Implant therapy has been applied to various clinical

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1 Implnt Length nd Dimeter, Bicorticl Anchorge, nd Sinus Augmenttion on Bone Stress Distriution: Three-Dimensionl Finite Element Anlysis Hiroyoshi Moriwki, DDS, PhD 1 /Stoshi Ymguchi, PhD 2 /Tmki Nkno, DDS, PhD 3 / Ysufumi Ymnishi, DDS, PhD 1 /Stoshi Imzto, DDS, PhD 4 /Hirofumi Ytni, DDS, PhD 5 Purpose: Clrifiction of the protocol for using short implnts is required to enle widespred use of short implnts s n ville tretment option. The purpose of this study ws to investigte the influences of implnt length nd dimeter, icorticl nchorge, nd sinus ugmenttion on peri-implnt corticl one stress y three-dimensionl finite element nlysis. Mterils nd Methods: For one models with one quntity A nd C in the mxillry molr region, three-dimensionl finite element nlysis ws performed using different lengths nd dimeters of implnt computer-ided design models, nd the degree of mximum principl stress distriution for ech model ws clculted. Results: For one quntity A models, the degree of stress distriution of the 4-mm-dimeter, 6-mm-length implnt ws the gretest. For one quntity C models, the degree of stress distriution of the 5-mm-dimeter, 6-mm-length implnt with icorticl nchorge ws much smller thn tht for the 4-mm-dimeter, 13-mm-length implnt with sinus ugmenttion. Conclusion: The results of this study suggest tht 6-mm-length implnts should e selected in cses with one quntity C where the one width permits incresing implnt dimeter from 4 mm to 5 mm. Int J Orl Mxillofc Implnts 2016;31:e84 e91. doi: /jomi.4217 Keywords: iomechnics, dentl implnts, finite element nlysis, sinus floor ugmenttion Implnt therpy hs een pplied to vrious clinicl cses s prosthetic tretment option ecuse of its positive clinicl performnce. 1,2 However, lveolr one quntity C, 3 cused y severe periodontl disese nd long-term edentulous rch, 4 restricts the use of implnt therpy. While one ugmenttion such s sinus ugmenttion 5,6 nd veneer grfting 7,8 nd lterliztion of the inferior lveolr nerve 9,10 re dopted to llow insertion of implnts, these therpies still hve disdvntges such s surgicl invsion nd 1 Resercher, Deprtment of Fixed Prosthodontics, Osk University Grdute School of Dentistry, Osk, Jpn. 2 Associte Professor, Deprtment of Biomterils Science, Osk University Grdute School of Dentistry, Osk, Jpn. 3 Assistnt Professor, Deprtment of Biomterils Science, Osk University Grdute School of Dentistry, Osk, Jpn. 4 Professor, Deprtment of Biomterils Science, Osk University Grdute School of Dentistry, Osk, Jpn. 5 Professor, Deprtment of Fixed Prosthodontics, Osk University Grdute School of Dentistry, Osk, Jpn. Correspondence to: Stoshi Ymguchi, Deprtment of Biomterils Science, Osk University Grdute School of Dentistry, Osk , Jpn. Fx: Emil: ymgu@dent.osk-u.c.jp 2016 y Quintessence Pulishing Co Inc. infection, risks of sensory nerve prlysis, prolonged tretment time, nd n increse in tretment cost. 11 Menwhile, short implnts hve chieved significnt mrket shre owing to the improvement in implnt surfce chrcteristics, 12 nd hve een relesed y vrious mnufcturers. An in silico study 13 reported tht there ws no difference in peri-implnt one stress cused y chnging the length of the implnt ody when mesured y finite element nlysis using simplified computer-ided design models. Clinicl studies hve reported tht the survivl rtes of stndrd-length nd short implnts were equivlent. 14,15 Recent clinicl evidence hs mentioned tht the use of short implnts my e considered n lterntive to more complicted one ugmenttion surgeries. 16 They hve concluded tht the use of short implnts gives ptients lower risk, shorter clinicl time, nd decresed cost when compred with one ugmenttion surgeries. 17 These dvntges of using short implnts s compred with one ugmenttion surgeries were lso mentioned in other studies, nd there were few differences in survivl rte etween short nd stndrd-length implnts. However, the risk for one resorption in osseointegrted implnts is greter for tretment involving short implnts. 8 Clinicins must follow certin protocols when using short implnts, eg, splinting to other implnts, 21,22 e84 Volume 31, Numer 4, 2016

