KNEE JOINT 413 



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, a middle pair, a distal pair, and a medial perpen- 

 dicular facet (Goodsir). 



The facetted appearance of the posterior surface of the patella indicates 

 that, in the movements of that 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 upwards, backwards, 

 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 that 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 middle facets, and, lastly, the two distal facets, come successively 

 into contact with the trochlear surface of the femur (Goodsir). In Fig. 182, 

 183, 187 the position of the patella in the fully extended knee is exhibited. 



Now examine the condylar surfaces of the femur (Fig. 178). 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 corresponding part. The obliquely 

 directed part of 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 of the menisci and the proximal end of the tibia until the surface 

 of the lateral condyle, and the corresponding part of the medial condyle, 

 are exhausted. This movement of the femoral condyles has been compared 

 to that of " a wheel partially restrained by a drag" (Goodsir). Any 

 additional movement must necessarily take place in connection with the 

 anterior oblique third of the medial condyle, and the result is a rotation or 

 screw-like motion of the femur medially. The medial condyle travels back- 

 wards round the intercondyloid eminence of the tibia, and the anterior part of 

 the intercondylar fossa comes into contact with the anterior cruciate ligament 

 and the medial tubercle of the intercondyloid eminence (Bruce Young). 

 The joint is now "screwed home" or locked. In the initial stage of 

 flexion the reverse movement must be accomplished. The unlocking of 

 the joint can be brought about only by a rotation medially of the tibia, 

 or a rotation laterally of the femur. 



When fully extended, the joint is locked, and the posterior part of the 

 capsule, the collateral ligaments, and the anterior cruciate ligaments are 

 tense. The limb is converted into a rigid column, and the upright posture 

 is thereby maintained with the smallest possible degree of muscular 

 exertion. 



The muscles which operate upon the bones of the leg so as to produce 

 flexion and extension of the limb at the knee joint are : (i) extensors, the 

 four parts of the quadriceps extensor ; (2) flexors^ the biceps femoris, 



