

SCREW-HINGE JOINTS. 499 



The Screw-hinge Joint is a modification of the simple hinge form (Langer, Henke), e.g., the 

 humero-ulnar articulation. Strictly speaking, simple flexion and extension do not take place 

 at the elbow-joint, but the ulna moves on the capitellum of the humerus like a nut on a bolt ; 

 in the right humerus, the screw is a right spiral, in the left, a left spiral. The ankle-joint is 

 another example ; the nut or female screw is the tibial surface, the right joint is like a left- 

 handed screw, the left the reverse, (b) The Pivot-Joint (rotatoria), with a cylindrical surface, 

 e.g., the joint between the atlas and the axis, the axis of rotation being around the odontoid 

 process of the axis. In the acts of pronation and supination of the fore-arm at the elbow- 

 joint, the axis of rotation is from the middle of the cotyloid cavity of the head of the radius 

 to the styloid process of the ulna. The other joints which assist in these movements are above 

 the joint, between the circumferential part of the head of the radius and the sigmoid cavity of 

 the ulna, and bcloio the joint, between the sigmoid cavity of the radius which moves over the 

 rounded lower end of the ulna. 



2. Joints with movements around two axes. (a) Such joints have two unequally curved 

 surfaces which intersect each other, but which lie in the same direction, e.g., the atlanto- 

 occipital joint, or the wrist-joint, at which lateral movements, as well as flexion and extension, 

 take place, (b) Joints with curved surfaces, which intersect each other, but which do not lie 

 in the same direction. To this group belong the saddle-shaped articulations, whose surface is 

 concave in one direction, but convex in the other, e.g., the joint between the metacarpal bone 

 of the thumb and the trapezium. The chief movements are (1) flexion and extension, (2) 

 abduction and adduction. Further, to a limited degree, movement is possible in all other 

 directions ; and, lastly, a pyramidal movement can be described by the thumb. 



3. Joints with movement on a spiral articular surface (spiral joints), e.g., the knee-joint 

 (Goodsir). The condyle of the femur, curved from before backwards, in the antero-posterior 

 section of its articular surface, represents a spiral (Ed. Weber), whose centre lies nearer the 

 posterior part of the condyle, and whose radius vector increases from behind, downwards and 

 forwards. Flexion and extension are the chief movements. The strong lateral ligaments arise 

 from the condyles of the femur corresponding to the centre of the spiral, and are inserted into the 

 head of the fibula and internal condyle of the tibia. When the knee-joint is strongly flexed, 

 the lateral ligaments are relaxed they become tense as the extension increases ; and when the 

 knee-joint is fully extended, they act quite like tense bands which secure the lateral fixation of 

 the joint. Corresponding to the spiral form of the articular surface, flexion and extension do not 

 take place around one axis, but the axis moves continually with the point of contact ; the axis 

 moves also in a spiral direction. The greatest flexion and extension cover an angle of about 

 145. The anterior crucial ligament is more tense during extension, and acts as a check liga- 

 ment for too great extension, while the posterior is more tense during flexion, and is a check 

 ligament for too great flexion. The movements of extension and flexion at the knee are further 

 complicated by the fact that the joint has a screw-like movement, in that during the greater 

 extension the leg moves outwards. Hence, the thigh, when the leg is fixed, must be rotated 

 outwards during flexion. Pronation and supination take place during the greatest flexion to 

 the extent of 41 (Albert) at the knee-joint, while with the greatest extension it is nil. It 

 occurs because the external condyle of the tibia rotates on the internal. In all positions during 

 flexion, the crucial ligaments are fairly and uniformly tense, whereby the articular surfaces are 

 against each other. Owing to their arrangement, during increasing tension of the anterior 

 ligament (extension), the condyles of the femur must roll more on to the anterior part of the 

 articular surface of the tibia, while by increasing tension of the posterior ligament (flexion), they 

 must pass more backwards. 



4. Joints toith the axis of rotation round one fixed 'point. These are the freely movable 

 arthrodial joints. The movements can take place around innumerable axes, which all inter- 

 sect each other in the centre of rotation. One articular surface is nearly spherical, the other is 

 cup-shaped. The shoulder- and hip-joints are typical "ball-and-socket-joints." We may 

 represent the movements as taking place around three axes, intersecting each other at right 

 angles. The movements which can be performed at these joints may be grouped as : (1) 

 pendulum-like movements in any plane, (2) rotation round the long axis of the limb, and (3) 

 circumscribing movements [circumduction], such as are made round the circumference of a 

 sphere ; the centre is in the point of rotation of the joint, while the circumference is described 

 by the limb itself. 



Limited arthrodial joints are ball joints with limited movements, and where rotation on the 

 long axis is wanting, e.g., the metacarpophalangeal joints. 



5. Eigid joints or amphiarthroses are characterised by the fact that movement may occur in 

 all directions, but only to a very limited extent, in consequence of the tough and unyielding 

 external ligaments. Both articular surfaces are usually about the same size, and are nearly 

 plane surfaces, e.g., the articulations of the carpal and the tarsal bones. 



II. Symphyses, synchondroses, and syndesmoses unite bones without the formation of a 

 proper articular cavity, are movable in all directions, but only to the slightest extent. Physio- 

 logically they are closely related to amphiarthrodial joints. 



III. Sutures unite bones without permitting any movement. The physiological importance 



