394 HUMAN ANATOMY. 



of muscles and fascia ; and (c) its backward curvature, which carries it to a plane 

 posterior to that of the tibia, which thus protects it both internally and anteriorly 

 from direct violence. 



Fractures about the middle of the lower third of the shaft, and especially those 

 about 7.5 centimetres (three inches) from the ankle, are so commonly produced by 

 leverage that, whatever their exact level, most of them may be grouped as instances 

 of Pott's fracture, although an effort has been made to draw between them distinc- 

 tions that are ordinarily academic rather than practical. 



These fractures usually result from over-abduction of the foot. When that 

 occurs suddenly, the weight of the body being upon the limb, the tension first comes 

 upon the deltoid ligament. This may stretch slightly or some of its fibres may be 

 torn, or there may be a small detachment from its malleolar origin. As a rule, such 

 a case ends in a more or less severe sprain. If the ligament ruptures, or the tip of 

 the malleolus is torn off, or the malleolus itself is fractured, the abduction of the 

 foot continues, and the astragalus is subluxated and carried against the inner surface 

 of the external malleolus. The fibula is thus converted into a lever of the first order. 

 The force is applied at its lower end ; the fulcrum consists of the stout tibio-fibular 

 ligaments, which are often stronger than the bone jtself and which are rarely com- 

 pletely ruptured, though often stretched and lacerated ; the weight or resistance is 

 in the body of the bone, which is prevented from moving inward by the articulation 

 of its upper end with the tibia. As soon, therefore, as its Hmit of elasticity is ex- 

 ceeded, it breaks at a weak (if not its weakest) point, and the upper end of the 

 lever — i.e., of the lower fragment — is forced in the direction opposite to that of the 

 lower end, — i.e., the malleolus (Fig. 410). The impact of the astragalus and the 

 pull of the ligaments may cause, in addition to the fracture of the tip of the malleolus, 

 fracture of the anterior or of the outer articular edge of the tibia. If the tibio- 

 fibular ligaments rupture, the fibula becomes a lever qf^the second order, the 

 fulcrum shifting to its upper end. The dislocation of, tke' astragalus outward will be 

 more marked. The bone may break at any point; but the fracture is still likely to 

 be within the limits of the lower third. 



Rose and Carless have adopted the following useful classification based on the 

 injury to the inner side of the foot or to the tibia itself. It divides these fractures 

 into four groups, the term Pott's fracture being correctly applied, according to these 

 authors, to the first two only. i. The internal lateral ligament is torn through ; 

 the intact internal malleolus can be felt projecting beneath the skin (Fig. 410, A). 

 2. The malleolus is torn of? and a distinct sulcus can be felt between it and the lower 

 end of the tibial shaft (Fig. 410, B). 3. The interosseous tibio-fibular ligament is 

 ruptured (or the flake of bone at the tibial attachment is torn off) ; the subluxation 

 outward is very marked ; either the inner malleolus or the deltoid ligament yields, — 

 " Dupuytren's fracture" (Fig. 410, C). 4. The tibia fractures transversely just 

 above the base of the malleolus ; the lower end of the upper fragment may be mis- 

 taken for the tip of the malleolus (Fig. 410, Z?). 



The less frequent accident of forcible over-inversion of the foot, if the external 

 lateral ligament holds, produces by the same mechanism a similar series of occur- 

 rences. The tip of the external malleolus is dragged violently inward, the tibio- 

 fibular ligaments act again as a fulcrum, and the bone is apt to break at about the 

 same level, — i.e., from 5 to 7.5 centimetres (two to three inches) above the joint, — 

 the upper end of the lower fragment being carried outward instead of inward. In 

 these cases there is a subluxation of the astragalus inward which not infrequently 

 results in a fracture of the inner malleolus. In all these forms of fracture the lacera- 

 tion of ligamentous structures loosening the connection of the foot to the leg, the 

 upward pull of the calf muscles, and the weight of the foot itself combine to produce 

 a subluxation of the foot backward which is often overlooked. 



The cardinal symptoms of the common form of Pott's fracture are eversion of 

 the foot, prominence of the inner malleolus, shortening of the distance from the front 

 of the ankle to the web of the great toe, increased width between the malleoli, and 

 tenderness over (a) the space between the tibia and the external malleolus anteriorly, 

 — i.e., over the strained or torn tibio-fibular ligaments ; (d) over the base or tip or 

 anterior border of the internal malleolus, — i.e., over a ruptured internal lateral 



