To observe the physiological displacement of mandibular first molar during growth and to explain this phenomenon.

11 untreated adolescents with normal occlusion were selected from the longitudinal sample with implants collected by Dr. J. R. Mathews, UCSF, in the 1970s. 7 males and 4 females are included in this sample, and all of them had lateral cephalograms annually taken from 8 to 16 years old. All the cephalograms are traced and superimposed with aid of the implants. The displacement and tipping of mandibular first molars are measured on the cephalograms.

Relative to the mandibular, the average mesial displacement of mandibular first molar mesial-buccal cusp was (3.63±1.29)mm from 8 to 16 years old, and the average vertical displacement was (5.92±1.78)mm(P<0.05). The tipping of mandibular first molar showed no significant change. Mandibular first molar moved at the fastest rate from 10 to 12 years old, during which time (1.52±1.25)mm mesial displacement was observed. Pearson correlation test showed that there was significant correlation(r=0.623, P<0.01) between vertical displacement of mandibular first molar and mandibular growth(Co-Gn).

For the individuals with growth potential, mandibular first molar will continue to move misally without mechanical force. The vertical displacement of mandibular first molar and mandibular growth(Co-Gn) are at the same pace, which implies an adaptive pattern of dental and skeletal vertical growth.

Dental and Skeletal Growth; Mandibular First Molar; Implant Study