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Stability of immediately loaded 3- and 6-mm miniscrew implants in beagle dogs—a pilot study
ORIGINAL ARTICLE
Stability of immediately loaded 3- and 6-mm miniscrew implants in beagle dogs—a pilot study Micah G. Mortensen,a Peter H. Buschang,b Donald R. Oliver,c Hee-Moon Kyung,d and Rolf G. Behrentse Surprise, Ariz, Dallas, Tex, St. Louis, Mo, and Daegu, Korea Introduction: We compared the stability of 3- and 6-mm long miniscrew implants (MSIs) loaded with orthopedic force levels. Methods: Using a split-mouth experimental design, we placed MSIs into the jaws of 5 mature beagle dogs and immediately loaded them for 6 weeks. Continuous forces were applied by reciprocally loading pairs of MSIs with nickel-titanium coil springs. The mandibles had the 3-mm MSIs randomly loaded with forces of 600 or 900 g. In the maxilla, the 3- and 6-mm MSIs were randomly assigned and loaded with 600 g of force. An unloaded, control MSI was placed in each quadrant. Overall success was defined as MSIs that remained intact; net success rates excluded MSIs that had sheared off and all implants placed in a dog that frequently chewed his run bars and food bowl. Results: The overall success rates of the loaded and the control 6-mm MSIs were 100%. Overall and net success rates for the 3-mm experimental MSIs were 66.7% and 95.2%, respectively. Similarly, the overall and net success rates of the 3-mm control MSIs were 66.7% and 81.8%, respectively. The overall success rates of the 3-mm mandibular MSIs loaded with 900 and 600 g of force were both 60%; their net success rates were 100% with 900 g and 85.7% with 600 g. Overall success rates of the 3-mm experimental MSIs in the maxilla and the mandible were 80% and 60%, repectively. The net success rates were 100% (maxilla) and 85.7% (mandible). There were no significant (P . 0.05) differences in stability associated with force or location. The loaded 3- and 6-mm MSI pairs demonstrated significant decreases in interimplant distance, averaging 2.2 and 1.8 mm, respectively; the 3-mm MSIs loaded with 900 g showed significantly more displacement than those loaded with 600 g. Conclusions: Success rates of immediately loaded 3-mm MSIs were significantly lower than those of immediately loaded 6-mm MSIs. Neither force nor location explained differences in the success rates. The linear displacements of the MSIs were associated with load amount rather than implant length. (Am J Orthod Dentofacial Orthop 2009;136:251-9)
A
lthough ‘‘absolute’’ anchorage is often pursued, it is seldom achieved. Auxiliary sources of anchorage, such as headgears, palatal buttons, and transpalatal or lingual arches, are typically used to preserve anchorage. These devices are limited because they do not allow for complete control over dental movements. Furthermore, they rely on patient compliance, which often is suboptimal.1
a
Private practice, Surprise, Ariz. Professor and director of Orthodontic Research, Department of Orthodontics, Baylor College of Dentistry, Texas A&M University System Health Science Center, Dallas, Tex. c Assistant professor, Department of Orthodontics, Center for Advanced Dental Education, St. Louis University, St. Louis, Mo. d Professor and chair, Department of Orthodontics, School of Dentistry, Kyungpook National University, Daegu, Korea. e Professor and chair, Department of Orthodontics, Center for Advanced Dental Education, St. Louis University, St. Louis, Mo. Supported by the Orthodontic Education and Research Foundation of Saint Louis University, St Louis, Mo. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Peter H. Buschang, 3302 Gaston Ave, Dallas, TX 75246; e-mail, [email protected]. Submitted, May 2007; revised and accepted, March 2008. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.03.016 b
Several types of devices are used to establish absolute anchorage, avoid unwanted tooth movement, and minimize the need for patient compliance. Restorative implants (approximately 4 mm in diameter and 7-18 mm long) have been used for orthodontic anchorage,2-6 but the surgery, arch space, healing times, and placement locations limit their use.7 Palatal implants (3.3-3.75 3 4-6 mm) also require invasive surgeries for placement and removal, and mechanical attachments to palatal implants are more complicated than their restorative counterparts.8 The effectiveness and efficiency of onplants that do not penetrate bone but also require surgical procedures and extended healing times have yet to be established.9,10 Recently, miniscrew implants (MSIs) have drawn orthodontists’ attention. Their small size (1.2-2 3 5-12 mm) allows them to be placed almost anywhere, with only minimal surgery for placement. MSIs are also much less expensive than restorative implants. Unfortunately, human studies have shown variable success rates with MSIs: ranging from 49% to 100%.11,12 Animal studies, which allow for greater control than human studies, perhaps are the best way to evaluate factors associated with MSI success. Owens et al13 and Carrillo et al14 reported 86% (18 of 21) and 99% (95 251
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of 96) success rates (defined as lack of gross mobility) after immediately loading 6-mm MSIs in beagles. A 94% success rate (remaining intact in bone) was recently demonstrated for 6-mm MSIs immediately loaded with 600 g of force.15 Asikainen et al,16 who placed 4.5-mm MSIs loaded with 250 to 350 g of force into the foreheads of sheep, had 100% success after 3 months. Ohmae et al17 placed 4-mm long MSIs into the jaws of beagles with 100% success after loading them with 150 g of force for 12 to 18 weeks. The shortest documented MSI is 4 mm long.17 However, Doi15 reported that many 6-mm MSIs that remained stable had only 3.5 mm of threads placed into the bone. This suggests that the cortical plate provides enough bone for MSI stability. Shorter MSIs might be expected to subject the patient to less risk and discomfort during placement, but their stability remains to be established. The purpose of this 6-week experimental study was to compare the difference in stability between 3- and 6-mm long MSIs immediately loaded with 600 g of force, as well as the difference in stability between 3-mm long MSIs immediately loaded with either 600 or 900 g of force. MATERIAL AND METHODS
The sample included 5 healthy male beagle dogs (ages, 10-15 months; weight, about 9 kg). They received a soft diet 3 weeks before MSI placement and were housed individually. All procedures were approved by the Animal Care Committee of Saint Louis University in St Louis, Mo. The dogs were injected with buprenorphine hydrochloride (Carpuject, Hospira, Lake Forest, Ill), 0.01 mg per kilogram subcutaneously, and carprofen (Rimadyl, Pfizer Animal Health, Exton, Pa), 4 mg per kilogram subcutaneously, for analgesia. Acepromazine maleate (VEDCO, St Joseph, Mo) was given, 0.04 mg per kilgram, subcutaneously as a preanesthetic agent, along with 10 mg per kilogram of ampicillin (Fort Dodge Animal Health, Fort Dodge, Iowa) as a prophylactic measure. Induction was performed with 3 to 4 mg per kilogram of propofol (Baxter Healthcare, Deerfield, Calif) intravenously and 1% to 2% isoflurane (IsoSol, VEDCO). An intravenous drip 0.9% sodium chloride (Baxter Healthcare) was given at a rate of 20 mg per kilogram per hour to maintain hydration. Initial periapical radiographs were taken to determine whether there was adequate space for MSI placement. The implant sites were first swabbed with 0.12% chlorhexidine gluconate (Acclean, Henry Schein, Melville, NY) and infiltrated with 0.25% bupivicaine hydrochloric acid (Abbott Laboratories, North Chicago, Ill). Six mandibular MSIs were placed through the alve-
