238 MECHANISM OF THE KNEE-JOINT. 



the leg, in passing from a position "of extension to flexion, 

 moves laterally outwards along the axis of rotation, to an extent 

 which is directly as the amount of rotation and the sine of 

 the angle of inclination of the thread — that is, in proportion 

 to the extent of flexion and the rapidity of the screw. 



In attempting by Langer's method to develope these arti- 

 cular screw-models, I found, that when two casts were united, 

 an apparently satisfactory helix M^as produced. But, in adding 

 to the series, the spire diminished, and the helix closed on 

 itself It appeared that not only the angle of inclination of 

 the thread, but also the radius of rotation, diminished. The 

 surfaces of two casts of the same articular surface coiild not 

 be accurately adapted to one another, even by placing them 

 together in the mould in which they were both cast. I found 

 that when one or the other of the lateral ridges which repre- 

 sent the thread of the screw was situated as a guide in joining 

 the models, the helix closed in one or the other direction. 

 In addition, I found that the models only fitted their moulds 

 in one position — viz. that in which they were cast. This 

 follows from what already has been stated regarding the forms 

 of articular surfaces. The tibio-astragaloid articular surfaces 

 cannot therefore be segments of a cylindrical series. It appears 

 extremely probable that, abstracting the terminal facets, the 

 acting areas on each surface consist each of a segment of a 

 conical screw — the convex portions of these two screws being 

 on the astragaloid, the concave on the tibial articular surface ; 

 the one screw coming into action in flexion, the other in 

 extension. 



In following up the subject, I was compelled to re-examine 

 the knee-joint from this new point of view. It now occurred 

 to me, that instead of attempting to procure co-ordinates by 

 direct admeasurement of the articular surfaces, data for an 

 approximate conception of the forms of these surfaces in any 

 joint might be reached by tracing the path or locus of any 



