THE PATELLA 



255 



The Patella (Knee Cap). 



the most efficient manner in which stresses are transmitted is by the arrangement of the resist- 

 ing material in lines parallel to the direction in which the stresses occur and in the paths taken 

 by the stresses. Theoretically the most efficient manner to attain these objects would be to pro- 

 long the innermost filaments of the bone as straight lines parallel to the longitudinal axis of 

 the bone, and gradually to flare the outer shell of compact bone outward, and continuing to give 

 of/ filaments of bone parallel to the longitudinal axis as the distal end of the femur is approached. 

 These filaments should be well-braced transversely and each should carry its proportionate 

 part of the total load, parallel to the longitudinal axis, transmitting it eventually to the 

 articular surface, and in a direction perpendicular to that surface." 



Referring to Fig. 249, it is seen that the large expansion of the bone is produced by the gradual 

 transition of the hollow shaft of compact bone to cancellated bone, resulting in the production 

 of a much larger volume. The trabeculse are given off from the shaft in lines parallel to the 

 longitudinal axis, and are braced transversely by two series of trabeculae at right angles to 

 each other, in the same manner as required theoretically for economy. 



Although the action of the muscles exerts an appreciable effect on the stresses in the femur, 

 it is relatively small and very complex to analyze and has not been considered in the above analysis. 



Ossification (Figs. 252, 253, 254). The femur is ossified from five centers: one for the body, 

 one for the head, one for each trochanter, and one for the lower extremity. Of all the long bones, 

 except the clavicle, it is the first to show traces of ossification; this commences in the middle of 

 the body, at about the seventh week of fetal life, and rapidly extends upwa*rd and downward. 

 The centers in the epiphyses appear in the following order : in the lower end of the bone, at the 

 ninth month of fetal life (from this center the condyles and epicondyles are formed) ; in the head, 

 at the end of the first year after birth; in the greater trochanter, during the fourth year; and 

 in the lesser trochanter, between the thirteenth and fourteenth years. The order in which the 

 epiphyses are joined to the body is the reverse of that of their appearance; they are not united 

 until after puberty, the lesser trochanter being first joined, then the greater, then the head, and. 

 lastly, the inferior extremity, which is not united until the twentieth year. 



The patella (Figs. 255, 256) is a flat, triangular bone, situated on the front of 



e knee-joint. It is usually regarded as a sesamoid bone, developed in the 

 tendon of the Quadriceps femoris, 

 and resembles these bones (1) in 

 being developed in a tendon; (2) in 

 it:> center of ossification presenting 

 a knotty or tuberculated outline; 

 (3) in being composed mainly of 

 dense cancellous tissue. It serves 

 tc protect the front of the joint, 

 and increases the leverage of the 

 Quadriceps femoris by making it 

 a(t at a greater angle. It has an 

 anterior and a posterior surface 

 tbree borders, and an apex. 



Surfaces. The anterior surface is convex, perforated by small apertures for the 

 passage of nutrient vessels, and marked by numerous rough, longitudinal striae. 

 T lis surface is covered, in the recent state, by an expansion from the tendon of 

 the Quadriceps femoris, which is continuous below with the superficial fibers of 

 the ligamentum patellae. ' It is separated from the integument by a bursa. The 

 posterior surface presents above a smooth, oval, articular area, divided into two 

 facets by a vertical ridge; the ridge corresponds to the groove on the patellar 

 surface of the femur, and the facets to the medial and lateral parts of the same 

 surface; the lateral facet is the broader and deeper. Below the articular surface 

 is a rough, convex, non-articular area, the lower half of which gives attachment 

 to the ligamentum patellae; the upper half is separated from the head of the tibia 

 by adipose tissue. 



Borders. The base or superior border is thick, and sloped from behind, down- 

 ward, and forward: it gives attachment to that portion of the Quadriceps femoris 



FIG. 255. Right patella. 

 Anterior surface. 



FIG. 256. Right patella. 

 Posterior surface. 