American Journal of Orthodontics and Dentofacial Orthopedics August 2009
olar mucosa, approximately 5 mm apically to the root furcations of the third and fourth premolars and the first molar. A tissue punch was used to prevent the mandibular mucosa from wrapping around the MSI during placement. Six maxillary MSIs were placed between the roots of the second and third premolars and the first molar, approximately 3 mm palatally to the gingival margins of the teeth. All MSIs were placed with a hand driver without predrilling. Periapical radiographs were taken after placement to verify that the roots had not been contacted. Two lengths of MSIs were used for the study: 3 and 6 mm (Fig 1). Although the 6-mm MSIs are commercially available as part of the AbsoAnchor system (Dentos, Daegu, Korea), the 3-mm MSIs were specially manufactured for this project by Dentos. All MSIs measured 1.3 mm in diameter, with pointed and notched tips. The coronal surface of each MSI was dimpled with a diamond bur; the dimples served as registration points for the interimplant measurements. Twelve MSIs were placed in each of the 5 dogs, including 8 experimental (loaded) and 4 control implants (Fig 2). The 3-mm MSIs in the mandible were randomly loaded with either 600 or 900 g of force. Loaded with 600 g of force, 3- and 6-mm MSIs were randomly assigned to the right and left sides of the maxilla. All MSIs were oriented perpendicularly to the bony surface. The maxillary MSIs were placed palatally because excessive tissue overgrowth around buccally placed MSIs has been reported.13-15 Two experimental MSIs were placed in each quadrant approximately 20 mm apart, depending on root location. A control MSI of the same length was placed slightly apically and anteriorly to the midpoint between the 2 experimental MSIs. Primary stability was initially confirmed by exerting manual pressure with the hand driver and a cotton forceps. Mobility of the MSIs was measured with the Periotest device (Medizintechnik Gulden, Lautertal, Germany).18,19 Linear measurements were made between pairs of experimental MSIs in each quadrant by using a digital caliper (Chicago Brand Industrial, Fremont, Calif). The interimplant distance and the mobility measurements were repeated 3 times, and the middle values were used for the statistical analyses. The experimental MSIs were loaded by ligating 2 9-mm nickel-titanium coil springs (428-650, The Orthodontic Store, Gaithersburg, Md) between each pair. The distances that the coil springs needed to be activated to deliver orthopedic force levels (600 and 900 g) were predetermined with a force gauge to be 13.4 and 18.2 mm, respectively. These distances were verified intraorally by using the digital caliper. The springs were ligated to the MSIs with 0.012-in stainless steel ligature
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Statistical analysis
Fig 1. MSIs, 6 and 3 mm.
wires. The ends of the wires were encapsulated with flowable composite (Natural Elegance, Henry Schein) to minimize soft-tissue irritation and ensure that the appliances remained intact. The control MSIs remained unloaded. The dogs were monitored daily for 6 weeks to verify that the appliances were intact and to evaluate the soft tissues. The study areas were irrigated daily with 0.12% chlorhexidine gluconate (Acclean) to flush food particles from the MSIs and springs. Subcutaneous injections of 10 mg per kilogram of aqueous ampicillin (Fort Dodge Animal Health) and 4 mg per kilogram of subcutaneous carprofen (Rimadyl) were given for 10 days after MSI placement to control inflammation and pain. Because of severe tissue inflammation and overgrowth around the mandibular implants and springs, 1 dog required an additional 21 days of carprofen administration. MSIs that were pulled out during the first 5 weeks were replaced within 1 to 7 days; those that failed during the last week of the study were not replaced. Replacement MSIs provided only an anchor for the force being placed on the original, intact, experimental MSI; no assessments of the replaced MSIs were performed. Six weeks after initial MSI placement, the dogs were killed with a lethal dose (3-5 mL) of pentobarbital (Euthanasia-5, Henry Schein). The distances between each pair of study MSIs were again measured 3 times with a digital caliper.
MSI success was defined as lack of MSI pullout. Chisquare tests were used to determine whether MSI success was associated with length (3 vs 6 mm), location (maxilla vs mandible), and force level (600 vs 900 g). When appropriate (ie, any cell N \5), the Fisher exact test was used to determine the statistical significance of the chi-square values. The phi coefficient was also used to evaluate the relationship between success and length. Intraclass correlations, described in terms of Cronbach alpha, were used to determine the reliability of the 3 successive Periotest measurements. The Wilcoxon signed-rank test for related samples was used to evaluate changes in interimplant distances (displacement). The 2 Periotest mobility values of the MSI pairs were averaged, and Pearson product-moment correlations were used to analyze the relationship between initial mobility and displacement. To determine the association between initial MSI mobility and success, the Periotest values were rescaled to 4 ordinal values as follows: 8 to 16, 17 to 121, 122 to 136, and 137 to 150. The Spearman rho correlation coefficient was used to analyze the correlation between MSI success and initial mobility. All statistical analyses were performed by using SPSS software (version 14.0, SPSS, Chicago, Ill). RESULTS
The overall success rates of the 3- and 6-mm experimental MSIs were 66.7% and 100%, respectively (Table I), representing a statistically significant (chisquare 5 4.03, P 5 0.045) difference. Of the 10 3-mm experimental MSIs that failed, the tips of 5 had sheared during placement (Fig 3). Sixty percent (6 of 10) of the 3-mm experimental MSIs that failed were from dog number 3 (2 of the 6 MSIs were sheared). This dog was described by the supervising veterinarian as unusually active and was regularly observed chewing on its food bowl and the run bars of its cage. The net success rate of 3-mm experimental MSIs, excluding those that sheared and all in dog number 3, was 95.2%. After these exclusions, the difference in success rates between 3- and 6-mm experimental MSIs was not statistically significant (chi-square 5 0.35, P 5 0.557). The overall success rates of the 3- and 6-mm control MSIs were 66.7% and 100%, respectively (Table I). Of the 5 3-mm control MSIs that failed, 1 had a sheared tip, and 1 belonged to dog number 3, resulting in a net success rate of 81.8%. Neither the overall (chi-square 5 1.98, P 5 0.160) nor the net (chi-square 5 0.93, P 5 .334) differences in success rates between 3- and 6-mm control MSIs were statistically significant.
