Tilting of Posterior Mandibular and Maxillary Implants for ... · tilted maxillary implants were placed in an angulation position to expand the prosthesis base without graft-ing or

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The International Journal of Oral & Maxillofacial Implants 405

Tilting of Posterior Mandibular and MaxillaryImplants for Improved Prosthesis Support

Leonard Krekmanov, DSS, DDS, PhD1/Mikael Kahn, DDS2/Bo Rangert, PhD, Mech Eng3/Håkan Lindström, MSc, Eng Physics3

Rehabilitation of atrophied edentulous arches with endosseous implants in the posterior regionsis often associated with anatomic problems such as jaw shape and location of the mental loop,mandibular canal, and maxillary sinuses. The purpose of this investigation was to modify themethod for implant placement in the posterior part of the jaws to extend fixed implant-con-nected prostheses further distally, and to reduce the length of cantilevers in complete-arch pros-theses without transpositioning the mandibular nerve or performing bone grafting in the maxilla.Forty-seven consecutive patients were treated with implants (25 patients/36 mandibularimplants, 22 patients/30 maxillary implants) placed in tilted positions. They were followed amean of 40 months (mandibles) and 53 months (maxillae). In the mandible, implants close tothe mental foramina were tilted posteriorly approximately 25 to 35 degrees. In the maxilla, theposterior implants were placed close to and parallel with the sinus walls and were titled anteri-orly/posteriorly approximately 30 to 35 degrees. Patients gained a mean distance of 6.5 mm ofprosthesis support in the mandible and 9.3 mm in the maxilla, as a result of implant tilting.There were no implant failures in mandibles. The cumulative success rates in the maxilla at 5years were 98% for tilted implants and 93% for non-tilted implants. Paresthesias of the mentalnerve were observed on 4 sides during the first 2 to 3 weeks after implant placement. Analysisof the load distribution in one mandibular case showed no significant difference between tiltedand the non-tilted implants, and the improved prosthesis support was confirmed. Satisfactorymedium-term results concerning osseointegration and significant extension of prosthesis sup-port show that the method can be recommended. This technique may allow for longer implantsto be placed with improved bone anchorage. (INT J ORAL MAXILLOFAC IMPLANTS 2000;15:405–414)

Key words: edentulous jaw, endosseous dental implantation, implant-supported dental prosthesis, maxillary sinus, mandibular nerve

Edentulous patients treated with prostheses sup-ported by osseointegrated implants can realize

improved masticatory function in terms of chewingefficiency and bite force.1 However, rehabilitationof edentulous posterior regions with endosseousimplants is often associated with problems of

anatomic origin, and bite forces are at their greatestfurther back in the dentition.2 According to theoriginal concept for the placement of BrånemarkSystem implants in an atrophied completely edentu-lous arch, the implants are placed in a fairly uprightposition.3,4 Consequently, it is often necessary tofabricate a bilateral cantilever that is up to 20 mm inlength so as to provide the patient with good chew-ing capacity in molar regions.

MANDIBULAR ANATOMY

An oval-shaped mandibular body, where the mentalforamina are located on each side of the horizontalcurvature of the arch, differs from the rectangulartype of mandible, where the foramina are located on

1Associate Professor, Department of Oral & Maxillofacial Surgery,Central Hospital, Västerås, Sweden.

2Associate Professor, Department of Prosthodontics, Central Hos-pital, Västerås, Sweden.

3Research Engineer, Nobel Biocare AB, Göteborg, Sweden.

Reprint requests: Dr Leonard Krekmanov, Centrallasarettet, 72189 Västerås, Sweden. Fax: +46 21 17 54 30. E-mail:[email protected]




406 Volume 15, Number 3, 2000

KREKMANOV ET AL

a frontal straight line in the incisive area (Figs 1aand 1b). Placement of implants between the mentalformina in oval-shaped mandibles, especially whenthe mental foramina are positioned posteriorly, per-mits favorable implant positions, without the needfor long posterior extensions, and masticatoryforces will be evenly distributed between theimplants.5 However, in patients with rectangularmandibles, the implants are forced into a more orless straight-line configuration. In these situationsthe load force on the cantilevers in the molarregions will lead to bending moments on theimplants, introducing high stress in both bone andimplant components.5 This situation may be morecritical in patients with natural teeth or stable pros-thetic restorations in the maxilla.

