Effect of miniscrew placement torque on resistance to miniscrew movement under load.

INTRODUCTION: The primary stability of orthodontic anchorage miniscrews is believed to result from mechanical interlock, with success based upon a number of variables, including screw diameter, angle of placement, monocortical vs bicortical placement, placement through attached or unattached soft tissue, presence or absence of a pilot hole, periscrew inflammation, and maximum placement torque. The purpose of this ex-vivo study was to further explore the relationship between maximum placement torque during miniscrew placement and miniscrew resistance to movement under load. METHODS: Ninety-six titanium screws were placed into 24 hemi-maxillae and 24 hemi-mandibles from cadavers between the first and second premolars by using a digital torque screwdriver. All screws were subjected to a force parallel to the occlusal plane, pulling mesially until the miniscrews were displaced by 0.6 mm. The Spearman rank correlation test was used to evaluate whether there was an increasing or a decreasing relationship between maximum placement torque of the screws, miniscrew resistance to movement, and bone thickness. A paired-sample t test and the nonparametric Wilcoxon signed rank test were used to compare maximum placement torque, bone thickness, and miniscrew resistance to movement between coronally positioned and apically positioned screws in the maxilla and the mandible, and between screws placed in the maxilla vs screws placed in the mandible. Additionally, 1-way analysis of variance (ANOVA) with the post-hoc Tukey-Kramer test was used to determine whether there was a significant difference in miniscrew resistance to movement for screws placed with maximum torque of <5 Ncm, 5 to 10 Ncm, and >10 Ncm. RESULTS: The mean difference in miniscrew resistance to movement between maximum placement torque groupings, <5 Ncm, 5 to 10 Ncm, and >10 Ncm, increased throughout the deflection range of 0.0 to 0.6 mm. As deflection increased to 0.12 to 0.33 mm, the mean resistance to movement for miniscrews with maximum placement torque of 5 to 10 Ncm was statistically greater than for screws with maximum placement torque <5 Ncm (P <0.05). As deflection increased to 0.34 to 0.60 mm, the mean resistance to movement for miniscrews with maximum placement torque of 5 to 10 Ncm and >10 Ncm was significantly greater than for screws with maximum placement torque <5 Ncm (P <0.05). At no deflection was there a significant difference in resistance to movement between the 2 miniscrew groups with higher placement torque values of 5 to 10 Ncm and >10 Ncm. CONCLUSIONS: Ex vivo, the mean resistance to movement of miniscrews with higher maximum placement torque was greater than the resistance to movement of those with lower maximum placement torque.

Effect of miniscrew angulation on anchorage resistance.

INTRODUCTION: Even though the use of titanium miniscrews to provide orthodontic anchorage has become increasingly popular, there is no universally accepted screw-placement protocol. Variables include the presence or absence of a pilot hole, placement through attached or unattached soft tissue, and angle of placement. The purpose of this in-vitro study was to test the hypothesis that screw angulation affects screw-anchorage resistance. METHODS: Three-dimensional finite element models were created to represent screw-placement orientations of 30°, 60°, and 90°, while the screw was displaced to 0.6 mm at a distance of 2.0 mm from the bone surface. In a parallel cadaver study, 96 titanium alloy screws were placed into 24 hemi-sected maxillary and 24 hemi-sected mandibular specimens between the first and second premolars. The specimens were randomly and evenly divided into 3 groups according to screw angulation (relative to the bone surface): 90° vs 30° screw pairs, 90° vs 60° screw pairs, and 30° vs 60° screw pairs. All screws were subjected to increasing forces parallel to the occlusal plane, pulling mesially until the miniscrews were displaced by 0.6 mm. A paired-samples t test was used to assess the significance of differences between 2 samples consisting of matched pairs of subjects, with matched pairs of subjects including 2 measurements taken on the same subject. One-way analysis of variance (ANOVA) with the post-hoc Tukey studentized range test was conducted to determine whether there were significant differences, and the order of those differences, in anchorage resistance values among the 3 screw angulations at maxillary and mandibular sites. RESULTS: The finite element analysis showed that 90° screw placement provided greater anchorage resistance than 60° and 30° placements. In the cadaver study, although the maximum anchorage resistance provided by screws placed at 90° to the cadaver bone surface exceeded, on average, the anchorage resistance of the screws placed at 60°, which likewise exceeded the anchorage resistance of screws placed at 30°, these differences were not statistically significant. CONCLUSIONS: Placing orthodontic miniscrews at angles less than 90° to the alveolar process bone surface does not offer force anchorage resistance advantages.

Mandibular "tripod" advancement of a Class II Division 2 deepbite malocclusion.