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Fig 2. MSI placement locations and forces applied.
The phi coefficient showed a low, significant association (f 5 0.32, P 5 0.045) between experimental MSI length and success. The control MSIs showed no significant association between length and success (f 5 0.33, P 5 0.160). The overall success rates of the 3-mm experimental MSIs were 80% in the maxilla and 60% in the mandible (Table II). After excluding those that sheared and those in dog number 3, the success rates were 100% for the maxilla and 85.7% for the mandible. The differences in success between 3-mm experimental MSIs in the maxilla and the mandible were not statistically significant either before (chi-square 5 1.20, P 5 0.273) or after (chi-square 5 0.65, P 5 0.421) the exclusions.
The overall success rates of the 3-mm control MSIs were also 80% in the maxilla and 60% in the mandible (Table II). After the exclusions, the success rates of the 3-mm control MSIs were 100% for those in the maxilla and 71.4% for those in the mandible. The differences in success between control MSIs in the maxilla and the mandible were not statistically significant either before (chi-square 5 0.60, P 5 0.439) or after (chi-square 5 1.07, P 5 0.301) the exclusions. The overall success rates of the 3-mm mandibular MSIs loaded with 600 and 900 g of force were both 60% (Table III). After the exclusions, the success rates were 100% for 900 g of force and 85.7% for 600 g of force. The differences in success rates between the 3-mm mandibular MSIs loaded with 600 and 900 g of
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Table I.
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Success rates by length of MSIs
Length Experimental MSIs 3 mm 6 mm Control MSIs 3 mm 6 mm Total 3 mm 6 mm
Number placed
Number failed
Overall success rate
Number failed that were sheared
Success rate excluding sheared implants
Number failed from dog number 3
Net success rate
30 10
10 0
66.7% (20/30) 100% (10/10)
5 0
80% (20/25) 100% (10/10)
6 0
95.2% (20/21) 100% (8/8)
15 5
5 0
66.7% (10/15) 100% (5/5)
1 0
71.4% (10/14) 100% (5/5)
2 0
81.8% (9/11) 100% (4/4)
45 15
15 0
66.7% (30/45) 100% (15/15)
6 0
76.9% (30/39) 100% (15/15)
8 0
90.6% (29/32) 100% (12/12)
Fig 3. Scanning electron microscope images of MSIs: A, normal MSI; B, 6-mm MSI that sheared during placement; C, sheared 3-mm MSI.
force were not statistically significant either before (chisquare 5 0.00, P 5 1.000) or after (chi-square 5 0.93, P 5 0.335) the exclusions. All loaded implant pairs showed significant decreases in interimplant distances during the 6-week experimental period. The mean displacement of the 8 3-mm MSI pairs that remained intact throughout the experiment was 2.2 mm, with a range of 0.4 to 4.4 mm. The 5 pairs of loaded 6-mm MSIs that remained intact showed a mean decrease in interimplant distance of 1.8 mm, with a range of 1.0 to 3.4 mm. The difference in linear displacement between the 3- and 6-mm MSI pairs was not statistically significant (P 5 0.369).
The mean displacement of the 3-mm MSI pairs loaded with 600 g of force was 2.0 mm (n 5 6), compared with a mean displacement of 3.1 mm for the 3-mm MSI pairs loaded with 900 g of force (n 5 2). This difference was statistically significant (P 5 0.012). The displacement of the 3-mm maxillary MSIs closely approximated the displacement of the 3-mm mandibular MSIs. The intraclass correlations showed good reliability between successive mobility measurements with the Periotest. The average Cronbach alpha was 0.97. There was no significant correlation between initial Periotest mobility and success of the experimental (r 5 0.10, P 5 0.506) or control (r 5 0.27, P 5 0.350) MSIs.
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Table II.
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Success rates of 3-mm MSIs by location
Length Maxilla Experimental MSIs 3 mm Control MSIs 3 mm Total 3 mm Mandible Experimental MSIs 3 mm Control MSIs 3 mm Total 3 mm
Table III.
