Descriptive classification of variations in primary mandibular first molars

Variability in tooth morphology can play a significant role in the achievable results of restorative treatment. This study identified the variations of morphology of the primary mandibular first molar (Type I and II coronal outline and Type A-F cusp alignment). Type I coronal outline was found more frequently (86 percent) than Type II (14 percent). 93 percent of the primary mandibular first molars comprise four cusps, 5 percent comprise five cusps, and 2 percent comprise 3 cusps (F). Among the four-cusp group, 70 percent had an oblique transverse ridge (C), 25 percent had a horizontal transverse ridge (B), and 5 percent had a poorly developed transverse ridge (A). Among the five-cusp-group, 67 percent had a distal cusp (E), and 33 percent had a fifth lingual cusp (D). The most frequent combination of coronal outline and cusp alignment was Type I-C (60 percent), and followed by Type I-B (22 percent), II-C (5 percent), and II-A (4 percent).