Assessment of orthodontic treatment efficacy of Clarity Aligners using the Peer Assessment Rating index and the American Board of Orthodontics Cast-Radiograph Evaluation.

INTRODUCTION: 3M Oral Care Solutions (St Paul, Minn) has recently introduced Clarity Aligners into the market. This cohort study evaluated the orthodontic treatment efficacy of this clear aligner system using the Peer Assessment Rating (PAR) index and the American Board of Orthodontics Cast-Radiograph Evaluation (CR-Eval). METHODS: Pretreatment and posttreatment dental models of 87 subjects who had undergone orthodontic treatment using Clarity Aligners in both arches to align their teeth to a target setup were independently evaluated by 4 examiners using the PAR index and the American Board of Orthodontics CR-Eval. Changes in CR-Eval and PAR scores from pretreatment to posttreatment were calculated, with PAR score reductions also expressed as percentages. RESULTS: Treatment with Clarity Aligners reduced the CR-Eval scores from 39.05 ± 14.98 to 30.34 ± 8.76, resulting in a statistically significant difference of 8.76 ± 11.45 between pretreatment and posttreatment scores. Similarly, aligner treatment reduced the weighted PAR scores from 13.40 ± 9.26 to 5.80 ± 4.84, resulting in a statistically significant difference of 7.50 ± 7.56 between pretreatment and posttreatment scores. The overall median PAR reduction was 53%, with 94% of the subjects having reduced PAR scores after treatment. Seventy-eight percent of subjects had >30% PAR reduction, 57% had >50% PAR reduction, and 33% had >70% PAR reduction. CONCLUSIONS: The results suggest that Clarity Aligners may be an effective treatment modality in mild to moderate malocclusions.

Midpalatal suture density ratio: Assessing the predictive power of a novel predictor of skeletal response to maxillary expansion.

INTRODUCTION: Rapid maxillary expansion (RME) is a common orthodontic treatment to correct maxillary transverse deficiency; however, the inability to determine the precise timing of fusion of the midpalatal suture creates difficulty for clinicians to prescribe the appropriate treatment, surgical or nonsurgical expansion. The purpose of this study was to assess the predictive power of the midpalatal suture density ratio (MPSD) for a skeletal response to RME. METHODS: Pre- and posttreatment cone-beam computed tomography scans were obtained from 78 orthodontic patients aged from 8 to 18 years treated with RME. MPSDs were calculated from pretreatment scans, and a prediction was made for the amount of skeletal expansion obtained at the level of the palate after comprehensive orthodontic treatment. Predicted values were compared with actual outcomes as assessed from posttreatment scans, followed by regression analyses to investigate correlations between MPSD and skeletal expansion and equivalence testing to analyze the performance of the predicted measurements. RESULTS: The MPSDs were not statistically significantly (P >0.05) correlated with the amount of skeletal expansion achieved. In addition, the predicted skeletal expansion using MPSD was not statistically equivalent to the skeletal expansion achieved using an equivalence margin of ±0.05. CONCLUSIONS: The results suggest that the MPSD obtained from pretreatment cone-beam computed tomography scans were not correlated well enough with the amount of skeletal expansion achieved to be an effective predictor of the amount of long-term skeletal expansion after RME.

Comparative assessment of bonding time and 1-year bracket survival using flash-free and conventional adhesives for orthodontic bracket bonding: A split-mouth randomized controlled clinical trial.