Length
Number placed
Number failed
Overall success rate
Number of failed implants that were sheared
Success rate excluding sheared implants
Number of failed implants from dog number 3
10
2
80% (8/10)
0
80% (8/10)
2
100% (8/8)
5
1
80% (4/5)
0
80% (4/5)
1
100% (4/4)
15
3
80% (12/15)
0
80% (12/15)
3
100% (12/12)
20
8
60% (12/20)
5
80% (12/15)
4
85.7% (12/14)
10
4
60% (6/10)
1
1
71.4% (5/7)
30
12
60% (18/30)
6
66.7% (6/9) 75% (18/24)
5
Net success rate
81% (17/21)
Success rates of mandibular MSIs by force level Number placed
Number failed
Overall success rate
Number of failed implants that were sheared
Success rate excluding sheared implants
Number of failed implants from dog number 3
Net success rate
10
4
60% (6/10)
3
85.7% (6/7)
2
100% (6/6)
10
4
60% (6/10)
2
75% (6/8)
2
85.7% (6/7)
900-g load 3 mm 600-g load 3 mm
There also was no statistically significant correlation between initial Periotest mobility and displacement of the experimental MSIs (r 5 0.01, P 5 0.993). DISCUSSION
During the study, 25% (10 of 40) of the experimental and 25% (5 of 20) of the control MSIs failed. Six of these had sheared tips. Since only 1 of the 30 MSIs that remained stable had a sheared tip, it appears that shearing might have contributed to those failures. The screw tip is designed to draw the rest of the screw into the bone. A damaged tip could cause the threaded barrel to strip the bone when turned. For that reason, success rates were also calculated after excluding the sheared MSIs. Success rates were also calculated after excluding all MSIs in dog number 3, which was regularly observed chewing on the run bars and food bowl. Although these exclusions complicate the interpretation of the results, they identify important external factors that might cause MSI failures. The study was based on the notion that MSIs gain most of their primary stability (mechanical retention) from the cortical plate. Due to the differences in success rates of the 3- and 6-mm MSIs, primary stability might have been increased by contact with the medullary bone. This could
explain why none of the 6-mm experimental MSIs failed. Because secondary stability is low during the first few weeks of healing (Fig 4), adequate primary stability is critical for short-term MSI success. The length of the MSI does not necessarily equal the depth of its placement. Caraway20 showed that the 6-mm MSIs used in our study were placed an average of 3.9 mm into the bone; the 3-mm MSIs were placed an average of 1.6 mm; the cortical plate thickness for the dogs in this study was approximately 2.1 mm. On that basis, the average 6-mm MSI passed through the cortical plate and into the medullary bone, whereas the average 3-mm MSI did not completely pass through the cortex. Since most (7 of 10) of the 3-mm experimental MSIs that failed did so within the first 2 weeks of the study (Fig 5), it is possible that they were not placed far enough into the cortical bone to attain primary stability. If the MSIs had been completely placed into bone, the success rate of the 3-mm MSIs might have been higher. There was no significant difference in success rates between the 3-mm MSIs immediately loaded with either 600 or 900 g of force; this is consistent with previous animal studies comparing lower forces. Owens et al13 found no differences in the success rates of
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 136, Number 2
Primary
Secondary
257
Total
100 90 80
Stability (%)
70 60 50 40 30 20 10 0 0
1
2
3
4
5
6
7
Time (weeks)
Fig 4. Stability patterns of endosseous dental implants (adapted from Raghavendra et al30 with permission by Quintessence Publishing Co Inc, copyright 2005).
Number of failed implants
4
3
2
1
0
*
*
*
0 1 2 3 4 5 6 7
* *
*
*
*
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
Day of failure Control
Experimental
Fig 5. Failure times for 3-mm MSIs: D indicates an MSI that was sheared; * indicates an MSI in dog number 3.
6-mm MSIs loaded with either 25 or 50 g of force; Carrillo et al14 also reported no significant differences in the success rates of 6-mm MSIs loaded with 25, 50, or 100 g; Doi15 found that the success rates of 6-mm MSIs loaded with 300 or 600 g were similar. This suggests that force levels simply must be low enough not to crush the surrounding bone before stability is affected. Such force levels are probably outside the range of those used in typical orthodontic situations, since the maxi-
mum pull-out force of 6-mm MSIs has been reported from 134 from 388 N (1 N 5 102 g).21,22 The 3-mm control MSIs had an overall success rate similar to that of the 3-mm experimental MSIs. However, the control MSIs failed both early and late (Fig 5). Because they were unloaded, force cannot be implicated in their failure. Although it is unclear exactly why they failed at the same rate as the loaded ones, both length and depth of placement could be contributing factors.
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The MSIs showed significant linear displacements during the study, depending on the amount of force applied. MSI displacement was reported in both immediately15 and delayed22,23 loaded MSIs. The 6-mm experimental MSIs in our study were displaced approximately 0.9 mm (1.8 mm interimplant distance divided by 2); this was more than previously reported by Doi.15 Because the 3-mm MSIs loaded with 900 g were displaced more than those loaded with 600 g, the increased force appears to be a contributing factor. Although the average displacement of the 3-mm MSI pairs was 2.2 mm, the apex was displaced only about 0.3 mm. Since this displacement could have been a step toward eventual MSI failure, more MSIs might have failed if the study had been longer. It is also possible that the displacement occurred during the first few weeks of the study, with the MSIs retained by the tangentially applied forces until healing occurred. Once bony healing establishes secondary stability, mobility can actually be reduced.24 There was no association between initial Periotest mobility and success or failure of MSIs, or displacement of the experimental MSIs. Further research is warranted to establish the validity of the Periotest with MSIs. The success rate of 3-mm experimental MSIs in the mandible was 20% lower than those placed in the maxilla. Although this difference was not statistically significant, other authors found lower success rates in the mandible.25-28 Mandibular bone is denser than maxillary bone, and density might affect the success rates of MSIs.29 All 6 failed MSIs that had sheared tips were in the mandible. It is possible that the 6-mm MSIs would have also sheared if they had been placed in the mandible. It appears that the increased torque required to place an MSI through denser bone might alter the integrity of MSIs that are 1.3 mm in diameter and lead to material failure. This supports the practice of predrilling sites in the mandible when smaller MSIs are used. This study was limited by the shearing of several MSIs and the behavior of dog number 3. A longer study period could have provided more information regarding the stability of the 3-mm MSIs. The major limitation of this study, however, was the lack of statistical power; this makes it impossible to say that the force level applied or the placement location played no role in stability. Thus, a larger sample size would most likely have allowed for a more meaningful interpretation of the results. Further studies are certainly warranted. CONCLUSIONS
1.
Overall, the success rates of immediately loaded 3-mm MSIs were significantly lower than those of
American Journal of Orthodontics and Dentofacial Orthopedics August 2009
2.
3.
4.
immediately loaded 6-mm MSIs. After the exclusions for plausible causes, however, the difference was not statistically significant. The difference in success rates between 3-mm mandibular MSIs loaded with 600 or 900 g of force was not significant. Both the 3- and 6-mm MSIs immediately loaded with 600 or 900 g of force experienced significant linear displacements during the study. There was no relationship between initial mobility and success or failure of MSIs or eventual displacement of the experimental MSIs.
We thank George Vogler for his assistance during this project, Heidi Israel for her statistical help, and Dentos for its support.
