ORIGINAL ARTICLE

Microdamage of the cortical boneduring mini-implant insertion withself-drilling and self-tappingtechniques: A randomized controlled trial

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Sumit Yadav, Madhur Upadhyay, Sean Liu, Eugene Roberts, William P. Neace, and Ravindra NandaFarmington and West Hartford, Conn, and Indianapolis, Ind

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538

Introduction: The purpose of this research was to evaluate microdamage accumulation after mini-implantplacement by self-drilling (without a pilot hole) and self-tapping (screwed into a pilot hole) insertiontechniques. The null hypothesis was that the mini-implant insertion technique would have no influence onmicrocrack accumulation and propagation in the cortical bones of the maxillae and mandibles of adulthounds. Methods: Mini-implants (n 5 162; diameter, 1.6 mm; length, 6 mm) were placed in the maxillae andmandibles of 9 hounds (12-14 months old) with self-drilling and self-tapping insertion techniques. Thetechniques were randomly assigned to the left or the right side of each jaw. Each hound received 18 mini-implants (10 in the mandible, 8 in the maxilla). Histomorphometric parameters including total crack length andcrack surface density were measured. The null hypothesis was rejected in favor of an alternate hypothesis:that the self-drilling technique results in more microdamage (microcracks) accumulation in the adjacentcortical bone in both the maxilla and the mandible immediately after mini-implant placement. A cluster levelanalysis was used to analyze the data on the outcome measured. Since the measurements were clusteredwithin dogs, a paired-samples t test was used to analyze the average differences between insertion methodsat both jaw locations. A significance level of 0.05 was used for both analyses. Results: The self-drillingtechnique resulted in greater total crack lengths in both the maxilla and the mandible (maxilla: meandifference, 18.70 6 7.04 mm/mm2; CI, 13.29-24.11; mandible: mean difference, 22.98 6 6.43 mm/mm2; CI,18.04-27.93; P \0.05), higher crack surface density in both the maxilla and the mandible (maxilla: meandifference, 10.39 6 9.16 mm/mm2; CI, 3.34-17.43; mandible: mean difference, 11.28 6 3.41 mm/mm2; CI,8.65-13.90; P \0.05). Conclusions: This study demonstrated greater microdamage in the cortical bones ofadult hounds in both the maxilla and the mandible by the self-drilling insertion technique compared with theself-tapping technique. (Am J Orthod Dentofacial Orthop 2012;141:538-46)

tant professor, Division of Orthodontics, Health Center, University of Con-ut, Farmington.tant professor, Division of Orthodontics, Health Center, University ofecticut, Farmington.tant professor, Department of Orthodontics, Indiana University-Purduersity, Indianapolis.ssor emeritus, Department of Orthodontics, Indiana University-Purduersity, Indianapolis.tant professor, Department of Biostatistics, University of Hartford, Westrd, Conn.ssor and head, Division of Orthodontics, Health Center, University ofecticut, Farmington.uthors report no commercial, proprietary, or financial interest in the prod-r companies described in this article.t requests to: Sumit Yadav, Division of Orthodontics, University of Con-ut Health Center, 263 Farmington Avenue, Room No L7063 MC 1725,ngton, CT 06053; e-mail, yadav_sumit17@yahoo.com.itted, March 2011; revised and accepted, December 2011.5406/$36.00ight � 2012 by the American Association of Orthodontists..1016/j.ajodo.2011.12.016

M ini-implants have gained enormous popu-larity in orthodontics and are now consid-ered sources for absolute osseous

anchorage.1 Their primary advantages are simple sur-gical insertion and removal, placement at numerousanatomic locations, economic viability, and minimalneed for patient compliance.2-7 However, before theuse of a mini-implant for orthodontic anchorage, itis necessary to ensure adequate stability.8 The successor failure of an implant largely depends on the degreeto which it integrates with the host bone. Overall, thesuccess of mini-implants is guided by the physical,chemical, and biologic events at the bone-implant in-terface.9,10 The initial stability of the mini-implant atits placement is determined by the quality and quan-tity of bone at the implant site, the geometry of the

Fig 1. Mini-implant design.

Yadav et al 539

implant, and the method of site preparation. Becausebone quality and volume are determined by local (pa-tient) factors, the initial stability of the fixture is pri-marily influenced by the implant design and theplacement technique.