INTRODUCTION: A new flash-free adhesive promises to eliminate the flash removal step in bonding and to reduce bonding time by as much as 40% per bracket, with a bond failure rate of less than 2%. The aim of this trial was to compare bonding time and bracket failure rate over a 1-year period between the flash-free adhesive and a conventional adhesive for orthodontic bracket bonding. METHODS: Forty-five consecutive patients had their maxillary incisors, canines, and premolars bonded with ceramic brackets (Clarity Advanced; 3M Unitek, Monrovia, Calif) using a flash-free adhesive (APC Flash-Free Adhesive Appliance System; 3M Unitek) on 1 side and a conventional adhesive (APCII Adhesive Appliance System; 3M Unitek) on the other side. The side allocation was randomized. Bonding was timed to the nearest second. Bond failure was recorded at standardized intervals of 4 weeks. The primary outcome was bonding time (average per tooth for each patient and per quadrant). Secondary outcomes were bracket failure rate within 1 year, time to first-time failure of a bracket, and bond failure type (adhesive remnant index score). Bonding times and adhesive remnant index scores upon bond failure were compared using paired t tests, with P <0.05 considered statistically significant. The adhesives were considered equivalent if the confidence interval for the difference between bracket failure rates fell within a margin of equivalence of ±5%. RESULTS: The bonding times were significantly shorter with the flash-free adhesive than with the conventional adhesive, both per tooth (P <0.001) and per quadrant (P <0.001). Compared with the conventional adhesive, the average bonding times per tooth and per quadrant with the flash-free adhesive were 37.3% and 32.9% shorter, respectively. The bracket failure rates at 1 year were 3.7% for the flash-free adhesive and 0.9% for the conventional adhesive. This was statistically equivalent. The average times to first-time failure of a bracket were 25 weeks for the flash-free adhesive and 11 weeks for the conventional adhesive. Although there were no significant differences in the adhesive remnant index scores upon failure (P >0.05), the flash-free adhesive tended to fail more often at the enamel-adhesive interface than did the conventional adhesive. CONCLUSIONS: The use of the flash-free adhesive may result in bonding time savings of approximately one third compared with the conventional adhesive. With regard to bracket survival, a statistically significant difference was not found between the 2 adhesives when ceramic brackets were bonded. REGISTRATION: This trial was registered on December 3, 2013 (ClinicalTrials.gov ID, NCT02030002). PROTOCOL: The protocol was not published before trial commencement.

Morphological interaction between the nasal septum and nasofacial skeleton during human ontogeny.

The nasal septal cartilage is thought to be a key growth center that contributes to nasofacial skeletal development. Despite the developmental influence of the nasal septum however, humans often exhibit a high frequency of septal deviation suggesting discordance in the growth between the septum and surrounding nasofacial skeleton. While there are numerous etiological factors that contribute to septal deviation, the surrounding nasofacial skeleton may also act to constrain the septum, resulting in altered patterns of growth. That is, while the nasal septum has a direct morphogenetic influence on aspects of the nasofacial skeleton, other nasofacial skeletal components may restrict septal growth resulting in deviation. Detailing the developmental relationship between these structures is important not only for understanding the causal determinants of nasal septal deviation, but also for developing a broader understanding of the complex interaction between the facial skeleton and chondrocranium. We selected 66 non-syndromic subjects from the University of Minnesota Orthodontic Clinic who ranged from 7 to 18 years in age and had an existing pretreatment cone-beam computed tomography (CBCT) scan. Using CBCT data, we examined the developmental relationship between nasal septal deviation and the surrounding nasofacial skeleton. We measured septal deviation as a percentage of septal volume relative to a modeled non-deviated septum. We then collected a series of coordinate landmark data in the region immediately surrounding the nasal septum in the midsagittal plane representing the nasofacial skeleton. First, we examined ontogenetic changes in the magnitude of nasal septal deviation relative to chronological age and nasofacial size. Next, using Procrustes-based geometric morphometric techniques, we assessed the morphological relationship between nasal septal deviation and nasofacial skeletal shape. Our results indicate that variation in the magnitude of nasal septal deviation was established in our earliest age group and maintained throughout ontogeny. Moreover, nasal septal deviation was correlated with non-allometric variation in nasofacial shape restricted to the region of the anterior sphenoid body. Ultimately, our results suggest that early developmental variation in midline basicranial components may act to alter or constrain patterns of nasal septal growth.

Midpalatal suture density ratio: A novel predictor of skeletal response to rapid maxillary expansion.

