First permanent maxillary molar morphology and ideal occlusion.

AIM. To assess if variations in the anatomy of the upper first permanent molar might prevent the establishment of an ideal molar occlusion after orthodontic treatment. DESIGN. Measurements on dental casts selected at random from initial orthodontic records. MATERIALS AND METHODS. Forty maxillary dental casts were selected using criteria that assured the anatomical integrity of the upper right first permanent molar. The casts were scanned and the position of the molar cusps was measured using computer software. MAIN OUTCOME MEASURES. a. Buccal width (BW): the width between the buccal cusps. b. Cusp angle (CA): the angle between the lines joining the buccal and palatal cusps. c. Dist: the distance of the mesiobuccal cusp from a line perpendicular to the line joining the palatal cusps and passing through the mesiopalatal cusp. RESULTS. BW: mean 4.88 mm (SD: 0.41 mm, range: 4.2 – 6.0 mm). CA: mean -2.43 degrees (SD: 6.01, range -12.7 – 13.8 degrees). Dist: mean 2.15 mm (SD: 0.69 mm, range 1.0 – 3.8 mm). A large variability was noted. The morphology of 20% of the molars was such, that, if the mesiopalatal cusp were properly positioned in the central groove of the lower molar, the mesiobuccal cusp would be more than one-fifth cusp mesial or distal to the mesiobuccal groove of the lower molar. CONCLUSIONS. In some cases, the anatomy of the upper molar may not allow an ideal molar occlusion, and compromises in molar positioning may be required. The results of this study are based on the assumption of an average lower molar morphology and cuspal relationship.