Another limitation for posterior placement ofmandibular implants is the mental nerve loop,which leads into the mandibular canal. This loop isusually situated 2 to 9 mm anterior to the mentalforamina. Thus, an upright distal implant may needto be placed anterior to the mental foramina andthe loop to avoid damage to the nerve.6 Transposi-tioning of the mandibular nerve is one possibilityfor placing long and stable implants in the molarregions.7 However, problems with paresthesias ofthe mental nerve have been reported.7–11

Another technique is to place short implantsabove the mandibular canal.10,12 However, suchimplants are anchored only in the superior cortex,which compromises the load-bearing capacity. Shortimplants are reported to fail more frequently thanlonger ones.13 Placement of implants lingual to thenerve canal, through both the superior and inferiorlingual cortex, has also been described.14,15

MAXILLARY ANATOMY

An important aspect in the rehabilitation of maxil-lary edentulous patients with endosseous implants isthe fact that in many cases there is sufficient alveolarcrest volume in the anterior region, while in the pre-molar and molar regions, severe bone resorption ispresent.16 Consequently, it is not possible to placelong implants that are equally distributed along thealveolar crest. When gathering the implants in theanterior region of such an arch, there is often a needfor a bilateral cantilever length of up to 20 mm ormore, so as to give the patient good chewing capac-ity in the premolar and molar areas.

In the literature there are many recommenda-tions for cantilever length.17 Cantilever length hasbeen related to marginal bone loss around implantsand mechanical failure of the components.18 Pros-theses with cantilevers of 15 mm or less survivedsignificantly better than fixed prostheses with a can-tilever length exceeding 15 mm.17 Calculation ofimplant load distribution in the maxilla clearlydemonstrates the advantage of well-distributedimplants in controlling implant loading.19 Simulta-neous bone grafting from the iliac crest20 or themandible21 has been reported, as well as postopera-tive problems with graft and host side morbidity.21

IMPLANT TILTING

From the literature it is thus well understood that aspread of implants along the alveolar crest is beneficialfor load distribution. The support advantage gainedby moving the implant head in the posterior directionis well known,5 and in this study, tilting of implants

Figs 1a and 1b Example of 2 extreme shapes of the mandible. (Left) Oval-shaped mandibular body. The mental foramina are located oneach side of the curvature of the anterior part of the arch. (Right) Rectangular mandible. The mental foramina are located on a transversestraight line in the incisive area.





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was used to gain such increased support. Posterior tilt-ing of the distal implant on each side in the mandibleand maxilla may reduce cantilever length and as a con-sequence give rise to better load distribution. In addi-tion to broadening the prosthetic base, the tilting mayalso allow for improved cortical anchorage and pri-mary stability as well as the use of a longer implant.

Laboratory tests on models22 and theoretical cal-culations23 have indicated that tilted (angled)implants may increase the stress to bone. Thesestudies were performed on single implants. Duringloading, a tilted single implant may be subjected tobending, leading to increased marginal bone stress.24

If such an implant is part of a multiple-implant–sup-ported prosthetic restoration, however, the spread ofthe implants and rigidity of the prosthesis willreduce bending of the implants.25 In this study thetilting was mostly in the direction of the prosthesisextension, and therefore the possible stress increaserelated to tilting is anticipated to be negligible.

In the literature, tilting of implants for engagingthe pterygoid plate in the posterior maxillae isreported, indicating that this is a predictable proce-dure for establishing end support for a maxillaryprosthetic restoration.26–29 In 15 patients followedfor a minimum of 3 years, one posterior implant ateach end of each complete-arch prosthesis wasangled to follow the sinus walls and to penetrateinto the cortical bone of the nasal cavity.30

This study was carried out to evaluate the clinicalsuccess and surgical and prosthodontic effects ofposterior extension of the prosthetic base by tiltingthe most posterior positioned implant on each sideof the restorations for both maxillae and mandibles.A biomechanical analysis was also performed on invivo measurements for implant load distribution.

