 

 forced to move. 



Should the hind limb slip in a backward direction the angle of the 

 joint becomes more obtuse and the lips of the trochlea are turned down- 

 wards, thus greatly favouring lateral displacement of the patella, which 

 under these circumstances is no longer immobilised on the trochlear 

 pulley; displacement outwards then occurs, constituting the condition 

 termed luxation. 



Various forms of mechanical violence, like blows, collisions of the stifle 

 with the jambs of doors, falls, etc., may also bring about this luxation. 



The symptoms of fully-developed accidental luxation are charac- 

 teristic. Immediately the accident occurs, the limb is immobilised in a 

 state of complete extension ; neither the stifle nor the hock joint can be 

 flexed, and only the fetlock joint retains any degree of mobility. 



Movement is very difficult. The hind limb appears rigid, as though 

 formed of one bone. The pastern is directed backwards and dragged 

 along the ground, and when weight is placed on the limb the anterior 

 surface of the pastern may almost be in touch with the ground. The 

 limb is advanced, but the foot cannot be placed properly on the ground. 



Locally the patella is found to be outside the external lip of the 

 trochlea, and its internal ligaments are extremely tense. 



If the luxation is spontaneous and of muscular origin, or a consequence 

 of relaxation of the tendons, it is usually noticeable immediately the 

 animal leaves the stall. The animal cannot move without great difficulty. 

 It grows steadily worse with the lapse of time, because the synovial mem- 

 brane becomes irritated and chronic arthritis is set up. 



The symptoms are identical with those of traumatic luxation, but are 

 only temporary. 



