Our objective was to determine whether the elevation of a full-thickness mucoperiosteal flap alone, without cortical cuts, decreases the amount of bone around teeth and accelerates mesial tooth movements.
The mandibular second premolars of 7 beagle dogs were extracted, and on a randomly selected side, a full-thickness mucoperiosteal buccal flap extending from the distal aspect of the third premolar to the mesial aspect of the first premolar was elevated. The other side did not receive flap surgery. The mandibular third premolars were protracted with orthodontic appliances. Tooth movements were analyzed biweekly over an 8-week period with calipers and radiographs. The amount and density of bone were analyzed using microcomputed tomography; bone remodeling was evaluated with histologic sections.
Experimental tooth movements measured intraorally between cusp tips were significantly greater (25.3%) than control tooth movements. The approximate center of resistance measured radiographically also moved significantly more (about 31%) on the experimental than on the control side. The experimental premolar tipped more than the control premolar (10.5° vs 8.7°), but the difference was not statistically significant. The medullary bone volume fraction mesial to the third premolar was significantly less (9.1%) and the bone was significantly less dense (9%) on the experimental side than on the control side. Histology showed no apparent side differences in the numbers of osteoclasts and osteoblasts evident in the medullary bone.
Elevation of a full-thickness mucoperiosteal flap alone (ie, without injury to bone) decreases the amount and density of medullary bone surrounding the tooth and accelerates tooth movement. Due to its limited effects, elevation of a flap alone to increase tooth movements may not be justified.
Full-thickness mucoperiosteal flap elevation increases the rate of orthodontic tooth movement.
Flap elevation decreases the bone volume fraction and density of medullary bone in the surgical area.
Numbers of osteoclasts and osteoblasts on each side were similar after 8 weeks.
Orthodontic treatment duration ranges from 21 to 27 months for nonextraction patients and from 25 to 35 months for extraction patients. Treatment time is influenced by many factors, including type of malocclusion, amount of tooth movement required, mechanics used, and patient compliance. Prolonged treatment times are problematic because they are associated with increased risks of root resorption, decalcification, and periodontal problems. To minimize these risks, orthodontists are continually trying to reduce treatment time, while providing treatment results equal to or better than those currently being delivered.
The best way to reduce treatment time is to increase the rate of tooth movement. Corticotomy surgeries are currently the most popular means of facilitating tooth movement. Experimental results show that corticotomies approximately double the rates of tooth movement. Rates are increased by injuring the bone and inducing the regional acceleratory phenomenon (RAP). Corticotomies injure the bone in 2 ways: by elevating a full-thickness mucoperiosteal flap and by cutting or perforating cortical bone. Unexpectedly, cortical bone damage to depths of 2 to 3 mm without mucoperiosteal flap elevation has no effect on tooth movements. Even when the flapless procedures produced the same amount of cortical bone damage as traditional corticotomies, tooth movements were not accelerated because the RAP effects were restricted to the cortical bone. This emphasizes the potential importance of flap elevation or the interaction of flap elevation and cortical damage.
In 1994, Yaffe et al showed that the RAP can be induced in rats by the elevation of full-thickness mucoperiosteal flaps alone (ie, without cortical bone cuts). However, they readapted the flaps in place without sutures, which could have confounded the results. Moreover, rats are not good models for humans because of differences in bone composition, density, quality, and turnover rates. In addition, performing flap surgery in animals the size of rats probably produces a noxious stimulus greater than that produced in humans. Experiments evaluating the effects of mucoperiosteal flap surgery on bone should be performed using larger animal models, such as dogs, which are superior to rodents, and even many larger mammals for studying bone. Because the effect of flap surgery alone on orthodontic tooth movement has not previously been evaluated, it remains unclear whether the effects extend to the bone surrounding teeth that need to be moved.
The null hypothesis of this study was that flap elevation alone has no effect on tooth movements or bone surrounding the tooth to be moved. Understanding the unique role that flap surgery plays in producing the RAP effect should provide important insights into the potential mechanisms responsible for accelerating tooth movements with corticotomies.
Material and methods
Seven skeletally mature male beagle dogs, weighing between 20 and 25 pounds, were used in this experiment. All animals had fully erupted dentitions and were healthy. Housing, care, and experimental protocols were approved by the Institutional Animal Care and Use Committee at Texas A&M University, Baylor College of Dentistry in Dallas. During the experiment, the dogs were fed a soft diet to minimize damage to the orthodontic appliances.
After a 10-day quarantine, the animals fasted for 12 hours and then were sedated with an intramuscular injection of ketamine (8-24 mg/kg) mixed with xylazine (0.22 mg/kg). Dental prophylaxis using an ultrasonic scalar irrigated with 0.12% chlorhexidine gluconate was performed, and bone markers (6-mm long Imtec miniscrew implants; 3M Unitek, Monrovia, Calif) were placed for radiographic reference. Four markers were placed in the mandible, 2 on each side. The heads of the screws were removed to the level of the gingival tissue for animal comfort.
