Predictable drill-free screw positioning with a graduated 3-dimensional radiographic-surgical guide:...

CLINICIAN’S CORNER

Predictable drill-free screw positioning witha graduated 3-dimensional radiographic-surgicalguide: A preliminary report

Sergio Estelita,a Guilherme Janson,b Kelly Chiqueto,a Marcos Janson,c and Marcos Roberto de Freitasd

Bauru, Brazil

Introduction: Mini-implants are placed in restricted sites, requiring an accurate surgical technique. However,no systematic study has quantified technique accuracy to reliably predict the surgical risks. Therefore, a grad-uated 3-dimensional radiographic-surgical guide (G-RSG) was proposed, and its inaccuracy and risk index(RI) were estimated. Methods: The sample consisted of 6 subjects (4 male, 2 female), who used mini-implantanchorage. Ten drill-free screws (DFS) were placed by using the G-RSG. The central point of the mesiodistalseptum width (SW) was the selected implant site on the presurgical radiograph. The distances between DFSand the adjacent teeth (5-DFS and 6-DFS) were measured to evaluate screw centralization and inaccuracy de-gree (ID). These distances were statistically compared by independent t tests, and inaccuracy was determinedby the expression ID 5 (5-DFS – 6-DFS)/2, which represents deviation of the mini-implant’s final position re-garding the central point initially selected. Then SW, ID, and screw diameter (SØ) were combined to estimatethe surgical risk with RI expressed by RI 5 SØ/SW – ID. Results: The 5-DFS and 6-DFS distances were notsignificantly different. The ID of the G-RSG was 0.17 mm. The low ID ensured a safe RI (\1) in spite of the re-stricted SW. Conclusions: The G-RSG accuracy allowed fine prediction of the final DFS position in the inter-radicular septum, with a low RI, which is a helpful tool to estimate surgical risks. (Am J Orthod DentofacialOrthop 2009;136:722-35)

Orthodontic anchorage with mini-implants hasbeen frequently used because it prevents un-wanted tooth movements and reduces the de-

pendence on patient cooperation, increasing thepredictability in attaining the treatment goals.1-3 How-ever, current mini-implant failure rates of 10% to 20%are significant efficacy limitation of this anchorage sys-tem,4-12 whereas the restricted anatomic dimensionsof implant sites can limit mini-implant applicability

a Graduate student, Department of Orthodontics, Bauru Dental School, Univer-

sity of Sao Paulo, Bauru, SP, Brazil.b Professor and head, Department of Orthodontics, Bauru Dental School,

University of Sao Paulo, Bauru, SP, Brazil.c Private practice, Bauru, Sao Paulo, Brazil.d Professor, Department of Orthodontics, Bauru Dental School, University of

Sao Paulo, Bauru, SP, Brazil.

The devices described in this article (G-RSG, CRP, and parallelometer) received

a national patent license with the following patent number PI0802308-5, which

was registered by INPI (National Institute for Industrial Property). The patent

was required by the University of Sao Paulo, including as inventors the authors

Sergio Estelita and Guilherme Janson. The other authors report no commercial,

proprietary, or financial interest in the products or companies described in this

article.

Reprint requests to: Sergio Estelita, Department of Orthodontics, Bauru Dental

School, University of Sao Paulo, Alameda Octavio Pinheiro Brisolla 9-75,

Bauru, Sao Paulo, 17012-901, Brazil; e-mail, [email protected].

Submitted, June 2007; revised and accepted, December 2007.

0889-5406/$36.00

Copyright � 2009 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2007.12.028

722

because of increased surgical risks and complications,unfavorably influencing the risk-benefit rate.11,13-18

It was demonstrated that a mini-implant placementtechnique without previous drilling is a surgical factorthat reduces operating time,19-21 improves primary sta-bility and holding power,19,20,22-25 increases the inter-face between bone and screw,22-24 and prevents boneoverheating,20,24,26-28 decreasing mini-implant mobilityand failure risk.24 From the point of view of this en-hanced clinical performance, drill-free screws (DFS)can be considered a significant advance in mini-implantdesign.21,24,27 Furthermore, the clinical procedure toplace DFS prevents tooth root damage from the surgicaldrill.21,26,27,29 Although this seems encouraging forscrew placement safety, it is well known that direct con-tact between self-drilling screws and a tooth root isunacceptable, because it can cause considerable cemen-tum and dentin injuries.1,29-32 Thus, DFS placementsuccess depends on correct positioning in the interradic-ular septum. Furthermore, injurious mini-implant place-ment and positioning are associated with screwfailure11,12,17,29,32-34 and possible inflammatory root re-sorption.35,36 For these reasons, improperly placedmini-implants should be removed, and a replacementshould be performed in another site.29,31,37,38 Damagedtooth roots should not be moved because additionalmovement could exacerbate the situation.39,40

mailto:[email protected]

Therefore, the significant advantages of the DFS designhave little or no clinical importance if it is mistakenlypositioned in the interradicular septum.

If complications and failures increase because ofimproperly positioned screws in the interradicular sep-tum, an accurate surgical guide should be used to directthe 3-dimensional (3D) course of the screw regardingadjacent tooth roots.5,41-43 Even though several 3D sur-gical guides for mini-implants have been described, nosystematic studies have measured their accuracy, and,consequently, surgical risks cannot be reliably pre-dicted.32,41-47 In this article, we propose a graduated3D radiographic-surgical guide (G-RSG) to predict thefinal DFS position in the interradicular septum, and itsaccuracy was evaluated to obtain a risk index (RI) forsafe mini-implant placement.

