IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015

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Use of Narrow-Diameter Implants in the Posterior Jaw: A Systematic Review André Assaf, BDS, DU, MBA,* Moustapha Saad, BDS, DU,† Marwan Daas, DDS, PhD,‡ Jihad Abdallah, BDS, MScD,§ and Rima Abdallah, BDS, DSck

ntil recently, the use of implants with a diameter smaller than #3.5 mm has been reserved for the replacement of teeth with narrow clinical crowns and/or for limited interdental or interimplant spaces such as in the upper lateral or lower incisor areas. The observed success rate being similar to that of standard-diameter implants (SDIs)1,2; it is suggested that implant success is not related to the implant diameter. Bone loss around narrow implants was within the same limits as those reported around SDI.3 Other clinical scenarios with limited ridge width have extended the indications of narrow implants as a suitable solution to avoid invasive ridge management techniques.4 This option is however avoided in the posterior jaw for prosthetic and biomechanical considerations. The emergence profile of posterior teeth is rarely harmonious with a narrow implant neck. Complications are expected to exceed those generally observed for the SDI, such as implant

U

*Adjunct Associate Professor, Department of Prosthodontics, Beirut Arab University, Beirut, Lebanon; Senior Lecturer, Department of Prosthodontics, Lebanese University, Beirut, Lebanon. †Private Practice, Periodontology and Implant Dentistry, Tyre, Lebanon; Abu Dhabi, United Arab Emirates; Doha, Qatar. ‡Associate Professor, Department of Prosthodontics, University Paris VI, Paris, France. §Clinical Professor, Head of Implantology Unit, Faculty of Dentistry, Beirut Arab University, Beirut, Lebanon. kAssociate Clinical Professor, Periodontology Department, Faculty of Dentistry, Beirut Arab University, Beirut, Lebanon.

Reprint requests and correspondence to: André Assaf, BDS, CES, DU, MBA, Sayegh Building, Orthodox Hospital Street, Geitawi-Achrafieh, Beirut, Lebanon, Phone: +9613541937, Fax: +9611446655, E-mail: [email protected] ISSN 1056-6163/15/00000-001 Implant Dentistry Volume 0  Number 0 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/ID.0000000000000238

Statement of Problem: Evidence is limited on the efficacy of narrowdiameter implants (NDIs) in the posterior jaw. Purpose: The purpose of this systematic review was to assess the survival of NDIs and provide guidelines for their safe use. Materials and Methods: Electronic search of the English-language literature enriched by hand search to identify suitable publications was made. Only peer-reviewed clinical studies published from January 1990 through March 2014 were included. Results: Seventeen studies with a total of 1644 implants met the

inclusion criteria, with an observation period from 1 up to 12 years. The mean survival rate of 98.6% was reported. Technical and other complications were observed. Conclusion: Short-term clinical data suggest that NDIs may serve in the posterior jaw as an alternative to standard-diameter implants. However, certain clinical conditions must be observed to assure longterm success. (Implant Dent 2015;0:1–13) Key Words: posterior ridge, smalldiameter implants, success rate, guidelines

fracture, abutment fracture,5–7 screw loosening or fracture, and ceramic fracture. The higher biomechanical risk is explained by (1) weaker mechanical properties of the components because of their smaller dimension and material composition. Indeed, pure titanium is a relatively soft material that may present fatigue problems when used for thin implants and implant components exposed to high constraints8 (2) the magnification of the occlusal forces by 1.5 to 2 times posteriorly9 compared with the anterior part of the jaw. As a result, the biologic response to the generated stress within the bone and implant body is expected to be different, particularly in lower bone densities often observed in the posterior maxilla. Moreover, an optimal implant length might also not be possible in the

posterior jaw because of anatomic structures such as the sinus or the inferior dental canal. Implant location on the arch is therefore a crucial factor to consider. Unfortunately, it is not well specified in most of the studies that evaluate the survival/success rate of narrowdiameter implants (NDIs). Moreover, the highest percentage of evaluated NDIs is placed in the area of lower incisor and upper lateral incisor.10 Only lately have data been published on NDIs used in the posterior jaws that demonstrate an equivalent success rate to SDIs.11 Splinting NDIs with wider implants or with natural abutments is one prosthetic modality reported in many studies. However, it has been observed that NDIs used alone could be a reliable treatment for