INTRODUCTION: During adolescence, increasing interdigitation of the midpalatal suture increases resistance to rapid maxillary expansion (RME); this decreases its skeletal effect. In this study, we aimed at determining whether a novel measure of midpalatal suture maturity, the midpalatal suture density ratio, can be used as a valid predictor of the skeletal response to RME. METHODS: The midpalatal suture density ratio, chronologic age, cervical vertebral maturation, and the stage of midpalatal suture maturation were assessed before treatment for 30 patients (ages, 12.9 ± 2.1 years) who underwent RME as part of comprehensive orthodontic treatment. Measurements on cone-beam computed tomography scans were used to determine the proportions of prescribed expansion achieved at the greater palatine foramina, the nasal cavity, and the infraorbital foramina. RESULTS: There was a statistically significant negative correlation between the midpalatal suture density ratio and both the greater palatine foramina and the infraorbital foramina (r = -0.7877 and -0.3647, respectively; P <0.05). In contrast, chronologic age, cervical vertebral maturation, and stage of midpalatal suture maturation were not significantly correlated to any of the assessed measures of skeletal expansion (r range, -0.2209 to 0.0831; P >0.05). CONCLUSIONS: The midpalatal suture density ratio has the potential to become a useful clinical predictor of the skeletal response to RME. Conversely, chronologic age, cervical vertebral maturation, and stage of midpalatal suture maturation cannot be considered useful parameters to predict the skeletal effects of RME.

Debonding and adhesive remnant cleanup: an in vitro comparison of bond quality, adhesive remnant cleanup, and orthodontic acceptance of a flash-free product.

BACKGROUND/OBJECTIVES: A new flash-free adhesive promises to eliminate the need to clean up excess adhesive upon orthodontic bracket bonding. This study evaluated this new adhesive with regard to microleakage at the enamel-bracket interface, amount of adhesive remaining on the tooth after bracket debonding, time required for adhesive remnant cleanup, and clinical practitioners' preference in comparison to a conventional adhesive. MATERIALS/METHODS: A total of 184 bovine incisors were bonded with ceramic brackets using either the flash-free adhesive (APC Flash-Free Adhesive Coated Appliance System, 3M Unitek [3M], Monrovia, California, USA) or a conventional adhesive (APCII Adhesive Coated Appliance System, 3M). Twenty-four of the teeth were scanned using microcomputed tomography to quantify microleakage into the adhesive layer. Twenty orthodontists debonded the brackets, removed the remaining adhesive, and then completed a survey regarding their preference for one of the two adhesives. The adhesive remnant was quantified and the time required for its removal recorded. Differences between the adhesives were tested for statistical significance. RESULTS: For both adhesives, the microleakage was minimal with no significant differences between the two adhesives. The adhesive remnant was significantly larger for the flash-free adhesive, whereas there was no significant difference in adhesive cleanup time. Fourteen out of the 20 orthodontists preferred the flash-free adhesive over the conventional adhesive. LIMITATIONS: In vitro testing cannot replicate the actual clinical situation during in vivo debonding. CONCLUSIONS: With regard to bond quality and adhesive remnant cleanup, the new flash-free adhesive performs just as well as the conventional adhesive, and, of the two products, is the one preferred by most orthodontists.

Clinical use of a direct chairside oral scanner: an assessment of accuracy, time, and patient acceptance.

INTRODUCTION: Chairside oral scanners allow direct digital acquisition of the intraoral situation and can eliminate the need for conventional impressions. In this study, we aimed to assess accuracy, scan time, and patient acceptance of a chairside oral scanner when used for full-arch scans; these are critical factors for acceptance of this technology in the orthodontic setting. METHODS: Fifteen patients had digital models made from both intraoral scans (Lava COS; 3M ESPE, St Paul, Minn) and alginate impressions. Each procedure was timed, and patient preference was assessed with a survey. In addition, digital models were made from 5 plaster model pairs using the intraoral scanner and an orthodontic model scanner. Model pairs were digitally superimposed, and differences between models were quantified. Accuracy was assessed using the Bland-Altman method. Time differences were tested for statistical significance with the Student t test. RESULTS: Digital models made using the chairside oral scanner and either impressions or the orthodontic model scanner did not differ significantly. The chair time required to take impressions was significantly shorter than the time required for the intraoral scans. When processing time was included, the time requirement did not differ significantly between methods. Although 73.3% of the patients preferred impressions because they were "easier" or "faster," 26.7% preferred the scan because it was "more comfortable." CONCLUSIONS: Despite the high accuracy of chairside oral scanners, alginate impressions are still the preferred model acquisition method with respect to chair time and patient acceptance. As digital technology continues to progress, intraoral scanning may become more accepted for use in orthodontics.

