PRACTICAL CONSIDERATIONS : THE PATELLA. 



417 



which is thus embedded, as it were, in a hood spread out over the front of the joint 

 and extending to the lateral ligaments and to the oblique lines running up from the 

 tubercle to the tuberosities (Fig. 424). The force causing the fracture in cases of 

 direct violence, or atmospheric pressure on the front of the knee if the fracture was 

 from muscular action, drives in between the fragments, as they separate, in the 

 shape of shreds or of an irregular fringe, portions of that part of the rectus tendon 

 which was inserted into the longitudinal grooves or stri^ on the anterior surface of 

 the bone. These ofier an obstacle to bony union. As the synovial membrane of 

 the knee-joint lies in contact with, and is attached to, the under surface of the 

 patella, it will usually be lacerated, — i.e., the knee-joint will be opened and the 

 fragments surrounded by bloody synovial fluid. The synovial membrane is re- 

 flected from the patella some distance above the apex of the bone ; hence a fracture 

 may occur at that level without involvement of the joint. The pad of fat on which 



Fig. 430. 



Patella 



Subpatellar tissue 



Showing position of patella in relation to condyles of femur with knee flexed at right angle. 



the tip of the bone rests, and over which the membrane is reflected, may aid in 

 saving the joint from injury. 



The common failure to get bony union by non-operative methods is thus seen 

 to be due to ( i) separation o\ the fragments by the quadriceps, (2) the interposition 

 of portions of the capsule, (3) the presence of blood- clot and synovial fluid, and is 

 supposed to be further favored by (4) the sesamoid character of the bone inclining 

 it to unite by fibrous rather than by bony tissue. It has been asserted, however 

 (Wirth), that the patella is a detached portion of the upper tibial epiphysis and not 

 a true sesamoid bone. 



As non-union is common on account of the above anatomical conditions, oper- 

 ative measures are often resorted to. In the open operations used in old united 

 fractures the fragments are drilled obliquely from a half-inch above and below the 

 line of fracture to just above the cartilaginous under surface, so that the wire used 

 to hold them together does not lie in the joint. 



27 



