4IO 



HUMAN ANATOMY. 



former usually and the latter invariably are incomplete, owing to the large superficial 

 areas of the joint-surfaces. In the great majority of cases dislocations of the knee 

 are due to indirect violence acting through the femur as a lever, — as, for example, in 

 falls forward, the foot and leg being fixed. The weight of the trunk carrying the 

 upper end of the thigh forward, brings the lower end with great power — the fulcrum 

 and the resistance, or weight, being so close to each other — against the posterior 

 ligament, a rupture of which permits the movement to continue and results in an 

 anterior dislocation of the knee, which is, regarded from an etiological stand point, 

 a displacement of the femur backward. 



If the fall is in the opposite direction, the femur may be displaced anteriorly, — 

 i.e., posterior dislocation of the knee may occur. Occasionally the anterior disloca- 

 tion has followed the fall of a weight upon the front of the femur. The application 

 of force to the front of the leg when the knee was flexed has produced a posterior 

 dislocation, the effect of the biceps, popliteus, and semimembranosus in reinforcing 

 the posterior ligament being minimized in that position. 



Lateral dislocations are caused by adduction or abduction of the leg, the thigh 

 being fixed, or by falls sideways when the foot and leg are fixed. The great width 



Fig. 424. 



Patella- 



Vastus internus 

 / 





^ 



Lateral 

 expansion of 

 quadriceps 

 tendon 



Tendon of 

 adductor magnus 



Tibiar^ 



/ 



Internal condyle 



Tendon of sartonus 

 Inner aspect of right knee-joint, showing expansion of quadriceps tendon. 



of the joint and the slight resistance offered by the interposition of the tibial spine 

 between the femoral condyles render them rarer than antero-posterior luxations. 



Forward dislocation is more common, possibly because of the greater laxity 

 of the capsule in front, and is more apt to be complete than the backward. The 

 knee is extended ; the tibial tubercle prominent ; the antero-posterior diameter 

 increased ; the anterior margin of the tibial tuberosities palpable in front ; the 

 rounded condyles may be felt, but less distinctly posteriorly ; the popliteal concavity, 

 is obliterated ; the aponeurotic expansion of the quadriceps is loose and lies in folds 

 about the upper border of the patella. The femoral vessels and nerves may be 

 bruised, compressed, or lacerated. 



In backward dislocation also the knee is in extension and the antero-posterior 

 diameter increased. The displaced bony prominences may be recognized by palpa- 

 tion. This dislocation is even less apt to be complete than the forward variety ; but 

 if it is, the vessels and nerves are oftener injured, as shown by the more frequent 

 occurrence of gangrene. This is probably due to the sharpness and prominence of 

 the backward projection of the upper edge of the tibial tuberosities, as compared 

 with the rounded depressed notch between the femoral condyles which receives the 

 vessels in forward dislocation. 



In lateral dislocation, in accordance with the direction of the displacement, 



