Morphological changes of human mandibular bone during fetal periods.

Standardized photographs (lateral and basal view) of fetal mandibles (n = 80) were taken. Outline segments (from gnathion to condylion, on the coronoid process and from infradentale to gnathion in lateral view; and on the inferior margin in basal view) were analyzed into sine curves, according to Fourier series. Using the calculated Fourier coefficients, we statistically examined how mandibular shape changed with fetal growth. Materials were classified into four groups on the basis of the body length (BL): 100 < or = BL < 200 mm (BL1, n = 20); 200 < or = BL < 300 mm (BL2, n = 20); 300 < or = BL < 400 mm (BL3, n = 20); and 400 < or = BL < or = 500 mm (BL, 4 = 20). Statistical analyses were all done on these stature groups. On the outline from gnathion to condylion, the constant and the first term phase showed statistically greater values in BL4 than in the other groups, as assessed by t-test. The first term phase for the coronoid process significantly decreased with increments in stature in the canonical disciminant analysis. On the outline from infradentale to gnathion, although the cumulative contributions of the first two terms were constant (about 85%) in all four groups, the relative contributions with growing stature decreased in the first term and increased in the second term. The mean of the phase in the first term indicated slightly more than -90 degrees in BL1, and was less than -90 degrees in BL2 to BL4. The averages of the second term phases presented little change from BL1 to BL3, and were close to -100 degrees. From the canonical discriminant analysis, the constant was great and the first term amplitude was small in BL1 and BL4, and the constant was small and the first term amplitude was great in BL2 and BL3. Also, the second term amplitude in BL4 indicated a significantly bigger volume than in BL3. The constant for the inferior margin manifested statistically significant increments in BL2 to BL4, as assessed by t-test. We interpret the results as follows: 1. On the outline from gnathion to condylion, anteroinferior shift occurs in BL4, resulting in an acute gonial angle. The change of angle during the prenatal period precedes the postnatal findings that bring about a progressive decrease in the gonial angle. 2. The tip of the coronoid process indicates a forward shift with increasing fetal body stature.(ABSTRACT TRUNCATED AT 400 WORDS)