This case report describes the treatment of a 25-year-old woman with a Class II malocclusion, secondary to mandibular skeletal deficiency, and mild overclosure. Inferior surgical repositioning of the maxilla is often the treatment of choice for patients with maxillary vertical deficiency; however, this patient had borderline vertical deficiency that was treated with a mandibular "tripod" advancement (leveling of the mandibular arch after surgery) coupled with a setback and down-grafting genioplasty. The surgical-orthodontic treatment plan, combined with cosmetic dentistry, resulted in dramatically improved facial esthetics and occlusal relationships.

Effect of screw diameter on orthodontic skeletal anchorage.

INTRODUCTION: Many case reports have documented the successful use of titanium miniscrews for orthodontic anchorage. However, the literature lacks a well-controlled study examining the effect of miniscrew diameter on anchorage force resistance. The purpose of this in-vitro study was to compare the force resistance of larger-diameter monocortical miniscrews to smaller-diameter monocortical miniscrews; and to compare the force resistance of larger-diameter monocortical miniscrews to smaller-diameter bicortical miniscrews. METHODS: Ninety-six titanium alloy screws were placed into 24 hemisected maxillary and 24 hemisected mandibular specimens between the first and second premolars. Specimens were randomly and evenly divided into 2 groups. In the first group, 24 large-diameter screws (2.5 x 17 mm) and with 24 small-diameter screws (1.5 x 15 mm) were placed monocortically. In the second group, 24 large-diameter screws (2.5 x 17 mm) were placed monocortically and 24 small-diameter screws (1.5 x 15 mm) were placed bicortically. All screws were subjected to tangential force loading perpendicular to the miniscrew with lateral displacement of 0.6 mm. Statistical analyses, including the paired-samples t test and the 2-samples t test, were used to quantify screw force-deflection characteristics. One-way analysis of variance (ANOVA) with the post-hoc Tukey studentized range test was used to determine any significant differences, and the order of those differences, in force anchorage values among the 3 screw types at maxillary and mandibular sites. RESULTS: Mean mandibular and maxillary anchorage force values of the 2.5-mm monocortical screws were significantly greater than those of the 1.5-mm monocortical screws (P <0.01). No statistically significant differences in mean mandibular anchorage force values were found between the 2.5-mm monocortical screws and the 1.5-mm bicortical screws. However, mean maxillary anchorage force values of the 1.5-mm bicortical screws were significantly greater than those of the 2.5-mm monocortical screws (P <0.01). Data analyzed with 1-way ANOVA with the post-hoc Tukey studentized range tests indicated that the mean mandibular and maxillary force values of the 2.5-mm monocortical screws and the 1.5-mm bicortical screws were significantly greater than those of the 1.5-mm monocortical screws (P <0.01). Based on the 2-samples t test, mean anchorage force values at mandibular sites were significantly greater than at maxillary sites for the 2.5-mm monocortical screws and the 1.5-mm monocortical screws. There were no statistically significant differences in mean anchorage force values between maxillary and mandibular sites for the 1.5-mm bicortical screws. CONCLUSIONS: In vitro, larger-diameter (2.5 mm) monocortical screws provide greater anchorage force resistance than do smaller-diameter (1.5 mm) monocortical screws in both the mandible and the maxilla. Smaller-diameter (1.5 mm) bicortical screws provide anchorage force resistance at least equal to larger-diameter (2.5 mm) monocortical screws. An alternative to placing a larger-diameter miniscrew for additional anchorage is a narrower bicortical screw.

Bicortical vs monocortical orthodontic skeletal anchorage.