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NARROW-DIAMETER IMPLANTS

IN THE

posterior jaw or for full-mouth rehabilitation.1,12,13 In addition to implant diameter, implant survival is affected by implant surface and length, the osseous quality, and the practitioner’s learning experience curve. Other factors contribute to the success of implants in general and NDIs in load-bearing areas in particular: (1) bone quantity, (2) absence of periimplant infection, (3) favorable occlusal scheme and biomechanical response to occlusal forces, and (4) good patient hygiene and compliance. It is therefore essential, when restoring the posterior jaw, to understand the complexity of the factors that enhance the durability of the treatment with small-diameter implants compared with implants of standard-diameter placed after bone augmentation. The aim of the study is to find if the success of NDIs in the posterior jaw can be predictable when clinical guidelines are followed and appropriate prosthetic restorations are provided. After a literature review, the factors that affect the success rate are discussed, and a patient selection guide for a careful but extended use of narrow implants will be proposed.

POSTERIOR JAW

MATERIALS

AND



ASSAF

ET AL

METHODS

Scarce studies exist for the use of NDIs in the posterior jaw despite their well-defined indications in specific clinical scenarios. Indeed, most studies evaluate the global survival of NDIs of different lengths used to treat all kinds of edentulism regardless of the location on the ridge. Search Strategy

Electronic search of the Englishlanguage literature enriched by hand searches was performed in PubMed databases to identify suitable publications. The keywords used were: “narrow-diameter implants” and “posterior jaw,” “narrow-diameter implants” and “posterior mandible,” “small diameter implants” and “crowns” and “dental,” “narrow-diameter implants” and “fixed partial dentures” and “posterior,” “narrow-diameter implants,” and “posterior mandible,” and “fixed partial denture” and “crowns.” Only peer-reviewed clinical studies published from January 1990 through March 2014 were included (Fig. 1). Study Types

Clinical studies on narrow-dental implant success and survival under functional loading and the radiographic

analysis of the marginal bone level including at least 10 treated patients and published in the English language were evaluated. The following study designs were included: 1. Prospective: randomized controlled, nonrandomized controlled, cohort studies. 2. Retrospective: controlled and casecontrolled studies. Inclusion Criteria

1. Implant diameter between 2.9 and 3.5 mm. 2. Restoration is fixed, either single or splinted to other implants (either narrow or wider) or splinted to natural teeth (fixed partial dentures or multiple splinted crowns). 3. Narrow implants in bounded saddles, that is, the presence of 1 natural tooth posterior to the implant. 4. Implant(s) position: in premolarmolar sites, unilateral or bilateral. 5. Implant distribution: might reach the anterior teeth with or without arch crossing. 6. Jaw: upper or lower or both. 7. Review articles providing a followup duration of at least 12 months following loading.

Fig. 1. Search strategy. Electronic search of the English-language literature enriched by hand searches was performed in PubMed databases to identify suitable publications. Only peer-reviewed clinical studies published from January 1990 through March 2014 were included.

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Author/Year/Study Design/ Patients Number

NDI Number/Min Length Implant Macrogeometry and Microgeometry

Follow-up Period

Tolentino et al 201415/prospective 42 Straumann (21 NDI made of Cylindrical shape, 1y monothread, SLA surface trial (NDI Ti vs NDI Ti-Zr)/42 pure Ti/21 NDI Roxolid made patients of Ti-Zr alloy) Ø: 3.3 mm/Min L: 8 mm Up to 5 y Geckili et al, 201331/retrospective 68 of 159/Ø: Straumann 3.3 mm, Screw-shape implant (SLA, TiO2, RBM, SLA) Astra 3.5 mm, Bio-Lok 3.45 (anterior and posterior)/71 mm, Xive 3.4 mm; L: NM patients Chiapasco et al12/prospective/ Up to 2 y 51/Straumann Roxolid Ø: Cylindrical shape, 18 patients 3.3 mm/Min L: 8 mm monothread, titaniumzirconium alloy, SLA surface Xive: tapered, self-taping, grit- Up to 12 y Lee et al, 201316/retrospective/ 155 (% of 541)/MkII, MkIII, blasted and high338 patients Groovy, Replace Select, temperature acid-etched Xive, Straumann, Dentium Ø: 3.3, 3.4, 3.5 mm/Min L: 8 mm Dentium: slightly conical SLA surface Replace Select: Ti unite Barter et al17/prospective pilot 22 implant/Straumann Roxolid Screw shape; Ti-Zr alloy; Up to 2 y study/22 patients Ø: 3.3 mm/Min L: 8 mm SLActive Málo and de Araújo Nobre, 201111/retrospective/147 patients

