4 2 4 THE INFERIOR EXTREMITY 



ligament, so that the articular surfaces can be separated and 

 examined. 



The Articular Surfaces of the Ankle Joint. The proximal 

 articular area is formed by the distal surface of the tibia, the 

 lateral surface of the medial malleolus, and the medial surface 

 of the lateral malleolus. The three surfaces together form 

 the boundaries of a socket. It is important to note that the 

 socket is wider in front than it is behind, and that it is 

 concave both from side to side and from before backwards. 

 The distal articular area is formed by the dorsal surface of 

 the body of the talus and by parts of its medial and lateral 

 surfaces. It also is broader in front than behind. It is 

 generally convex both from before backwards and from side 

 to side, and it fits into the socket formed by the bones of 

 the leg. When the joint is dorsi-flexed, that is, when the toes 

 are turned upwards, the broad part of the distal articular 

 .area rotates backwards into the narrow part of the proximal 

 articular area and the joint becomes locked. When the joint 

 is plantar-flexed, that is, when the toes are turned downwards, 

 the narrow part of the distal articular area moves forwards 

 into the wide part of the proximal articular area and a small 

 amount of side to side movement becomes possible. 



Movements. The movements which take place at the ankle joint are 

 (i) dorsal-flexion (sometimes called flexion) ; (2) plantar-flexion (some- 

 times called extension) ; and (3) a very limited degree of lateral movement 

 when plantar-flexion is complete. The two principal movements (dorsi- 

 flexion and plantar-flexion) take place around a horizontal axis, which is 

 not transverse, but which is directed laterally and posteriorly, so that it 

 is inclined to the median plane of the body at an angle of about 60 

 (Krause). This horizontal axis passes through or near the interosseous 

 canal between the calcaneus and talus (Henle). As the articular cavity 

 formed by the tibia and fibula, and also the part of the talus which 

 plays in it, are broader in front than behind, it follows that the 

 more completely the ankle joint is dorsi-flexed, the more tightly will the 

 talus be grasped between the two malleoli. In the erect position the talus 

 is held firmly in the bony socket, and portions of its articular surface project 

 both in front of and behind the tibia. The line of the centre of gravity 

 falls anterior to the ankle joint, and as a result the bones are kept firmly 

 locked. When, on the other hand, the ankle joint is fully plantar-flexed 

 (as when we rise on tip-toe) the narrower posterior part of the talus is 

 brought into the socket, and thus a limited amount of lateral movement is 

 allowed. In dorsi-flexion the calcaneo-fibular and posterior talo-fibular 

 bands, the greater part of the deltoid ligament, and the posterior part of 

 the capsule are put on the stretch. In plantar-flexion the anterior talo- 

 fibular ligament, the anterior fibres of the deltoid ligament, and the anterior 

 part of the capsule are rendered tense. 



The Muscles principally concerned in producing dorsi-flexion of the 