MATERIAL AND METHODS

Twenty-five consecutive mandibular patients (10 maleand 15 female, mean age 63.1 years, range 35 to 80)and 22 consecutive maxillary patients (10 male and 12female, mean age 61.2 years, range 42 to 72) were fol-lowed for 40 months (range 35 to 54 months) and 53months (range 35 to 60 months), respectively. Inthese patients, 36 tilted mandibular implants and 40tilted maxillary implants were placed in an angulationposition to expand the prosthesis base without graft-ing or transposition of the alveolar nerve.

All patients were operated under local anesthesiausing epinephrine with adrenaline (xylocaine, adren-aline 2%, 3.6 mL) injected on each side of the arch.Postoperatively, the patients were given phe-noxymethylpenicillin 1.5 g twice daily for 5 days.The patients rinsed their mouths with chlorhexidine(Corsodyl, SmithKline Beecham Consumer Health-care, Philadelphia, PA) twice daily for 1 week.

Surgical TechniqueIn completely edentulous mandibular situations,incisions were made on top of the alveolar crest,from the first molar on one side to the first molaron the contralateral side with bilateral releasingincisions. Subperiosteal dissection on the lingualand vestibular surfaces was carried out. In partiallyedentulous situations, similar local flaps were made.The mental foramina were located, and the lengthand tilting of the mental loop were noted. The mostposterior implant was placed close to the anteriorwall of the mental loop and parallel with it. Thus,this implant was tilted distally approximately 25 to35 degrees (Fig 2). This technique provides the fol-lowing 3 advantages: (1) implant support is moved

Fig 2 Tilted versus non-tilted implant posi-tioning in the mandible. Note that the tiltedimplant enables more distal support for theprosthesis. The implant is tilted approximately25 to 30 degrees.





KREKMANOV ET AL

posteriorly, (2) implant length is increased, and (3)the implant follows a dense bony structure, theanterior wall of the mental loop, enhancing primarystability. After placement of the posterior implantsbilaterally, additional implants were placed in theanterior space between. Flap adaptation and sutur-ing were performed in the usual manner.

For completely edentulous maxillary patients,incisions were made on the alveolar crest from thefirst molar on one side to the first molar on the con-tralateral side with bilateral releasing incisions. Sub-periosteal dissection on the palatal and vestibularsurfaces was carried out. Angulation of the anteriorsinus wall was visualized via a hole drilled in the lat-eral sinus wall. The most posterior implant wasplaced close to and parallel with the anterior sinuswall. Thus, this implant was tilted distally approxi-mately 30 to 35 degrees. This technique providesthe following 3 advantages: (1) implant support ismoved posteriorly, (2) implant length is increased,and (3) the implant follows a dense bony structure,the anterior sinus wall, thereby enhancing primarystability. Where a patient had sufficient bone in thetuberosity, a similar procedure was performed forthe placement of an anteriorly tilted implant closeto the posterior sinus wall (Fig 3). Then the place-ment of implants in the anterior part of the maxillawas performed. Flap adaptation and suturing wereperformed in the usual manner.

Abutment placement was performed 3 monthsafter implant placement in the mandible and after 6months in the maxilla. Healing abutments wereused after second-stage surgery in both arches.Later, the prosthodontist changed these to standardor angulated abutments. When implants diverged

more than 30 degrees, angulated abutments wereused. Otherwise, standard abutments or Estheti-Cone abutments (Nobel Biocare, Yorba Linda, CA)were applied. At this stage, implant stability waschecked by the surgeon; it was checked again laterby the prosthodontist. Panoramic radiographicexamination was conducted immediately after abut-ment connection.

Prosthodontic TreatmentProsthodontic treatment began from 1 day to 3weeks after the abutment connection. Partial pros-theses were fabricated of precious metal and porce-lain, while complete-arch prostheses were fabricatedeither with a gold framework or with a titaniumframework combined with acrylic resin teeth (Fig 4).

The gained distance of prosthesis support, as aconsequence of the tilted implants, was measured onan orthopantomogram as the mesiodistal distancefrom the point at which the tilted implant supportedthe prosthesis to the point where an upright implantwould have provided support (Fig 5).