Mini-implants can be inserted by either the self-tapping or the self-drilling technique. Self-tapping re-quires predrilling of a pilot hole followed by screwingin of the implant body. This technique has some inher-ent disadvantages, particularly for interradicular areas,such as damage to adjacent tooth roots, drill-bit break-age, and, most importantly, thermal necrosis of the sur-rounding bone.11-13 Necrosis of the bone contributes toosteolysis around the implant and loss of stability.14,15

On the other hand, the self-drilling procedure is simpler,allowing insertion of the mini-implant into bone with-out predrilling for most clinical applications. In spite ofthis advantage, if the bone is dense or the screw diam-eter is large with a tapered shape, excessive placementtorque can cause overcompression of the corticalbone, leading to microdamage. Microdamage is a per-manent deformation of the microstructure of loadedcortical bone in the form of fatigue and creep, andmanifests histologically as microcracks that are discon-tinuities of the calcium-rich bone matrix around im-plants.16,17 It has been suggested that microcracksinitiate “targeted remodeling,” wherein osteoclaststarget and remove the region of bone matrix that hassustained the microdamage, and osteoblasts replacethe area with new bone.

Although a few studies have reported the effects ofdiameter and shape of mini-implants on microdamageto the cortical bone and on the stability of self-drillingand self-tapping implants, no study has evaluated themicrodamage accumulation in the cortical bone aftermini-implant placement by different placement tech-niques.18,19 Moreover, studies involving some kind ofloading force to the implants can affect the earlymicrodamage, making it difficult to differentiate theeffect of implantation methods from that of loadingforces on the bone. Therefore, the purpose of thisin-vivo study was to evaluate microdamage accumu-lation in the cortical bone of the maxillae and themandibles of adult hounds, immediately after mini-implant placement, by the self-drilling (without pilothole) and self-tapping (predrilled pilot hole) insertiontechniques. The null hypothesis was that mini-implantinsertion technique would have no influence onmicrocrack accumulation and propagation in the cor-tical bones of the maxilla and the mandible of thehounds. It was our intention to create evidence for cli-nicians regarding a better insertion technique formini-implants.

American Journal of Orthodontics and Dentofacial Orthoped

MATERIAL AND METHODS

Nine adult male hounds were selected for this study.They were obtained from an unrelated study duly ap-proved by the Institutional Animal Care and Use Com-mittee of the University of Connecticut Health Centerat Farmington, Conn. They were 12 to 14 months oldwith all permanent teeth present and jaw growth com-plete. A total of 162 mini-implants (Dentaurum, Isprin-gen, Germany) were placed bilaterally on the buccalsides of the mandibles and maxillae of the hounds.The mini-implants were placed immediately after thehounds were killed. The outer thread diameter of themini-implant was 1.6 mm, and the inner core diameterof the shank was 1.3 mm (ie, the threads were 0.3 mmdeep). The screw shank and threads were cylindrical forthe top two thirds, and the lower third was tapered (Fig1). The total length was 6 mm. On the basis of insertiontechnique and the site of placement, the mini-implantswere divided into 4 groups: group A, self-drilling in themandible; group B, self-drilling in the maxilla; group C,self-tapping in the mandible; and group D, self-tapping in the maxilla. Each hound received 18 im-plants: 10 in the mandible and 8 in the maxilla (Fig2). The insertion techniques for the mini-implantswere randomized between the maxilla and the mandi-ble of each hound, and between the left and right sidesof each jaw. The statistician (W.N.) on this project usedcomputer-generated random numbers (Rand function,Excel 2007; Microsoft, Redmond, Wash) for allocationof the sequence. This ensured even distributions ofthe mini-implants placed by each technique (Fig 3).

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Fig 2. Placement of mini-implants in the jaws of an adult hound.

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The mini-implants in groups A and B were insertedwithout drilling a pilot hole. In groups C and D, pilotholes were drilled with an internally irrigated twist drill0.3 mm in length and 1 mm in diameter. The drillingrotation was 1000 rpm, and the rotational axis wasclockwise. The implants were screwed into the preparedholes by using a screwdriver specifically made for thispurpose. All mini-implants were inserted perpendicularto the cortical bone. One operator (Y.S.) inserted allmini-implants. The initial stability was measured withdental tweezers.