2 Moriwki et l no use for single-tooth replcement in molr sites, 22 nd the use of wider dimeter for short implnts. 21 In cses tht cnnot stisfy these protocols, one ugmenttion surgery is selected, especilly in the mxill. The purpose of this study ws to investigte the influences of implnt length nd dimeter, icorticl nchorge, 23 nd sinus ugmenttion on peri-implnt corticl one stress using three-dimensionl finite element nlysis. For one quntity A 3 nd C models in the mxillry molr region, three-dimensionl finite element nlysis ws performed using two-piece implnt computer-ided design models composed of n implnt ody, utment, nd utment screw MATERIALS AND METHODS The models composed of n implnt ody, utment, nd utment screw were creted using computerided design softwre (SolidWorks Premium 2011, SolidWorks) s shown in Fig 1. The short-length nd regulr-pltform implnt ody ws defined s ϕ4 6-mm. 24 The long length nd regulr pltform implnt ody ws defined s ϕ4 13-mm. 24 The short-length nd wide-pltform implnt ody ws defined s ϕ5 6-mm. 24 The implnt-utment joints comprised n internl joint. The pitch of threds with 0.66-mm intervls nd the shpe of the threds were the sme in ll implnts. The height of the utment ws 7.0 mm. The implnt nd utment were connected y the utment screw. Two computer-ided design models of posterior mxillry one were creted with missing premolrs nd molrs (Fig 2). Alveolr one quntity A nd C models were designed to enle insertion of implnts 13.0 mm nd 6.0 mm in length, respectively. The overlying corticl one of oth models ws designed to e 1.0 mm thick. 12,25 The remining res were designed s cncellous one. The ϕ4 6-mm, ϕ4 13-mm, nd ϕ5 6-mm implnts were plced on the one quntity A model (Fig 3). Although not used in clinicl sitution, these models were prepred s controls to investigte the influence of implnt length, icorticl nchorge, nd implnt dimeter. Similrly, 6-mm-length implnts were plced on the one quntity C model with icorticl nchorge (Fig 4). The 13-mm-length implnt ws plced on the one quntity C model with sinus ugmenttion, which ws composed of mxillry one nd grft mterils (Fig 5). As control model, the ϕ4 6-mm implnt ws plced on the one quntity C model with icorticl nchorge nd sinus ugmenttion, nd the ϕ4 13-mm implnt ws plced on the one quntity C model without sinus ugmenttion (Fig 6) Fig 1 Geometry of computer-ided design models of ϕ4 6-mm implnt (left), ϕ5 6-mm implnt (middle), nd ϕ4 13-mm implnt (right) (mm). Sufficient Fig 2 Computer-ided design models of mxillry lveolr one. () Bone quntity A. () Bone quntity C. Insufficient Fig 3 Computer-ided design models of ech implnt in the cse of one quntity A. () ϕ4 6-mm implnts. () ϕ4 13-mm implnts. (c) ϕ5 6-mm implnts. c The Interntionl Journl of Orl & Mxillofcil Implnts e85

3 Moriwki et l Corticl one Grft mteril Cncellous one Fig 4 Computer-ided design models of ech implnt in the cse of one quntity C. () ϕ4 6-mm implnts with icorticl nchorge. () ϕ5 6-mm implnts with icorticl nchorge. Fig 5 One of the computer-ided design models of ech implnt in the cse of one quntity C with sinus ugmenttion, which ws composed of mxillry one nd grft mterils. Fig 6 Two control models were prepred for compring the influences of implnt length nd dimeter, icorticl nchorge, nd sinus ugmenttion. () ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge nd sinus ugmenttion. () ϕ4 13-mm implnts plced on the one quntity C without sinus ugmenttion. 