Comparison of the crown dimensions between the maxillary second deciduous molar and the first permanent molar.

The morphologic relationship of the crown structures of the maxillary second deciduous molar (m2) and the first permanent molar (M1) was investigated with odontometric methods. Materials used were 124 male dental casts taken from Chinese living in Kaohsiung (Taiwan). The mesiodistal and buccolingual diameters, cusp sizes, and intercusp distances were measured. The mean crown diameters were larger in M1 than in m2. The tooth crown proportions were similar in m2 and M1. The intercusp distance was compressed buccolingually in m2. With respect to the relative cusp size, the paracone was not significantly different between m2 and M1. The metacone was relatively larger in m2 than M1, while lingual cusps (protocone and hypocone) were relatively smaller in m2 than in M1. Principal component analysis was performed to investigate factors influencing the variation in crown dimensions of the two teeth. Four components were extracted: (1) buccolingual intercusp distances, (2) crown proportion (mesiodistal and buccolingual diameters), (3) mesiobuccal and distolingual compression, and (4) protocone and mesiodistal intercusp distance. Analysis of principal component scores indicated that the crown proportions of m2 and M1 were similar. The m2 had a smaller protocone and more buccolingually compressed intercusp distances than did M1. The morphologic characteristics of the crown of m2 indicate that it is more primitive and has developed less than M1.