Accuracy, reproducibility, and time efficiency of dental measurements using different technologies.

INTRODUCTION: Historically, orthodontists have taken dental measurements on plaster models. Technological advances now allow orthodontists to take these measurements on digital models. In this study, we aimed to assess the accuracy, reproducibility, and time efficiency of dental measurements taken on 3 types of digital models. METHODS: emodels (GeoDigm, Falcon Heights, Minn), SureSmile models (OraMetrix, Richardson, Tex), and AnatoModels (Anatomage, San Jose, Calif) were made for 30 patients. Mesiodistal tooth-width measurements taken on these digital models were timed and compared with those on the corresponding plaster models, which were used as the gold standard. Accuracy and reproducibility were assessed using the Bland-Altman method. Differences in time efficiency were tested for statistical significance with 1-way analysis of variance. RESULTS: Measurements on SureSmile models were the most accurate, followed by those on emodels and AnatoModels. Measurements taken on SureSmile models were also the most reproducible. Measurements taken on SureSmile models and emodels were significantly faster than those taken on AnatoModels and plaster models. CONCLUSIONS: Tooth-width measurements on digital models can be as accurate as, and might be more reproducible and significantly faster than, those taken on plaster models. Of the models studied, the SureSmile models provided the best combination of accuracy, reproducibility, and time efficiency of measurement.

Mental wellbeing of frontline health workers post-pandemic: lessons learned and a way forward.

Objective: To assess the state of mental wellbeing among medical and dental frontline health workers as the COVID-19 pandemic transitions to an endemic phase and to determine what employer-provided intervention strategies these workers perceive as effective and desirable to improve their mental wellbeing. Methods: An anonymous online survey distributed to frontline health workers in a hospitalist program of a tertiary care medical center and a university dental school in Minnesota in September 2022. The survey contained validated tools to measure depression severity, levels of perceived stress, and mental health status as well as questions to determine effective strategies to improve emotional wellbeing among these health workers. Data was evaluated on an aggregate level as well as stratified by level (e.g., physician, staff) and field (e.g., medicine, dentistry). Results: On average, all groups of health workers suffered from moderate to moderately severe depression, had a much higher perceived stress level than average, and had a fair mental health status. There were no significant differences in depression severity, stress level, or mental health status among physicians, dentists, medical staff, and dental staff. The majority of the respondents perceived adjusted work hours, rewards and incentives, and teamwork as the most effective and desirable strategies to improve their mental wellbeing. Conclusion: The current mental wellbeing of frontline health workers is poor. Many are dissatisfied with healthcare and consider leaving the industry. To improve their employees' mental wellbeing, healthcare employers might want to consider adjusted work hours, rewards, and teamwork as these intervention strategies are perceived as most effective and desirable by the intended recipients.

Radiographic Evaluation of Predictors for Spontaneous Space Closure and Supereruption After the Loss of Permanent First Molars in Yup'ik Children.

PURPOSE: The purpose of this study was to evaluate pre-extraction variables associated with spontaneous space closure between the permanent second molar (PSM) and the second premolar following early extraction of the permanent first molar (PFM). Additionally, this study aimed to assess supereruption in compensated and uncompensated maxillary molars to determine if compensating extraction increases the odds of spontaneous space closure. METHODS: A total of 134 patients aged six to 12 years with extracted PFM(s) were evaluated for spontaneous man- dibular space closure. Panoramic radiographs were reviewed to assess pre-extraction variables. Bitewing radiographs of 156 patients aged six to 13 years with previous PFM extractions were used to measure supereruption in compensated and uncompensated extractions. Both compensated and uncompensated extractions were evaluated for complete mandibular space closure. RESULTS: Statistically significant predictors of space closure included extraction at age eight to 10 years (P=0.04; 95 percent confidence interval [95% CI] equals 0.08 to 0.91), permanent third molar presence (P=0.02; 95% CI equals 1.16 to 4.9), and follow-up time (P=0.001; 95% CI equals 1.16 to 1.69). The odds of uncompensated PFM super- eruption were greater than those of compensated PFM supereruption (P<0.001; 95% CI equals 1.86 to 6.92). Additional follow-up increased the odds of supereruption (P<0.001; 95% CI equals 1.08 to 1.30). Uncompensated extractions did not reduce the odds of spontaneous space closure (P=0.54; 95% CI equals 0.56 to 3.08). CONCLUSIONS: Permanent first molar extraction later than age 10 years is a negative predictor for spontaneous space closure, while permanent third molar presence is a positive predictor. Uncompensated maxillary PFMs do not inhibit spontaneous per- manent mandibular second molar space closure; however, uncompensated extractions are more likely to supererupt.