150 N The mechnicl properties of one, titnium, nd the grft mteril used for the three-dimensionl finite element nlysis re shown in Tle 1. In this study, the properties of the grft mteril were equlized for cncellous one y ssuming 100% sustitution. For simultions of osseointegrted implnts, fixed Fixed surfce Buccl side Distl side Mesil side Occlusl surfce Fig 7 Assemled computer-ided design model of one nd implnt models. The mesil nd distl section surfces of the one were fixed. Sttic lod of 150 N ws pplied to the sl ridge surfce of the utment t 30 degrees in direction olique to the long xis of the implnts. Tle 1 Mechnicl Properties of Ech Component Used for Finite Element Anlysis Components Young s moduli (MP) Poisson s rtios Corticl one 13, Cncellous one 1, Implnt components 117, Grft mteril 1, ond condition ws set t the interfce etween the one or grft mteril nd the implnt ody. 26 A contct condition with sttic friction coefficient of 0.2, 31 which ccepts possile microscopic sliding, ws set t the interfces mong components of the implnts. 24 The mesil nd distl surfces of the mxillry one were fixed, nd sttic lod of 150 N 32 ws pplied to the sl ridge surfce of the utment t 30 degrees in direction olique to the long xis of the implnts 30 (Fig 7). The elements for three-dimensionl finite element nlysis were tetrhedrons with 16 nodes. To determine the mesh size tht offers n ccurte result in resonle mount of computtion time (less thn 40 minutes), the numer of elements ws incresed until the mximum principl stress converged. The results of convergence nlysis 30 re shown in Tle 2. The mesh size ws stndrdized to 0.3 mm in ll models. Threedimensionl finite element nlysis ws performed using the dd-in function of the computer-ided design softwre. The degree of mximum principl stress distriution to peri-implnt corticl one, which ws greter thn or equl to the solute vlue of the threshold, ws extrcted using computer-ided design softwre (Fig 8). The threshold for ech vlue otined y threedimensionl finite element nlysis ws set once every 10 MP from 40 to 40 MP. To investigte the influence of implnt length, icorticl nchorge, sinus ugmenttion, nd implnt e86 Volume 31, Numer 4, 2016

4 Moriwki et l Tle 2 Totl Numers of Elements for Ech Model Totl no. of elements 155, , , , , , , ,545 1 = ϕ4 6-mm implnts; 2 = ϕ4 13-mm implnts; 3 = ϕ4 6-mm implnts with icorticl nchorge; 4 = ϕ4 13-mm implnts with sinus ugmenttion; 5 = ϕ5 6-mm implnts; 6 = ϕ5 6-mm implnts with icorticl nchorge; 7 = ϕ4 6-mm implnts with icorticl nchorge nd sinus ugmenttion; 8 = ϕ4 13-mm implnts without sinus ugmenttion. S X A Fig 8 Clcultion procedure of the degree of mximum principl stress distriuted to peri-implnt corticl one. The mximum principl stress distriution to peri-implnt corticl one greter thn or equl to the solute vlue of the threshold ws extrcted y computer-ided design softwre. Totl volume of extrcted prts ws clculted. c Fig 9 four fctors: implnt length, icorticl nchorge, sinus ugmenttion, nd implnt dimeter. d dimeter, the degree of loss of mximum principl stress distriution in four groups with: (1) different implnt lengths (ϕ4 6-mm implnts plced on the one quntity A ϕ4 13-mm implnts plced on the one quntity A, ϕ4 6-mm implnts plced on the one quntity C with sinus ugmenttion ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion, nd ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge ϕ4 13-mm implnts plced on the one quntity C with icorticl nchorge; (2) the implementtion of icorticl nchorge (ϕ4 6-mm implnts plced on the one quntity A ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge, ϕ4 13-mm implnts plced on the one quntity A ϕ4 13-mm implnts plced on the one quntity C with icorticl nchorge); (3) the implementtion of sinus ugmenttion (ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge ϕ4 6-mm implnts plced on the one quntity C with sinus ugmenttion, ϕ4 13-mm implnts plced on the one quntity C with icorticl nchorge ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion), nd (4) different implnt dimeters (ϕ4 6-mm implnts plced on the one quntity A ϕ5 6-mm implnts plced on the one quntity A, ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge ϕ5 6-mm implnts plced on the one quntity C with icorticl nchorge) were compred (Fig 9). RESULTS Degree of Mximum Principl Stress Distriution in Peri-implnt Corticl Bone Figure 10 shows the degree of mximum principl stress distriution in peri-implnt corticl one for the one quntity A model. The degree of mximum principl stress distriution for ϕ4 6-mm implnts ws greter thn