ARTICULATIONS 317 



this groove widens out into a distinct depression. In full extension the 

 lateral depression rests upon the anterior part of the lateral meniscus, 

 whilst the medial depression rests upon the anterior border of the lateral 

 tubercle of the spine of the tibia (Bruce Young). The line of demarcation 

 between the trochlea and the distal surface of the medial condyle of the 

 femur is not so distinct. Close to the medial margin of the bone there is a 

 depression which, in full extension, rests upon part of the anterior horn of 

 the medial meniscus (Bruce Young) : but lateral to this the trochlear 

 surface is prolonged posteriorly for a certain distance along the anterior 

 and medial margin of the intercondylar fossa. A portion of the medial 

 condyle is thus included in the trochlear surface, viz. the portion skirting 

 the medial border of the anterior part of the intercondylar fossa, and this 

 is termed the " crescentic facet " of the medial condyle. 



The posterior surface of the patella may next be examined (Fig. 114), and 

 its movements in connection with flexion and extension of the knee-joint 

 studied. A high vertical ridge divides its deep surface into a large lateral 

 and a smaller medial area. Each of these is still further subdivided by faint 

 ridges on the cartilage which coats the surface. A faint line upon the medial 

 area of the patella descends in a vertical direction so as to mark off a narrow 

 strip close to the medial border of the bone. This strip is called the medial 

 perpendicular facet. Two horizontal lines extend laterally from the lateral 

 border of the medial perpendicular facet to the lateral border of the bone, 

 and subdivide the remainder of the medial area and the whole of the lateral 

 area into three facets each. In a well-marked patella, therefore, the 

 posterior cartilage-covered surface shows seven facets, viz. a proximal 

 pair, an intermediate pair, a distal pair, and a medial perpendicular facet 

 (Goodsjr). 



The faceted appearance of the deep surface of the patella indicates that, 

 in the movements of this bone upon the trochlear surface of the femur, the 

 entire articular surface is never in contact with the femur at the same time. 

 In flexion and extension of the knee, the patella moves distally and proxi- 

 mally in a curved path, the concavity of which looks proximally, posteriorly, 

 and laterally. The different facets come into contact and break contact 

 with the femur in regular succession. Let us suppose the knee-joint to be 

 acutely flexed : in this condition of the limb the medial perpendicular facet 

 of the patella rests upon the crescentic facet of the medial condyle of the 

 femur, while the lateral of the two proximal patellar facets is in contact with 

 the lateral lip of the trochlear surface of the femur. No part of the patella 

 touches the medial lip of the trochlear surface. As the leg is moved from 

 the fully flexed to the fully extended position, the two proximal facets, then 

 the two intermediate facets, and, lastly, the two distal facets, come 

 successively into contact with the trochlear surface of the femur (Goodsir). 

 In Fig. 117 the position of the patella in the fully extended knee is 

 exhibited. 



Now examine the condylar surfaces of the femur (Fig. 1 14). The posterior 

 two-thirds of the medial condyle will be seen to be of equal extent with, 

 and parallel to, the lateral condyle. The anterior third of the medial 

 condyle, however, turns obliquely laterally to join the trochlear surface. 

 The lateral condylar surface has no part corresponding with this, and its 

 presence in connection with the medial condyle gives rise to the "screw- 

 home " movement, which is so characteristic of the knee-joint when fully 

 extended. At the commencement of flexion and at the completion of 

 extension there is a screw movement, or a movement of rotation of the 

 tibia and femur on each other. As the leg is moved forwards from the 

 condition of acute flexion, the condyles of the femur roll and glide over the 

 surfaces on the proximal end of the tibia until the surface of the lateral 



