Chap, xxi.] THE REGION OF THE KNEE. 443 



fracture by -muscular action 'is that Oi nbxlcri. "When 

 the knoe is bent, the patella rests upon the femoral 

 condyles' nlpn rts transverse kxiH only. Nieady the 

 whole of its 4 upper" half is unsupported behind, and 

 the extensor muscle acts in a line nearly at right 

 angles to the vertical axis of the bone. Thus, by 

 violent contraction of the quadriceps, the patella may 

 be snapped across the condyle 

 as a stick is snapped across 

 the knee (Fig. 48). As the 

 fracture usually causes the 

 patient to fall, it has been sup- 

 posed that the contact with the 

 ground, rather than any pre- 

 vious muscular action, may 

 have caused the lesion. But, 

 as Hamilton has pointed out, Fig . 4 8.-Dia g ram to show 



if a person falls UDOll the beilt Mechanism of Fracture of 

 i -i ,1 ! i i the Patella "by muscular 



knee when the limb also is action. 



fl^vorl nTkr fVa 4 -mm IT- f"Ko a > ^ ne f action of quadriceps 

 Upon tne trunK, tne mu scle ; &, femur ; c, tibia. 



part that comes in contact 



with the ground is not the patella, but the tubercle 



of the tibia. 



In the great majority of cases, the lesion not only 

 involves the bone, but also the cartilage and fibrous 

 structures that cover it respectively behind and in 

 front ; the synovial membrane also is torn, and the 

 patellar bursa opened up. Thus the synovial contents 

 may come in actual contact with the skin. "It is 

 anatomically possible, if the fracture involve only the 

 lower and non-articular portion of the. patella, and if 

 the amount of separation of the fragments is slight, 

 that the fatty tissue behind the apex of the patella, 

 over which the synovial membrane is reflected, may 

 save the latter from injury " (Henry Morris). In all 

 cases where there is much separation of the fragments, 

 the fibrous expansion attached to either side of the 