Right and left periapical radiographs were taken before and after bone marker placement using size 4 film. A custom holder was designed to standardize film and x- ray tube angulations and distances. To standardize intraoral measurements, notches were cut into the cusp tips of the canines and third premolars and at the most gingival aspect of the mesiobuccal groove of the first molars. Digital calipers were used to measure the distance between the canines and third premolars, as well as the distance between the third premolars and first molars. Three replicate caliper measurements were made for each distance and averaged. Triad custom tray material (Dentsply, York, Pa) was used to make mandibular impression trays, and alginate impressions of the mandibles were taken. The impressions were poured in die stone, and the models were used for appliance fabrication.
Appliances were designed based on an established protocol. Orthodontic band material (Dentaurum, Ispringen, Germany) was custom pinched and welded to fit the mandibular canines and third premolars. Headgear tubes (diameter, 0.051 indiameter) (3M Unitek) were soldered to the orthodontic bands on the third premolars.
Orthodontic wire (diameter, 0.045 in) was soldered to the canine bands and inserted through the headgear tubes on the bands of the third premolars ( Fig 1 , A ). The wire was designed to have a loop at the distolingual aspect of the canine band to attach a spring at the time of appliance delivery. The third premolars were able to move freely along the wire. The interior aspect of each band was microabraded before cementation. For animal comfort, a ball of solder was placed on the distal end of the wire.
After initial sedation with the ketamine and xylazine cocktail previously described, the dogs were intubated and administered 1% to 1.5% isoflurane in oxygen at a rate of 1 L per minute. Vital signs were monitored throughout. Local anesthetic (2% lidocaine with 1:100,000 epinephrine) was administered at the surgical sites via regional infiltration and an inferior alveolar block.
Both mandibular second premolars were sectioned, elevated, and extracted. On the experimental side, chosen using an electronically generated random number table, a full-thickness mucoperiosteal flap was elevated from the distal aspect of the third premolar to the mesial aspect of the first premolar. A vertical releasing incision was made at the mesial limit of the flap that went past the keratinized gingiva and into the level of the buccal mucosa ( Fig 1 , B ). The tissue was reapproximated with simple interrupted 4-0 vicryl resorbable sutures.
A 12-fluted carbide bur was used to remove calculus and debris from the teeth and to cut retention grooves around the canines and third premolars. The teeth were etched with 37% phosphoric acid gel for 15 seconds. After irrigating and drying the teeth, the appliance was cemented as 1 unit on each side of the arch with light-cured RelyX Unicem (3M ESPE, St Paul, Minn) resin cement. Excess cement was removed to prevent gingival irritation.
The appliances were activated by attaching a 9-mm heavy nickel-titanium coil spring (3M Unitek) from the third premolar headgear tube attachment to the soldered wire loop on the canine using 0.012-in stainless steel ligatures ( Fig 1 , C ). The springs were activated to 200 g, which was verified with a Correx force gauge (Haag-Streit, Bern, Switzerland). Postoperative periapical radiographs were taken.
Immediately after surgery, the dogs were given 1 dose of ketoprofen (1 mg/kg intramuscularly). For the next week, they were also given postoperative analgesic (nalbuphine, 2 mg/kg intramuscularly) and antibiotics (clindamycin, 11 mg/kg intramuscularly) twice daily. Intraoral caliper measurements and periapical radiographs were taken every 2 weeks for 8 weeks after surgery. At each of these occasions, the nickel-titanium spring was checked and retied to ensure that it was delivering 200 g of force.
After 9 weeks of third premolar protraction (day 56), the animals were again sedated with the previously described ketamine and xylazine cocktail, and final records were obtained, including periapical radiographs, caliper measurements, and impressions. Surgical plane anesthesia was then confirmed, and the common carotid arteries were both cannulated, and the external jugular veins severed. An intracardial injection of 2 cm 3 of beuthanasia-D was given. After cessation of heart function was confirmed, approximately 1.5 L of saline solution followed by 1 L of 70% ethanol was flushed through the cannulas. The mandible was then harvested and stored in 70% ethanol.
Periapical radiographs were imported into Viewbox 4.0 (DHAL Software, Kifissia, Greece). The mesial and distal alveolar crest of the fourth premolar, mesial and distal root apices of the third premolar, and furcation of the third premolar were digitized ( Fig 2 ). The long axis of the third premolar was defined by a line extending from the midpoint between the root apices to the furcation. The angle formed by the long axis of the third premolar and the alveolar crest landmark was used to assess tipping. The midpoint of the furcal bone of the third premolar was calculated and used to assess translation of the tooth’s center of resistance. This midpoint was projected perpendicularly onto the plane defined by the alveolar crest landmarks, and the distance to the mesial crest of the fourth premolar was measured.