MATERIAL AND METHODS

The sample consisted of 6 subjects (4 male, 2female) with complete Angle Class II malocclusionand mean age of 18.68 6 5.68 years (range, 12.91-27.33 years). Four patients had bilateral Class II maloc-clusion, and 2 had Class II subdivision malocclusion.Orthodontic treatment was performed with 0.0223 0.028-in preadjusted edgewise appliances. The2-maxillary premolar extraction protocol was used tocorrect the bilateral Class II patients, and the Class IIsubdivision malocclusion patients were treated with 1maxillary premolar extraction.48 The Class II antero-posterior severity required absolute anchorage withmini-implants to obtain a Class I canine relationship.Thus, a total of 10 premolars were extracted in the sam-ple, and a corresponding number of DFS were placed inthe buccal alveolar bone between the maxillary first mo-lar and second premolar to anchor the anterior retractionforces. Six mini-implants were placed in the right side(4 male, 2 female), and 4 were placed in left side (3male, 1 female).

All DFS (SH 1514-07, Absoanchor, Dentos, Daegu,Korea) had the same dimensions (diameter, 1.5 mm;length, 7 mm) and were placed by the same surgeon,(S.E.), using a G-RSG and a coaxial radiographic posi-tioner (CRP) to obtain accurate and safe screw position-ing in the interradicular septum. The operation principlesand clinical procedures of G-RSG/CRP use are describedbelow.

Device principles

The G-RSG is a 0.045-in stainless steel telescopictube with a graduated horizontal scale soldered to theend of a vertical arm, which is attached to a horizontalarm by a Gurin lock. Both arms are made of 0.021 3

American Journal of Orthodontics and Dentofacial Orthopedics

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0.025-in stainless steel wire, allowing the G-RSG to beplaced in fixed orthodontic appliances (Fig 1, A and B).

The CRP uses bitewing positioner principles and hasa short free end of 5-mm, 0.040-in stainless steel wirethat is perpendicular to the film plane and parallel tothe cone locator arm (Fig 1, C and D). The CRP is usedto obtain the presurgical radiograph (G-RSG/CRP),guiding the x-ray through the tissues according to theG-RSG telescopic tube direction (Fig 1, E and F).

A parallelometer of 0.040-in stainless steel wire sol-dered parallel to a 0.12-in stainless steel tube is fittedinto the G-RSG telescopic tube to guide the screwdriver(G-RSG/parallelometer), making the screw placementpath coincident with the x-ray path (Fig 1, G and H).

Briefly, the narrow link established by the G-RSG be-tween radiographic (G-RSG/CRP) and surgical (G-RSG/parallelometer) procedures summarizes the G-RSGprinciples.

Clinical procedures

The implant site should be initially chosen accord-ing to mechanical needs and bone availability,49 requir-ing a diagnostic radiograph of the intended site.31 If thesurgical technique used to place the mini-implant ishighly accurate, the minimal mesiodistal width of theinterradicular septum should be about 2.5 to 3mm,14,38 because the DFS diameter should not besmaller than 1.5 mm,21,22,24,50 which is half or moreof the available space. However, sufficient interradicu-lar space is often considered improper because of an in-adequate radiographic technique, when dentalstructures are obliquely projected on the interradicularseptum32 (Fig 2). To avoid these mistaken interpreta-tions, bitewing principles should be preferentially ap-plied to evaluate bone availability in the interradicularseptum.51,52 Thus, an accurate radiographic evaluationof the intended interradicular septum should alwaysbe the first step of mini-implant placement.14

The sequential steps of DFS placement with G-RSGand CRP in the maxillary or the mandibular arch are asfollows.

1. The surgical materials are previously autoclaved,including the G-RSG, CRP, and parallelometer.The patient has presurgical prophylaxis with a so-dium bicarbonate jet for dental plaque removaland 0.12% chlorhexidine mouth rinse to reduce sur-gery contamination by oral microorganisms.39,53

2. A diagnostic radiograph is taken (Fig 3, A). How-ever, to place the screw with only 1 x-ray, a diagnos-tic radiograph can be taken with the G-RSG alreadypositioned. Thus, bone availability evaluation andplacement site selection can be done with the

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same radiograph. The G-RSG is attached to thefixed appliance, introducing the 0.021 3 0.025-inhorizontal arm in the 0.022 3 0.028-in bracket slots(Fig 3, B and B0); this allows mesiodistal adjust-

ments of the G-RSG position so that the centralpoint of the horizontal graduated scale is centeredon the interradicular septum according to clinicaljudgment. Scale position can be vertically changed