Tolentino 201114/prospective/ trial: NDI vs SDI/22 patients Yaltirik et al18; retrospective; 28 patients Arisan et al19/retrospective/ 139 patients

247/Brånemark MkII, MKIII, NobelSpeedy Ø: 3.3 mm; Min L: 10 mm

33 of 48/10 mm Straumann

SLA screw shape

160 of 316 implants 8 mm Straumann 3.3 Xive 3.4

SLA, screw shape, Xive

Parafunction

95.2%

95.2%

No

98.74%

98.52%

No

100%

100%

Yes

98.1%/91.8%

98.7%

No

95.2%

95.2%

No

95.1%/NM

95.1%/NM

Yes

1y

100%

100%

No

5y

93.75%

96.9%

Yes

Up to 9.1 y

92.3%, 91.4%

86.25%

No

Nobel speedy: slightly taper- Up to 11 y shape, self-tapping, TiUnite

MKIII: cylindrical shape, selftapping, TiUnite MkII: screw-shape implant, turned surface 54 (54 NDI, 54 SDI)/Straumann Cylindrical shape, Ø: 3.3 mm/Min L: 8 mm monothread, SLA surface

Survival/Success Survival/Success Rate of NDI Rate of NDI Placed in Placed in the the Anterior and Posterior Jaw Posterior Jaws

(continued on next page)

IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015

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Table 1. Studies Analyzing the Success of Narrow Implants in the Posterior Jaw Highlighting the Specifics Related to the Used Implants, the Prosthesis Design, and Patient Characteristics

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Author/Year/Study Design/ Patients Number 20

Degidi et al /retrospective/ 237 patients

Veltri et al21/prospective/ 12 patients

319 of 510/8.5 mm/multiple Brånemark, Ankylos, Xive, implant systems (3–3.5 mm) Frialit, IMZ, Maestro and Maximus (BioHorizon), Restore 73/9 mm/Astra 3.5 mm Osseospeed, TiO2-blasted surface

7y

99.4%

99.37%

No

1y

100%

100%

Yes

98.7% (posterior success of NDI)

No

91.66%

Yes

75 (of 122)/Straumann Ø: 3.3 mm/Min L: 10 mm

Cylindrical shape, monothread, TPS surface

Up to 7 y

Brånemark MkII (12 of 22) Ø: 3.3 mm/Min L: 10 mm 99 of 298/8 mm Straumann (3.3 mm) 74 (of 192)/3idØ: 2.9 and 3.25 mm/Min L: 8.5 mm 160 of 182 Straumann Ø: 3.3 mm/Min L: 8 mm

Screw-shape implant, turned surface

Up to 5 y

98.1% maxilla, 96.9% mandible/ 96.1% maxilla, 92% mandible 96% 96.6%

98.99%

Yes

Screw-shape implant, osseotite surface

Up to 7 y

95.3% (overall)

93.24%

Yes

Screw-shape implant, TPS

Up to 1 y

99.4%/96.4%

99.3%/96.2%

Yes



Up to 6 y

Loading Protocols

Notes

(2 failed) not mentioned

No

SC

6 wk

1 NM

No

All implants restored with single crowns Marginal bone resorption around NDI made of pure Ti or Ti-Zr was not statistically different No relation between implant location, length, type of prosthesis, and marginal bone level Implant survival, periimplant bone resorption, and prosthetic complication rate comparable with those reported for SDI

14 implants placed with sinus lifting and/or GBR

FPD and overdenture

NM

Immediate or delayed FPD, SC, full-fixed crowns, overdenture (2–12 mo)

No

NM

No

(continued on next page)

ET AL

Type of Prosthesis

ASSAF

Splinted to Wider Diameter Implant

Bone Management

0

Parafunction

POSTERIOR JAW

Number and Length of Failed NDI Placed in the Posterior Jaw

Follow-up Period

IN THE

Romeo et al1/comparison between NDI vs SDI used in different location (anterior, posterior)/ retrospective/68 patients Comfort et al22/prospective (anterior, posterior)/9 patients Zinsli et al4/retrospective/ 149 patients Vigolo et al23/retrospective (anterior and posterior)/ 165 patients Hallman24/prospective/40 patients