Follow-upAt the follow-up visits, the prostheses wereremoved, and implant stability was determined bythe prosthodontist. Radiographs were taken toascertain integration and marginal bone level. Theimplant was judged successful if it was stable anddid not show more than 2 mm of marginal bone losssubsequent to prosthesis connection at any giventime. If the implant was deemed stable and func-tional but bone loss was greater, the implant wasclassified as surviving. In the calculation of successrates, surviving implants were excluded.

Fig 3 Tilted versus non-tilted implant con-figuration in the maxilla. Note that 3 tiltedimplants provide optimal distribution of thesupporting base, as opposed to 4 implants inthe non-tilted configuration. Tilting implantsalso allows for longer implants to be placed,thus further increasing the load-carrying abil-ity. These implants are tilted approximately25 to 30 degrees.





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Implant Load AssessmentStrain gauge measurements of implant loading wereperformed on a patient (Figs 4a and 4b) who hadused a distally cantilevered prosthesis supported by3 implants (1 tilted and 2 non-tilted) in the left halfof the mandible for approximately 4 years. Thestrain gauge technique, described by Glantz et al,31

relates the strains from 3 strain gauges mounted onthe lateral surface of the abutment to axial force andbending moments acting on the implant abutment(axial force is defined as the force acting along thelong axis of the abutment/implant). The measuringprocedure,32 including calibration of the individualstrain-gauged abutments, was monitored by a spe-

cially designed LabView program (National Instru-ments, Austin, TX).

At the time of registration, all of the patient’s origi-nal abutments were removed and replaced withstrain-gauged abutments. The patient was asked toapply maximum bite force on a specially designed bitefork,33 which also was equipped with strain gauges,thus enabling measurements of bite force. Repetitionof this procedure while the bite fork was moved overthe prosthesis gave rise to a series of registrations offorces and moments on the supporting implants, aswell as the force applied through the bite fork.

The patient’s implant/prosthesis reconstructionwas theoretically simulated by a model (Fig 6) based

Figs 4a and 4b Illustrations of the patient who was the subject of strain gauge measurements. In this patient, the posterior implant,placed anterior to the right mental foramina, was tilted posteriorly approximately 42 degrees. The cantilever was approximately 15 mmlong to cover the molar region. Additional implants were placed in the right and left lateral incisor areas. The patient’s natural dentitionwas present in the maxilla. (Left) Panoramic radiograph. (Right) Intraoral photograph of the measurement situation. The patient’s originalabutments were replaced by strain-gauged measuring abutments. Note the tilted distal implant/abutment.

Fig 5 Panoramic radiograph of a patient who received implantsin the maxilla and mandible. Note the tilting of the mental canalsand their location in relation to the implants. In the maxilla, pos-terior implants were placed parallel to the anterior and posteriorsinus walls, which provided the possibility for appropriateprosthodontic rehabilitation.

Fig 6 Theoretical model. The uppermost picture shows theprosthesis with the tilted implant (the upright implant alternativeis indicated as a black contour). The lower illustration shows thetheoretical model of the same situation. The picture shows abeam resting on bars, with circular cross-section.

on a straight bar supported by 3 rigid hinges.34 Thismodel, like the Skalak model,35 will predict onlyaxial forces at the supports but not bendingmoments. Calculations with more refined models,including bending moments and implant flexibil-ity,25 have shown that this simpler approach is oftensufficient for estimations of force distribution. Withthis simple model, theoretical calculations were per-formed to estimate the effect on force distributionfrom the change of support positions caused by tilt-ing of the implants. Because of the simplicity of themodel, the results of these calculations should beinterpreted as relative differences only.

RESULTS

The cumulative implant success rates are shown inTables 1 and 2. The success rate was 100% for themandibular implants, regardless of whether theywere tilted or non-tilted. After loading in the max-illa, 1 tilted implant (n = 40) was lost between 3 and4 years; for the non-tilted implants (n = 98), 2 werelost between 1 and 2 years, 3 were lost between 3and 4 years, and 1 was lost between 4 and 5 years.This provided a 4-year cumulative success rate of95.7% for the tilted implants and 92.5% for thenon-tilted implants. In the tilted group, 2 implants

were classified as surviving after 5 years, based onthe bone loss criteria; for the non-tilted implants,this number was 5.

