Distal Cantilevers and Implant Dentistry

Distal Cantilevers and Implant Dentistry Georgios E. Romanos, DDS, PhD, Prof Dr med dent1/Bhumija Gupta, BDS2/Steven E. Eckert, DDS, MS3 Purpose: The use of distal cantilevers in fixed implant-supported prostheses is often discussed in dental literature in relation to implant restorations. The aim of this study was to conduct a systematic review of the literature in an effort to identify factors related to the long-term success of distal cantilevers. In addition, a more traditional review was conducted to determine complications associated with this type of prosthetic design. Material and Methods: A systematic literature review was conducted of all published articles identified through a search of MEDLINE from January 1980 through June 2011 using the terms “crestal bone loss,” “dental implants,” “distal cantilevers,” “marginal bone loss,” and “periodontal disease.” An initial review was conducted of titles and a second review was performed using full abstracts. Articles were thus identified to assess studies that met the inclusion criteria that described fixed complete dental prostheses utilizing distal cantilevers. In vivo studies were included for data extraction while in vitro and animal studies were used for descriptions of clinical or mechanical performance. Results: Following the method described, from an initial identification of 49 studies, a total of 14 human clinical studies were found that met the inclusion criteria and provided data associated with clinical performance. An additional 5 studies detailed in vitro research. It was noted that the majority of studies lacked specific data but, nonetheless, provided subjective and objective information regarding the use of cantilevers. The follow-up period of human studies ranged from 1 to 10 years. Weighted averages demonstrate a prosthetic survival rate of 95% during a follow-up period of 7.3 ± 2.6 years. The descriptive studies demonstrate that the most frequent complication was screw loosening, which was observed equally in virtually all studies, even if controlled torque was used to connect abutments and prostheses. Porcelain chipping and esthetic veneer fracture were described, although data associated with these occurrences were not available for systematic review. Conclusions: Based on 19 articles (including 14 human studies), it was difficult to derive data-based conclusions regarding the use of cantilever prostheses other than an approximate 95% prosthetic survival rate. Subjective and objective information from these studies demonstrated a distinct preference for the use of distal cantilever prostheses, as few risks are associated with these prostheses. Careful evaluation of the studies shows that cantilevers provide a treatment option without a high risk of complications. Int J Oral Maxillofac Implants 2012;27:1131–1136 Key words: complications, crestal bone loss, distal extension, implant-retained prostheses

D

istal cantilevers have been discussed in fixed prosthodontics as a method to reestablish occlusion when there are no posterior teeth to support a prosthesis.1–7 Extrapolation of the findings in natural dentition toward implant dentistry has led to considerable debate within the dental implant community.1–9

1Professor

and Associate Dean for Clinical Affairs, Department of Dental Medicine, Stony Brook University, School of Dental Medicine, Stony Brook, New York. 2Resident, Advanced Education in General Dentistry (AEGD), Division of General Dentistry, Eastman Institute for Oral Health, University of Rochester, Rochester, New York. 3 Professor Emeritus, Mayo Clinic, College of Medicine, Rochester, Minnesota. Correspondence to: Dr Georgios E. Romanos, Department of Dental Medicine, Stony Brook University, School of Dental Medicine, 184C Sullivan Hall, Stony Brook, NY 11794-8705. Fax: 631-632-3116. Email: [email protected]

There are various implant dentistry treatment concepts using distal cantilever prostheses with reports of long-term success.1–5,8,9 Some of the reasons for incorporating the distal cantilever design in implantsupported prostheses include a reduction in the number of implants used to support a prosthesis, which also has a concomitant reduction in the cost of the restorations; a decreased need for surgical intervention; and a simplification of surgery in those anatomical areas that are associated with the inferior alveolar nerve vascular bundle or the sinus floor. When planning a distal cantilever, factors such as abutment selection, control of the magnitude of forces, and rigidity/strength of connectors are not fully understood.7 The purpose of the present systematic review was to determine what data exist to support the use of distal cantilever prostheses on implants and to demonstrate complications that can be encountered. The International Journal of Oral & Maxillofacial Implants 1131

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Romanos et al

Table 1  Studies with Complete Dental Prostheses Author (study type)

Aim/objectives

Implant total and type

Gallucci et al8 (clinical ­prospective)

To evaluate the primary complications associated with mandibular fixed implant-supported rehabilitations with distal cantilevers over 5 years of function.

