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Fiber-reinforced composite fixed dental prostheses in the anterior area: A 4.5-year follow-up
Fiber-reinforced composite fixed dental prostheses in the anterior area: A 4.5-year follow-up Cornelia Frese, DDS,a Petra Schiller, PhD,b Hans Joerg Staehle, MD, DDS,c and Diana Wolff, DDSd School of Dental Medicine and Institute of Medical Biometry and Informatics, Ruprecht Karls University Heidelberg, Heidelberg, Germany Statement of problem. Currently, fiber-reinforced fixed dental prostheses (FRC FDPs) are a reliable treatment option for the restoration of single missing teeth in the anterior area. Purpose. The purpose of this study was to evaluate the survival of direct and semidirect fabricated FRC FDPs in the anterior area and to rate the quality of the outcome. Material and methods. Twenty-four participants (12 men, 12 women) were included in the investigation. The prostheses were made of preimpregnated, unidirectional fiber-reinforced composite (FRC) (everStick, GC) by using a direct (n¼18) or semidirect (n¼6) technique. Eleven FRC FDPs had been placed in the maxilla and 13 had been placed in the mandible. Follow-up intervals were recorded, and the prostheses were classified as success (successful), survival (unfavorable event but still in vivo), or failure (lost). Quality was rated according to the modified United States Public Health Services (USPHS) or Ryge criteria. Results. The FRC FDPs evaluated in this study showed an overall survival rate (success) of 72.6% and a functional survival rate (success þ survival) of 85.6% (median follow-up 54 months). According to the USPHS/Ryge criteria, most of the restorations displayed excellent or good quality. Survival analysis was performed by the Kaplan-Meier method. Conclusions. The FRC FDPs evaluated in this study showed promising survival rates and good quality after a median follow-up period of 4.5 years (quartile range 3.5 to 6.3 years), thus indicating that FRC FDPs are reliable treatment options for the restoration of single missing teeth in the anterior area. The use of preimpregnated FRC materials with higher fiber content might improve the clinical fabrication of FRC FDPs but does not influence their long-term clinical survival. (J Prosthet Dent 2014;-:---)
Clinical Implications
Fiber-reinforced fixed dental prostheses (FRC FDPs) made of fully preimpregnated FRC materials showed promising clinical survival data. They may provide a reliable treatment option for the restoration of single missing teeth in the anterior area.
The development of metal-free restorations and prostheses for the replacement of missing teeth continues to interest both dentists and patients. In the anterior area, the use of a resin-
bonded fixed dental prosthesis (FDP) contributes to a conservative treatment but also requires a choice of material. During the past decades, cast metal FDPs have been successfully used in
clinical dentistry and are reported to have a 5-year survival rate of 87.7%.1 In the early 1990s, glass-infiltrated alumina ceramics were introduced and proposed as metal-free alternatives for
This study was supported by the German Association of Dental and Maxillofacial Medicine (DGZMK). a
Senior Dentist, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. Statistician, Institute of Medical Biometry and Informatics, Ruprecht Karls University Heidelberg. c Department Head, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. d Assistant Professor, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. b
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Volume FDPs in the anterior dentition. The 10year survival rate was reported to be 73.9% for 2-retainer FDPs and 94.4% for single-retainer FDPs.2 Frequent technical complications such as loss of retention or fracture of veneering or framework material are caused by imbalances between biomechanical stress in the oral cavity and material properties.1 Consequently, a framework material with a lower modulus of elasticity than that of cast metal alloys or ceramic restorations might be useful to reduce the chipping of veneering material from a comparatively rigid framework. Furthermore, because of their lower modulus of elasticity, fiberreinforced composites (FRCs) should minimize the stress level at the toothrestoration interface and enable an adjustment of the framework’s mechanical properties by inserting adequate fiber volumes.3,4 The incorporation of glass fibers into composite resin material was an earlier variation of today’s FRC and was considered promising in diverse fields.5,6 After 30 years of experience, FRCs have been successfully applied in clinical dentistry.4,7-16 Various studies have shown that framework design, flexural and fatigue properties, and load-bearing capacities of FRCs have improved.17-21 Direct and semidirect fabricated FRC FDPs now compete successfully with conventional cast metal resinbonded FDPs. Having shown that the short-term survival and quality outcomes of FRC FDPs are promising,16 studies should now focus on exploring outcome after longer follow-up periods. However, midterm and long-term survival data with regard to the clinical outcome and number of patients suitable for such observations are limited. The aging of composite resin and the disintegration of veneering composite and fiber material leading to fatigue fractures might affect long-term survival and occur only after several years of clinical use. Therefore, the purpose of this study was to reevaluate a cohort of patients who had received FRC FDPs in the anterior area between 2002 and 2007.