INTRODUCTION: Case reports have documented the use of titanium miniscrews in providing skeletal anchorage for orthodontic tooth movement. Success rates as low as 50% have been reported for screw retention in either the facial or the lingual cortical plates (monocortical placement). The purpose of this in-vitro study was to test the hypothesis that bicortical miniscrew placement (across the entire width of the alveolus) gives the orthodontist superior force resistance and stability (anchorage) compared with monocortical placement. METHODS: Forty-four titanium alloy screws, 1.5 x 15.0 mm, were placed in 22 hemi-sected maxillae and mandibular specimens between the first and second premolars. Half were placed monocortically, half were placed bicortically, and all were subjected to tangential force loading perpendicular to the miniscrew through a lateral displacement of 1.5 mm. Bone samples were sectioned and bone thickness at the screw sites measured. Statistical analyses, consisting of paired samples t tests, 2-samples t tests, Spearman rank correlation tests, and Fisher exact tests, were used to compare monocortical with bicortical screw force-deflection characteristics and stability. Additionally, 2-dimensional plane-stress finite-element models of bicortical and monocortical screw placement subjected to similar loading were analyzed. RESULTS: As hypothesized, deflection force values were significantly greater for bicortical screws than for monocortical screws placed in both the maxilla and the mandible (P <0.01 in each instance). Furthermore, force values at mandibular sites were significantly greater than those at maxillary sites for both types of screws. No significant differences in deflection force values were found between the right and left sides of the jaws, or between coronal and apical alveolar-process screw positions. A significant increasing relationship was found between mandibular buccal bone thickness and deflection force for monocortical screws only, and no relationship was found between maxillary bone thickness and deflection force for monocortical or bicortical screws. Monocortical screws were significantly more mobile after force application than bicortical screws. Finite-element analysis indicated lower cortical bone stresses with bicortical placement than with monocortical placement, and these results were consistent with in-vitro experimental findings. CONCLUSIONS: Bicortical miniscrews provide the orthodontist superior anchorage resistance, reduced cortical bone stress, and superior stability compared with monocortical screws.

Friction does not increase anchorage loading.

Conventional wisdom suggests that orthodontists must apply added force to overcome friction during canine retraction (sliding mechanics), the result of which can be increased anchorage loading and anchorage loss. However, for a frictional force to be exerted mesially by the archwire against a canine during retraction, the archwire must be compressed between the canine and the anchor molar, and an equal but opposite force must be applied distally against the molar by the archwire. In other words, the frictional force that reduces the force of retraction on the canine must also reduce the protraction force on the molar. Emphasis on employing reduced-friction (eg, self-ligating) brackets during sliding mechanics to prevent added posterior anchorage loading is unwarranted and based more on bracket salesmanship than on orthodontic biomechanics.

Longitudinal study of facial skeletal growth completion in 3 dimensions.

INTRODUCTION: Previous authors have suggested that transverse facial skeletal growth is completed before either anteroposterior or vertical growth and that anteroposterior growth is completed before vertical growth. Our purpose in this study was to examine this concept. METHODS: Twenty-four subjects (11 male, 13 female) who had annual lateral and posteroanterior cephalograms up to and including age 17 or 18 and again at age 25 or older were identified from the Iowa Facial Growth Study. Transverse, anteroposterior, and vertical facial dimensions were measured longitudinally into adulthood by using key skeletal landmarks. Descriptive statistics were calculated, and nonparametric Wilcoxon signed rank tests were performed separately for the sexes to determine the age at which each anteroposterior, vertical, and transverse growth variable reached adult size. RESULTS: For both sexes, an overlap exists at any age in the amount of growth completed for the various measurements in the transverse, anteroposterior, and vertical dimensions. Although some transverse measures (cranial width and interjugal width) attain adult size before any anteroposterior or vertical ones, there is evidence of continued growth for other transverse parameters (interzygomatic width and intergonial width). A similar overlap is seen in the anteroposterior and vertical dimensions. CONCLUSIONS: A distinct separation, by time and dimension, is not seen in the amount of facial growth completed during development. Instead, a dramatic spread and an overlap of growth curves are observed throughout the developing years.

The effects of altered maxillary growth on patterns of mandibular rotation in a pig model.

OBJECTIVES: A thorough understanding of influence of maxillary growth on patterns of mandibular rotation during development is important with regard to the treatment of skeletal discrepancies. In the present study, we examined whether experimentally altered maxillary position has a significant influence on patterns of mandibular rotation in a pig model. DESIGN: Maxillary growth was altered in a sample of n=10 domestic pigs via surgical fixation of the circummaxillary sutures. We compared the experimental group to control and surgical sham samples and assessed the effects of altered maxillary growth on mandibular form using geometric morphometric techniques. We tested for significant differences in mandibular shape between our samples and examined axes of morphological variation. Additionally, we examined whether altered mandibular shape resulting from altered maxillary position was predictably associated with morphological changes to the condylar region. RESULTS: There was a statistically significant difference in mandibular shape between the experimental and control/sham groups. As a result of vertical displacement of the snout, mandibles in the experimental sample resulted in greater anterior rotation when compared to the control/sham pigs. Variation in rotation was correlated with morphological changes in the condyle including the shape of the articular surface and condylar orientation indicative of greater anterior mandibular rotation. CONCLUSIONS: Vertical displacement of the maxilla had a significant effect on mandibular shape by encouraging anterior mandibular rotation. This result has important implications for understanding the effects of altered mandibular posture on condylar remodeling the treatment of skeletal discrepancies such as the correction of hyperdivegent mandibular growth.