237 Straumann implants; hybrid prostheses with ­distal extension cantilevers

Semper et al9

To retrospectively determine whether a relationship exists between the length of the distal bar extension and the amount of marginal bone loss around implants supporting cantilevered bar-retained dentures.

313 implants; many different systems

Maló et al10 (clinical study)

To develop and document a simple, safe, and effective surgical and prosthetic protocol for immediate function.

176 Nobel Biocare immediately loaded implants

Wolfinger et al11

To suggest a simple, reliable, and documented method for immediate implant loading of complete-arch mandibular prostheses.

144 Brånemark System ­implants

Maló et al12 ­ (retrospective ­clinical study)

To evaluate immediate function of four implants supporting a fixed prosthesis in the completely edentulous maxilla.

128 Brånemark System TiUnite implants

Katsoulis et al13 ­ (2-year clinical trial)

To analyze maintenance service of fixed maxillary prostheses and overdentures based on conventional titanium bars and CAD/CAM.

193 Nobel Biocare implants

Material and Methods The following PICO (patient problem, intervention, comparison, and outcome) question was developed: In patients with anatomical limitations that prevent the placement of dental implant retainers on the most distal aspect of the anticipated dental prosthesis, would distal implant-supported cantilever prostheses provide acceptable clinical performance when compared with traditional tooth-retained distal cantilever prostheses? The following focused questions were addressed regarding implant-supported prostheses: (1) Are distal cantilevers a viable option for implant-supported prostheses? and (2) Are there specific complications that can be anticipated with distal cantilever prostheses? A review of the dental literature was conducted in an effort to identify articles that would address the PICO question. A MEDLINE search was used to identify appropriate publications. The database was searched from January 1980 through June 2011 using the following terms in different combinations: “crestal bone loss,” “dental implants,” “distal cantilevers,” “marginal bone loss,” and “periodontal disease.” Articles identified in the initial search were then categorized as experimental studies (in vitro or laboratory studies), in vivo human studies, or in vivo animal studies. For the literature to be included, the following eligibility criteria were used: case reports, retrospective studies, randomized clinical trials, articles with a reference list of potentially relevant original and review articles, and articles published only in English in peer-reviewed journals. Data extraction was to be considered only for

case series, cohort studies, and randomized clinical trials. Case reports and experimental studies were analyzed for descriptive information. Titles and abstracts of articles were independently screened by two authors (GER and BG) and checked for agreement. Since the search terms applied to both implant- and natural tooth–supported prostheses, the identified articles were then sorted using the category of implant or natural tooth to identify the method of support. The full text of the articles that were judged by title and abstract to be relevant was read, and data or descriptive material were abstracted and compiled.

Results Based on the inclusion criteria, 14 articles were identified that presented clinical material that could be abstracted.

Clinical Studies with Complete Dental Prostheses

A total of six studies were identified that presented information on performance of complete dental fixed restorations with distal cantilevers in human clinical studies.8–13 The implant survival rate ranged from 80% to 100%.8,10–12 Weighted averages demonstrated a prosthesis survival rate during a 7.3 ± 2.6–year follow-up period of 95%. Complications were observed in the categories of prosthetic complications and bone loss. There was no clearly documented etiology for the different complications. The lack of consistent reporting of information

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Romanos et al

Patients (n)

Follow-up

Complications reported

45

5y

7 cases of screw loosening/­fracture; 20 events of fracture of the acrylic resin base or teeth

Implant survival rate: 100% (237/237) Prosthesis survival rate: 95.5% (43/45)

48

4y

No correlation shown between use of cantilever design and bone loss

No statistically significant bone loss was related to cantilever length

44

NA (retrospective)

5 implants lost

Prosthesis survival rate: 100%

24

5y

No prosthesis failure reported; Implant loss seen with immediate loading

Prosthesis survival rate: 100% Implant cumulative survival rate: 97%

32

6 and 12 mo, retrospectively

3 implants lost

Implant survival rate: 97.6%

Fracture of matrices and bar extensions occurred only in gold bar overdenture group

High success reported for Titanium bar overdenture group; fractures of the gold bar extensions; no implants failed

41 (16 gold bar implant overdentures, 12 titanium, 13 CAD/CAM)

2y

led to an inability to accumulate data regarding complications and, consequently, data are presented as they relate to each individual study (Table 1). Examples of the complications reported are screw loosening/ fracture, fracture of the acrylic resin denture base, as well as fracture of bars at the solder joint in gold versus titanium restorations. There is no clear information in these studies about the thickness of titanium or goldalloy frameworks.