The clinical quality rating of esthetic, biological, and functional parameters of composite resin restorations was mostly obtained by the original Ryge and Snyder methods.22 Because the veneering material of FRC FDPs consists entirely of composite resin, the clinical performance of these prostheses was rated with a quality index similar to that used for composite resin restorations. The standard protocol recently recommended by Hickel et al23,24 (modified United States Public Health Services [USPHS]/Ryge or FDI World Dental Federation [FDI] clinical criteria) was used. The purpose of this study was to evaluate the survival of FRC FDPs in the anterior area and to report on the quality of their outcomes. The null hypothesis of this investigation was that FRC FDPs are not a recommendable treatment alternative for the anterior area after a longer follow-up period.
MATERIAL AND METHODS Patients who had received a FRC FDP in the anterior area in the Department of Conservative Dentistry of the University Hospital Heidelberg between 2002 and 2007 were contacted. Twenty-four of them met the inclusion criteria and could be included in the study. Survival analysis was carried out by the Kaplan-Meier method after success, survival, or failure of the prostheses was recorded. Additionally, 17 of those FRC FDPs underwent quality rating according to modified USPHS/Ryge or FDI clinical criteria. The study was approved by the local ethics committee of the Medical Faculty of the University of Heidelberg (S-011/ 2009). All participants who had received an FRC FDP in the anterior area in the Department of Conservative Dentistry of the University Hospital Heidelberg between 2002 and 2007 and who met the inclusion criteria provided written informed consent. The 8 inclusion criteria are described as follows: participant had received an FRC FDP in the anterior area between January 1, 2002 and December 31,
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2007 in the Department of Conservative Dentistry, University Hospital Heidelberg, Germany; participant was not a member of dental staff or a dental student; participant was not physically restricted in practicing oral hygiene; participant was between 18 and 85 years old; participant was not pregnant or nursing; participant had not received antibiotic therapy within the last 30 days; participant did not require antibiotic prophylaxis before dental treatment; and participant was in good general health. Names and telephone numbers were extracted from the records, and the participants were contacted by telephone. Twenty-four participants were scheduled for inclusion in the analysis of survival rates. Seventeen participants were able to take part in the clinical examination at University Hospital Heidelberg. The clinical investigation of the 17 scheduled participants followed a previously published protocol.16 Seven individuals could not take part because they had either moved to another city or were no longer willing to participate. To obtain at least the information necessary for survival analysis, these 7 individuals were contacted by telephone. The investigator asked the following 4 questions: Is the FRC FDP still in place? When did the failure event take place? Was the FRC FDP repaired? Did you lose the FRC FDP? By including these 7 individuals in the analysis of survival rates, as much information as possible was obtained to minimize bias. However, because of the small study size and limited access to complete clinical data, a biased outcome (nonexamined restorations not representative of group as a whole) cannot be excluded.