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22. Huja SS, Litsky AS, Beck FM, Johnson KA, Larsen PE. Pull-out strength of monocortical screws placed in the maxillae and mandibles of dogs. Am J Orthod Dentofacial Orthop 2005;127:307-13. 23. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic forces? Am J Orthod Dentofacial Orthop 2004;126:42-7. 24. Roberts WE, Marshall KJ, Gongloff RK. Rigid endosseous implants for orthodontic and orthopedic anchorage. Angle Orthod 1989;59:247-56. 25. Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2006;130:18-25. 26. Tseng YC, Hsieh CH, Chen CH, Shen YS, Huang IY, Chen CM. The application of mini-implants for orthodontic anchorage. Int J Oral Maxillofac Surg 2006;35:704-7. 27. Cheng SJ, Tseng IY, Lee JJ, Kok SH. A prospective study of the risk factors associated with failure of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Implants 2004;19:100-6. 28. Deguchi T, Takano-Yamamoto T, Kanomi R, Hartsfield JK Jr, Roberts WE, Garetto LP. The use of small titanium screws for orthodontic anchorage. J Dent Res 2003;82:377-81. 29. Park HS, Lee YJ, Jeong SH, Kwon TG. Bone density of the alveolar and basal bone in the maxilla and the mandible. Am J Orthod Dentofacial Orthop 2009 (in press). 30. Raghavendra S, Wood MC, Taylor TD. Early wound healing around endosseous implants: a review of the literature. Int J Oral Maxillofac Implants 2005;20:425-31.
Stability of immediately loaded 3- and 6-mm miniscrew implants in beagle dogs—a pilot study Micah G. Mortensen,a Peter H. Buschang,b Donald R. Oliver,c Hee-Moon Kyung,d and Rolf G. Behrentse Surprise, Ariz, Dallas, Tex, St. Louis, Mo, and Daegu, Korea Introduction: We compared the stability of 3- and 6-mm long miniscrew implants (MSIs) loaded with orthopedic force levels. Methods: Using a split-mouth experimental design, we placed MSIs into the jaws of 5 mature beagle dogs and immediately loaded them for 6 weeks. Continuous forces were applied by reciprocally loading pairs of MSIs with nickel-titanium coil springs. The mandibles had the 3-mm MSIs randomly loaded with forces of 600 or 900 g. In the maxilla, the 3- and 6-mm MSIs were randomly assigned and loaded with 600 g of force. An unloaded, control MSI was placed in each quadrant. Overall success was defined as MSIs that remained intact; net success rates excluded MSIs that had sheared off and all implants placed in a dog that frequently chewed his run bars and food bowl. Results: The overall success rates of the loaded and the control 6-mm MSIs were 100%. Overall and net success rates for the 3-mm experimental MSIs were 66.7% and 95.2%, respectively. Similarly, the overall and net success rates of the 3-mm control MSIs were 66.7% and 81.8%, respectively. The overall success rates of the 3-mm mandibular MSIs loaded with 900 and 600 g of force were both 60%; their net success rates were 100% with 900 g and 85.7% with 600 g. Overall success rates of the 3-mm experimental MSIs in the maxilla and the mandible were 80% and 60%, repectively. The net success rates were 100% (maxilla) and 85.7% (mandible). There were no significant (P . 0.05) differences in stability associated with force or location. The loaded 3- and 6-mm MSI pairs demonstrated significant decreases in interimplant distance, averaging 2.2 and 1.8 mm, respectively; the 3-mm MSIs loaded with 900 g showed significantly more displacement than those loaded with 600 g. Conclusions: Success rates of immediately loaded 3-mm MSIs were significantly lower than those of immediately loaded 6-mm MSIs. Neither force nor location explained differences in the success rates. The linear displacements of the MSIs were associated with load amount rather than implant length. (Am J Orthod Dentofacial Orthop 2009;136:251-9)
A
lthough ‘‘absolute’’ anchorage is often pursued, it is seldom achieved. Auxiliary sources of anchorage, such as headgears, palatal buttons, and transpalatal or lingual arches, are typically used to preserve anchorage. These devices are limited because they do not allow for complete control over dental movements. Furthermore, they rely on patient compliance, which often is suboptimal.1
a
Private practice, Surprise, Ariz. Professor and director of Orthodontic Research, Department of Orthodontics, Baylor College of Dentistry, Texas A&M University System Health Science Center, Dallas, Tex. c Assistant professor, Department of Orthodontics, Center for Advanced Dental Education, St. Louis University, St. Louis, Mo. d Professor and chair, Department of Orthodontics, School of Dentistry, Kyungpook National University, Daegu, Korea. e Professor and chair, Department of Orthodontics, Center for Advanced Dental Education, St. Louis University, St. Louis, Mo. Supported by the Orthodontic Education and Research Foundation of Saint Louis University, St Louis, Mo. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Peter H. Buschang, 3302 Gaston Ave, Dallas, TX 75246; e-mail, [email protected]. Submitted, May 2007; revised and accepted, March 2008. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.03.016 b
Several types of devices are used to establish absolute anchorage, avoid unwanted tooth movement, and minimize the need for patient compliance. Restorative implants (approximately 4 mm in diameter and 7-18 mm long) have been used for orthodontic anchorage,2-6 but the surgery, arch space, healing times, and placement locations limit their use.7 Palatal implants (3.3-3.75 3 4-6 mm) also require invasive surgeries for placement and removal, and mechanical attachments to palatal implants are more complicated than their restorative counterparts.8 The effectiveness and efficiency of onplants that do not penetrate bone but also require surgical procedures and extended healing times have yet to be established.9,10 Recently, miniscrew implants (MSIs) have drawn orthodontists’ attention. Their small size (1.2-2 3 5-12 mm) allows them to be placed almost anywhere, with only minimal surgery for placement. MSIs are also much less expensive than restorative implants. Unfortunately, human studies have shown variable success rates with MSIs: ranging from 49% to 100%.11,12 Animal studies, which allow for greater control than human studies, perhaps are the best way to evaluate factors associated with MSI success. Owens et al13 and Carrillo et al14 reported 86% (18 of 21) and 99% (95 251
