The Lower Extremity and Pelvis 



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ligament connecting the plantar surfaces of the heads of the bones, with the Adductor 

 trans versus muscle. 



ASTRAGALUS (Figs. 144 and 145). 



Consists of body, neck and head. The body carries a concavo-convex upper articular 

 surface for tibia, continuous with inner and outer malleolar facets on the sides : the outer 

 facet, for the fibula, is longer and more vertically directed. 



Lower aspect of body rests, through an oblique concave articular surface, on the 

 os calcis : internal to this and in front of it is the interosseons groove which completes 

 the sinus tarsi, separating the articular 

 under surface of the body from the articular 

 head. The depth of this groove causes the 

 constriction of the neck to be more apparent 

 below and externally : many vascular canals 

 mark the bone in this sulcus. 



Now place the bone on the calcaneum 

 and articulate it with the bones of the leg : 

 the tibia rests on its upper surface and by 

 its malleolus, with a small curved articular 

 area directed forwards and downwards on its 

 inner side, whereas the fibula has a larger 

 triangular facet, concave from above down- 

 wards, on its outer side only. Evidently, 

 therefore, the weight of the body is trans- 

 mitted by the tibia, and the fibula is only a 

 guard to strengthen the joint and prevent 

 displacement of the foot outwards. 



Every time the foot comes to the ground, 

 in making a step forward, there would be a 

 tendency for the leg bones to slip forward 

 on to the dorsum of the foot, if there were 

 no special mechanism to prevent it : this 

 is provided by the shape of the upper 

 articular (or trochlear) surface of the astra- 

 galus, which is broader in front than behind, 

 so that any forward movement of the leg 

 bones on it is checked by the increasing 

 breadth of the surface. The anterior and posterior ligaments of the joint are feeble, 

 for they do not limit movement : the dorsi-flexion of the joint is limited by the broader 

 part of the articular surface becoming engaged between the tibia and fibula, and exces- 

 sive dorsi-extension is checked by tension of the posterior fibres of the lateral ligament. 

 The lateral ligaments hold the bones in apposition. The internal lateral ligament, 

 fastened to the margin of the malleolus, has its middle fibres attached to the susten- 

 taculum tali, which is about three-quarters of an inch immediately below the malleolus : 

 its most anterior and posterior fibres reach the astragalus, so that we can find markings 

 for these (Fig. 145) on the neck and inner side of the astragalus respectively. 



The external lateral ligament has three long bands, between which there are short 

 feeble fibres attached round the articular margin. The anterior and posterior bands 

 go to the astragalus, and the middle one to the os calcis in a downward (and slightly 

 backward) direction. We can find the attachment of the first two bands on the astra- 



Oi 



FIG. 144. Inferior surface of left astragalus. 

 I. and II. are facets for the front part of 

 os calcis which may be continuous or 

 separated more or less ; a., surface lying 

 on inferior calcaneo-scaphoid ligament 

 which extends between sustentaculum 

 tali and scaphoid ; 6., the inner con- 

 tinuation of the same surface moulded 

 by the internal lateral ligament which 

 passes into the other, and by tendon of 

 Tibialis posticus lying over it. The 

 tendinous expansion and ligaments 

 exhibit a fibre-cartilaginous thickening 

 here which plays over and covers this 

 part of the bone, and completes the 

 capsule for its head. 



