DISLOCATIONS OF THE PATELLA. 123 



the femur leaves the acetabulum, it passes most commonly above 

 the tendon of the obturator internus muscle. Many of the 

 muscles about the joint are torn, but to a different degree in 

 various cases. The gluteus maximus often has its deeper fibres 

 lacerated, and frequently the gluteus medius, pyriformis and 

 even the obturator internus and gemelli are damaged. The ilio- 

 femoral ligament, as has been mentioned, remains intact, 

 although its external or upper fibres will be found very tense. 



The head of the bone being displaced backward, the great 

 trochanter comes to look forwards, and there will be marked 

 shortening of the limb, the actual amount depending upon the 

 distance to which the head of the bone has ascended upon the 

 dorsum ilii. The foot is inverted so that the ball of the great toe 

 rests against the instep of the sound foot, the long axis of the 

 dislocated femur being directed across the lower third of the 

 sound thigh. 



In the reduction of this form of dislocation by manipulation, 

 flexion of the thigh upon the abdomen relaxes the ilio-femoral 

 ligament, and some abduction still further induces this condition. 

 Slight internal rotation disengages the head from behind the 

 acetabulum, then traction upon the limb in the line of the femur 

 brings the head of this bone over the socket, into which it will 

 generally slip. 



Dislocations of the Patella. Again, dislocation of the patella 

 is a rare displacement, the only comparatively common variety of 

 the accident being a dislocation outwards. The line of contrac- 

 tion of the quadriceps muscle has a tendency to drag the patella 

 outwards, seeing that the direction of the large bulk of the fibres 

 of this composite muscle is downwards and inwards. This 

 obliquity of action is naturally considerably increased in cases of 

 genu valgum, in some instances of which the patella may slip 

 outwards over the external condyle whenever the knee is flexed. 



Dislocations at the Knee. The great strength of the liga- 

 ments entering into the knee joint explains the rarity of dis- 

 location at this articulation, and even when it occurs, it is 

 usually but partial. 