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of 96) success rates (defined as lack of gross mobility) after immediately loading 6-mm MSIs in beagles. A 94% success rate (remaining intact in bone) was recently demonstrated for 6-mm MSIs immediately loaded with 600 g of force.15 Asikainen et al,16 who placed 4.5-mm MSIs loaded with 250 to 350 g of force into the foreheads of sheep, had 100% success after 3 months. Ohmae et al17 placed 4-mm long MSIs into the jaws of beagles with 100% success after loading them with 150 g of force for 12 to 18 weeks. The shortest documented MSI is 4 mm long.17 However, Doi15 reported that many 6-mm MSIs that remained stable had only 3.5 mm of threads placed into the bone. This suggests that the cortical plate provides enough bone for MSI stability. Shorter MSIs might be expected to subject the patient to less risk and discomfort during placement, but their stability remains to be established. The purpose of this 6-week experimental study was to compare the difference in stability between 3- and 6-mm long MSIs immediately loaded with 600 g of force, as well as the difference in stability between 3-mm long MSIs immediately loaded with either 600 or 900 g of force. MATERIAL AND METHODS
The sample included 5 healthy male beagle dogs (ages, 10-15 months; weight, about 9 kg). They received a soft diet 3 weeks before MSI placement and were housed individually. All procedures were approved by the Animal Care Committee of Saint Louis University in St Louis, Mo. The dogs were injected with buprenorphine hydrochloride (Carpuject, Hospira, Lake Forest, Ill), 0.01 mg per kilogram subcutaneously, and carprofen (Rimadyl, Pfizer Animal Health, Exton, Pa), 4 mg per kilogram subcutaneously, for analgesia. Acepromazine maleate (VEDCO, St Joseph, Mo) was given, 0.04 mg per kilgram, subcutaneously as a preanesthetic agent, along with 10 mg per kilogram of ampicillin (Fort Dodge Animal Health, Fort Dodge, Iowa) as a prophylactic measure. Induction was performed with 3 to 4 mg per kilogram of propofol (Baxter Healthcare, Deerfield, Calif) intravenously and 1% to 2% isoflurane (IsoSol, VEDCO). An intravenous drip 0.9% sodium chloride (Baxter Healthcare) was given at a rate of 20 mg per kilogram per hour to maintain hydration. Initial periapical radiographs were taken to determine whether there was adequate space for MSI placement. The implant sites were first swabbed with 0.12% chlorhexidine gluconate (Acclean, Henry Schein, Melville, NY) and infiltrated with 0.25% bupivicaine hydrochloric acid (Abbott Laboratories, North Chicago, Ill). Six mandibular MSIs were placed through the alve-
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olar mucosa, approximately 5 mm apically to the root furcations of the third and fourth premolars and the first molar. A tissue punch was used to prevent the mandibular mucosa from wrapping around the MSI during placement. Six maxillary MSIs were placed between the roots of the second and third premolars and the first molar, approximately 3 mm palatally to the gingival margins of the teeth. All MSIs were placed with a hand driver without predrilling. Periapical radiographs were taken after placement to verify that the roots had not been contacted. Two lengths of MSIs were used for the study: 3 and 6 mm (Fig 1). Although the 6-mm MSIs are commercially available as part of the AbsoAnchor system (Dentos, Daegu, Korea), the 3-mm MSIs were specially manufactured for this project by Dentos. All MSIs measured 1.3 mm in diameter, with pointed and notched tips. The coronal surface of each MSI was dimpled with a diamond bur; the dimples served as registration points for the interimplant measurements. Twelve MSIs were placed in each of the 5 dogs, including 8 experimental (loaded) and 4 control implants (Fig 2). The 3-mm MSIs in the mandible were randomly loaded with either 600 or 900 g of force. Loaded with 600 g of force, 3- and 6-mm MSIs were randomly assigned to the right and left sides of the maxilla. All MSIs were oriented perpendicularly to the bony surface. The maxillary MSIs were placed palatally because excessive tissue overgrowth around buccally placed MSIs has been reported.13-15 Two experimental MSIs were placed in each quadrant approximately 20 mm apart, depending on root location. A control MSI of the same length was placed slightly apically and anteriorly to the midpoint between the 2 experimental MSIs. Primary stability was initially confirmed by exerting manual pressure with the hand driver and a cotton forceps. Mobility of the MSIs was measured with the Periotest device (Medizintechnik Gulden, Lautertal, Germany).18,19 Linear measurements were made between pairs of experimental MSIs in each quadrant by using a digital caliper (Chicago Brand Industrial, Fremont, Calif). The interimplant distance and the mobility measurements were repeated 3 times, and the middle values were used for the statistical analyses. The experimental MSIs were loaded by ligating 2 9-mm nickel-titanium coil springs (428-650, The Orthodontic Store, Gaithersburg, Md) between each pair. The distances that the coil springs needed to be activated to deliver orthopedic force levels (600 and 900 g) were predetermined with a force gauge to be 13.4 and 18.2 mm, respectively. These distances were verified intraorally by using the digital caliper. The springs were ligated to the MSIs with 0.012-in stainless steel ligature
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Statistical analysis
Fig 1. MSIs, 6 and 3 mm.
wires. The ends of the wires were encapsulated with flowable composite (Natural Elegance, Henry Schein) to minimize soft-tissue irritation and ensure that the appliances remained intact. The control MSIs remained unloaded. The dogs were monitored daily for 6 weeks to verify that the appliances were intact and to evaluate the soft tissues. The study areas were irrigated daily with 0.12% chlorhexidine gluconate (Acclean) to flush food particles from the MSIs and springs. Subcutaneous injections of 10 mg per kilogram of aqueous ampicillin (Fort Dodge Animal Health) and 4 mg per kilogram of subcutaneous carprofen (Rimadyl) were given for 10 days after MSI placement to control inflammation and pain. Because of severe tissue inflammation and overgrowth around the mandibular implants and springs, 1 dog required an additional 21 days of carprofen administration. MSIs that were pulled out during the first 5 weeks were replaced within 1 to 7 days; those that failed during the last week of the study were not replaced. Replacement MSIs provided only an anchor for the force being placed on the original, intact, experimental MSI; no assessments of the replaced MSIs were performed. Six weeks after initial MSI placement, the dogs were killed with a lethal dose (3-5 mL) of pentobarbital (Euthanasia-5, Henry Schein). The distances between each pair of study MSIs were again measured 3 times with a digital caliper.
