Residual stress and osmotic swelling of the periodontal ligament.

Osmotic swelling and residual stress are increasingly recognized as important factors in soft tissue biomechanics. Little attention has been given to residual stress in periodontal ligament (PDL) biomechanics despite its rapid growth and remodeling potential. Those tissues that bear compressive loads, e.g., articular cartilage, intervertebral disk, have received much attention related to their capacities for osmotic swelling. To understand residual stress and osmotic swelling in the PDL, it must be asked (1) to what extent, if any, does the PDL exhibit residual stress and osmotic swelling, and (2) if so, whether residual stress and osmotic swelling are mechanically significant to the PDL's stress/strain behavior under external loading. Here, we incrementally built a series of computer models that were fit to uniaxial loading, osmotic swelling and residual stretch data. The models were validated with in vitro shear tests and in vivo tooth-tipping data. Residual stress and osmotic swelling models were used to analyze tension and compression stress (principal stress) effects in PDL specimens under external loads. Shear-to-failure experiments under osmotic conditions were performed and modeled to determine differences in mechanics and failure of swollen periodontal ligament. Significantly higher failure shear stresses in swollen PDL suggested that osmotic swelling reduced tension and thus had a strengthening effect. The in vivo model's first and third principal stresses were both higher with residual stress and osmotic swelling, but smooth stress gradients prevailed throughout the three-dimensional PDL anatomy. The addition of PDL stresses from residual stress and osmotic swelling represents a unique concept in dental biomechanics.

Resistance to sliding of orthodontic archwires under increasing applied moments.

BACKGROUND: Orthodontic treatment with fixed appliances involves sliding of brackets along archwires. These movements involve friction, which causes resistance to sliding. In addition, moments cause teeth to tip until binding occurs between the bracket and archwire. The manufacturer of a new TiMolium®Titanium archwire claims material properties superior to ß-Titanium, potentially leading to reduced resistance to sliding. OBJECTIVE: To compare TiMolium archwires with ß-Titanium and stainless steel archwires as the current gold standard for sliding mechanics under application of an increasing moment. MATERIALS AND METHODS: A total of 120 stainless steel (Smartclip, 3M, Monrovia, CA) and ceramic self-ligating 0.022″-slot brackets (Clarity SL, 3M) were divided into six equal-sized groups. Resistance to sliding was tested with 0.019″ × 0.025″ TiMolium (TP Orthodontics, La Porte, IN), ß-Titanium (3M), and stainless steel (3M) archwires using a custom-designed apparatus to simulate sliding mechanics and application of moments of 1000, 2000, and 3000 g-mm. RESULTS: Using stainless steel brackets, the TiMolium archwires had significantly higher resistance to sliding than stainless steel archwires at all moments tested while there was no difference between TiMolium and ß-Titanium. Using ceramic brackets, the resistance to sliding with TiMolium archwires was no different than with stainless steel archwires. Both TiMolium and stainless steel archwires showed significantly lower resistance to sliding than ß-Titanium. CONCLUSION: TiMolium archwires have resistance to sliding intermediary to stainless steel and ß-Titanium archwires when clinically relevant moments are applied. Used with the stainless steel brackets, they behave like ß-Titanium, whereas used with the ceramic brackets, they behave more like stainless steel.

Effect of applied moment on resistance to sliding among esthetic self-ligating brackets.

OBJECTIVE: To determine the effect of mode of ligation and bracket material on resistance to sliding (RS) by comparing various esthetic brackets of conventionally ligated and self-ligating (SL) designs under an increasing applied moment in the second-order dimension. MATERIALS AND METHODS: Eight different commercially available esthetic brackets of SL and conventional elastomeric-ligated (CL) designs were mounted on a testing apparatus to simulate canine retraction using sliding mechanics and the application of a moment on 0.019″×0.025″ stainless steel archwire. The samples examined were the CL brackets Clarity™, Inspire Ice™, SpiritMB™, and Mystique™, and the SL brackets ClaritySL™, In-OvationC™, In-OvationR™, and Smartclip™. The RS at calculated moments of 2000 g-mm and 4000 g-mm was determined and compared between the various brackets. Descriptive measures and one-way analysis of variance were used to calculate means and statistical differences among the bracket types. RESULTS: The CL monocrystalline bracket displayed significantly greater (P < .05) RS than all other brackets tested. Among the other brackets, the range of RS values was 145.8-191.7 g and 291.9-389.2 g at moments of 2000 g-mm and 4000 g-mm, respectfully, though these differences were not significant (P < .05). All brackets tested displayed greater levels of RS (P < .05) at 4000 g-mm than at 2000 g-mm. CONCLUSION: With the exception of the CL monocrystalline bracket, all brackets displayed comparable amounts of RS regardless of mode of ligation or bracket slot material.

A comparison of resistance to sliding of self-ligating brackets under an increasing applied moment.