Fig 1. G-RSG/CRP principles. A, The G-RSG has a 0.040-in telescopic tube with a graduated scale.The vertical arm to which the telescopic tube is soldered is attached to a horizontal arm by a Gurinlock. B, The horizontal arm is made of a 0.021 3 0.025-in stainless steel wire, which allows attach-ment of the G-RSG to the fixed appliances. C, The CRP standardizes the presurgical radiographs ac-cording to bitewing positioner principles. D, A short free end of the 0.040-in stainless steel wire of 5mm in length, perpendicular to the film plane and parallel to the cone locator arm, allows the link be-tween the CRP and the G-RSG telescopic tube. E, The CRP free end is placed in the G-RSG tele-scopic tube, establishing coaxiality between the x-ray path and the telescopic tube direction. F,The G-RSG image is used to select an adequate implant site, where the graduated scale is not super-imposed over critical anatomic structures in this x-ray path. G, The screwdriver fits into the 0.12-inparallelometer tube, and its 0.040-in stainless steel wire is placed into the G-RSG telescopic tube,making screw placement direction coincident to the x-ray path. H, The radiographically selected im-plant site is clinically determined on the graduated scale by rotating the parallelometer around the G-RSG telescopic tube until the screw tip reaches the intended site, keeping the screwdriver directionunchanged. Subsequently, screwdriver rotation inside the parallelometer tube allows accurate DFSplacement in the selected implant site, according to the x-ray path.

American Journal of Orthodontics and Dentofacial Orthopedics Estelita et al 725Volume 136, Number 5

by sliding the vertical arm through the Gurin lock.Tube position can also be adjusted by torquingthe vertical or horizontal arm, although these mod-ifications are usually unnecessary when the archhas been aligned.

3. With the G-RSG centrally positioned on the interra-dicular septum, the short free end of the CRP is in-troduced into the telescopic tube, and the CRPspatial position, defined by the telescopic tube, isused to guide the x-ray tube to obtain the presurgi-cal radiograph (Fig 3, C and C0). Sequentially, theradiographic image of the G-RSG graduated scaleis taken as the reference to determine the centralpoint of the interradicular septum (Fig 3, D andD0). The scale mark coincident to the interradicularcentral point represents the most adequate implantsite, which can be clinically identified and trans-ferred to the alveolar mucosa with high accuracy.Briefly, the horizontal graduated scale is a radio-graphic and clinical reference to determine withonly 1 radiograph the exact site where the DFScan be safely placed.

4. After implant site selection, the stainless steel wireof the parallelometer is placed into the G-RSG tele-scopic tube, and the parallelometer tube is used toguide a screwdriver in a coincident direction withthe telescopic tube. Next, the screwdriver positionis adjusted horizontally on the scale marks, rotatingthe parallelometer around the telescopic tube to po-sition the DFS tip coincident with the selected im-plant site. When the screw tip is correctlypositioned in the selected site, the DFS is placedin the safe path defined by telescopic tube (Fig 3,E-F0). According to the surgeon’s preference, if

Fig 2. The periapical radiographic image shows inade-quate interradicular space for mini-implant placementbecause of an obliquely projected image. When thissame anatomic region had bitewing evaluation, itsmore orthogonal x-ray path prevented oblique root pro-jection on interradicular bone, showing the actual mesio-distal width of the interradicular septum. In this case, thesignificant changes in interproximal overlapping andtranspalatal bar position confirm the distinct x-ray pathsused to obtain the periapical and bitewing radiographs.

more than half of the screw length has alreadybeen placed, use of the G-RSG/parallelometer canbe suspended because the implant site and screwplacement path will not change.

5. Immediately after DFS placement, the CRP/G-RSG can be used again to obtain a standardizedfinal radiograph to evaluate the position of themini-implant regarding the adjacent anatomicstructures in this specific x-ray path (Fig 3, G andG0). Then the G-RSG is disconnected from the fixedappliance (Fig 3, H and H0), and the patient is in-structed to maintain satisfactory oral hygiene.9

Methods

To evaluate the accuracy of the G-RSG/CRP, post-surgical radiographs were digitized with a SprintScan35 Plus scanner (Polaroid, Boston, Mass) at 100% scaleand high resolution (675 dpi), allowing magnification of300% without image quality loss. Digital images werestored in tagged image file format (TIFF), and Photoshopsoftware (version 7.0, Adobe Systems, San Jose, Calif)was used for radiographic image measurements.54

The central point of the mesiodistal septum width(SW) is the most adequate and safe site for mini-implants. For this reason, placement of the DFS withthe G-RSG/CRP was aimed at the central point. Thescrew centralization degree regarding mesiodistal SWwas taken as the parameter to evaluate the accuracy ofthe G-RSG/CRP in predicting the postsurgical screwpositioning in the interradicular septum.

To evaluate screw centralization, the distances be-tween the mini-implant and the periodontal ligaments ofadjacent tooth roots were measured. The DFS distanceto the maxillary second premolar (5-DFS) and the firstmolar (6-DFS) were calculated (Fig 4, A). Furthermore,5-DFS and 6-DFS variables were taken to classify themini-implants into 2 groups according to their placementdeviation toward adjacent teeth. Otherwise, mini-im-plants in opposite deviation directions would canceleach other regarding 5-DFS and 6-DFS discrepancy,masking their actual differences. Group 1 consisted ofmini-implants displaced toward the premolar, and group2 consisted of mini-implants displaced toward the molar.The 5-DFS and 6-DFS distances were statistically com-pared in each group and in the whole sample.

The arithmetic difference between 5-DFS and6-DFS was divided by 2, and the absolute value wasthe G-RSG/CRP inaccuracy degree (ID), because it rep-resents screw deviation regarding the ideal centralizedposition in the interradicular septum.