The maxillae and the mandibles of the houndswere dissected into separate bone blocks containingthe specimen (mini-implant) with about 5 mm of ad-jacent supporting bone. Each block was assigned anidentifying number by the laboratory technician onthis project to which the principal investigator (Y.S.)was blinded. They were then transferred to 70% ethylalcohol and held until processing was started. All boneblocks were stained with 1% basic fuchsin (catalog no.B660-03; J. T. Baker, Phillipsburg, NJ) as previouslydescribed and embedded in methyl methacrylate con-taining 0.5% initiator (Perkodox 16; AKZO, Chicago,Ill), oriented in their labeled containers and polymer-ized in a water bath that started at room temperatureand was gradually increased over 24 hours to 34C� topolymerize the blocks.20 The specimens were then 2dimensionally x-rayed with microcomputed tomogra-phy (model 1072; Skyscan, Aartselaar, Belgium) to de-termine each implant’s orientation in the bone block.The bone block was ground on a model trimmer to

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prepare a flat surface that was parallel to the planeof the mini-implant. Each specimen was thenmounted on a plastic slide for further grinding withvarious grinding papers (K320, K500, K800, K1000,and K1200) on a grinding (milling) system (ExaktMedical Instruments, Oklahoma City, OK) until 1side of the implant was exposed completely (Fig 4).The exposed side was then polished on the grindingmachine with a polishing paper and observed underthe microscope to quantify any scratches. It wasthen mounted to a second slide by using light-curedresin (Exakt Medical Instruments). The first slide waspopped off, and the block was ground on the grindingmachine until the second side was exposed com-pletely. Once the section reached the desired thicknessof 50 to 70 mm, it was polished as described aboveand readied for microscopic analysis (Fig 4). The mill-ing on each side of the embedded implant was ad-justed so that the final specimen was a centralsection of the implant interface with its supportingbone.

The histomorphometric parameters, microcracklength (mm) and crack surface density (mm2) wereevaluated (at 20 times magnification) with a micro-scope (model 59920; Nikon, Tokyo, Japan) and soft-ware (Osteo; Bioquant Image Analyses, Nashville,Tenn) by using the appropriate filters (Fig 5). All mea-surements were made at 20 times magnification. Dif-fuse microdamage was visualized as pooled basicfuchsin that formed a diffuse staining pattern adja-cent to the mini-implant (Fig 6). A diffuse pattern

Journal of Orthodontics and Dentofacial Orthopedics

Fig 3. Flowchart describing the experimental procedure.

Yadav et al 541

was defined as one in which clusters of microcrackswere too small to be distinguished from one another(ie, \1-2 mm).

Statistical analyses

A power analysis was conducted to determine thesample size needed to detect significant differences incrack length (measured in micrometers per 2 mm) andcrack surface density (total crack length divided by 2) be-tween the 2 insertion methods based on a paired-samples t test for data analysis. The power analysis was

American Journal of Orthodontics and Dentofacial Orthoped

based on an estimated effect size (Cohen’s d) of 1.10, aconventional a level of .05, and a desired power(1 – b) of .80. Cohen’s d is an effect size estimate derivedfrommeasuring the difference betweenmeans relative tothe standard deviation of the mean difference, and thusis a standardized measurement. With the specified pa-rameters, significant differences in both dependent vari-ables were detectable with a sample of 9 hounds. Thedata from this study indicated that the smallest effectsize achieved was 1.13; thus, this sample size was suffi-cient to afford adequate statistical power to detect

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Fig 4. Sample preparation and final 100-mm section of mini-implant with the surrounding bone.

Fig 5. Fluorescence imaging (20 times magnificatiion) ofbone adjacent to the implant outlining the microcracks inthe cortical bone (arrows).

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differences in crack length and crack density between the2 insertion methods tested on the same subjects. All cal-culations were performed with the computer applicationG-Power (Department of Experimental Psychology,Heinrich-Heine-University, Dusseldorf, Germany), whichis based on the formulas of Cohen. A cluster level analysiswas used to analyze the data on crack length (mm/2 mmof area). The average of the 4 maxillary measurementsfor the self-drilling technique and the average of the 5mandibular measures for the self-tapping techniquewere calculated, yielding a pair of averages, one foreach insertion method, for each hound. The same prep-aration was used to produce a pair of averages for eachhound for both insertion methods on the mandible.

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Since the measurements were clustered within dogs,a paired-samples t test was used to analyze the averagedifferences between insertion methods at both jaw loca-tions. A significance level of 0.05 was used for both anal-yses. A second cluster analysis was conducted on cracklength density (total crack length divided by 2). Thedata were prepared in the same way as described above.Paired-samples t tests were then used to analyze themean differences in crack density between the 2 inser-tion methods, both conducted by using a significancelevel of 0.05. An investigator (Y.S.) measured the micro-crack lengths twice. The method error was calculated byusing Dahlberg’s formula, Se25S d2=2n, where d is thedifference between duplicate measurements, and n is thenumber of double measurements.21 On average, themethod error for the microcrack length measurementwas 0.11 mm (P\0.05).