OBJECTIVE: To test the null hypotheses that at clinically relevant amounts of applied moment, there are no differences in the amount of resistance to sliding (RS) between self-ligating (SL) and conventionally ligated (CL) brackets on both stainless steel (SS) and nickel-titanium (NT) archwire. MATERIALS AND METHODS: Three different SL brackets and one CL bracket, all 0.022″ slot, were tested on a custom-built device to simulate canine retraction mechanics in the second-order dimension. The setup allowed for simultaneous and continuous measurement of RS and applied moment at the bracket-archwire interface. The brackets tested were Damon3, In-Ovation R, Smartclip, and Victory, all of which were tested with 0.019″ × 0.025″ SS and NT archwires. The RS at calculated moments of 2000 g-mm and 4000 g-mm was determined and compared between the various brackets and both archwire types. Descriptive measures, analysis of variance, and Tukey-Kramer post-test comparisons were used to calculate results. RESULTS: All brackets displayed a greater amount of RS with NT than with SS archwires. At the higher moment levels (4000 g-mm), no significant reduction in RS was found between CL and SL brackets on both SS and NT archwires. At lower levels of applied moment (2000 g-mm), reductions in RS of 18% (42.7 g) and 18% (38.5 g) were found between the CL bracket and the best performing SL bracket on NT and SS, respectively. CONCLUSION: At low values of applied moment, some statistical differences were found; however, in general, the differences in RS amongst the various SL and CL brackets tested may not be clinically relevant.

Effect of neonatal capsaicin treatment on orthodontic tooth movement in male Sprague-Dawley rats.

INTRODUCTION: In this study, we examined the effect of neonatal administration of capsaicin on the magnitude of orthodontic tooth movement in rats. METHODS: Twelve timed pregnant Sprague-Dawley rats were randomized between the capsaicin group and the vehicle group. The pups received treatment with either capsaicin or vehicle on day 2 of life. Capsaicin treatment has been shown to produce a selective destruction of fine myelinated and unmyelinated Aδ and C sensory nerve fibers, causing an inhibition of the effects from neurogenic inflammation. Tooth-movement experiments began at 12 weeks of age. A mesial tipping force was applied to the maxillary first molar by using a 3-mm length of Sentalloy closed-coil spring (Dentsply GAC Intl, Bohemia, NY) activated from a bonded molar cleat to the maxillary incisors; this appliance delivers a constant tipping force of 50 g. Diastema measurements between the first and second molars were made at 2 and 4 weeks after appliance placement. Measurements were made indirectly from stone models by using a charge-coupled device microscope camera and Optimas 5.2 measurement software (Media Cybernetics, Bethesda, Md). Two-way repeated-measures analysis of variance (ANOVA) was used to analyze the differences between the groups. RESULTS: The capsaicin-treated rats and the controls did not differ in the amount of tooth movement at the collected time points (P >0.05). Similarly, the magnitude of change of tooth movement from 2 to 4 weeks did not differ between the groups (P >0.05). An increase in average diastema size was observed between 2 and 4 weeks after appliance activation in both treatment groups (P <0.0001). CONCLUSIONS: These results suggest that neonatal capsaicin desensitization in the rat does not affect the rate of orthodontic tooth movement after the application of a 50-g tipping force to the maxillary first molar. This might be due in part to the development of compensatory mechanisms in the chronically desensitized rat. Further studies are necessary to determine the reproducibility and histologic characteristics of this treatment.

Effect of alendronate on orthodontic tooth movement in rats.

INTRODUCTION: Osteoclastic activity is required for orthodontic force to move teeth through alveolar bone. Bisphosphonates are drugs that inhibit osteoclast maturation, function, and survival. The aim of this study was to assess orthodontic tooth movement in rats receiving bisphosphonate treatment. METHODS: Two groups of Sprague-Dawley rats were used. The rats in the treatment group received 7 mg per kilogram of body weight per week of alendronate sodium, and those in the control group received no drugs. A coil spring exerting a constant 50-g force was activated across the span from the central incisors to the first molar. As the first molar tipped mesially, a diastema between the first and second molars was created. Vinyl polysiloxane impressions were poured in die stone, and the diastema was measured indirectly with a charged-couple device microscope camera and Optimas software (Media Cybernetics, Newburyport, Mass). RESULTS: Statistical analysis with repeated-measures analysis of variance showed less orthodontic tooth movement in the alendronate group compared with control group (0.06 vs 0.24 mm at 2 weeks, and 0.45 vs 1.06 mm at 4 weeks; P = 0.0004 for the alendronate vs control main effect). CONCLUSIONS: This study demonstrated an inhibitory effect of alendronate administration on orthodontic tooth movement in a rat model.

Enamel loss associated with orthodontic adhesive removal on teeth with white spot lesions: an in vitro study.

Teeth with white spot lesions (WSL) might be more prone to enamel loss during bracket debonding. This in vitro study compared enamel loss from teeth with (n = 14) and without (n = 14) WSL after polishing with low-speed finishing burs or disks (Sof-Lex, 3M ESPE, St Paul, Minn). Debonded surfaces were analyzed with a contact stylus profilometer, and digitized data were compared with baseline readings by using AnSur NT software (Regents, University of Minnesota, Minneapolis, Minn). Specimen surfaces were also examined with a scanning electron microscope. Two-way analysis of variance was performed to analyze the data. In teeth without WSL, the volume losses were 0.16 mm(3) for the bur group and 0.10 mm(3) for the disk group; the mean maximum depths were 47.7 microm for the bur group and 54.3 microm for the disk group. In teeth with WSL, the volume losses were 0.06 and 0.17 mm(3), and the mean maximum depths were 35.1 and 48.7 microm for the bur and disk groups, respectively. There were no significant differences in enamel loss between the 2 groups of teeth without WSL (P =.12). However, in teeth with WSL, the burs removed less enamel than the disks (P = 0.006). Scanning electron microscope examination showed that any damage on the enamel surface was usually located in the cervical third of the teeth. On most specimens, even though tooth surfaces appeared resin-free to the naked eye, there were remnants of it. The differences between groups were so small that they might be clinically insignificant.