The interradicular SW was considered the smallestdistance between the periodontal ligaments of adjacent


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Fig 3. Clinical procedures with the G-RSG/CRP in the maxillary and mandibular arches: A, bitewingdiagnostic radiograph; B-B00, the G-RSG is attached to the fixed appliances; C-C00, the CRP is placedin the G-RSG telescopic tube, and a standardized presurgical radiograph is taken; D-D00, the radio-graphic image of the G-RSG graduated scale is taken as a reference to determine the implant site;E-E00, the screwdriver fits into the parallelometer tube, and the DFS is placed in the selected implantsite according to the telescopic tube direction; F-F00; the G-RSG is used to take a postsurgical radio-graph with the CRP; G-G00, the final screw position regarding adjacent tooth roots is evaluated on thepostsurgical radiograph; H-H00, the G-RSG is disconnected from the fixed appliances.


Fig 4. Measurements of the postsurgical radiograph. A, 5-DFS and 6-DFS: the smallest distance be-tween DFS and the periodontal ligament of the maxillary second premolar (5-DFS) and the first molar(6-DFS). B, Interradicular SW: the smallest distance between periodontal ligaments of adjacent toothroots, measured at the screw placement site. C, SIW: the greater width of the screw radiographicimage.

tooth roots, measured at the screw placement site (Fig 4,B). SW was used to evaluate bone availability for mini-implant placement between the maxillary second pre-molar and the first molar, and to calculate the risk index(RI).

Additionally, the greater screw image width (SIW)was measured to indirectly evaluate whether the place-ment path of the screw was coincident with the directiondetermined by the telescopic tube (Fig 4, C). Consider-ing that the actual screw head diameter is 1.9 mm andthat the radiographic image produced with CRP hasa 10% magnification factor, the greater width of thescrew’s radiographic image should be 2.09 mm, whichis equivalent to the radiographic diameter of the screwhead when its long axis and telescopic tube have coinci-dent directions. The discrepancies between the tele-scopic tube direction and the screw’s placement pathwere determined by a greater SIW on the radiographicimage.

Risk index

Knowledge of the ID of the surgical technique, thescrew diameter (SØ), and the interradicular SW allowscorrect and safe mini-implant placement. These vari-ables should be related to each other. The RI proposedin this study can be calculated for a specific surgical pro-cedure, relating these variables according to the follow-ing formula:

MinRI 5 SB=SW� ðID� 2SDÞ

MaxRI 5 SB=SW� ðID12SDÞIf the minimum RI (MinRI) and maximum RI (MaxRI)are less than or equal to 1, the surgical procedure is safe,because the SØ is smaller than the SWeven when a bone

safety margin, concerning the inaccuracy range (ID 6 2SD), is subtracted from the SW.

If MinRI #1 \MaxRI, then the surgical procedureis risky, because the SØ is smaller than the SW onlywhen the minimal ID value is considered (ID – 2 SD).

If MinRI and MaxRI .1, then the surgical proce-dure is not indicated, because the SØ is greater thanthe SW even when the minimal ID value is considered(ID – 2 SD). In this case, surgical technique, SØ, andimplant site should be reconsidered.

The RI is a rational and simple form to relate the 3most important variables of the surgical technique,mini-implant characteristics, and the patient’s anatomiclimitations, ensuring safer and more versatile mini-implant use. If the RI is unfavorable for mini-implantplacement, the surgeon can still change his surgicaltechnique, the SØ, or the SW to put the RI into a saferrange.

All radiographs were remeasured by the same exam-iner (S.E.). The casual error was calculated according toDahlberg’s formula, S2 5 Sd2/2n, where S2 is the errorvariance, d is the difference between repeated measure-ments, and n is the number of duplicate determina-tions.55 The systematic error was evaluated by paired ttests.56

The same radiographic measurements were made bya second examiner (K.C.), and the paired t test was usedto evaluate interexaminer error. A P value smaller than0.05 was considered statistically significant.56

Statistical analysis

The distances 5-DFS and 6-DFS were compared inthe whole sample and in groups 1 and 2 with t tests.

Means and standard deviations were obtained forthe variables ID, SW, SIW, MinRI, and MaxRI.


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RESULTS

The casual errors were within acceptable levels, andthere were no significant systematic or interexaminer er-rors. Because of interexaminer agreement, only the firstexaminer’s measurements were used in this study.

The positioning of the DFS in the interradicular sep-tum had favorable centralization because there was nostatistically significant difference between 5-DFS and6-DFS in either the whole sample or separately in thegroups (Table I).

Means and standard deviations for ID, mesiodistalSW, SIW, MinRI, and MaxRI are shown in Table II.

