THE KNEE-JOIXT. 373- 



~ 



of motion is not a fixed one. If the joint is examined while in a condition of 



extreme flexion, the posterior part of the articular surfaces 



of the tibia will be found to be in. contact with the posterior 



rounded extremities of the condyles of the femur ; and if 



a simple hinge-like movement were to take place, the 



axis, round which the revolving movement of the tibia 



occurs, would be in the back part of the condyle. If the 



leg is now brought forward into a position of semiflexion, 



the upper surface of the tibia will be seen to glide over 



the condyles of the femur, so that the middle part of the 



articular facers are in contact, and the axis of rotation 



must therefore have shifted forward to nearer the centre FlG y>2 _ View of t] 



of the condvles. If the leg is now brought into the posterior surface of the pa- 



,, .. -ii/-i TT i i -i tella, showing diagrammat- 



extended position, a still further gliding takes place and icaiiy the areas of contact 

 a further shifting forward of the axis of rotation. This ^jtions of ttie'kjnee'^ 5 

 is not. however, a simple movement, but is accompanied 



by a certain amount of rotation outward round a vertical axis drawn through the 

 centre of the head of the tibia. This rotation is due to the greater length of the 

 internal condyle, and to the fact that the anterior portion of its articular surface 

 is inclined obliquely outward. In consequence of this it will be seen that toward 

 the close of the movement of extension that is to say. just before complete 

 extension is effected the tibia glides obliquely upward and outward over this 

 oblique surface of the inner condyle, and the leg is therefore necessarily rotated 

 outward. In flexion of the joint the converse of these movements takes place: 

 the tibia glides backward round the end of the femur, and at the commencement 

 of the movement the tibia is directed downward and inward along the oblique 

 curve of the inner condyle, thus causing an inward rotation to the leg. 



During flexion and extension the patella moves on the lower end of the femur, 

 but this movement is not a simple gliding one ; for if the articular surface of this 

 bone is examined, it will be found to present on each side of the central vertical 

 ridge two less marked transverse ridges, which divide the surface, except a small 

 portion along the inner border, which is cut off by a slight vertical ridge into 

 six facets (see Fig. 262), and therefore does not present a uniform curved sur- 

 face, a* would be the case if a simple gliding movement took place. These six 

 facets three on each side of the median vertical ridge correspond to and denote 

 the parts of the bone respectively in contact with the condyles of the femur during 

 flexion, semiflexion. and extension. In flexion only the upper facets on the patella 

 are in contact with the condyles of the femur ; the lower two-thirds of the bone 

 rests upon the mass of fat which occupies the space between the femur and tibia. 

 In the semiflexed position of the joint the middle facets on the patella rest upon 

 the uv>st prominent portion of the condyles. and thus afford greater leverage to 

 the Quadriceps by increasing its distance from the centre of motion. In complete 

 extension the patella is drawn up, so that only the lower facets are in contact with 

 the articular surfaces of the condyles. The narrow strip along the inner border 

 is an exception to this, and would appear to be in contact with the internal condyle 

 throughout its whole extent in every position of the joint. As in the elbow, so it 

 is in the knee the axis of rotation in flexion and extension is not precisely at 

 right angles to the axis of the bone, but during flexion there is a certain amount 

 of alteration of plane ; so that, whereas in flexion the femur and tibia are in the 

 same plane, in extension the one bone forms an angle of about ten degrees with 

 the other. There is, however, this difference between the two extremities : that 

 in the upper, during extension, the humeri are parallel and the bones of the 

 forearm diverge; in the lower, the femora converge below and the tibia are 

 parallel. 



In addition to the slight rotation during flexion and extension, the tibia enjoys 

 an independent rotation on the condyles of the femur in certain positions of the 

 joint. This movement takes place between the interarticular fibro-cartilages and 