MSI success was defined as lack of MSI pullout. Chisquare tests were used to determine whether MSI success was associated with length (3 vs 6 mm), location (maxilla vs mandible), and force level (600 vs 900 g). When appropriate (ie, any cell N \5), the Fisher exact test was used to determine the statistical significance of the chi-square values. The phi coefficient was also used to evaluate the relationship between success and length. Intraclass correlations, described in terms of Cronbach alpha, were used to determine the reliability of the 3 successive Periotest measurements. The Wilcoxon signed-rank test for related samples was used to evaluate changes in interimplant distances (displacement). The 2 Periotest mobility values of the MSI pairs were averaged, and Pearson product-moment correlations were used to analyze the relationship between initial mobility and displacement. To determine the association between initial MSI mobility and success, the Periotest values were rescaled to 4 ordinal values as follows: 8 to 16, 17 to 121, 122 to 136, and 137 to 150. The Spearman rho correlation coefficient was used to analyze the correlation between MSI success and initial mobility. All statistical analyses were performed by using SPSS software (version 14.0, SPSS, Chicago, Ill). RESULTS
The overall success rates of the 3- and 6-mm experimental MSIs were 66.7% and 100%, respectively (Table I), representing a statistically significant (chisquare 5 4.03, P 5 0.045) difference. Of the 10 3-mm experimental MSIs that failed, the tips of 5 had sheared during placement (Fig 3). Sixty percent (6 of 10) of the 3-mm experimental MSIs that failed were from dog number 3 (2 of the 6 MSIs were sheared). This dog was described by the supervising veterinarian as unusually active and was regularly observed chewing on its food bowl and the run bars of its cage. The net success rate of 3-mm experimental MSIs, excluding those that sheared and all in dog number 3, was 95.2%. After these exclusions, the difference in success rates between 3- and 6-mm experimental MSIs was not statistically significant (chi-square 5 0.35, P 5 0.557). The overall success rates of the 3- and 6-mm control MSIs were 66.7% and 100%, respectively (Table I). Of the 5 3-mm control MSIs that failed, 1 had a sheared tip, and 1 belonged to dog number 3, resulting in a net success rate of 81.8%. Neither the overall (chi-square 5 1.98, P 5 0.160) nor the net (chi-square 5 0.93, P 5 .334) differences in success rates between 3- and 6-mm control MSIs were statistically significant.
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Fig 2. MSI placement locations and forces applied.
The phi coefficient showed a low, significant association (f 5 0.32, P 5 0.045) between experimental MSI length and success. The control MSIs showed no significant association between length and success (f 5 0.33, P 5 0.160). The overall success rates of the 3-mm experimental MSIs were 80% in the maxilla and 60% in the mandible (Table II). After excluding those that sheared and those in dog number 3, the success rates were 100% for the maxilla and 85.7% for the mandible. The differences in success between 3-mm experimental MSIs in the maxilla and the mandible were not statistically significant either before (chi-square 5 1.20, P 5 0.273) or after (chi-square 5 0.65, P 5 0.421) the exclusions.
The overall success rates of the 3-mm control MSIs were also 80% in the maxilla and 60% in the mandible (Table II). After the exclusions, the success rates of the 3-mm control MSIs were 100% for those in the maxilla and 71.4% for those in the mandible. The differences in success between control MSIs in the maxilla and the mandible were not statistically significant either before (chi-square 5 0.60, P 5 0.439) or after (chi-square 5 1.07, P 5 0.301) the exclusions. The overall success rates of the 3-mm mandibular MSIs loaded with 600 and 900 g of force were both 60% (Table III). After the exclusions, the success rates were 100% for 900 g of force and 85.7% for 600 g of force. The differences in success rates between the 3-mm mandibular MSIs loaded with 600 and 900 g of
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Table I.
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Success rates by length of MSIs
Length Experimental MSIs 3 mm 6 mm Control MSIs 3 mm 6 mm Total 3 mm 6 mm
Number placed
Number failed
Overall success rate
Number failed that were sheared
Success rate excluding sheared implants
Number failed from dog number 3
Net success rate
30 10
10 0
66.7% (20/30) 100% (10/10)
5 0
80% (20/25) 100% (10/10)
6 0
95.2% (20/21) 100% (8/8)
15 5
5 0
66.7% (10/15) 100% (5/5)
1 0
71.4% (10/14) 100% (5/5)
2 0
81.8% (9/11) 100% (4/4)
45 15
15 0
66.7% (30/45) 100% (15/15)
6 0
76.9% (30/39) 100% (15/15)
8 0
90.6% (29/32) 100% (12/12)
Fig 3. Scanning electron microscope images of MSIs: A, normal MSI; B, 6-mm MSI that sheared during placement; C, sheared 3-mm MSI.
force were not statistically significant either before (chisquare 5 0.00, P 5 1.000) or after (chi-square 5 0.93, P 5 0.335) the exclusions. All loaded implant pairs showed significant decreases in interimplant distances during the 6-week experimental period. The mean displacement of the 8 3-mm MSI pairs that remained intact throughout the experiment was 2.2 mm, with a range of 0.4 to 4.4 mm. The 5 pairs of loaded 6-mm MSIs that remained intact showed a mean decrease in interimplant distance of 1.8 mm, with a range of 1.0 to 3.4 mm. The difference in linear displacement between the 3- and 6-mm MSI pairs was not statistically significant (P 5 0.369).
The mean displacement of the 3-mm MSI pairs loaded with 600 g of force was 2.0 mm (n 5 6), compared with a mean displacement of 3.1 mm for the 3-mm MSI pairs loaded with 900 g of force (n 5 2). This difference was statistically significant (P 5 0.012). The displacement of the 3-mm maxillary MSIs closely approximated the displacement of the 3-mm mandibular MSIs. The intraclass correlations showed good reliability between successive mobility measurements with the Periotest. The average Cronbach alpha was 0.97. There was no significant correlation between initial Periotest mobility and success of the experimental (r 5 0.10, P 5 0.506) or control (r 5 0.27, P 5 0.350) MSIs.
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Table II.
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Success rates of 3-mm MSIs by location
Length Maxilla Experimental MSIs 3 mm Control MSIs 3 mm Total 3 mm Mandible Experimental MSIs 3 mm Control MSIs 3 mm Total 3 mm
Table III.