DISCUSSION

Asscherickx et al33 emphasized that placement ofmini-implants in the alveolar process between toothroots is a critical procedure. For them, even if preventivemeasures are taken, such as an apical radiograph beforescrew placement, root damage can occur. To supply ac-curacy limitations during this critical procedure, a surgi-cal guide has been frequently indicated as the onlyalternative to obtain safe mini-implant place-ment.1,32,39,41-47,57,58

Before DFS, several surgical guides were proposedto direct the surgical drill during bone perforation, be-cause adequate final positioning of the mini-implantsregarding adjacent anatomic structures depends on thecorrect drill path into the bone, ensuring anatomic integ-rity.41,45-47 Because DFS placement does not requireprevious bone drilling, the final DFS position dependsonly on the screwdriver’s spatial position during place-ment. Thus, the need for a screwdriver guide is irrefut-able to ensure the integrity of anatomic structures withthis placement procedure.32,42-44

Accidents and complications

Frequently, conflicting reports about risks and com-plications of mini-implants are found in the literature.Recently, Wu et al32 showed that screw placement with-

Table I. Results of independent t tests between 5-DFSand 6-DFS distances

5-DFS 6-DFS

Group Mean SD Mean SD df P

Total sample (n 5 10) 0.22 0.41 0.16 0.33 18 0.725

Mini-implants displaced

toward premolar

(n 5 5)

0.02 0.33 0.30 0.33 8 0.225

Mini-implants displaced

toward molar (n 5 5)

0.42 0.41 0.02 0.29 8 0.116

out an accurate surgical guide results in a 20% rate ofinjuries during positioning. On the other hand, Melsenand Verna29 and Melsen53 do not use a surgical guideto place self-drilling mini-implants; they stated thatmini-implants placed in the periodontal ligament orthe tooth root should be removed and replaced in a dif-ferent site. Perhaps, it would be better not to drill in thepatient until the surgeon hits the interradicular septumbecause this is a questionable clinical procedure forplacing mini-implants. Mini-implant placement in theinterradicular septum is a critical procedure11-17,33,34

that usually requires an accurate surgical technique in-stead of an attempt/mistake method.32,41-47

Although DFS have made mini-implant placementsimpler,24 drill absence does not ensure tooth root integ-rity, and surgical complications and screw failures fromimproperly placed screws are difficulties still to be over-come.11,17,18 Wu et al32 demonstrated that the failure ratefrom direct contact between screw and root is high(20%), when less accurate surgical techniques are used.Cementum and dentin can be significantly damagedwhen mini-implants are screwed against the root surface(Fig 5). In these cases, the screw should be immediatelyremoved,29 and the tooth should not be moved for 3 to 4months until complete root and periodontium repair roreduce root resorption risks, although this increases treat-ment time.18,33,39 The screw should be replaced in the pa-tient, although root damage can happen again without anaccurate surgical technique.32,43 Screws placed in theperiodontal ligament have a greater failurerate.11,16,17,32,33 If a screw is lost and replacement is nec-essary, additional x-ray exposure is usually required, withadded surgery risks and radiation doses for the patient.

Correct screw positioning in the interradicular septumdepends on adequate selection of the implant site and thescrew placement path; otherwise, the integrity of adjacentanatomic structures cannot be ensured32,41-47 (Fig 6, Aand B). When the implant site is mistakenly selectedand the root surface is tangentially impinged by theDFS, the surgeon cannot always feel this injurious con-tact, and considerable root damage can occur with no no-ticeable clinical sign as shown in Figure 5, and tooth

Table II. Means and standard deviations of the variables

n 5 10

Variable Mean SD

ID 0.17 0.11

SW 2.45 0.65

SIW 2.07 0.06

MinRI 0.64 0.17

MaxRI 0.80 0.26


injury can be seen only on the postsurgical radiographicimage.59-61 However, if greater tangential root contactproduces a perceptible screw placement difficulty, someauthors believe that the screw path can be redirected toavoid the detrimental contact1,5,31,32,37 (Fig 6, C). Never-theless, additional care should always be used with anychange in the screw path to prevent root injury, becausethe path change can save 1 tooth but compromise an ad-jacent tooth (Fig 6, C), or it can make the fit of themini-implant not tight enough, requiring a new site.31

Furthermore, the method of preventing root damage—with screw placement resistance and the surgeon’s feelas references—is questionable,31,35,36,59,60,62,63 becausecortical bone thickness and bone density can change sig-nificantly in different patients and anatomic sites, with

Fig 5. Root damage caused by iatrogenic screw place-ment. Postsurgical radiographic image suggests screwinjury on the distal root surface of the maxillary first pre-molar because of inaccurately selected implant site andscrew path. The root injury occurred with no noticeableclinical signs, and the professional could not feel the drillor screw contact with the tooth root, because the rootwas tangentially impinged, allowing drill and screw pen-etration without significant effort. The impingement sus-picion makes this tooth chosen for Class II extractiontreatment. After extraction, root damage can be clini-cally visualized. The damaged area was observed witha stereoscope (sv6 Stereozoom microscope, Zeiss,Thornwood, NY; original magnification 40 times), show-ing that, regardless of drill injury, direct contact betweenscrew thread and root during placement is enough toproduce marked lesions on the root surface.

various amounts of resistance even when the screw is be-ing correctly placed into the interradicularbone.8,16,49,64,65 On the other hand, when the root surfaceis radially impinged, DFS placement can be impeded orgreatly hampered, and the surgeon can easily feel theroot damage (Fig 6, D). In this case, the implant sitemust be redefined because path changes alone probablywill not solve the problem, and root damage and screwbreakage can occur if the surgeon forces the screw66

(Fig 6, D). The attempt to avoid root damage by just anes-thetizing the soft tissues around the implant site does notseem reliable,31,67 since false-positive signals are fre-quently described by patients during the surgical proce-dure, perhaps because of the close proximity betweenthe screw path and the periodontal tissues,11 which haveabundant sensory innervation.68 Clinical signs such asscrew resistance, surgeon feeling, and patient sensitivityare not negligible, but they depend on experience, skill,and personal judgment, which are not accurate for ensur-ing root integrity.13,43 Therefore, safe DFS placement inthe interradicular septum highly depends on an accuratesurgical technique. If inadequate DFS placement can in-crease external root resorption risks,18,33,39 screw failurerates,11,16,17,32,33 screw breakage risks,66 need of replace-ment,29 and, thus, increased radiation doses and treatmenttimes, then screw placement without an accurate methodshould be avoided, especially in restricted sites. Based onthese reasons, the G-RSG was proposed to place DFS inthe interradicular septum, and its accuracy and RI weredetermined.

