Abstract: PURPOSE: To establish the 3-D finite element models of maxillary first molar, and calculate the stress magnitude and distribution within the PDL of maxillary first molar when loaded with intrusion force. METHODS: Date from CT scanning image of the maxillary first molar was used to establish the 3-D finite element model of maxillary first molar. By simulating the horizontal unbalanced bone resorption model sequentially, the magnitude and distribution of PDL stress at these models were calculated under intrusion force. RESULTS: The 3-D finite element models of maxillary first molar were constructed. With constant decrease of alveolar bone height, stress of the PDL increased non-linearly. The stress in PDL changed slowly when alveolar bone height reduced from normal level to 3.5 mm, but it increased dramatically when the height of alveolar bone resorption exceeded 6.0 mm and root furcation was exposed. The high-stress concentrated in root furcation of maxillary first molar when root furcation had not been exposed, and it would be transfered to apex or one third of root length when root furcation had been exposed. CONCLUSIONS: With the sustained decrease of alveolar bone height (less than 6 mm), orthodontic force value should be reduced according to the degree of periodontal bone loss. When the alveolar bone loss reduces to more than 6 mm and the root furcation exposes completely, orthodontic force value should be reduced significantly.

Key words: Maxillary first molar, 3-D finite element method, Alveolar bone loss, Stress distribution