244 MECHANISM OF THE KNEE-JOINT. 



as being accomplished without any contact of the opposite 

 surfaces up to the completion of the one and from the com- 

 mencement of the other ; and the contact takes place simul- 

 taneously all over from base to vertex in the one case, and is 

 as simultaneously broken in the other. However irregular 

 the movements may be by which a conical screw-combination 

 may be screwed on or off, there is one general direction by 

 which the movements must be guided namely, a rotation of 

 one or other, or of both screws in opposite directions, round 

 the common axis of the system. The amount of this rotation 

 will be directly as the rapidity of the thread in any given 

 combination ; but, however rapid the thread may be in any 

 such combination, there must always be a certain amount of 

 rotation to admit of complete separation of the two elements. 

 It is evident, therefore, that in the application of a conical 

 screw- combination in the construction of a joint required to 

 move in flexion and extension, the most advantageous direc- 

 tion in which the axis of the combination could be placed 

 would be coincident with the axis of flexion and extension, 

 and the most disadvantageous that which most nearly opposes 

 the axis of the limb -; for if coincident with that axis, there 

 would be no apparent hinge-movement at all. We have 

 exemplifications of the permissible limits of such an arrange- 

 ment in the elbow and knee joints. In the former the axis of 

 the screw is nearly at right angles to the axis of the limb ; 

 in the latter, nearly coincident with it. 



The arrangements by which the screw-combinations at the 

 knee-joint are adjusted for the general movements of flexion 

 and extension are as follows : The diameters of the funda- 

 mental cones considerably exceed their vertical height. They 

 are double-threaded, deep cut, with an obliquity of thread so 

 proportioned to the cone that they form little more than half- 

 spires. The upper portions of the taps of these screw-systems 

 consist of ligamentous texture (crucial ligaments), the basal 

 portions of bone and cartilage (tibial condyles and spine), 