Wear and fatigue resistance: An in-vitro comparison of three polyethylene terephthalate glycol and thermoplastic polyurethane materials for vacuum-formed retainers.

OBJECTIVE: To test the wear and fatigue resistance of three materials (Essix ACE®, Taglus®, and Zendura A®) for the fabrication of vacuum-formed retainers in an artificial oral environment. MATERIAL AND METHODS: Wear resistance was tested by subjecting 21 retainers of each Essix ACE®, Taglus®, and Zendura A® to 12,000 wear cycles at 75N to simulate one year of retainer wear with moderate nighttime bruxing. Post-wear retainer thickness was compared to baseline measurements to calculate wear depth. Fatigue resistance was tested by flexing 15 retainers of each material at an angle of 25 degrees for 1,825 cycles to simulate one year of removing and reinserting a retainer five times per day. Retainers were visually inspected for fractures. Pairwise t-tests with correction using Tukey's method were used to determine significant differences between materials. RESULTS: The mean wear depths were 0.155±0.021mm, 0.168±0.031mm, and 0.096±0.033mm for Essix ACE®, Taglus®, and Zendura A®, respectively. The wear depth of Zendura A® was significantly lower than that of both Essix ACE® (P<0.001) and Taglus® (P<0.001). There was no significant difference in wear depth between Essix ACE® and Taglus® (P=0.312). Under the parameters set for the fatigue resistance test, fractures did not occur on any of the tested materials. CONCLUSIONS: Under the assumption of moderate nighttime bruxing for one year, Zendura A® is the most wear-resistant among the materials tested. With the assumption of retainer removal and reinsertion five times per day for one year, all three materials tested have the same ability to resist fatigue.

Dosimetry of a cone-beam computed tomography machine compared with a digital x-ray machine in orthodontic imaging.

INTRODUCTION: Cone-beam computed tomography (CBCT) has become a routine imaging modality for many orthodontic clinics. However, questions remain about the amount of radiation patients are exposed to during the scans. This study determined the amounts of radiation potentially absorbed by a patient during orthodontic imaging with a CBCT machine with various scan settings compared with a conventional 2-dimensional digital x-ray machine. METHODS: The radiation exposures delivered by a next generation i-CAT CBCT machine (Imaging Sciences International, Hatfield, Pa) at various scan settings and orthopantomograph OP100/OC100 digital x-ray machine (Instrumentarium Dental, Tuusula, Finland) during panoramic and cephalometric radiography were recorded using thermoluminescent dosimeters placed inside a head and neck phantom. The manufacturer-recommended settings for an average adult male were used for both types of machines. Effective doses were calculated using the tissue-weighting factors recommended by the 2007 International Commission on Radiological Protection. RESULTS: The effective doses at various voxel sizes and field of view settings ranged from 64.7 to 69.2 µSv for standard resolution CBCT scans (scan time 8.9 s) and 127.3 to 131.3 µSv for high resolution full field of view scans (scan time 17.8 s), and measured 134.2 µSv for a high-resolution landscape scan with a voxel size as would be used for SureSmile (OraMetrix, Richardson, Tex) therapy (scan time 26.9 s). The effective doses for digital panoramic and lateral cephalometric radiographs measured 21.5 and 4.5 µSv, respectively. CONCLUSIONS: CBCT, although providing additional diagnostic and therapeutic benefits, also exposes patients to higher levels of radiation than conventional digital radiography.

Dynamic force decay evaluation of latex and non-latex orthodontic elastics.