ComplicationsPostoperatively, suture dehiscence was observed in4 of 25 mandibular cases and in 3 of 22 maxillarypatients. In these cases the cover screws were visi-ble. These patients were instructed to carry outcareful oral hygiene and to brush the exposed metal.

Paresthesias of the mental nerve were observedon 4 sides during the first 2 to 3 weeks after implantplacement. No cases of nerve disturbance werenoted at the time of abutment connection.

From a prosthetic point of view, tilted implantsresult in somewhat less accessibility for the restora-tive dentist. Otherwise, there were no specificprosthodontic problems found in this study. Withrespect to maintenance, patients initially seemed tohave problems with cleaning distally in theirmouths, regardless of whether the implant wastilted or not.

Prosthesis Base ExtensionCalculation of the gained distance of prosthesis sup-port, as a consequence of the tilted implants,showed a slight variation between the left and rightside in the mandible. The mean length on the left


KREKMANOV ET AL




Table 1 Cumulative Success Rate in Mandibular Cases

Tilted Non-tilted

Time not Time notTime Successful Surviving reached Failed CSR Successful Surviving reached Failed CSR

Less than 1 year 36 0 0 0 100.0% 32 0 0 0 100.0%1 to 2 years 34 1 1 0 100.0% 32 0 0 0 100.0%2 to 3 years 23 3 10 0 100.0% 21 0 11 0 100.0%3 to 4 years 22 0 14 0 100.0% 20 0 12 0 100.0%4 to 5 years 5 0 31 0 100.0% 8 0 24 0 100.0%More than 5 years 2 0 34 0 100.0% 3 0 29 0 100.0%

Table 2 Cumulative Success Rate in Maxillary Cases

Tilted Non-tilted

Time not Time notTime Successful Surviving reached Failed CSR Successful Surviving reached Failed CSR

Less than 1 year 40 0 0 0 100.0% 98 0 0 0 100.0%1 to 2 years 40 0 0 0 100.0% 98 0 0 2 98.0%2 to 3 years 36 0 4 0 100.0% 85 1 10 0 98.0%3 to 4 years 23 0 17 1 95.7% 54 6 36 3 92.5%4 to 5 years 16 0 23 0 95.7% 40 2 51 1 90.2%More than 5 years 5 2 32 0 95.7% 13 5 74 0 90.2%





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side was 6.6 mm (range 3 to 12 mm) and on theright side it was 6.5 mm (range 5 to 10 mm). Themean distance gained in the maxilla was 9.3 mm(range 5 to 15 mm).

Load MeasurementStrain gauge measurements on the one patient sub-jected to in vivo registration (Figs 4a and 4b),showed no significant differences in forces or bend-ing moments on the tilted implant, in contrast tothe non-tilted implants supporting the prosthesis.32

Figure 7a shows the influence of the bite forkforce on the load on each implant. The graphdepicts the relative amount of applied axial forcecounteracted by each implant when bite force wasapplied at different positions along the prosthesis.As seen in Fig 7a, the maximum amplification for allthe implants was approximately 130%. The corre-sponding 30% amplification is the result of theleverage effects within the structure and is consid-ered to be a small number.5 The bending momentsmeasured during the experiment were all relativelysmall, rarely above 20 N-cm, corresponding to alever arm of about 3 mm for the acting force.

To test the accuracy of force registration, thesum of reaction forces was compared to the appliedbite force on a percent level. The sum did notalways add up to 100%. However, since the sum isderived from the difference of rather large numbers,a tolerance in the measured parameters of 10% isprobably sufficient to explain the discrepancy in thediagram. Considering that the implants were notplaced vertically, this deviation was thought to beacceptable and the accuracy of the measuring tech-nique was considered confirmed.

The theoretical model, with the implants posi-tioned as in the clinical situation, showed the samegeneral features as the actual measurements (Figs7b and 7c). The highest force influence with themodel was 175%. When the distal implant supportin this model was placed in a position correspond-ing to that of a non-tilted implant (Fig 7c), the dis-tal cantilever increased substantially, and a threefoldmagnification of the implant forces was found.32

This result confirmed that tilting the implant andincreasing the supporting prosthesis base may givethe desired reduction in implant forces.