RESULTS

All mini-implants showed primary stability after in-sertion; however, 8 specimens were lost during prepara-tion (on the grinding system): 2 from the mandible(1 each from the self-drilling and self-tapping groups)and 6 from the maxilla (2 from self-drilling and 4 fromself-tapping groups). On average, 200 to 300 mm of dif-fuse damage (extensive damage) of cortical bone adja-cent to the screw was present on either side of theimplants in all groups. This zone was excluded fromanalyses (Fig 6). Microdamage in the form of discontinu-ities was observed in all specimens in both the maxillaand the mandible with the 2 insertion techniques (Fig 5).

Total crack lengths in the mandible and the maxillawere 75.326 9.13 mm and 76.786 10.82 mm, respec-tively, for the self-drilling technique, and 52.33 6

Journal of Orthodontics and Dentofacial Orthopedics

Fig 6. Fluorescence imaging (20 times magnification) of bone adjacent to the implant outlining the dif-fuse damage in the cortical bone.

Table I. Mean differences, confidence intervals, and Pvalues for maxillary and mandibular total crack lengthmeasurements for the 9 hounds

mm SD t P 95% CIMaxillaSelf-drilling 76.78 10.82Self-tapping 58.07 7.19Difference 18.70 7.04 7.97 0.001 13.29-24.11

MandibleSelf-drilling 75.32 9.13Self-tapping 52.33 6.78Difference 22.98 6.43 10.71 0.001 18.04-27.93

Means and standard deviations are averages of the measurements ofeach insertionmethod for each subject. The differences are the meandifferences between insertion methods; t is the result of paired-samples t tests on the mean differences between insertion methods;CI is the 95% confidence interval for the mean differences betweeninsertion methods. An a level of .05 was used to reject the null hy-pothesis.

Yadav et al 543

6.78 mm and 58.07 6 7.19 mm, respectively, for theself-tapping technique (Table I). The differences be-tween the techniques were statistically significant(P \0.05). The crack surface density in the mandiblewith the self-drilling technique was 36.97 6 5.94mm/mm2, and the self-tapping technique yielded25.69 6 4.31 mm/mm2. In the maxilla, the crack sur-face density with the self-drilling technique was36.37 6 6.89 mm/mm2; with the self-tapping tech-nique, it was between 12 and 30 mm/mm2, witha mean of 25.99 6 5.86 mm/mm2 (Table II). In boththe maxilla and the mandible, total crack lengths andcrack surface densities were significantly less for theself-tapping technique (P \0.05) compared with theself-drilling technique (Tables I and II).

DISCUSSION

In this study, microdamage only at the time of mini-implant placement was evaluated. Although it is not cer-tain that dead bone is identical to live bone, this studyprovides data concerning microdamage in bone duringmini-implant placement. Adult hounds were chosen asan experimental model because they have been usedother studies on dental implants and orthodonticmini-implants, and their bone morphometry closely re-sembles the human maxilla and mandible.22

The null hypothesis was rejected in favor of an alter-nate hypothesis that the self-drilling technique resultsin more microdamage (microcracks) accumulation inthe adjacent cortical bone for both the maxilla andthe mandible immediately after mini-implant place-ment. Surprisingly, our results showed no differencein microdamage between the maxilla and the mandiblewith either insertion technique. Previously, it was

American Journal of Orthodontics and Dentofacial Orthoped

shown that the insertion torque of self-drilling screwsis appreciably greater than that of self-tapping screws,since a higher force is needed for insertion with the self-drilling technique.18,23 The resulting stress can causeovercompression of the cortical bone and deformationof the microstructure surrounding the implant in theform of fatigue, creep, and eventual cracking.16,17,24

Deeply placed mini-implants can also create a similarsituation of overcompression of cortical bone and ex-tensive crack formation.25 Microcracks have been in-vestigated as a parameter that simultaneously affectsthe mechanical and structural properties of corticalbone as well as its toughness, and have been implicatedin the pathomechanics of mini-implant failure. Sodenet al26 also showed gross bone deformation and greater

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Table II. Mean differences, confidence intervals, and P values for maxillary and mandibular crack surface densitymeasurements for the 9 hounds

mm SD t P 95% CIMaxillaSelf-drilling 36.37 6.89Self-tapping 25.99 5.86Difference 10.39 9.16 3.40 0.009 3.34-17.43

MandibleSelf-drilling 36.97 5.94Self-tapping 25.69 4.31Difference 11.28 3.41 9.91 0.001 8.65-13.90

Means and standard deviations are averages of the measurements of each insertion method for each subject. The differences are the mean differ-ences between insertion methods; t is the result of paired-samples t tests on the mean differences between insertion methods; CI is the 95% con-fidence interval for the mean differences between insertion methods. An a level of.05 was used to reject the null hypothesis.