NDI Number/Min Length Implant Macrogeometry and Microgeometry

Survival/Success Survival/Success Rate of NDI Rate of NDI Placed in Placed in the the Anterior and Posterior Jaw Posterior Jaws

NARROW-DIAMETER IMPLANTS

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Table 1. (Continued)

Number and Length of Failed NDI Placed in the Posterior Jaw Two: 1 (3.3 3 11.5); 1 (3.3 3 13)

Bone Management GBR and ridge splitting

Type of Prosthesis

Loading Protocols

Notes

FPD

NM

No significant effect of bone quality, implant location, and length on implant survival (bone density subjectively evaluate during surgery) Most of implant failures related to mechanical nature NDI placed between incisor and 2nd premolar splinted to a standard or wide diameter implant Fenestration .2 mm excluded MBL and PD around NDI and SDI/WDI were not statistically different Implant type has a positive effect on survival rate

SC 1 (3.3 3 10)

12: 9 machined; 3 (3.3 3 10); 2 (3.3 3 11.5); 4 (3.3 3 15); 3 oxidized; 1 (3.3 3 10); 1 (3.3 3 13); 1 (3.3 3 15)

Xenograft if 1–2 threads were exposed

No

0

FPD

FPD, SC, full-fixed crowns

SC

1 (0.3), (3 3 10) 22 failed implants

No No

2, (3.0 3 13), (3.4 3 15)

No

Splinted to Wider Diameter Implant

SC PFD SC, PFD, OD

12 wk

Different loading protocol including immediate loading

6 wk

3–6 mo 3 mo mandible, 6 mo maxilla Different loading protocol including immediate loading

Type of prosthetic design would affect implant survival All implants were restored with single crowns Marginal bone resorption around SDI and NDI was almost the same 8 mm length were used Body fracture of 2 implants Smoking and posterior localization of NDI were associated with increased risk of failure Many of NDIs were placed in postextraction socket

Yes

Yes

No

No

NM NM

NM

Better results in terms of MBL were observed with wider and longer NDI (3,4 and 13) (continued on next page)

IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015

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Table 1. (Continued)

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Number and Length of Failed NDI Placed in the Posterior Jaw

Bone Management

0

No

3 (3.3 3 10)

No

No

4: (3.25 3 10)

No No

No Yes

Yes

ASSAF ET AL

No

Notes

Opposing arch: Fixed (3) natural teeth (9) Heavy smoker excluded FPD, SC Based on bone quality Class 1 and 2 Kennedy No combination between NDI and SDI Severe clenchers and heavy smokers excluded Type 4 bone, a determining failure factor Cumulative survival rate of NDI and SDI not statistically different MBL, PPD, and MBI comparable between SDI and NDI FPD, fixed Full crowns 6 mo Clenching habits included Failed implant was placed in Type 4 bone FPD, overdenture, fixed 3–6 mo 2 implant fractures full mouth Most of prostheses screw retained SC, FPD 2 mo Occlusal surface designed to avoid premature contact during protrusive and lateral movement Failure related to poor bone quality and occlusal problems Gold occlusal surface Heavy smoker included Almost half of the failed implants were lost before loading SC, FPD, overdenture, 3–6 mo Heavy smokers included fixed full mouth The failed implant was lost before loading Opposing dentition: fixed or natural teeth



3, 3 3 8

6 mo

POSTERIOR JAW

3, 3 3 8 (fractured implant)

Loading Protocols

IN THE

1 NM

Type of Prosthesis Full-fixed crowns

Splinted to Wider Diameter Implant

NARROW-DIAMETER IMPLANTS

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Table 1. (Continued)

No

Although in most of the mentioned studies, the NDIs were not exclusively placed in the posterior jaw and the reported success/survival rate was established for all regions; our meta-analysis estimated the implant number and the implant success/survival rate for those placed only in the posterior jaw. Therefore, 2 success/survival rates were considered, the 1 mentioned in each study and the 1 corresponding to our assessment. The implant macrogeometry and microgeometry were also analyzed, such as implant form (tapered/cylindrical) and the type of surface preparation (SLA/Ti unit, etc.). The annual or the overall MBL was also mentioned as calculated in each study. MBL indicates marginal bone loss; SLA, sandblasted, large grit acid etched; NM, not mentioned; SC, single crown; RNM, resorbable blast media; FPD, fixed partial denture; GBR, guided bone regeneration; MBL, marginal bone loss; PD, probing depth; WDI, wide diameter implant; OD, over denture; TPS, titanium plasma spray.