PURPOSE: To evaluate the force decay over time of latex and non-latex orthodontic elastics subjected to either static or dynamic stretching under simulated intraoral conditions. MATERIALS AND METHODS: Four types of elastics (1/4-inch 4.5 ounces and 1/4-inch 6.5 ounces, each latex and non-latex) were subjected to either static stretching to 3 times internal diameter (ID) or dynamic stretching from 3 to 4.5 times ID in artificial saliva at 37 °C for 24 h. Forces generated by the elastics were measured at 0, 1, 2, 4, 8, 12, and 24 h. Differences among elastic types, time points, and between stretching regimens were tested for statistical significance (P < 0.05). RESULTS: Both stretching regimens caused rapid force decay in all elastic types, which was significantly higher in the non-latex elastics than in the latex elastics. In contrast, there were no differences between elastic types made of the same material. With both stretching regimens, the force decay was significant only after the first hour for the latex elastics, whereas it remained significant up to 24 h for the non-latex elastics. All elastic types generated significantly lower forces after dynamic stretching than after static stretching with 70.2, 68.8, and 66.1% of the initial force remaining after 4, 8, and 24 h for latex elastics and 48.0, 40.8, and 29.5% for non-latex elastics. CONCLUSION: Latex elastics retained significantly more force over time than their non-latex equivalents. Because of the higher force decay in a dynamic environment, it is important that non-latex elastics be changed more frequently.

Transfer accuracy of 3D-printed trays for indirect bonding of orthodontic brackets.

OBJECTIVES: To evaluate the transfer accuracy of 3D-printed indirect bonding trays constructed using a fully digital workflow in vivo. MATERIALS AND METHODS: Twenty-three consecutive patients had their incisors, canines, and premolars bonded using fully digitally designed and 3D-printed transfer trays. Intraoral scans were taken to capture final bracket positioning on teeth after bonding. Digital models of postbonding scans were superimposed on those of corresponding virtual bracket setups, and bracket positioning differences were quantified. A total of 363 brackets were evaluated. One-tailed t-tests were used to determine whether bracket positioning differences were within the limit of 0.5 mm in mesiodistal, buccolingual, and occlusogingival dimensions, and within 2° for torque, tip, and rotation. RESULTS: Mean bracket positioning differences were 0.10 mm, 0.10 mm, and 0.18 mm for mesiodistal, buccolingual, and occlusogingival measurements, respectively, with frequencies of bracket positioning within the 0.5-mm limit ranging from 96.4% to 100%. Mean differences were significantly within the acceptable limit for all linear dimensions. Mean differences were 2.55°, 2.01°, and 2.47° for torque, tip, and rotation, respectively, with frequencies within the 2°-limit ranging from 46.0% to 57.0%. Mean differences for all angular dimensions were outside the acceptable limit; however, this may have been due to limitations of scan data. CONCLUSIONS: Indirect bonding using 3D-printed trays transfers planned bracket position from the digital setup to the patient's dentition with a high positional accuracy in mesiodistal, buccolingual, and occlusogingival dimensions. Questions remain regarding the transfer accuracy for torque, tip, and rotation.

Orbital volume changes during growth and development in human children assessed using cone beam computed tomography.

OBJECTIVES: To measure growth-related changes in orbital volume from childhood to the late teenage years using cone-beam computed tomography (CBCT) scans. METHODS: This retrospective cohort study involved 65 (24 male, 41 female) healthy Caucasian children (ages 6-18 years) with existing serial craniofacial CBCT scans. CBCT scans were available for 292 orbits. Each orbit was transformed into a closed space with well-defined boundaries, and orbital volume was measured using manual segmentation. A novel statistical analysis was applied to extract the maximum amount of longitudinal information from the data. Intra- and inter-operator correlation coefficients were calculated from replications performed on a random subset of 10% of the sample. RESULTS: Orbital volume increased at a rate of 1-2% annually until the late teenage years. Intra- and inter-operator agreement between repeated measurements were >90%. CONCLUSIONS: Orbital volume increases by 1-2% per year throughout childhood continuing until the late teenage years. This annual increase is large enough to be clinically relevant as it may lead to less-than-optimal long term surgical outcomes when reconstructive surgery for the pediatric anophthalmic socket is required.

The masticatory system under varying functional load. Part 2: Effect of reduced masticatory load on the degree and distribution of mineralization in the rabbit mandible.