544 Yadav et al

microdamage accumulation in the cortical bone withself-drilling mini-implants than with self-tappingones. This accumulation of microdamage can producelocal ischemia, bone necrosis, bone remodeling, andpremature loss of orthodontic mini-implants. However,most studies to date have consistently shown a greatersuccess rate with the self-drilling technique comparedwith the self-tapping technique in spite of greater mi-crodamage.18,27,28 One explanation can be thatmicrocracks tend to coalesce at higher stress levels,thereby increasing the resistance to fracture of thebone, whereas at lower stress levels, microcracksmight increase the fatigue resistance of corticalbone.29,30 The contradictory reports in the literatureraise an important question, “do microcracks reallycause mini-implant failure?” This is interesting becauseremodeling sites in cortical bone have been shown tooccur in conjunction with microcracks.31 Nevertheless,accumulations of microcracks are considered agentsof stress distribution, thus enhancing fatigue life.

Previously, we showed that, after implant place-ment, both remodeling and modeling (resorption) ofthe bone take place.32,33 Also, the remodeling activityis greatest at the bone-implant interface up to 1mm ad-jacent to the implant (1 mm5 1000 mm) and decreasessignificantly with increasing distance from the bone-implant interface. The increased remodeling activitymight be induced by the accumulation of microcracksin and around the bone-implant interface during im-plant placement (although bone remodeling can alsobe due to metabolic and hormonal fluctuations).30,34

This increased remodeling activity at the bone-implant interface is essential to prevent fatigue fracture,directly affecting the mini-implant’s stability and lon-gevity. However, if bone-remodeling foci are intensiveand broad, tissue compliance will be tough to maintain,and propagation of the microcracks might continue tocompromise the stability of the mini-implant.

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Studies have also shown that osteocyte apoptosis dueto microcrack formation and accumulation is responsi-ble for sending signals to initiate osteoclastic resorp-tion.35,36 Furthermore, targeted remodeling helps tomaintain the viability of the osteocytes and preventsthe accumulation of old bone through the repair of themicrodamage.37 Therefore, microdamage in many wayshelps to maintain bone around the implant. This raisesan important question to be answered in subsequentstudies: “does targeted remodeling adjacent to themini-implant, as a result ofmicrodamage, occur at a sim-ilar time and pace as resorption of the bone, thus ensur-ing implant stability?”

An interesting finding of this study was the presenceof diffusely stained areas of bone immediately adjacentto the mini-implants, related to the insertion of the im-plant with both techniques. They were nondirectional,randomly oriented, and located at a maximum of 300mm from the mini-implant on either side. These smallcrack processes are collectively known as diffuse matrixmicrodamage, because of the diffuse pooling of histo-logic stains seen under the light microscope in suchdamaged regions.38 Recent studies have shown that lin-ear microcracks and diffuse damage have differentialbiomechanical responses and outcomes: diffuse damageis a hallmark of bones with better fracture resistance; ie,unlike linear microcracks, diffuse damage allows bonesto dissipate energy and postpone fracture.39 This canbe an interesting area for further research, because dif-fuse microdamage has not been previously explored inthe orthodontic literature and might provide interestingevidence about the mechanical stability of mini-implants. Further studies are also required to investigatethe influence of the depth of the threads and other mini-implant design variables on the accumulation of micro-cracks and diffuse damage in the cortical bones of themaxilla and the mandible. Issues related to bone healingafter microdamage due to excessive compressive force

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exerted by the self-drilling technique and necrosiscaused by excessive heat generated by the self-tappingtechnique also await investigation.

Since this was an in-vitro study, clinicians should becautious about directly extrapolating our results becausethe densities of the hounds’maxillae and mandibles dif-fer from those of humans.

CONCLUSIONS

1. This study showed that the self-drilling techniquecaused greater microdamage of the surroundingbone immediately after placement of a mini-implant. All histomorphometric parameters (crackdensity, crack length, and number of cracks) weresignificantly elevated for the self-drilling techniquecompared with the self-tapping technique. How-ever, there were no differences in the microdamagecaused by either technique between the maxilla andthe mandible.

2. Diffusely stained areas adjacent to the mini-implants (diffuse microdamage) were noted in allsections with both the self-drilling and the self-tapping insertion techniques.

3. No definite relationship could be established be-tween the increased microdamage noted with theself-drilling technique and the greater stability ofthe mini-implants. Further studies are needed to ex-plore this relationship in detail.

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