From Surgical and Prosthetic Perspectives Implant microgeometry and macrogeometry

Implant number

Implant stability

Healing time

Implant material

Background

Recommendation

Lower survival rate of screw-shaped NDI with smooth surface than surface-treated surface and taper-shaped implant (Málo and de Araújo Nobre11) Increasing implant number provides favorable stress distribution for smalldiameter implants and lessens the biomechanical risks (Petrie and Williams32; Qian et al33) Contributes to implant success (Meredith et al37)

Implants with rough surface are highly recommended

Largely dependent not only on bone density but also on implant design and surface and on the technique and accuracy of the osteotomy preparation (Cordaro et al38) The reduction of implant diameter is accompanied with a reduced bone-toimplant contact (Romeo et al1) Allow extended healing time to improve bone-implant support capacity

Stability of the NDI must be carefully assessed

The reduced cross-section of the implant is accompanied by a reduction of the fatigue strength leading to a greater risk of fatigue failure (Quek et al5; Allum et al6)

Reflection

Replacing each missing tooth with an implant (Lindhe et al36 book, p 1199)

Use implant systems of favorable microgeometry and macrogeometry

Protect implant from full loading until it has proven its strength (Friberg et al39; Rangert et al40) New implant material may represent one of the possible solutions

Usually, better outcomes are found in the posterior mandible than in the posterior (Morris et al30)

Alloying titanium to zirconium provides favorable mechanical properties (Kobayashi et al41; Assis et al42). This alloy has been found highly biocompatible with the absence of potential release of Al and V into the tissues that might lead to any adverse effects

The 4 grades of pure titanium are weak enough to fulfill high demands on mechanical stability (Al-Nawas et al13) Even the Ti-6AL-4V alloy has inherent weaknesses that must be compensated (AlNawas et al13) (continued on next page)

IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015

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Table 2. Guidelines for the Successful Use of NDIs in the Posterior Jaw: From the Surgical and Prosthetic Perspectives

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From Surgical and Prosthetic Perspectives Occlusal considerations

Recommendation

Reflection

Marginal ridge contacts should be avoided (unless the implants are splinted) as they may be most damaging by creating cantilever effects and bending moments To reduce the potential lateral force on the implants

Group-function occlusion is preferred when anterior teeth are periodontally compromised8,17,34 (Bidez and Micsh43; Hobo et al44; Chapman45) Working and nonworking interferences should be avoided18 (Lundgren and Laurell46) Ref: As the delivered load in a narrow ridge is often offset, the occlusal scheme is crucial for the success of NDIs

Establish anterior guidance in excursions

Reduce crown contour with a compromise on harmonization to natural teeth Eliminate cantilevers (Misch,49 p 200) Splint implant restorations

ET AL

Narrow the size of the prosthetic occlusal table (Misch48; Rangert et al40)

ASSAF

The crown-implant ratio increases and more angulations for the abutments are needed.

Sharing the load will decrease the force moments, minimize the risks of fracture, decrease occlusal forces to the crestal bone, and reduce mechanical complication such as abutment screw loosening (Lindhe et al,36 book, p 1190) Lateral forces are reduced, particularly if implants are more than 2 and placed in a tripod or cross-arch situation8,9



Narrower implant bodies are more vulnerable to occlusal table width and offset loads (Misch,47 p 493)

POSTERIOR JAW

Prosthetic design

Place initial occlusal contacts on natural dentition Reduce cusp inclination to eliminate angled load Occlusal contacts are centrally oriented within a 1–1.5 mm flat area The prosthetic design must be accommodated to reduce the angled forces on the prosthesis

IN THE

Background With the gradual atrophy of the ridge, the implant sites shift more lingually than their natural predecessors creating excessive bending moments and overcontours Primary occlusal contacts cannot be placed within the implant diameter

NARROW-DIAMETER IMPLANTS

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Table 2. (Continued)

IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015

Table 3. Guidelines for the Successful Use of NDIs in the Posterior Jaw: Patient-Related Factors Patient-Related Factors