A reduction in mechanical loading of the mandible brought about by mastication of soft food is assumed to decrease the remodelling rate of bone, which, in turn, might increase the degree of bone mineralization. The effect of a reduction in masticatory functional load on the degree and distribution of mineralization of mandibular bone was investigated in male juvenile New Zealand White rabbits. The experimental animals (n=8) had been raised on a diet of soft pellets from 8 to 20 weeks of age, while the controls (n=8) had been fed pellets of normal hardness. The degree of mineralization of bone (DMB) was assessed at the attachment sites of various jaw muscles, the condylar head, and the alveolar process. Differences between groups and among sites were tested for statistical significance using a Student's t-test and one-way analysis of variance, respectively. The DMB did not differ significantly between the experimental and control animals at any of the sites assessed. However, in the rabbits that had been fed soft pellets, both cortical bone at the attachment sites of the temporalis and digastric muscles and cortical bone in the alveolar process had a significantly higher DMB than cortical bone at the attachment site of the masseter muscle, while there were no significant differences among these sites in the control animals. The results suggest that a moderate reduction in masticatory functional load does not significantly affect the remodelling rate and the DMB in areas of the mandible that are loaded during mastication but might induce a more heterogeneous mineral distribution.

The masticatory system under varying functional load. Part 1: Structural adaptation of rabbit jaw muscles to reduced masticatory load.

Skeletal muscle fibres can change their myosin heavy-chain (MyHC) isoform and cross-sectional area, which determine their contraction velocity and maximum force generation, respectively, to adapt to varying functional loads. In general, reduced muscle activity induces transition towards faster fibres and a decrease in fibre cross-sectional area. In order to investigate the effect of a reduction in masticatory load on three functionally different jaw muscles, the MyHC composition and the corresponding cross-sectional area of fibres were determined in the superficial masseter, superficial temporalis, and digastric muscles of male juvenile New Zealand White rabbits that had been raised on a soft diet (n=8) from 8 to 20 weeks of age and in those of normal diet controls (n=8). Differences between groups were tested for statistical significance using a Mann-Whitney rank sum test. The proportion and cross-sectional area of fibres co-expressing MyHC-I and MyHC-cardiac alpha were significantly smaller in the masseter muscles of the animals that had been fed soft food than in those of the controls. In contrast, the proportions and cross-sectional areas of the various fibre types in the temporalis and digastric muscles did not differ significantly between the groups. The results suggest that reducing the masticatory load during development affects the contraction velocity and maximum force generation of the jaw-closing muscles that are primarily responsible for force generation during chewing. These muscles adapt structurally to the reduced functional load with changes in the MyHC composition and cross-sectional area mainly within their slow fibre compartment.

Buccal alveolar bone changes following rapid maxillary expansion and fixed appliance therapy.

OBJECTIVES: To assess factors that may be associated with buccal bone changes adjacent to maxillary first molars after rapid maxillary expansion (RME) and fixed appliance therapy. MATERIALS AND METHODS: Pretreatment (T1) and posttreatment (T2) cone-beam computed tomography scans were obtained from 45 patients treated with RME and preadjusted edgewise appliances. Buccal alveolar bone thickness was measured adjacent to the mesiobuccal root of the maxillary first molar 4 mm, 6 mm, and 8 mm apical to the cementoenamel junction, and anatomic defects were recorded. Paired and unpaired t-tests were used to compare alveolar bone thickness at T1 and T2 and to determine whether teeth with posttreatment anatomic defects had thinner initial bone. Correlation analyses were used to examine relationships between buccal alveolar bone thickness changes and amount of expansion, initial bone thickness, age at T1, postexpansion retention time, and treatment time. RESULTS: There was a statistically significant reduction in buccal alveolar bone thickness from T1 to T2. Approximately half (47.7%) of the teeth developed anatomic defects from T1 to T2. These teeth had significantly thinner buccal bone at T1. Reduction in alveolar bone thickness was correlated with only one tested variable: initial bone thickness. CONCLUSIONS: RME and fixed-appliance therapy can be associated with significant reduction in buccal alveolar bone thickness and an increase in anatomic defects adjacent to the expander anchor teeth. Anchor teeth with greater initial buccal bone thickness have less reduction in buccal bone thickness and are less likely to develop posttreatment anatomic defects of buccal bone.