PubMed · 14680016
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Stanley P Kessel. 2003. Smile analysis.. https://doi.org/10.1016/j.ajodo.2003.10.007
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A new geometry of orthodontic retraction spring was experimentally studied through an electronic device (platform for measuring forces), using strain gauges that were adapted to cantilever beams. The sample consisted of 36 titanium-molybdenum springs, divided into three groups of 12 springs each. The springs were produced with different cross sections of 0.016 x 0.022 inch and 0.017 x 0.025 inch and with different angles between the extremities (120 degrees and 130 degrees). The springs were adapted to the platform in three different positions so that the force system developed by them could be known (horizontal forces, vertical forces, alpha-beta moments, and moment-to-force ratio M:F). The analysis of factorial variance and the Tukey honestly significant difference test were applied to verify the differences between the averages caused by three possible variation sources and the interactions between them. Regression analysis was also performed to obtain the spring rate. The results show the interactions between the three geometric variables, force magnitudes, and also the spring rates, which are compatible with the ones mentioned in the literature related to the subject. The spring rate was within the levels that are appropriate for clinical use (varying from beta = 33.1 gf/mm to beta = 43.9 gf/mm).
Exposed root surfaces frequently exhibit non-carious notches representing material loss by abrasion, erosion, and/or abfraction. Although a contribution from mechanical stress is often mentioned, no definitive proof exists of a cause-effect relationship. To address this, we examined dimensional changes in dentin subjected to cyclic fatigue in two different pH environments. Human dentin cantilever-beams were fatigued under load control in pH = 6 (n = 13) or pH = 7 (n = 13) buffer, with a load ratio (R = minimum load/maximum load) of 0.1 and frequency of 2 Hz, and stresses between 5.5 and 55 MPa. Material loss was measured at high- and low-stress locations before and after cycling. Of the 23 beams, 7 withstood 1,000,000 cycles; others cracked earlier. Mean material loss in high-stress areas was greater than in low-stress areas, and losses were greater at pH = 6 than at pH = 7, suggesting that mechanical stress and lower pH both accelerate erosion of dentin surfaces.
PURPOSE: The motions and stress distributions of mandibular complete dentures during use were compared and investigated in terms of denture rigidity using 3D finite element method (FEM) stress analysis. METHODS: Four models for analysis of mandibular complete dentures with different elastic modulus (1. A resin base denture (C), 2. A resin base denture with reinforcement wire (R), 3. A metal base denture (M), 4. A metal plate denture consisting of a double structure (T)) and measured using 3D FEM. Modes were separated into two elastic bodies: the dentures and residual ridge. Analyses were executed under the same conditions total loads of 20 kgf and contact. RESULTS: 1. The total equivalent-potential strain of dentures was high beneath the load point for C and R (2.38 approximately 2.86 x 10(-3)epsilon at buccal-shelf of working side and 0.55 approximately 0.68 x 10(-3)epsilon at posterior residual ridge). The entire dentures barely distorted for M and T (0.98 approximately 1.17 x 10(-3)epsilon at buccal-shelf of working side and 0.21 approximately 0.25 x 10(-3)epsilon at posterior residual ridge). 2. In the main stress distribution of mucosa beneath denture bases, high loads were located beneath the loading point of dentures, at lingual distal areas of the working side in C and R. In M and T, increasing the rigidity of dentures reduced the stress beneath the loading point of dentures. Stress was distributed widely and evenly. CONCLUSIONS: When the rigidity of dentures is high, the strain of dentures decreases, the stress of the residual mucous membrane will evenly become distributed, and the distance of the balancing side from the denture border to the residual mucous membrane at the time of unilateral balanced position will decrease.