After the animals were killed, all specimens were stripped of soft tissue, sectioned to fit into 27-mm wide microcomputed tomography tubes, and scanned using a microcomputed tomography 35 scanner (Scanco Medical, Basserdorf, Switzerland). The specimens were stabilized so that the occlusal plane was approximately perpendicular to the long axis of the specimen tubes, which were filled with 70% ethanol and sealed with Parafilm (Pechiney Plastic Packaging, Chicago, Ill).
The specimens were imaged at 30 μm resolution, 55 kVp, 145 μA, and 600 ms integration time. The images were smoothed with a Gaussian filter of sigma equal to 0.9 and support equal to 1. With the operator blinded, a medullary bone volume (1.4 × 1 × 3 mm) mesial to the third premolar was segmented (using a minimum threshold of 270) and analyzed with the microcomputed tomography software (version 6.0; Scanco). The medullary bone segment was located within the area of the surgical flap. It was the bone into which the third premolar was being moved. The inferior limit of the volume was located 25 slices apical to the third premolar distal root apex; the superior limit was 50 slices coronal from the inferior limit. It was within the center of the medullary bone, parallel to the lingual cortex ( Fig 3 ). Bone volume/total volume, apparent bone density, and material bone density were measured on both the control and experimental sides.
The specimens were cut to include bone immediately mesial to the mandibular third premolar and distal to the first premolar, from the alveolar crest to the level of the mental foramina. The specimens were decalcified in ethylenediaminetetraacetic acid, dehydrated in graded alcohol, cleared with xylene, and then infiltrated and embedded in paraffin. Using a coronal orientation, the block was sectioned into slices 5 to 10 μm thick, starting with the section closest to the mesial root of the third premolar. Every sixth slice was mounted, producing a total of 6 sections. These sections were mounted on glass slides and stained with hematoxylin and eosin to evaluate osteoclast populations. The slides were visualized and photographed under a microscope (Eclipse 80i; Nikon, Tokyo, Japan).
Multilevel statistical models were used to statistically determine the shape of the curve describing tooth movement and to evaluate differences between the experimental and control sides. The multilevel models were developed using the MLwiN software (version 2.01; Center for Multilevel Modeling, Institute of Education, London, United Kingdom). The iterative generalized least squares method was used to estimate model parameters.
Each model’s fixed portion allowed for determination of the polynomial that best fit the repeated measurements of tooth movement as a function of time. The terms of the polynomial provided information about the tooth movements at day 56 (intercept terms), rates of tooth movements (intercept terms), and acceleration or deceleration of tooth movements (quadratic term). The terms of the polynomial were derived by initially fitting third-order polynomials and testing the terms statistically. The statistical significance of the terms, as well as the group differences in tooth movements, were assessed using confidence intervals derived from their associated standard errors. The higher-order terms of the polynomial were rejected in a sequential fashion until statistical significance ( P <0.05) was attained. The constant terms described the tooth movement at day 56, the linear terms described the rate of change (velocity), and the quadratic terms described the change in rate (acceleration).
Data from the microcomputed tomography analysis were normally distributed and described using means and standard deviations (SPSS software version 22.0; Armonk, NY). The experimental and control sides were compared using 1-tailed paired t tests and a significance level of P <0.05.
After surgery, healing proceeded normally with no signs of swelling or infection. Four dogs had damaged appliances during the experiment, due to either bond failure or spring detachment ( Table I ). All appliances were repaired within 48 hours of when the damage occurred. Tooth movements of dog F were not evaluated, because of the number of appliance breakages that occurred and the aberrant tipping of the third premolars.
|Debonded appliance||Detached spring||Total|
Intraoral caliper measurements showed that the third premolar was protracted mesially. Tooth movements followed a quadratic pattern, with significant side differences in the amount ( P <0.01) and rate of tooth movement ( P <0.05), for both intraoral measurements ( Table II ). Relative to the first molars, the third premolars moved 4.6 and 5.6 mm on the control and experimental sides, respectively ( Fig 4 , A ). Relative to the canines, the third premolars moved 3.1 mm on the control side and 3.9 mm on the experimental side ( Fig 4 , B ). The differences amounted to approximately 21.7% and 25.8% when measured relative to the first molars and canines, respectively. The relative overall difference in tooth movement, based on the average of the 2 estimates, was 25.3%.
|Experimental-side tooth movement||Differences between experimental and control side tooth movement|
|Third premolar to canine||5.58||3.40e-1||1.89||2.19e-1||1.22e-1||5.05e-2||−9.30e-1||2.93e-1||−2.36e-1||1.20e-1||NS||NS|
|Third premolar to first molar||3.83||2.28e-1||1.38||1.47e-1||9.65e-2||3.38e-2||−6.37e-1||1.96e-1||−1.97e-1||8.01e-2||NS||NS|