G-RSG/CRP principles and applications

Screw placement accuracy is directly associatedwith radiographic standardization and surgical orienta-tion.32,41 A radiographic-surgical guide can establisha standardized link between radiographic and surgicalprocedures, assisting predictable positioning of thescrew in the interradicular septum.41 In the G-RSG,this link is a telescopic tube (Figs 1, A and B, and 3, Band B0) that guides x-ray and screw placement throughcoincident paths.

The coaxial relationship between the telescopic tubeand the radiographic positioner (G-RSG/CRP) duringthe presurgical radiograph makes the x-ray path coinci-dent to the telescopic tube direction (Figs 1, E, and 3, Cand C0). The radiographic image standardization ob-tained with this fixed relationshp between x-ray path,object, and film position shows the true spatial arrange-ment between the anatomic structures and the G-RSG inthis x-ray path, avoiding oblique projections and dis-torted images that can cause mistaken interpretations.32

Thus, with the G-RSG radiographic image as the


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Fig 6. Usual errors during screw placement: A, the screw has an adequate insertion path, but rootdamage occurs because of inadequate implant site selection; B, in this case, the implant site is cor-rect, but an improper screw path was chosen, compromising root integrity; C, tangential root im-pingement can be corrected by changing the screw path, but root integrity of the adjacent toothcan be compromised; D, radial root impingement is a serious error that cannot be solved only bychanging the screw path; an adequate implant site must also be determined.

reference, an adequate implant site can be selectedwhere the graduated scale image is not superimposedon critical anatomic structures (Figs 1, F, and 3, D andD0). If a presurgical radiograph is taken without stan-dardization, the image cannot show the actual spatial ar-rangement between the surgical guide and the anatomicstructures, causing the surgeon to make a mistake aboutthe screw placement site (Fig 5).69 Thus, radiographicimage standardization, as provided by the G-RSG/CRP (Figs 1, E and F, and 3, C-D0), is essential in select-ing a correct implant site. Contrary to other usual radio-graphic positioners, the patient does not need to bite oruse muscular effort to stabilize the CRP during x-ray ex-posure; it is maintained in position by the G-RSG tele-scopic tube. The clinical procedures described toassemble the unit would be unlikely to produce signifi-cant forces to bend or distort the wires with such a trans-verse dimension and short length. Thus, significant G-RSG/CRP deformation does not occur, and radio-graphic image accuracy and reproducibility is ensured.

However, the correct selection of the implant site ona standardized radiographic image represents only a2-dimensional orientation, which is insufficient to en-sure safe placement because the screw path into thebone is undefined; for each 8� of variation from the ad-equate screw path, the tip of an 8-mm screw deviatesabout 1.04 mm (Fig 6, B). In a narrow interradicular sep-tum, this deviation amount can cause root damage andsurgical failure.11,17,32,33 With the G-RSG, the directlink between radiographic and surgical procedures al-lows the x-ray path in the presurgical radiograph toalso be used to place the DFS. Thus, the presurgical ra-diographic image can be used to predict final screw po-sitioning because the x-ray and the screw penetrate thetissues in the same path through adjacent anatomicstructures.32,41

For this reason, the parallelometer was created tomake the screwdriver direction coincident with thex-ray path defined by the G-RSG telescopic tube (Figs1, G, and 3, E and E0). Thus, the screw can be placed


in the selected site according to an ideal x-ray path, pro-viding accurate and safe 3D surgical orientation withonly 1 radiographic exposure (Figs 1, G and H, and 3,E and E0). Moreover, the parallelometer rotation aroundthe telescopic tube allows slight variations in horizontalscrewdriver position to maintain its direction. The place-ment site previously chosen on the G-RSG image can al-ways be clinically selected on the graduated scale(Fig 1, H).

Some authors have suggested 2-dimensional surgicalguides for mini-implants.31,38,57,70 Because it is unlikelyto obtain surgical accuracy with only a 2-dimensionalsurgical guide orientation,5,43,71 a guide with 3D orienta-tion should be preferentially used.32,41-47 The surgicaltechniques suggested by Kim et al,43 Kitai et al,46 andFreudenthaler et al45 require computed tomographyand stereolithography machines, and increased com-plexity, cost, and radiation exposure to the patient. Cous-ley and Parberry,44 Morea et al,47 and Suzuki andBuranastidporn42 used surgical guides with 3D orienta-tion, although without presurgical radiographic stan-dardization. Wu et al32 suggested a surgical guide withan acrylic template to standardize the presurgical radio-graph, and the presurgical radiograph was cropped tomake a radiographic template for mini-implantplacement. More recently, Estelita et al41 developedthe 3D radiographic-surgical guide principle to placemini-implants in the interradicular septum, but that guidedid not allow placement of DFS. The G-RSG/CRP hasthe following differential characteristics: fine presurgicaland postsurgical radiographic standardization, accuratelink between radiographic and surgical procedure, grad-uated scale to select an adequate implant site with only 1standardized x-ray exposure, 3D screw placement orien-tation according to the x-ray path, low radiation dose tothe patient, satisfactory cost and risk-benefit rate, andhandy technology.