Fabrication of FRC FDPs The FRC FDPs had been fabricated by dentists experienced in restorative dentistry and were either surface retained or embedded in minimally invasive cavity preparations of the abutment teeth. They replaced 1 incisor or 1 canine. Retention on the abutment
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teeth was either of fixed-fixed design (3-unit FRC FDP, Fig. 1) or cantilever design (2-unit FRC FDP, Fig. 2). The fabrication and insertion processes were standardized following direct or semidirect technique protocols.16 If necessary, small inlay-cavity preparations were carried out on abutment teeth. For semidirect fabrication, an impression with irreversible hydrocolloid (HS-Alginate; Henri Schein Dental Depot) was made. A cast was prepared with a fast-setting A-silicone material (KwikkModel nature; R-dental Dentalerzeugnisse), and the fabrication of the FRC FDP was done chairside. Fiber frameworks were made of fully impregnated unidirectional FRC (everStick C&B, diameter 1.2 mm, total number of fibers: 4000; Stick Tech) and subsequently covered with viscous composite resin. The pontics were made with a multilayer technique from commercially available hybrid composite resin (Herculite XRV; KerrHawe SA; Enamel HFO plus; Tetric Evo Ceram; Ivoclar Vivadent). The FRC FDPs were then bonded with flowable composite resin (Tetric Flow; Ivoclar Vivadent) after enamel etching. Following a direct technique, the complete fabrication process was done intraorally with the same materials. The abutment teeth were isolated with rubber dam (Hygienic Dental Dam; Coltène/Whaledent). After enamel etching and the application of adhesive resin (Optibond FL; KerrHawe), fiber frameworks were inserted with flowable composite resin (Tetric Flow; Ivoclar Vivadent). The pontics were then formed with a multilayer technique as described above. Following the manufacturer’s instructions, single-tooth replacement was done with 1 strand of unidirectional E-glass FRC material (everStick C&B, diameter 1.2 mm, total number of fibers: 4000; Stick Tech). With regard to the recommendations for framework design in the anterior area, the fibers were placed within the incisal third of the abutment teeth curving buccally within the gap to offer maximal resistance to forces (Figs. 1 and 2).25
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1 Schematic drawing for anterior FRC FDP, fixed-fixed design.
2 Schematic drawing for anterior FRC FDP, cantilever design.
Clinical examination Each examination followed a distinct protocol, including medical history, oral examination, standardized intraoral photography, clinical quality rating according to the modified USPHS or FDI criteria, gingival bleeding index (Mühlemann and Son),26 Turesky plaque index,27 professional tooth cleaning, and oral hygiene instructions. The clinical parameters were evaluated by a single examiner with a conventional diagnostic optical light (Sirolux F; Sirona Dental Systems GmbH), diagnostic probes, and binocular loupes (magnification 2.5). To obtain as much validity as possible, the examiner completed the calibration modules on the basis of the rating of intraoral photographs of composite resin restorations on the recommended Web site (http://www.e-calib.info).23,24 When an FRC FDP was completely lost, making repair impossible, it was registered as a failure. When less severe unfavorable events occurred, such as unilateral or bilateral debonding, chipping fractures, or delamination, the restorations were repaired at a subsequent recall appointment. Unfavorable events were documented in the patient’s records. These restorations were defined as survival. Restorations with no failure or with unfavorable events were classified as success.
As a result of the descriptive character of this study, categorical data were described by absolute and relative frequencies. Continuous data were described by arithmetic mean and standard deviation. The median observation time was calculated on the basis of the observation times of all participants. The analysis of survival rates of the prostheses was performed by the Kaplan-Meier method.28 Asymptotic pointwise 95% confidence intervals were provided for the median observation time of 54 months. Two survival times were defined. Functional survival was the time from treatment to complete failure (that is, clinical events that could be repaired were not regarded as failure in this analysis), and overall survival was the time from treatment to any failure (that is, complete failure or clinical event with repair [defined as survival], whichever came first). Observations with no event in the respective analysis were censored at the end of the individual observation period. Statistical analysis was performed by SAS statistical software (v9.2; SAS Institute Inc).
RESULTS In the group of 24 participants, there were 12 men and 12 women with a mean (standard deviation) age
4
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Survival data Data are shown in Figures 3 and 4. The median follow-up time, calculated on the basis of the observation time of patients, was 54 months (lower to upper quartile: 42.4-75.1). During the follow-up period, 3 FRC FDPs were severely damaged and were lost; they were therefore considered as failures. A total of 16 FRC FDPs required no intervention (repair) and were classified as successful. Five FRC FDPs had unfavorable events (n¼8), required intervention, and were classified as survival. Two FRC FDPs from the survival category had more than 1 unfavorable event. After the first unfavorable event, 2 more failure events took place. In each of them, a successful repair could be carried out. Details of FRC FDP performance can be found in an explicit life-cycle diagram according to van Heumen et al4,15 (Fig. 3). The overall survival rate for the FRC FDPs was 72.6% after 54 months (95% confidence interval 53.2 to 91.9) (Fig. 4). The functional survival rate for the FRC FDPs was 85.6% after 54 months (95% confidence interval 70.1 to 100) (Fig. 4). The null hypothesis tested in this investigation could be rejected.
