Mandibular rotation during the transitional dentition

Abstract

Objective: To determine whether dentoalveolar changes or vertical condylar growth is more closely related to the true forward rotation of the mandible during the transition between the late primary and early mixed dentition stages of development.

Materials and methods: The sample included 50 subjects (25 males and 25 females) with Class I (N = 25) and Class II (N = 25) molar relationships. They were selected based on the availability of lateral cephalograms at two developmental stages: T1: last film with complete primary dentition (5.8 ± 0.4 years) and T2: first film with permanent incisors and permanent molars fully erupted (8.0 ± 0.2 years). Seventeen landmarks were identified and 22 measurements were calculated. The mandibles at T1 and T2 were superimposed using natural reference structures in order to measure true mandibular rotation.

Results: The mandible underwent -2.4° ± 2.6° of true rotation, 1.9° ± 2.4° of remodeling, and -0.6° ± 1.8° of apparent rotation. There were no significant sex or Class differences in true rotation, remodeling, and apparent rotation. There was a moderate correlation (r = 0.76) between true rotation and remodeling and a moderately low correlation (r = 0.40) between true rotation and apparent rotation. There was a weak correlation between true rotation and SNA (r = 0.28). True rotation was most closely associated with the increases in U1/S-N (r = -0.34), increases in U1/PP (r = -0.36), and decreases in Id-Me (r = 0.36).

Conclusions: Independent of sex and Class, the true mandibular rotation that occurred between the late primary and early mixed dentition was mostly masked by angular remodeling, resulting in limited amounts of apparent rotation. True rotation was significantly related to anterior dentoalveolar changes but not to the vertical growth changes that occurred.

Figure 1.

Mean and standard deviation of mandibular rotation from T1 to T2.