tht for ϕ4 13-mm implnts (Figs 10 nd 10). The degree of mximum principl stress distriution for ϕ5 6-mm implnts ws smller thn tht for ϕ4 6-mm implnts (Figs 10 nd 10). However, the degree of compressive stress (negtive vlue of the mximum principl stress) distriution for ϕ5 6-mm implnts ws greter thn tht for ϕ4 13-mm implnts (Fig 10), nd the degree of tensile stress (positive vlue of the mximum principl stress) distriution for ϕ5 6-mm implnts ws smller thn tht for ϕ4 13-mm implnts (Fig 10). For the one quntity C model, the degree of tensile stress distriution for ϕ5 6-mm implnts with icorticl nchorge ws much smller thn tht for ϕ4 13-mm implnts with sinus ugmenttion (Fig 11), while the degree of compressive stress distriution for The Interntionl Journl of Orl & Mxillofcil Implnts e87

5 Moriwki et l Degree of mximum principl stress distriution (mm 3 ) ϕ 4 6-mm ϕ 4 6-mm implnts ϕ 4 13-mm ϕ implnts ϕ 5 6-mm implnts ϕ 5 6-mm implnts 40 Threshold of mximum principl stress (MP) Fig 10 Degree of stress distriution in the one quntity A model for ϕ4 6-mm implnts, ϕ4 13-mm implnts, nd ϕ5 6-mm implnts (mm 3 ): () Compressive stress. () Tensile stress. Degree of mximum principl stress distriution (mm 3 ) ϕ 4 6-mm ϕ 4 6-mm implnts ϕ 4 13-mm ϕ implnts ϕ 5 6-mm implnts ϕ 5 6-mm implnts Threshold of mximum principl stress (MP) Degree of mximum principl stress distriution (mm 3 ) ϕ 4 6-mm implnts with icorticl nchorge ϕ 4 6-mm implnts with icorticl nchorge ϕ 4 13-mm ϕ implnts 4 13-mm with implnts sinus with sinus ugmenttion ϕ 5 6-mm implnts ϕ 5 6-mm with implnts icorticl with icorticl nchorge Threshold of mximum principl stress (MP) Degree of mximum principl stress distriution (mm 3 ) ϕ 4 6-mm implnts with icorticl nchorge ϕ 4 6-mm implnts with icorticl nchorge ϕ 4 13-mm implnts with sinus ugmenttion ϕ 4 13-mm implnts with sinus ugmenttion ϕ 5 6-mm implnts ϕ 5 6-mm with implnts icorticl with icorticl nchorge Threshold of mximum principl stress (MP) Fig 11 Degree of stress distriution in the one quntity C model for ϕ4 6-mm implnts with icorticl nchorge, ϕ4 13- mm implnts with sinus ugmenttion, nd ϕ5 6-mm with icorticl nchorge (mm 3 ). () Compressive stress. () Tensile stress. ϕ4 6-mm implnts with icorticl nchorge ws greter thn tht for ϕ4 13-mm implnts with sinus ugmenttion (Fig 11). Degree of Loss of Mximum Principl Stress Distriution Figure 12 shows the results of the degree of loss of mximum principl stress distriution in ech model. In terms of compressive stress distriution (Fig 12), the implnt dimeters highly influenced the degree of loss of mximum principl stress distriution for ll thresholds. The implnt lengths nd the use of icorticl nchorge were similr mong thresholds of 20, 30, nd 40 MP. The decrese in mximum principl stress distriution relted to sinus ugmenttion ws less when compred with the other three fctors. In terms of tensile stress distriution (Fig 12), the implnt dimeter gin highly influenced the degree of loss of mximum principl stress distriution, similr to the compressive stress distriution, nd the implementtion of icorticl nchorge ws second in line. DISCUSSION For ϕ4 13-mm implnts plced on the one quntity A, the degree of mximum principl stress distriution of the peri-implnt corticl one ws reduced in comprison with ϕ4 6-mm implnts plced on the one quntity A. This result ws cused y otining lrger cncellous one surfce re in ϕ4 13-mm implnts plced on the one quntity A. Occlusl forces were e88 Volume 31, Numer 4, 2016