Clinical evaluation data The data are shown in Table I. A standard protocol for the clinical
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Success (S) (n=16)
Survival (SR) (n=5, FRC FDPs) n=8, unfavourable events occurred: 2 FRC FDPs had more than one unfavourable event and repair
24 FRC FDPs
3 failed FRC FDPs 8 unfavourable events: 2x debonding 4x chipping 2x delamination
21 FRC FDPs in function
Failure (F) (n=3) 3 Life cycle and failure analysis of FRC FDPs. 1.0 Functional survival
Survival Probability
of 35.5 (20.4) years. Seven of the 24 participants were interviewed by telephone with no clinical examination. Each participant had 1 FRC FDP. Eleven FRC FDPs were located in the maxilla, and 13 FRC FDPs were inserted in the mandible. Retention on the abutment teeth was either surface retention (n¼13) or minimally invasive cavity preparation (n¼11). Twenty prostheses had a fixed-fixed design (3-unit FRC FDP) with extension wings on both adjacent teeth. For 4 prostheses, a cantilever design (2-unit FRC FDP) was used for attachment.
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0.8 Overall survival
0.6 0.4 0.2
Censored 0.0 0
20
40
60
80
Time (months) 4 Survival rates of FRC FDPs. Median observation time was 54 months. Functional survival at 54 months was 85.6%; 95% confidence interval 70.1 to 100; n¼24, 3 failures. Overall survival was 72.6%, 95% confidence interval 53.2 to 91.9, n¼24, 8 unfavorable events.
testing of restorative materials and procedures according to the modified USPHS/Ryge or FDI clinical criteria was used.23,24 Seven individuals did not attend the evaluation appointment and had to be excluded from the assessment of the modified USPHS/Ryge or FDI clinical criteria. Seventeen participants were evaluated. The clinical quality rating (modified USPHS/Ryge or FDI clinical criteria) demonstrated that most of the restorations displayed excellent or good quality. Only a few prostheses were rated clinically sufficient, 1 as clinically unsatisfactory (category surface staining), and none as poor.
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DISCUSSION The null hypothesis tested in this investigation was rejected. Clinical midterm and long-term data on the performance of FRC FDPs are still limited.4,8,13,15,29 This study reports an overall survival rate of 72.6% after 54 months for FRC FDPs in the anterior area (Fig. 4). With regard to the authors’ previous investigation, data from the present study confirm expectations on survival data after a follow-up period of 4.5 years.16 The results of the present study, however, are comparable with the survival rate of 73.4% at 4.5 years reported by van Heumen
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Table I.
5 Modified USPHS/Ryge criteria for evaluated fiber-reinforced fixed dental prostheses (n¼17)
Clinically Clinically Clinically Clinically Clinically Excellent Good Sufficient Unsatisfactory Poor
Parameter
Variable
Esthetic
Surface luster
4 (24%)
11 (65%)
2 (12%)
-
-
Surface staining
7 (41%)
7 (41%)
2 (12%)
1 (6%)
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Color stability and translucency
11 (65%)
4 (24%)
2 (12%)
-
-
Anatomic form
14 (82%)
2 (12%)
1 (6%)
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-
12 (71%)
4 (24%)
1 (6%)
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Marginal adaptation (at abutment teeth)
11 (65%)
4 (24%)
2 (12%)
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-
Wear
11 (65%)
6 (35%)
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-
-
Patient’s opinion
11 (65%)
5 (29%)
1 (6%)
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-
16 (94%)
1 (6%)
-
-
-
Recurrence of caries, erosion, abfraction
10 (59%)
6 (35%)
1 (6%)
-
-
Tooth integrity (abutment teeth)
12 (71%)
5 (29%)
-
-
-
Periodontal response (abutment teeth)
6 (35%)
9 (53%)
2 (12%)
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-
Adjacent mucosa (pontic)
6 (35%)
8 (47%)
3 (18%)
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Oral and general health
12 (71%)
5 (29%)
-
-
-
Functional
Biological
Fracture and retention
Postoperative hypersensitivity (abutment teeth)
USPHS, United States Public Health Services.
