430 THE INFERIOR EXTREMITY 



to the manipulations. Subsequently, the plaster case is renewed 

 every three months, and on each occasion the limb is slightly 

 lowered and the amount of lateral rotation similarly decreased. 

 At the end of fourteen to twenty months the limbs should be in 

 a normal position and walking may be resumed. 



(2) TRAUMATIC DISLOCATION of the hip is comparatively rare, 

 owing to the fact that the arrangement of the bones renders 

 the joint osseously strong. For the same reason, fractures are 

 relatively much more frequent. 



In " regular dislocations " the ilio -femoral ligament is not 

 damaged or only its lateral limb is torn. When both limbs 

 of the ilio-femoral ligament are injured, or when portions of the 

 head of the femur or the acetabulum are fractured, the disloca- 

 tion is termed " irregular." 



(a) Regular Dislocations. The tear in the capsule is usually 

 inferior or postero -inferior, and the position which the head of 

 the femur takes up, after passing through it, depends partly 

 on muscular action and partly on the position of the limb 

 relative to the pelvis at the time of the accident. If the injury 

 is produced by a heavy weight falling on the sacrum when the 

 subject is in a stooping attitude with the lower limbs flexed 

 and rotated medially the common position at the time of the 

 accident the head of the femur passes posteriorly on to the 

 dorsum ilii. The rarer anterior dislocation occurs when the 

 limbs are abducted and rotated laterally. 



1. The Sub-acetabular Dislocation is the first stage in all 

 varieties of regular dislocations, but the head seldom remains 

 in this position. On leaving the lower part of the capsule the 

 femoral head at once encounters the obturator externus, and it 

 may pass backwards, either above or below the tendon, or it 

 may pass forwards on to the surface of the muscle. 



2. Dorsal Dislocation. In the commonest form of this 

 variety, the femoral head passes backwards above the obturator 

 externus and then ascends, either tearing through or passing 

 in front of the obturator internus and the piriformis. It comes 

 to rest on the dorsum ilii between the piriformis and the glutseus 

 medius. Owing to the slope of the ilium, the articular head is 

 directed backwards and the greater trochanter forwards, i.e. 

 the limb is inverted, and it is maintained in this position by 

 the tension of the lateral limb of the ilio-femoral ligament. In 

 addition, the thigh is slightly flexed. 



The flexion, medial rotation, shortening, and constrained 