Although some authors advocate vertical angulationof the mini-implant regarding bone surface to avoid rootdamage and increase screw stability, systematic studieshave not sustained these speculations.31,72,73 Accordingto Park et al,9 there is no significant correlation betweena vertical placement angle of the screw and success rate.On the other hand, Kwak et al74 demonstrated that themean bone tissue thickness, which covers the buccalroot of the maxillary first molar and second premolar,was not greater than 1.8 and 3.5 mm in the midpointand the apex of the root, respectively. Thus, if the im-plant site is mistakenly defined regarding adjacent toothroots, and an 8-mm screw is vertically placed at 30� to40� with the tooth’s long axis, it would be unlikely toavoid root damage because the screw placement depthwould reach 4 to 5.12 mm.31 Furthermore, this vertical

placement angle is empirically determined, and, accord-ing to Herman et al,34 it is nearly impossible to deter-mine, causing inaccurate screw positioning that can beassociated with screw failure.11,17,32-34 Additionally,the thickest cortical bone was observed near the coronaland apical regions,64 where vertical placement is limitedby damage risks to the maxillary sinus.1,14,74 However,if the implant site is correctly defined and screw place-ment follows an adequate horizontal path, there is noreason to worry about vertical angulation. Therefore,a vertical placement angle does not seem to be a relevantfactor to ensure safe screw placement. Although thesuccess rate is independent of the vertical placement an-gle,9 it can depend on the horizontal placement angleand site, because these 2 variables are the most impor-tant in defining mini-implant position regarding adja-cent tooth roots11 (Fig 6, A and B). Thus, accuratesurgical techniques to place mini-implants should in-clude the correct selection of the site and the horizontalplacement path.

G-RSG/CRP accuracy

There is scientific evidence that the accuracy of thesurgical technique in placing mini-implants providessignificant advantages for success rate, stability, ortho-dontic mechanics, available implant sites, and surgicalrisks. The attached gingiva and the thickest corticalbone are 2 factors that increase success rate and stabilityof mini-implants,6,8,9,39 whereas screw proximity totooth roots is associated with screw failure.11,17,32-34

However, the attached gingiva and thicker corticalbone are in the coronal region, where the interradicularseptum is narrower,14-16,64 making screw placementa more critical procedure.11,17,32-34 The coronal regionof the interradicular septum is the most intended ana-tomic region for mini-implants because it allows ortho-dontic forces with a greater horizontal component,favoring mesiodistal tooth movement without excessiveload on the mini-implant.1 Therefore, accuracy is an es-sential requisite for mini-implants. The surgical risksare inversely proportional to accurancy of the surgicaltechnique; thus, accuracy can allow screw placementin more restricted sites,14 avoiding presurgical toothmovements to augment bone availability that increasetreatment time.15,29,31

If the interradicular septum is a restricted anatomicregion for a mini-implant, then it is correct to use the cen-tral point of the septum as the safest implant site, be-cause, with less available space, the screw should bemore centralized or equidistant regarding adjacent rootsto reduce surgical risks and screw failures.11,17,32,33 Ifthe mini-implant has a small displacement with the


November 2009

orthodontic load,75 and proximity between the screw andthe periodontal ligament is directly associated withscrew failure,11,17,32,33 then the central point of the sep-tum is the best placement site.11 For this reason, the cen-tral point of the interradicular septum was used in thisstudy, and the accuracy of the G-RSG/CRP in predictingthe final screw position was evaluated by measuring thedistance between the mini-implant and the adjacent peri-odontal ligament (5-DFS 5 0.22 mm, 6-DFS 5 0.16mm). The results showed that screws were placed closeto the central point, with no significant difference be-tween 5-DFS and 6DFS, indicating high accuracy (TableI). However, this result does not represent the actualtechnique accuracy because 5-DFS and 6-DFS alwaystend to have convergent mean values when mini-im-plants displaced toward the molar are evaluated withthose displaced toward the premolar. For this reason,mini-implants displaced in opposite directions wereseparated into different groups, and, even so, intragroupcomparisons show no significant differences between5-DFS and 6-DFS, confirming satisfactory accuracy(Table I). Nevertheless, the lack of significant statisticaldifference does not mean that 5-DFS and 6-DFS have co-incident values and that inaccuracy technique is null.There was low ID (0.17 mm, Table II) that was notenough to make 5-DFS and 6-DFS significantly differ-ent. However, even knowing that the ID is low, it canbe clinically significant when the mesiodistal SW isrestricted. Thus, the 0.17-mm ID of the G-RSG/CRPtechnique should be understood as a necessary addi-tional bone safety margin that must always be presentto compensate for implant site error in the inaccuracyrange (0.17 mm 6 2 SD).

