THE KNEE-JOINT 



293 



the flexed position, the posterior part of the articular surface of the tibia is in contact with the 

 rounded back part of the femoral condyles; in the semiflexed position the middle parts of the 

 tibial facets articulate with the anterior rounded part of the condyles; while in the fully extended 

 position the anterior and middle parts of the tibial facets are in contact with the anterior flat- 

 tened portion of the condyles. So with the patella: in extreme flexion the medial articular facet 

 rests on the lateral part of the medial condyle of the femur; in flexion the upper pair of facets 

 rests on the lower part of the trochlear surface of the femur; in mid-flexion the middle pair 

 rests on the middle of the trochlear surface; while in extension the lower pair of facets on the 

 patella rests on the upper portion of the trochlear surface of the femur. 



Fig. 325. — The Collateral Ligaments of the Knee in Flexion and Extension. 



This difference may be described as the shifting of the points of contact of the articular 



fill 1*1 fl Pf-* 



2. it differs from a true hinge in that, in passing from a state of extension to one of flexion, 

 the tibia does not revolve round a single transverse axis drawn through the lower end of the 

 femur, as the ulna does round the lower end of the humerus. The articular surface ottne 

 tibia slides forward in extension and backward in flexion; thus the axis round which the tibia 

 revolves upon the femur is a shifting one, as is seen by reference to fig. 325, B, C, D. 



3. Another point of difference is that extension is accompanied by lateral rotation, ana 

 flexion by medial rotation. This rotation occurs round a vertical axis drawn through the mifja^e 

 of the lateral condyle of the femur and the lateral condyle of the tibia, and is most marked at 

 the termination of extension and at the commencement of flexion. This rotation ot the leg at 



■ the knee is a true rotation about a vertical axis, and thus differs from the obliquity oi the flexion 



