Korean J Orthod.  2016 May;46(3):155-162. 10.4041/kjod.2016.46.3.155.

Effect of labiolingual inclination of a maxillary central incisor and surrounding alveolar bone loss on periodontal stress: A finite element analysis

• Choi SH
• Kim YH
• Lee KJ
• Hwang CJ

• KMID: 2164259
• DOI: http://doi.org/10.4041/kjod.2016.46.3.155

Abstract

OBJECTIVE
The aim of this study was to investigate whether labial tooth inclination and alveolar bone loss affect the moment per unit of force (M(t)/F) in controlled tipping and consequent stresses on the periodontal ligament (PDL).
METHODS
Three-dimensional models (n = 20) of maxillary central incisors were created with different labial inclinations (5°, 10°, 15°, and 20°) and different amounts of alveolar bone loss (0, 2, 4, and 6 mm). The M(t)/F necessary for controlled tipping (M(t)/F(cont)) and the principal stresses on the PDL were calculated for each model separately in a finite element analysis.
RESULTS
As labial inclination increased, M(t)/F(cont) and the length of the moment arm decreased. In contrast, increased alveolar bone loss caused increases in M(t)/F(cont) and the length of the moment arm. When M(t)/F was near M(t)/F(cont), increases in M(t)/F caused compressive stresses to move from a predominantly labial apical region to a palatal apical position, and tensile stresses in the labial area moved from a cervical position to a mid-root position. Although controlled tipping was applied to the incisors, increases in alveolar bone loss and labial tooth inclination caused increases in maximum compressive and tensile stresses at the root apices.
CONCLUSIONS
Increases in alveolar bone loss and labial tooth inclination caused increases in stresses that might cause root resorption at the root apex, despite the application of controlled tipping to the incisors.

Keyword

Labial tooth inclination; Alveolar bone loss; Controlled tipping; Periodontal stress

MeSH Terms

Alveolar Bone Loss*
Arm
Finite Element Analysis*
Incisor*
Periodontal Ligament
Root Resorption
Tooth

Figure

• 

  Figure 1 The basic model of an incisor. A, Tooth length, 24.2 mm; tooth root, 13.2 mm; bracket position midpoint, 4.5 mm. The alveolar bone (dark blue) is situated 1 mm above the cementoenamel junction (CEJ). B, Normal inclination of an incisor 60° from the occlusal plane. C, A tetrahedral mesh covers the tooth surface and bracket. M, Mesial; D, distal; B, buccal; P, palatal.

• 

  Figure 2 Models of incisors with different inclinations and different amounts of alveolar bone loss. A total of 20 models were created (5 different inclinations and 4 levels of bone loss). A, Experimental incisor models with labial inclinations of 5°, 10°, 15°, and 20°. B, Models demonstrating 0, 2, 4, and 6 mm of alveolar bone loss.

• 

  Figure 3 Forces associated with the 3 different loads applied to the tooth models. A, The first load (1) was a retraction force (100 gf) applied parallel to the occlusal plane at the midpoint of the bracket. The second load was a coupled force (2, 3) that created a counter-tipping moment (Mt). B, The third load was a coupled force (4, 5) that created a rotation moment (Mr). C, Calculation of the three-dimensional displacement (Δ) of the apex.

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  Figure 4 Changes in the length of the moment arm (the perpendicular distance between the line of action of the force and the center of resistance) according to changes in maxillary central incisor inclination and surrounding alveolar bone loss.

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  Figure 5 Relationship between the Mt/F ratio and principal stresses in tooth models with 20° of labial inclination. The minimum principal stress levels are indicated by triangles.

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  Figure 6 Incisor root stress distribution patterns under conditions with Mt/F near Mt/Fcont (red area, maximum tensile stress; blue area, maximum compressive stress). A, A tooth model with no alveolar bone loss and an incisal inclination of 15°; controlled tipping occurs at Mt/F = 6. B, A tooth model with 6 mm of alveolar bone loss and an incisal inclination of 15°; controlled tipping occurs at Mt/F = 8.

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