Clinical Studies with Partial Dental Prostheses

A total of eight studies discussed the clinical performance of prostheses in partially edentulous patients when distal cantilever prostheses were used. The partially edentulous studies did not provide sufficient data for extraction. The publications did not report time-dependent data of the different complications. In addition, some studies reported an implant and prosthesis survival rate of 100% after 10 years14 or implant and prosthesis success rates of 100%.15 Other reports16 showed an implant survival rate of 96.9%. Studies that evaluated prosthetic performance in patients who had distal cantilevers in the management of partial edentulism demonstrated no specific complications related to the cantilevered design (Table 2). Each of the studies14–21 subjectively reported distal cantilevers to be a successful prosthetic approach with no predictable adverse events related to the cantilever design. None of the studies demonstrated the incidence or prevalence of complications, providing information regarding the infrequency of these observations. Similarly, fixed toothsupported partial dentures, with periodontally involved

Results

teeth as abutments, including distal cantilevers, did not show any failures after occlusal adjustment and comprehensive treatment planning.

In Vitro Studies

In general, biomechanical studies represented the majority of in vitro studies assessing the performance of cantilever prostheses.22–26 Most of these studies appeared to demonstrate that the implant associated with the most distal retainer of the prosthesis is placed under compressive force when the forces are limited to the cantilever portion of the prosthesis. Implants that are located anterior to this distal supporting implant are generally under tensile force when loads are applied to the distal cantilever. Additional biomechanical studies identified increased moments of force related to longer (more than one tooth) cantilevers. If the cantilever was longer, it would create increased compressive strength on the most distal abutment site, along with greater tensile load on the medial implant sites. In addition, variations in angulation of the implants may affect the magnitude of force. However, there is no evidence regarding negative clinical outcomes of implants placed with angulations (tilted implants) and their distal cantilevers,1 even if the load distributions were unfavorable. Photoelastic models23 of a moderately resorbed edentulous human mandible were used to assess the effects of cantilever length on load transfer to the mandible. Disproportionate increases in maximum stress occurred when cantilever length was increased. Minor variations in implant angulation had a distinct effect The International Journal of Oral & Maxillofacial Implants 1133

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Table 2  Studies with Partial Dental Prostheses Implant total and type

No. of implant cantilevers

Author (study type)

Aim/objective

Becker14 (clinical ­prospective)

To evaluate 10 years of implant-retained fixed prostheses, primarily in the maxilla.

115 Straumann implants

60

Hälg et al16 (clinical follow up)

To analyze whether a cantilever extension on an FPD supported by implants increased the amount of peri-implant bone loss or technical complications compared with reconstructions without cantilevers.

78 Straumann implants

27

Eliasson et al17

To evaluate and compare the long-term performance of FPDs supported by 2 or 3 implants.

375 Brånemark System implants

Brägger et al18

To evaluate incidences of technical/biologic complications and failures occurring in a cohort of consecutive partially edentulous patients with fixed reconstructions on implants.

33 Straumann implants

22

Romeo et al15 (clinical prospective cohort)

To assess the clinical outcomes of patients treated with implantsupported FPDs with cantilevers after a mean follow-up of 8 years.

116 Straumann implants

8 in maxilla 41 in mandible

Wennström et al19 ­(retrospective clinical)

To analyze whether the inclusion of cantilever extensions increased the amount of marginal bone loss at free-standing, implantsupported, FPDs over a 5-year period of functional loading.

48 Astra Tech implants

48

Kreissl et al20 ­(prospective)

To report incidences of technical problems, screw loosening, screw fracture, fracturing of veneering porcelain, and framework fracture in implant-supported FPDs and assess the survival and success rates (event-free survival) after 5 years of function.

205 Biomet 3i implants

23

Johansson and Ekfeldt 21 (retrospective)

To present treatment outcomes and patient reactions to rehabilitation with implant-supported FPDs.

