PRACTICAL CONSIDERATIONS : THE HIP-JOINT. 



375 



seen through it (Morris). Fig. 391 represents this diagrammatically. In addition, 



the greater elevation and thickness of the upper and outer rim of the acetabulum, 



and the pressure against the trochanter exerted by the ilio-tibial band of the fascia 



lata (Allis) in adduction of the thigh (which means an outward movement of the 



upper extremity of the femur) , should 



be mentioned among the factors that Fig 



resist displacement. The ligamen- 



tum teres is of little value, as its 



bony attachment to the femoral head 



is easily separated by a force less 



than that required to rupture the 



ligament. 



A line drawn from the anterior 

 spine to the tuber ischii will approxi- 

 mately bisect the acetabulum and 

 will divide each half of the pelvis 

 into two planes, the pubo-ischiatic, 

 inner or anterior, and the ilio-ischi- 

 atic, outer or posterior (Fig. 392). 

 When the head of the femur escapes 

 from the acetabulum it must lie on 

 the surface of one or other of these 

 planes. All dislocations are, there- 

 fore, either (i) outward — i.e., pos- 

 terior — or (2) inward — i.e., anterior. 



I. Outzvcwd or Posterior Luxations. — Traumatisms in which the force is 

 expended upon the region of the hip result, as a rule, in children in epiphyseal sep- 

 aration (page 361), in old persons in fracture of the neck of the femur (page 363). 

 In 173 cases of dislocation of the hip, 138 were between fifteen and forty-five years 

 of age. 



In practically all positions of the hip in which luxation is probable the force 

 acts through some form of leverage which brings the short arm of the lever — always 



the head and neck of the femur — 



Diagram indicating strong and weak portions of capsule of hip- 

 joint. {Allis.) 



Fig. 392. 



Superior spine 



against a weak portion of the cap- 

 sule. If it does this with the aid of 

 a bony fulcrum, the power is ex- 

 erted to the greatest possible ad- 

 vantage. Thus, in hyperextension 

 of the thigh, the acetabular rim acts 

 as a fulcrum, but the head of the 

 bone is brought against the anterior 

 part of the capsule, — the ilio-femoral 

 ligament, — which is usuallystronger 

 than the bone itself. Hyperflexion 

 is arrested by the contact of the 

 soft parts of the front of the thigh 

 with the abdomen ; hyperadduction 

 by the contact of the shaft with the 

 pubes. Hyperabduction, however, 

 brings the greater trochanter against 

 the prominent outer lip of the ace- 

 tabulum, while the head is carried 

 downward against the thin inner 

 and lower part of the capsule ; the 

 ilio-femoral and ischio-femoral liga- 

 ments are relaxed, and the weak pubo-femoral ligament offers but little resistance ; 

 the head, being opposite the shallowest part of the acetabulum, projects half its 

 bulk out of that cavity ; the weight — i.e., the resistance of the capsule — is very 

 close to the fulcrum, greatly increasing the power of the leverage. 



Diagram sliowing dividing line (X, V) between outer and inner 

 pelvic planes. (Allis.) 