Because Liou et al75 recommend 2 mm of safetyclearance between mini-implant and dental root, a1.5-mm screw diameter should require 5.5 mm ofinterradicular SW to ensure root integrity, making mini-implants impossible in most sites. Fortunately, a signifi-cant displacement or tipping of mini-implants is notusual.2,76 In this study, the mean width of the interradic-ular septum between the maxillary second premolar andthe first molar was 2.45 mm (Table II); this was notmuch different from the 2.9 mm of SW in a study onbone availability in the interradicular septum with com-puterized tomography.14 This same study showed thatthe mean width of the interradicular septum on the max-illary buccal side was about 3 mm. The authors statedthat at least a 3.5-mm SW is necessary to safely placea screw with a 1.5-mm diameter, leaving a criticalbone safety margin of only 1 mm between the screwand the periodontal ligament. Considering only thesedata and the fact that adequate DFS resistance requiresat least a 1.5-mm screw diameter, the maxillary buccal

side should be considered improper for DFS.21,22,24,50

However, the authors emphasized that surgical riskscan be significantly decreased with accurate radio-graphic control and optimal surgical skill.14 The stan-dardized radiographic technique with CRP wasessential in this study to obtain safe mini-implant place-ment. Conversely, surgical skill is a questionable condi-tion to ensure safe screw placement and should not beconfused with surgical technique because skill can beconsidered an unmeasurable variable inherent in eachsurgeon,13 whereas technique can be systematically ap-plied and reproduced, and its accuracy can be numeri-cally quantified.32 This is a pertinent considerationbecause, according to this investigation, the low ID(0.17 mm) of the G-RSG/CRP technique allowed an in-terradicular septum with a 2.45-mm mean width to besafely used for a 1.5-mm diameter screw, maintainingan acceptable RI, in spite of the restricted bone safetymargin between the screw and the periodontal ligament(Table II). Furthermore, surgical complications can oc-cur even with experienced surgeons using an inaccuratesurgical technique,35,36,59,60,62,63 highlighting the im-portance of surgical technique on the surgeon’sskill.13,43

The measurement of the greater SIW (2.07 mm)showed a similar value regarding known screw headdiameter (2.09 mm) (Table II). This result demon-strates that x-ray and screw had a coincident pathdefined by the telescopic tube. Thus, the screw’s ra-diographic image only shows the exact contour ofthe screw head because the long axis of the screw,which represents its path in the interradicular sep-tum, is coincident with the x-ray path (Figs 1, Eand G, and 3 C, C0, E, and E0). This coincidence be-tween x-ray and screw path, by using the telescopictube, allows the screw to be safely placed in theinterradicular septum according to the presurgicalradiographic image, preventing root damage froman improperly selected site and placement angle(Fig 6, A and B).

Therefore, if the G-RSG/CRP can accurately defineimplant site and placement angle of the screw, its 3Dorientation can significantly reduce surgical risks andfailures.

Risk index

The risk-benefit rate of a procedure should be themost important parameter to indicate its use. If a surgicalprocedure has a high risk for the patient, an unfavorablerisk-benefit rate cannot justify it. However, this logicalthought has no clinical application if the risk of a surgi-cal procedure cannot be quantified. For this reason, the


RI was proposed in this study to evaluate the surgicalrisks of mini-implant placement.

The primary requirement for mini-implant place-ment in the interradicular septum should be sufficientmesiodistal width to accommodate the screw thread di-ameter. Nevertheless, the greater the screw thread diam-eter in relation to the SW, the greater the surgical risk,requiring a surgical technique with a low ID. The ID ofthe surgical technique was combined with the data onSW and SØ to formulate an RI. The RI was considereddirectly proportional to screw diameter and inverselyproportional to the difference between SW and ID.

In this study the ID was 0.17 mm 6 0.11, and 2 SDwere used to determine maximum and minimum RIvalues. This low ID produced minimum and maximumRI values less than 1, despite the restricted SW. Thus,the RI shows that the accuracy of the surgical techniqueis an advantage in avoiding root damage because SW isinherent to the patient, and the DFS diameter cannot besignificantly changed. The DFS diameter should be be-tween 1.5 and 2 mm to prevent screw breakage withoutexcessively increasing screw diameter.21,22,24,50,77

Therefore, a surgical technique should be selected ac-cording to its accuracy, and the RI should be calculatedbefore mini-implant placement, informing the surgeonabout the risks of the procedure.

Although this preliminary report showed high accu-racy with the G-RSG/CRP technique, studies with morerepresentative samples should be performed. Addition-ally, we think that more studies should be performed toevaluate the accuracy of several surgical techniques cur-rently used to place mini-implants, because these sitesusually have restricted dimensions, and the RI is a ratio-nal option to reduce surgical accidents and complica-tions, increasing efficiency and success rate of thisemerging anchorage system. We are undertaking an ad-ditional study to evaluate DFS stability when placedwith the G-RSG/CRP.

CONCLUSIONS

The link of the G-RSG between the radiographicand surgical procedures ensures screw placement ac-cording to a safe x-ray path established in the presurgi-cal radiograph. As a result of this application, low IDand RI values were observed, allowing accurate predic-tion of the final DFS position in the interradicular sep-tum. The RI proposed in this study is a rewardingclinical and scientific tool for individual and systematicevaluations of the surgical risks.

We thank Dr. Hee-Moon Kyung, Dentos Inc, Daegu,Korea, for the generous donation of the mini-implantmaterials.

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