4i6 



HUMAN ANATOMY. 



pouches (which in infectious cases are usually involved), the direction of the articular 

 line (with which the saw cut should be parallel), and sometimes the possibility of 

 infection of the neighboring bursae. 



Landmarks. — The synovial membrane rises from four to five centimetres 

 (one and a half to two inches) above the upper border of the patella ; it is higher 

 on the inner than on the outer side of the thigh ; its upper limit descends in flexion 

 of the knee. 



The bony points have been described in connection with the femur and tibia 

 (pages 367, 390) ; the bursae will be described later. 



The Patella. — Congenital absence of the patella on one or both sides has been 

 noted in a number of instances, and has in some cases been observed in several 

 members of the same family. The functional disability was slight or altogether 

 unnoticeable. 



Ft-acture by muscular action is more common in this bone than in any bone of 

 the skeleton. It occurs usually with the leg in partial fle.xion upon the knee. In 

 this position fracture is favored because (i) the ligamentum patellae is then taut 

 and fixes the lower edge of the bone ; (2) the patella is in contact only through the 

 upper third of its convex under surface with the most prominent part of the articu- 

 lar surface of the condyles (Fig. 429); and (3) at this time the quadriceps extensor 



Fig. 429. 



Rectus muscle 



Patella 



Subpatellar tissue 

 Tendo patellae 



Tibia 



Femur 



Showing position of patella in relation to condyles of femur with knee partially flexed. 



has the greatest advantage of leverage upon the patella, as when the knee is fully 

 bent the muscle gets its leverage for the beginning of extension through the projec- 

 tion of the front of the condyles, and the patella lies on the pad of fat between the 

 femur and tibia (Fig. 430), and when the knee is almost or quite extended, the 

 patella — or three-fourths of it — occupies the depression of the trochlea, or even that 

 just above it. As a result of the cross-strain brought to bear in the partially flexed 

 position the bone usually breaks transversely a little below its mid-line, — i.e., through 

 the area unsupported by the femur beneath (Fig. 429). Occasionally it gives way 

 at a higher level. 



The accident may happen as the result of a fall, but the fall is more apt to 

 follow than to precede the fracture. In ordinary falls upon the knee the force is 

 received upon the tubercle of the tibia, not upon the patella. 



Direct violence often causes an irregular, comminuted, or stellate fracture. 



Fracture never occurs in children and is extremely rare before adult life. 

 When the bone is broken the fragments are immediately separated by the action 

 of the quadriceps upon the upper one. The degree of their separation will de- 

 pend upon the amount of laceration of the lateral aponeurotic expansions of the 

 conjoined tendon. Unless that fibrous structure is torn, no great separation of 

 the fragments can occur, as it is inserted into the borders and front of the patella, 