Background

Recommendation

Bone volume

Reducing the implant diameter reduces the available surface for osseointegration Romeo et al1)

Bone quality

Considered as the most critical factor for implant success at both surgical and functional stages It varies between different regions of the jaws: The density in posterior mandible is almost equal to that of the anterior maxilla and 20% less than the anterior mandible (Turkyilmaz et al55) The posterior maxilla has the lowest bone density (Zarb and Zarb56; Misch57)

1 mm of thickness must surround the entire implant surface (Davarpanah et al52; Spray et al53) A minimum of 10 mm of available bone height is advised (Lazzara et al54) Use rough surface implants

Increase number of implants Proper surgical technique

Prolong healing time Proper distribution of occlusal load

Force factor Age and gender

Opposing arch

Condition of the remaining teeth (periodontally or occlusal scheme wise)

Microtraumatic occlusion

High correlation between (maximum bite force [MBF] and marginal bone loss [MBL]) (Geckili et al58; Lindquist et al27,59; Wang et al60; Van Oosterwyck et al28) Are not factors of concern unless considering the force that is engendered by the patient (Geckili et al61) Load factor varies according to the nature of the opposing occlusion, increasing from a removable prosthesis until fixed restorations whether supported by natural teeth or by implants

Teeth adjacent to the implant restorations can have either a protective or an overloading effect depending on both the degree of tooth mobility and the extent of the implant restoration While teeth are relatively mobile and may also change in position over time, the stiffer implants may be subjected to an increased percentage of load6 (Lundgren and Laurell46) When forces during mastication and parafunctional activities are greater than normal, the implant would be subjected to a correspondingly higher load (Misch and Nieva62)

Special considerations should be given to patients who would be considered “strong chewers”

Take into consideration the anticipated loading capacity of the patient when designing the prosthesis

The occlusal plane and tipped or extruded teeth should be evaluated closely and restored to provide a favorable environment in terms of occlusion and force distribution Mobility of the natural remaining teeth must be assessed

One implant for each missing root is advised

A protective occlusal scheme must be planned Understand the occlusal condition of the patient by evaluating the primary reason for loss of natural teeth (Rangert et al40)

Any indicator of load factor risk should be considered such as a history of bruxism or the presence of broken teeth related to heavy occlusal forces (continued on next page)

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IN THE

POSTERIOR JAW



ASSAF

ET AL

Table 3. (Continued) Patient-Related Factors Type and extent of edentulism

Maintenance of hygiene

Background The adjacent teeth can have a relative protective effect When the 2 premolars are also missing, there is a decrease in the anterior support of the occlusal load (Misch49) When complete edentulism is restored, load is distributed for all the implants, reducing the load on posterior implants (Cehreli and Akca51) In unilateral partial edentulism, the available bone remains adequate for NDIs despite the long-term edentulism. However, the local bone density may be decreased (Misch49) The overcontours generated by the lingualized bone-driven position of the implants make the maintenance of oral hygiene more difficult

8. Studies dealing with simultaneous bone augmentation Exclusion Criteria

1. Review articles relating exclusively to overdentures. 2. Mini-implants for orthodontic anchorage. 3. Small-diameter implants that were not meant for permanent use, that is, fixation of temporary crown and bridges. Outcome

Outcome criteria were defined with respect to the existing reviews, and the main outcome parameters of the included studies were: 1. Dental implant survival with a follow-up period of at least 12 months. Survival was defined as a functioning implant that was not removed at the time of clinical follow-up. 2. Implant success was defined as an implant in function with no signs of periimplantitis and the evaluation of the marginal periimplant bone level under functional loading.

RESULTS Seventeen studies met the inclusion criteria. The total number of

Recommendation Examine the type of edentulism whether unilateral or bilateral, distal, or bounded in terms of the protective effect provided by the natural teeth Consider the degree of tooth mobility and the number of missing teeth to be replaced When an independent restoration is not possible, narrow implants can be splinted to a natural tooth but under strict conditions

Patients selected for this modality must be able to maintain good oral hygiene, mainly around the implant restoration using appropriate hygiene accessories

screened implants is 1644 restored with fixed restorations, either fixed partial prostheses or single replacements. The observation period ranged from 1 year up to 12 years, a median of 5-year follow-up. When screened and analyzed for only posterior rehabilitations, these studies reported a mean survival rates of 98.6%, with 2 of the studies reporting survival rates of 100%.12,14 Although the success rate was fairly high and comparable with regular-diameter implants, some complications were observed including technical complications such as decementation screw loosening; screw fracture, porcelain fracture, and limited implant fractures were noticed (Table 1).

