A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 9813.2.3 MandibleDuring postnatal growth in the mandible of monkeys, a large increase in overall size of the mandible,occurs as a result of extensive growth and remodeling processes that take place from infancy toadulthood (32). Essentially, bone growth occurs on all surfaces of the primate mandible duringpostnatal development (33, 34).The following discussion includes information taken primarily from a study conducted byMcNamara and Graber (32) to investigate the postnatal development of the mandible in rhesusmonkeys. In this study, 4 groups of monkeys (at various ages) were evaluated for 6 consecutivemonths. The groups included infant monkeys ages 5.5 to 7 months, juvenile monkeys ages 18 to24 months, adolescent monkeys ages 45 to 54 months, and young adult monkeys over the age of72 months (32).In rhesus monkeys, the mandible grew most rapidly during infancy (32). Growth at the mandibularcondyle was most rapid in infant monkeys and successively slowed as animals aged.Specifically, during the 6-month observation period, growth of the condyle in the infant, juvenile,adolescent, and young adult monkeys was 5.92, 4.47, 3.00, and 1.07 mm, respectively.During postnatal development, deposition of bone occurs along the posterior border of theramus and resorption of bone occurs along the anterior border of the ramus. The largest increasein the width of the ramus was seen in infant monkeys (5.5 to 13 months). When compared toanimals over the age of 18 months, the mandibular ramus of infant monkeys had 4 times as muchbone deposited along the posterior border than was resorbed from the anterior border. Bonedeposition on the posterior border of the ramus decreased as the animals aged. Changes in thewidth of the mandibular ramus in the adult monkey were slight (32). In addition, bone depositionoccurred along the posterior border of the condyle (32).In the infant, juvenile, and adolescent animals, remodeling of the mandibular angle was noted;however, remodeling of the mandibular angle was not seen in the adult monkeys (32). During the6-month observation period, the largest change in the mandibular angle occurred in infant monkeys.The average decrease in mandibular angle over the 6 month observation period in infant, juvenile,and adolescent monkeys was 6.2, 2.4, 1.7 degrees, respectively.3.3 DogAppearance of secondary ossification centers and fusion of the proximal and distal epiphyses withthe diaphysis is summarized in Table 6.3.3.1 HumerusThe humerus of the dog develops from five secondary centers of ossification: one found in the diaphysisthat is ossified at birth, one in the head and tubercles of the proximal epiphysis, and three in the distalepiphysis (one each for the medial condyle, lateral condyle, and medial epicondyle) (35, 36).A secondary ossification center appears in the head of the humerus at 1 to 2 weeks after birth (37,35, 38). At 5 months after birth, partial fusion of the epiphysis to the diaphysis begins. Closure of theepiphyseal plate occurs at approximately 10 - 12 months of age; however, in some dogs, the fusionline may still be evident at 12 months, but usually disappears completely by 14 months (35, 37, 30).In the distal epiphysis of the humerus, the ossification centers in the medial and lateral condylesappear during the 2 nd , 3 rd , or 4 th postnatal week (35, 37, 36). The center in the medial epicondyleappears during postnatal weeks 5, 6, 7, 8, or 9 (36, 38, 35). The center of the medial epicondyleunites with the medial condyle during postnatal months 4, 5, or 6, and the whole distal extremityfuses with the diaphyseal shaft during the 6 th , 7 th , or 8 th month after birth (35, 37, 36). At approximately10 months of age, the fusion line in the distal epiphysis of the humerus is barely visible (37).© 2006 by Taylor & Francis Group, LLC