io6 THE SURGICAL ANATOMY OF THE HORSE 



Weakness of the internal lateral ligament of the patella or any- 

 thing which favours or contributes to its rupture, will, therefore, be a 

 predisposing or contributory cause of external lateral luxation of the 

 patella. 



The quadriceps extensor cruris muscles act upon the patella by 

 pulling it upwards on the trochlea. When the patella is displaced the 

 mechanism of these muscles is subjected to considerable interference, and 

 consequently symptoms are presented which are not very unlike some of 

 those present in crural paralysis. To a slight extent, however, the animal 

 is still occasionally able to extend the stifle, but it will be evident that the 

 amount of space through which the foot can be drawn forward is 

 materially diminished on account of the lack of leverage at the stifle, 

 i.e., the stride becomes very much shortened, but usually the luxated limb 

 is projected backwards and fixed from the stifle downwards and the foot 

 rests on the toe. 



Diagnosis of lateral luxation of the patella is not difficult, since the 

 displaced patella may be easily felt, and in some cases even seen. Lame- 

 ness, of course, exists, and as in the case of any other inability at the 

 stifle, all the joints inferior to it become affected. The patellar 

 capsular ligament bulges out prominently below the stifle. 



To replace the patella the horse should be made to move suddenly 

 forward, the operator meanwhile pushing the bone upwards and in- 

 wards. Another method is to attach a sideline to the pastern of the 

 affected limb and lead the free end of it through a collar on the animal's 

 neck. The assistant should endeavour to draw the leg forwards by 

 pulling at the sideline, whilst the operator pushes the patella upwards 

 and inwards as before. 



It is an easy matter to return the bone to its normal position, but 

 much trouble arises in the prevention of recurrence of the luxation. 

 This is due to the stretching of the ligaments which has necessarily 

 occurred to admit of the bone being displaced in the first instance. As 

 a precaution, some operators who use the sideline in reducing the 