DISCUSSION Only few comparative prospective clinical studies, especially randomized ones, are available to document survival or success rate of NDIs used in the posterior jaws. Therefore, observational studies were also included in the review, most of which lacked a clear distinction between the outcomes of treatment of narrow implants placed in the anterior and posterior regions. Only 3 articles studied the placement of

narrow implants in the posterior jaw exclusively.11,14,15 The complications were pooled, and there was no specific segmentation with regard to anterior or posterior narrow implant complications. Ten of the 17 studies excluded implants less than 10 mm in length. The median survival rate of posterior NDI was found to be 98.6%, similar to regular-diameter implant survival rates. This might suggest a reliable therapy option, but the determination of implant survival alone is not a sufficient proof for the validity of the modality under any clinical situation. Indeed, when comparing their effectiveness versus SDIs in terms of available surface of osseointegration, initial stability, and mechanical strength, the conclusions are in favor of the wider implant.4,6 This explains why surgeons naturally choose a standard diameter when enough width is available. However, randomization trials between SDIs versus NDIs in the posterior region do not exist in the literature. Many factors seem to affect failure rates of implants in general and of NDIs in particular, namely implant type, implant restoration including the biomechanical conditions, bone quality and quantity affecting implant length, and resistance to torque,

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IMPLANT DENTISTRY / VOLUME 0, NUMBER 0 2015 as well as the healing time, infection, and periimplantitis. Implant type, mainly the surface microstructure and macrostructure, seems to have an impact on implant survival rate and the implant restoration design. These 2 factors were underlined by Maló and de Araújo Nobre11 who observed significantly more failures for screw-shaped implants with smooth surface compared with surface-treated and taper-shaped implants. They also identified a strong risk factor for implant failure with partial rehabilitations when compared with single-tooth and complete edentulous rehabilitation. However, their study has some shortcomings: different implant lengths (from 10 to 15 mm) were used as well as different loading protocols including immediate loading. A reduced diameter means a reduction in the bone-to-implant contact surface, and one might ask whether osseointegration is sufficient to withstand loading forces. Studies have demonstrated higher removal torque and pull-out forces with wider implant diameters.25,26 Poor bone quality is designated as one additional risk factor behind NDI failures because the available surface for osseointegration is reduced.1,27,28 A significant positive correlation exists between pull-out resistance and bone density for narrow- and reduced-diameter implants,4,26 with no relationship between early narrow implant failure and the osseous quality.2 The pull-out force seems to be correlated also to implant length.26 Implant length plays a critical role in conditioning small implant prognosis. All studies agree on a minimal implant length to be used: 8 mm in 7 studies12,15,16,23 and 10 mm in 9 studies.1,11,22,29 Degidi et al20 reported greater bone loss with NDIs of less than 10 mm length. Although it is proven that spongy bone (type 4) may increase the implant failure,30 a clear positive relationship between NDI location and failure is not defined.31 Arisan et al19 observed that the posterior localization of NDI would increase the risk of late-term complications and failure as well as marginal bone resorption. This was explained by

progressive periimplant bone loss and periimplantitis, the most common prosthetic complications being cement loosening and porcelain veneer fracture. No implant fracture was reported. Romeo et al1 reported similar findings. However, they attributed the problem mainly to periimplant bone infection rather than to biomechanical factors. The recent retrospective study conducted by Lee et al16 on 541 implants showed similar results concerning main early implant failure modality that is due to infection. However, it contradicted the observations concerning the effect of implant location; comparable survival rate of NDIs in the posterior and anterior region were reported, and most failed implants were placed in the anterior region. Seven of the 9 failed implants were categorized as early failures and 2 as late failures due to screw fractures. This study was limited by the use of many implant systems and the presence of guided bone regeneration procedures for 214 implants.16 Tolentino14 came to an advanced conclusion by considering the use of NDIs with single prostheses in the posterior jaw as reliable in selected patients. This prospective controlled study was the only one comparing NDIs of 3.3 mm adjacent to SDIs of 4.1 mm in the posterior jaw in terms of survival rate and marginal bone resorption. After 1 year, the survival rate was 100%with no statistical differences in marginal bone resorption. Interestingly, both implant types were used in the same area, with identical loading protocol and same length of 8 mm. However, there was no randomization because only bone width guided the selection of either NDI in sites of 5 mm width and SDI in sites of more than $7 mm width. The risk of implant fracture is higher with the reduction in implant diameter due to reduced mechanical stability and increased risk of overload.4 Significantly lower fracture resistance is reported for 3.0- and 3.3-mm-diameter implants compared with 4.1 mm.6 Fatigue fracture may occur after a long period of function. Although short-term outcomes of NDIs are quite promising, long-term survival rate studies are few and further research is needed to elucidate any existing controversy.6 A correlation is also