285 Brånemark System implants

on stress magnitude with maximal apical stresses in the distal tilted implants. Naturally occurring masticatory forces were assessed in dentitions with mandibular implant-supported fixed prostheses with bilateral distal two-unit cantilevers opposing complete dentures.26 Based on this study, closing and masticatory forces increased distally. It was also observed that premature occlusal contacts resulted in a significant redistribution of forces. There were also stress contributions from each cantilever unit, but the maximal vertical bending stress in the cantilever joint with the distal implant was not significantly influenced by the occlusal interferences.

Discussion There were eight studies related to partially edentulous patients and six studies in completely edentulous patients. These studies present results with a follow-up

146 implant-­ supported FPDs

285

period ranging from 1 to 10 years regarding prosthetic or implant survival. Descriptive studies demonstrate that the most frequent complication was screw loosening, while porcelain fracture and esthetic veneer fracture were also observed. With a prosthetic survival rate of 95% for the use of distal cantilevers in cases of complete edentulism, this treatment approach appears to be a predictable treatment modality. These figures compared favorably with previous descriptions of distal cantilevers retained by natural anterior teeth. A previous study by Nyman et al27 with distal cantilever fixed prostheses using periodontally compromised teeth as abutments showed a high long-term success rate with extensive documentation. Fixed tooth-supported prosthetic restorations, including distal cantilevers, did not demonstrate any failures when the occlusion was well adjusted after comprehensive treatment planning.28 This study does not evaluate the prosthetic restoration per se since the focus of the study was the long-term prognosis of periodontally

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Patients (n)

Follow-up

Complications reported

36

10 y

No implant fractures, abutment fractures, porcelain fractures, prosthesis fractures, soft tissue recession, or radiographic bone loss reported

All prostheses were satisfactory and functional; 100% success rate

54

3 to 12.7 y (mean, 5.3 y)

18.5% technical complications in cantilever group

95.7% survival rate; no significant bone loss

123

10.5 y

2 veneer fractures and 7 screw loosenings

100% success rate

14

10 y

Porcelain fracture

Implant-supported FPDs had a better success rate compared with tooth- and implant-supported FPDs; 3 failures

45

8.2 y

Esthetic veneer fracture; no prosthetic failure observed during the follow-up examinations

Implant success and survival rates were 90.5% and 100%, respectively; Prosthetic success and survival rates were 57.7% and 100%, respectively

24

5y

Bone level change at FPDs placed in the maxilla was significantly greater than that for FPDs in the mandible

No correlation between cantilever and bone loss

76

5y

6.7% screw loosening; 3.9% screw fracture; 8 veneer fractures

Cantilever FPDs showed the lowest success rate of 68.6%

83

53.9 mo

12% of gold screws and 17% of abutment screws showed “not acceptable” loosening; prostheses made in gold acrylic and titanium acrylic had more complications and showed greater need for repair than metal-ceramic restorations

96% success rate.

compromised teeth. When comparing implant retention with tooth retention, it is possible that the mobility of the natural tooth compensates for posterior occlusal contact since tooth mobility allows prosthetic mobility. Conversely, with distal cantilevers supported by implants in the anterior portion of the arch, there will be very limited mobility associated with framework flex. It may be testimony to the superior anchorage provided by the implant that allows these cantilever prostheses to function as they do. With the advent of the Brånemark approach utilizing complete-arch implant-supported cantilever prostheses, the distal cantilever has gained acceptance in implant dentistry.29,30 Technical complications such as fracture of the acrylic resin teeth and prosthesis base were causes for failures for mandibular fixed implantsupported rehabilitations with distal cantilevers.8 Some authors31–44 attempted to summarize the causes of failures/complications of implants in association with distal cantilevers. Insufficient metal thickness, in-

Results

ferior solder joints, parafunctional habits of patients, incorrect framework design, excessive cantilever length, and inadequate strength of alloys have all been reported as causes of prosthesis failure.

Conclusions After a systematic review of the literature and a traditional literature review, it can be concluded that, based upon 2,444 implants used to support distal cantilever prostheses, a 95% implant survival rate may be achieved. Prosthetic survival reported in these articles demonstrates a 95% prosthesis survival rate. These results are similar to previous publications where distal cantilevers were used with natural tooth retention. It appears that the use of distal cantilevers provides predictable and reliable clinical outcomes. Descriptive studies failed to present compelling evidence of increased rates of mechanical complications. The International Journal of Oral & Maxillofacial Implants 1135

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Romanos et al

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