Length
Number placed
Number failed
Overall success rate
Number of failed implants that were sheared
Success rate excluding sheared implants
Number of failed implants from dog number 3
10
2
80% (8/10)
0
80% (8/10)
2
100% (8/8)
5
1
80% (4/5)
0
80% (4/5)
1
100% (4/4)
15
3
80% (12/15)
0
80% (12/15)
3
100% (12/12)
20
8
60% (12/20)
5
80% (12/15)
4
85.7% (12/14)
10
4
60% (6/10)
1
1
71.4% (5/7)
30
12
60% (18/30)
6
66.7% (6/9) 75% (18/24)
5
Net success rate
81% (17/21)
Success rates of mandibular MSIs by force level Number placed
Number failed
Overall success rate
Number of failed implants that were sheared
Success rate excluding sheared implants
Number of failed implants from dog number 3
Net success rate
10
4
60% (6/10)
3
85.7% (6/7)
2
100% (6/6)
10
4
60% (6/10)
2
75% (6/8)
2
85.7% (6/7)
900-g load 3 mm 600-g load 3 mm
There also was no statistically significant correlation between initial Periotest mobility and displacement of the experimental MSIs (r 5 0.01, P 5 0.993). DISCUSSION
During the study, 25% (10 of 40) of the experimental and 25% (5 of 20) of the control MSIs failed. Six of these had sheared tips. Since only 1 of the 30 MSIs that remained stable had a sheared tip, it appears that shearing might have contributed to those failures. The screw tip is designed to draw the rest of the screw into the bone. A damaged tip could cause the threaded barrel to strip the bone when turned. For that reason, success rates were also calculated after excluding the sheared MSIs. Success rates were also calculated after excluding all MSIs in dog number 3, which was regularly observed chewing on the run bars and food bowl. Although these exclusions complicate the interpretation of the results, they identify important external factors that might cause MSI failures. The study was based on the notion that MSIs gain most of their primary stability (mechanical retention) from the cortical plate. Due to the differences in success rates of the 3- and 6-mm MSIs, primary stability might have been increased by contact with the medullary bone. This could
explain why none of the 6-mm experimental MSIs failed. Because secondary stability is low during the first few weeks of healing (Fig 4), adequate primary stability is critical for short-term MSI success. The length of the MSI does not necessarily equal the depth of its placement. Caraway20 showed that the 6-mm MSIs used in our study were placed an average of 3.9 mm into the bone; the 3-mm MSIs were placed an average of 1.6 mm; the cortical plate thickness for the dogs in this study was approximately 2.1 mm. On that basis, the average 6-mm MSI passed through the cortical plate and into the medullary bone, whereas the average 3-mm MSI did not completely pass through the cortex. Since most (7 of 10) of the 3-mm experimental MSIs that failed did so within the first 2 weeks of the study (Fig 5), it is possible that they were not placed far enough into the cortical bone to attain primary stability. If the MSIs had been completely placed into bone, the success rate of the 3-mm MSIs might have been higher. There was no significant difference in success rates between the 3-mm MSIs immediately loaded with either 600 or 900 g of force; this is consistent with previous animal studies comparing lower forces. Owens et al13 found no differences in the success rates of
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 136, Number 2
Primary
Secondary
257
Total
100 90 80
Stability (%)
70 60 50 40 30 20 10 0 0
1
2
3
4
5
6
7
Time (weeks)
Fig 4. Stability patterns of endosseous dental implants (adapted from Raghavendra et al30 with permission by Quintessence Publishing Co Inc, copyright 2005).
Number of failed implants
4
3
2
1
0
*
*
*
0 1 2 3 4 5 6 7
* *
*
*
*
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
Day of failure Control
Experimental
Fig 5. Failure times for 3-mm MSIs: D indicates an MSI that was sheared; * indicates an MSI in dog number 3.
6-mm MSIs loaded with either 25 or 50 g of force; Carrillo et al14 also reported no significant differences in the success rates of 6-mm MSIs loaded with 25, 50, or 100 g; Doi15 found that the success rates of 6-mm MSIs loaded with 300 or 600 g were similar. This suggests that force levels simply must be low enough not to crush the surrounding bone before stability is affected. Such force levels are probably outside the range of those used in typical orthodontic situations, since the maxi-
mum pull-out force of 6-mm MSIs has been reported from 134 from 388 N (1 N 5 102 g).21,22 The 3-mm control MSIs had an overall success rate similar to that of the 3-mm experimental MSIs. However, the control MSIs failed both early and late (Fig 5). Because they were unloaded, force cannot be implicated in their failure. Although it is unclear exactly why they failed at the same rate as the loaded ones, both length and depth of placement could be contributing factors.
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The MSIs showed significant linear displacements during the study, depending on the amount of force applied. MSI displacement was reported in both immediately15 and delayed22,23 loaded MSIs. The 6-mm experimental MSIs in our study were displaced approximately 0.9 mm (1.8 mm interimplant distance divided by 2); this was more than previously reported by Doi.15 Because the 3-mm MSIs loaded with 900 g were displaced more than those loaded with 600 g, the increased force appears to be a contributing factor. Although the average displacement of the 3-mm MSI pairs was 2.2 mm, the apex was displaced only about 0.3 mm. Since this displacement could have been a step toward eventual MSI failure, more MSIs might have failed if the study had been longer. It is also possible that the displacement occurred during the first few weeks of the study, with the MSIs retained by the tangentially applied forces until healing occurred. Once bony healing establishes secondary stability, mobility can actually be reduced.24 There was no association between initial Periotest mobility and success or failure of MSIs, or displacement of the experimental MSIs. Further research is warranted to establish the validity of the Periotest with MSIs. The success rate of 3-mm experimental MSIs in the mandible was 20% lower than those placed in the maxilla. Although this difference was not statistically significant, other authors found lower success rates in the mandible.25-28 Mandibular bone is denser than maxillary bone, and density might affect the success rates of MSIs.29 All 6 failed MSIs that had sheared tips were in the mandible. It is possible that the 6-mm MSIs would have also sheared if they had been placed in the mandible. It appears that the increased torque required to place an MSI through denser bone might alter the integrity of MSIs that are 1.3 mm in diameter and lead to material failure. This supports the practice of predrilling sites in the mandible when smaller MSIs are used. This study was limited by the shearing of several MSIs and the behavior of dog number 3. A longer study period could have provided more information regarding the stability of the 3-mm MSIs. The major limitation of this study, however, was the lack of statistical power; this makes it impossible to say that the force level applied or the placement location played no role in stability. Thus, a larger sample size would most likely have allowed for a more meaningful interpretation of the results. Further studies are certainly warranted. CONCLUSIONS
1.
Overall, the success rates of immediately loaded 3-mm MSIs were significantly lower than those of
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2.
3.
4.
immediately loaded 6-mm MSIs. After the exclusions for plausible causes, however, the difference was not statistically significant. The difference in success rates between 3-mm mandibular MSIs loaded with 600 or 900 g of force was not significant. Both the 3- and 6-mm MSIs immediately loaded with 600 or 900 g of force experienced significant linear displacements during the study. There was no relationship between initial mobility and success or failure of MSIs or eventual displacement of the experimental MSIs.
We thank George Vogler for his assistance during this project, Heidi Israel for her statistical help, and Dentos for its support.
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