DISCUSSION

The clinical results of this study indicate that implanttilting per se does not induce any biologic disadvan-tage. On the contrary, it seems to be both clinicallyand biologically advantageous, and a tilted implant as

a member of a prosthesis configuration can be welljustified from a biomechanical point of view.

The only implant losses were in the maxilla, andonly 1 tilted implant (n = 40) was lost after loading,but 6 non-tilted (n = 98) implants were lost. Therewere also more non-tilted implants classified as surviving because of greater bone resorption (14non-tilted versus 2 tilted). The reason for theimproved situations for the tilted implants may bethat they were longer and had a larger contact areawith cortical bone, particularly in the posteriorregion. Both these improvements were made possi-ble by the tilting per se.

The strain gauge measurements in one patientshowed no significant differences in either axialforce or bending moment between tilted and non-tilted implants within the same prosthesis. Thismeans that implant loading was not influenced bytilting. The axial force (ie, following the long axisof the implant) of the tilted implant, however, maycause a lateral force component relative to thebone. If this were the case, the same lateral forcemust be counteracted by the prosthesis by reasonof force equilibrium and would consequently havebeen registered as bending moments at the abut-ment level. Since the bending moments were smalloverall, this potential lateral force is of a relativelylow magnitude. A possible explanation is that therigidity of the prosthesis may counteract suchbending, provided the implant is part of a restora-tion structure.

The theoretical calculations suggest that theincreased supporting prosthetic base results in athreefold decrease of the forces acting on theimplants in the particular case studied. Thus, from abiomechanics point of view, the position of thecoronal end of the implant, rather than whether theimplant is tilted or not, is most important.

The in vivo measurements were carried out ononly one prosthesis in one patient. However, gen-eral conclusions may be drawn. Repeating the mea-surement in other patients would have introducedvariations in geometry and structure flexibility. Thegeometric variations were considered in the theo-retic model, confirming the conclusions on forcedistribution from the measurement and being inaccordance with other similar in vivo measure-ments.36 The bending moments were not consid-ered in the model, but the measurements indicatethat with the order of magnitude in the flexibility ofprosthesis and bone, there is no measurable influ-ence from the tilting. To alter this situation, achange in order of magnitude of the prosthesis andbone flexibility must be at hand, a situation that isunlikely in practice.





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Figs 7a to 7c Influence coefficients. The horizontal axis indicates position of load application, and the vertical axis indicates the propor-tional recorded force (as a percentage) in each of the 3 supporting implants.

Fig 7a The measured case (note the differentscale of this graph compared to Figs 7b and 7c).

Fig 7b Theoretical model with tilted distalimplant. This corresponds to the measured case.

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–150First Second First Canine Lateral Central Central Lateralmolar premolar premolar incisor incisor incisor incisor

Distal Position of load application Mesial

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The clinical implication of this study is that morepatients can be successfully treated with dentalimplants without more complex techniques, such asnerve transposition in the mandible or grafting ofthe maxilla. Treatment of some of the patients inthis study would not have been possible with con-ventional placement without grafting or other moredemanding procedures. Tilting per se is not consid-ered to be more complicated than conventionalimplant placement, as experienced by the authors.

CONCLUSIONS

The method of tilting implants described for treat-ment of edentulous arches represents an alternativeor complementary technique to others mentionedin the literature. It leads to an improved position ofthe support and allows for placement of longerimplants and/or improved anchorage in dense bone.Biomechanical measurements show that the tiltingdoes not have a negative effect on the load distribu-tion when it is a part of prosthesis support.

The advantages are further extension of theprosthesis in a posterior direction, possible use oflonger posterior implants, and improved boneanchorage. The technique is relatively easy to per-form in any outpatient setting by a surgeon who isnot familiar with transpositioning of the mandibularnerve or bone grafting of the maxillary sinus. Fur-thermore, it eliminates the need for such advancedtechniques for some patients.

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Fig 7c Theoretical model with uprightimplant. This corresponds to the black contoursshown in Fig 6. Note the principal similaritybetween Fig 7a and Fig 7b and the large effectof the upright implant (Fig 7c) in contrast to thetilted implant (Fig 7b).

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molar premolar premolar incisor incisor incisor incisor


MesialMiddleDistal

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