6 Moriwki et l implnt length sinus ugmenttion icortl nchorge implnt dimeter implnt length sinus ugmenttion icortl nchorge implnt dimeter Degree of loss of mximum principl stress distriution (mm 3 ) Threshold of mximum principl stress (MP) Degree of loss of mximum principl stress distriution (mm 3 ) Threshold of mximum principl stress (MP) Fig 12 Degree of loss of compressive nd tensile stress distriution (mm 3 ). 1 = ϕ4 6-mm implnts plced on the one quntity A ϕ4 13-mm implnts plced on the one quntity A; 2 = ϕ4 6-mm implnts with icorticl nchorge nd sinus ugmenttion on the one quntity C ϕ4 13-mm implnts with sinus ugmenttion on the one quntity C; 3 = ϕ4 6-mm implnts with icorticl nchorge on the one quntity C ϕ4 13-mm implnts without sinus ugmenttion on the one quntity C; 4 = ϕ4 6-mm implnts with icorticl nchorge on the one quntity C ϕ4 6-mm implnts with icorticl nchorge nd sinus ugmenttion on the one quntity C; 5 = ϕ4 13-mm implnts without sinus ugmenttion on the one quntity C ϕ4 13-mm implnts with sinus ugmenttion on the one quntity C; 6 = ϕ4 6-mm implnts on the one quntity A ϕ4 6-mm implnts with icorticl nchorge on the one quntity C; 7 = ϕ4 13-mm implnts on the one quntity A ϕ4 13-mm implnts without sinus ugmenttion on the one quntity C; 8 = ϕ4 6-mm implnts on the one quntity A ϕ5 6-mm implnts on the one quntity A; 9 = ϕ4 6-mm implnts with icorticl nchorge on the one quntity C ϕ5 6-mm implnts with icorticl nchorge on the one quntity C. () Compressive stress. () Tensile stress dispersed within the cncellous one holding the implnt. Therefore, stress distriution to the peri-implnt corticl one, cusing peri-implnt one resorption, decresed. For the sme reson, the degree of mximum principl stress distriution to the peri-implnt corticl one for ϕ5 6-mm implnts plced on the one quntity A ws reduced in comprison with ϕ4 6-mm implnts plced on the one quntity A. In ddition, the 5-mm-dimeter implnt could e well stilized y cncellous one ginst occlusl loding long the long xis of the implnt ecuse of the lrge surfce re of the implnt pex. On the sis of these fctors, ϕ5 6-mm implnts plced on the one quntity A showed n equivlent degree of compressive stress distriution to ϕ4 13-mm implnts plced on the one quntity A, nd the degree of tensile stress distriution for ϕ5 6-mm implnts plced on the one quntity A ws smller thn tht for ϕ4 13-mm implnts plced on the one quntity A. The ultimte strength of humn one under tension is lower thn under compression. 33 These results suggest tht the 5-mm-dimeter implnt my e resistnt to one resorption when compred with the 4-mm-dimeter implnt. It is thought tht the degree of mximum principl stress distriution for ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion incresed in comprison with tht for ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge, ecuse while ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge were only influenced y icorticl nchorge s stressreducing fctor, ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion were influenced y the implnt length in ddition to icorticl nchorge. The implementtion of icorticl nchorge could reduce the stress on peri-implnt one. 27 The success rte douled for monocorticlly nchored implnts, especilly in the mxill, in prospective clinicl short-term study of icorticl nchorge. 34 On the sis of these two fctors, ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion displyed reduced degree of stress distriution. However, the degree of stress distriution for ϕ4 6-mm implnts plced on the one quntity C with icorticl nchorge ws less thn tht of ϕ4 6-mm implnts plced on the one quntity A, ecuse the use of icorticl nchorge leds to the dispersion of occlusl forces to the corticl one t the sinus floor. In conventionl finite element nlysis study, the plcement of ϕ5 6-mm implnt reduced stress The Interntionl Journl of Orl & Mxillofcil Implnts e89

7 Moriwki et l on the peri-implnt one, cusing lrge surfce re of contct with the wide implnt, nd stiff corticl one. 29 They concluded tht the plcement of ϕ5 6-mm implnts ws more effective thn tht of longlength implnts with sinus ugmenttion in terms of tretment cost, tretment length, nd the risk of dditionl surgeries for ptients. 