11

found between the design of abutment connection and strength of the implant. For example, a 3.3-mm-diameter implant with an external connection design had shown greater strength than the same diameter implant with internal connection.5,6,32,33 In grafted site, the survival rates of SDI are found to be similar to NDI in the mandible and less than 90% in the maxilla.34 It can therefore be concluded that the use of NDIs to avoid augmentation or invasive procedures is an effective therapy. However, randomized studies comparing NDIs with augmentation and SDIs are not available. A clear definition of the indications and reporting the success and follow-up rates is mandatory.35 Guidelines to Reduce the Risks Inherent to the Use of NDIs in the Posterior Jaw

In the posterior jaw, multiple parameters influence the success of implant therapy: (1) the occlusal forces are higher, (2) eventuality of less bone quality, (3) the lack of vertical bone height below the sinus or above the dental canal, and (4) the need for an adequate restoration emergence profile.5,35 Because posterior partial edentulism is usually long dated and often preceded by bone resorption around the teeth before their loss, a restorative-driven implant selection and placement becomes quite challenging or can even exclude the placement of SDIs because of horizontal bone loss. NDIs become the only choice if bone manipulation is to be avoided. However, when the implants are restored, they are submitted to higher stresses than they would be in the anterior sites, giving a more critical role to biomechanical considerations for the success of reconstructions on NDIs35 (Table 2). Therefore, every implant carries a high strategic importance where both the surgeon and prosthodontist must be highly aware when dealing with implant selection and placement, as well as with providing both restoration and follow-up. The surgeon must make sure that bone density and volume will allow high implant stability and that the osteotomy technique is accurate and safe. Implant number, material, and geometry have to be conducive to an

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12

NARROW-DIAMETER IMPLANTS

IN THE

optimal bone-to-implant contact and a low incidence of mechanical failure. When designing the restoration, the prosthodontist must take every compensatory measure to control bending overload. An additional risk factor must be accounted for. It is related to the increased susceptibility of partial posterior implant restorations to implant bending, mainly due to the narrowing of the ridge obliging an in-line placement of the implants.50 With regard to patient selection, the extent of the posterior partial edentulism has a strong influence on the biomechanical behavior by the relative protective effect of the teeth adjacent to the implant restorations, more so for single-tooth implant restorations than in partial rehabilitations. The degree of tooth mobility must therefore be assessed as it might be of an overloading nature. When complete edentulism is restored, load is more broadly distributed, reducing the load on the posterior implants. This contrasts with partial rehabilitations where the effect does not occur with such efficacy.51 A judicious patient selection for narrowimplant modality is therefore an essential part of the planning. Positive bone volume and quality as well as the capacity to provide a favorable environment in terms of occlusion and force distribution are predictors of the restoration outcome (Table 3).

CONCLUSION The short-term use of NDI in the posterior jaw is a reliable treatment. Success rate and periimplant bone resorption appear to be similar compared with regular-diameter implants (.3.5 mm). Both the mid- and longterm use is well documented and has a high survival rate. However, to envisage this modality as a clinically evident treatment option for posterior edentulism, the evaluation of patient risk factors, the suitable selection of the implant system, and the management of occlusal factors are keys for success. Moreover, randomized controlled trial studies comparing the use NDIs versus SDIs in load-bearing regions are needed.

POSTERIOR JAW



ASSAF

ET AL

DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.

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Use of narrow-diameter implants in the posterior jaw: a systematic review.

Evidence is limited on the efficacy of narrow-diameter implants (NDIs) in the posterior jaw...
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