30 In this study, the degree of mximum principl stress distriution for ϕ5 6-mm implnts plced on the one quntity C with icorticl nchorge ws much smller thn tht for ϕ4 13-mm implnts plced on the one quntity C with sinus ugmenttion. It is thought tht the comintion of implnt dimeter nd icorticl nchorge hd igger influence in reducing mximum principl stress distriution thn implnt length, implnt dimeter, nd sinus ugmenttion. Considering tht overloding is included s one of the cuses of peri-implnt one resorption, ll results suggest tht selecting longer implnt is cliniclly desirle when there is lveolr one quntity A. When the one quntity C is present, ϕ5 6-mm implnts my e useful in cses where the one width remins sufficient to permit incresing the implnt dimeter from 4.0 mm to 5.0 mm. There re vrious tretment methods ville to reduce the stress on peri-implnt one. Nevertheless, there re few reports of the comprison mong these methods in terms of stress reduction. Thus, this comprison study is expected to clrify the guideline for clinicl implnt tretment where n lveolr one quntity C is present. Additionlly, the long-term prognosis of clinicl implnt tretment cn e expected s n outcome of this study. The degree of loss of mximum principl stress distriution y incresing implnt length nd tht y implementing icorticl nchorge were similr. This is ecuse the dispersion of the occlusl forces to the cncellous one cused y incresing implnt length nd the dispersion to the corticl one t the implnt pex cused y implementing icorticl nchorge were equl. A conventionl finite element nlysis study reported tht extensive one ugmenttion y sinus ugmenttion reduced the stress on peri-implnt one. 35 In this study, the influence of sinus ugmenttion ws less significnt thn implnt length, dimeter, nd icorticl nchorge. This occurred ecuse the contct re with the mxillry one ws not incresed even if longer implnt ws inserted fter sinus ugmenttion. Thus, much of the occlusl loding on the implnt hd spred to the mxillry one rther thn the sinus ugmenttion grft mteril. Implnt dimeter ws more influentil design prmeter of the implnt for stress on the peri-implnt one thn implnt length nd thred shpe, especilly for short implnts. 36 Additionlly, the implnt dimeter influenced stress levels, nd the wider-dimeter implnt could help to reduce one stress. 37 In this study, the degree of loss of mximum principl stress distriution y incresing implnt dimeter ws much higher thn the effect of implnt length, icorticl nchorge, nd sinus ugmenttion. This result is cused y otining strong support, which resists susidence of implnts, nd retention, which resists rolling of implnts, s result of incresing the contct re with corticl one, which hs etter mechnicl strength properties when compred with cncellous one. The increse in implnt length in this study ws 7.0 mm. In comprison with 6- nd 10-mm-length implnts, the influence of icorticl nchorge my ecome greter thn the influence of implnt length ecuse the increse in implnt length is 4.0 mm. Anlyses of implnts of vrious lengths re ongoing. The lck of simultion of the inhomogenous nd isotropic mteril properties of humn one nd of the grft mteril is one of the limittions of the threedimensionl finite element nlysis in this study. In ddition, the evlution of primry mechnicl stility nd secondry iologic stility 38 is not possile y sttic three-dimensionl finite element nlysis without the simultion of the one-implnt interfce using the vrious rtios of osseointegrtion in this study. However, the results of the present study with homogenous nd isotropic mteril properties definitively clrified the influence of implnt length, icorticl nchorge, sinus ugmenttion, nd implnt dimeter, even if these results were ised. CONCLUSIONS The results of this study suggest tht 4-mm-dimeter implnts with incresed length should e selected to reduce the mximum principl stress of peri-implnt corticl one when one quntity A is ville. When there is one quntity C, 6-mm-length implnts should e selected if the one width is sufficient to permit incresing the implnt dimeter from 4.0 mm to 5.0 mm. The 6-mm-length implnts with icorticl nchorge hve the potentil to ecome useful tretment in chieving reduced risk of surgicl invsion, shortening clinicl time, nd presenting lower cost to ptients. ACKNOWLEDGMENTS This reserch ws supported y Grnts-in-Aid for Scientific Reserch (Nos nd 15K11195) from the Jpn Society for the Promotion of Science. e90 Volume 31, Numer 4, 2016

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