et al14 in their systematic review including 435 individuals with FRC FDPs. The explorative character nature of this investigation must be considered as one of the limitations because preliminary hypotheses could be tested, but data could only be described descriptively. With regard to the preferable esthetic aspects and the minimally invasive approach of FRC FDPs in the anterior area, it seems appropriate to compare this type of restoration with ceramic FDPs, for which Kern and Sasse2 found a 10-year survival rate of 94.4% in the single-retainer group. Implant-supported single-tooth crowns are further treatment alternatives to FRC FDPs and have a 5-year survival rate of 94.5%.30 However, in relation to treatment time and cost, they are hardly comparable. The unfavorable events, such as the delamination, debonding, and chipping fracture of veneering composite found in the restorations, are in accordance with events described in the literature.4,8,13,15 Here, 8 unfavorable events occurred in 5 FRC FDPs and could be repaired subsequently, and the FRC FDPs remained in situ (Fig. 3).
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The authors observed that unilateral debonding often occurs in patients with periodontal attachment loss and increased tooth mobility. In such clinical situations, the choice of a cantilever design for fabricating FRC FDPs (Fig. 2) seems reasonable.16 Further reasons for failure might include faulty framework design. Low-structure frameworks are known to lead to higher failure modes because weak frameworks do not support restorations sufficiently. Another crucial parameter in framework design is positioning the fibers; optimal fiber position augments framework strength and increases the mechanical properties of FRC FDPs.18,19,31 In accordance with reports of anterior FRC FDPs, the authors recommend placing unidirectional FRC strands within the incisal third of abutment teeth curving out buccally within the gap (Figs. 1 and 2). In this way, declining stress and incisal loading can be sufficiently opposed by the FRC framework. Contemporary, fully preimpregnated, unidirectional fiber bundles are known to have higher fiber fractions than partially preimpregnated glass fiber bundles (65% versus 30%).4,13,15 For FRC FDPs made of partially
preimpregnated glass fibers, van Heumen et al showed a survival rate of 64% for anterior prostheses15 and 71% for posterior prostheses4 after 5 years, and Vallittu et al13 showed 75% survival after 55 months’ median follow-up. Data from the present study (72.6% overall survival and 85.6% functional survival after 4.5 years’ median follow-up; Fig. 4) showed similar survival rates after using fully preimpregnated unidirectional FRCs. Consequently, the use of fully preimpregnated fiber bundles with higher fiber fraction might improve the clinical fabrication of FRC FDPs, although it does not influence long-term clinical survival in the anterior area. Although unfavorable events occur even after longer observation times, the mechanical properties of FRC FDPs can be improved by the appropriate choice of materials, proper design, and fabrication. The aging of the FRC material due to biological and mechanical influences in the oral cavity might compromise the restorations’ survival. Lassila et al32 showed that in vitro water sorption of FRC is linearly dependent on the volume fraction of E-glass fibers. Göhring et al33 found a
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Volume decrease in flexural strength in all tested FRC materials after short-term water storage and thermocycling, regardless of filler content or resin matrix composition. The effect of water storage on FRC was described by Vallittu,34 who noted that the effect of short-term water storage was reversible after specimen drying. Long-term water storage, however, caused irreversible degradation and hydrolysis of the siloxane network that bonds the glass fibers to the polymer matrix. After 10 years of in vitro aging, the reduction in flexural strength and modulus of elasticity of E-glass fibers was 25% for autopolymerized and 23% for heatpolymerized FRC. These findings verify that the clinical survival of FRC FDPs depends on adequate fabrication and design and that it is also limited by material characteristics. Because this was a retrospective evaluation, the authors have no baseline data on the clinical quality rating according to the modified USPHS/Ryge or FDI clinical criteria. All FRC FDPs were assumed to have classifications of clinically excellent. After a period of 4.5 years, the functional parameters displayed slightly more clinically excellent ratings than biological or esthetic parameters (Table I). However, the bestrated clinical quality parameter (94% clinically excellent ratings) was postoperative hypersensitivity on abutment teeth within the group of biological parameters. In the surface staining category, 1 FRC FDP was rated clinically unsatisfactory. The findings of the clinical quality rating show that FRC FDPs provide a minimally invasive and comfortable treatment option for patients needing single-tooth replacement. In the case of unfavorable events, or if corrections of esthetic parameters (surface luster, surface staining, or anatomic form) are indicated, an immediate repair in a single treatment session is possible. In addition, the clinical outcome of FRC FDPs is limited by mechanical stresses, hydrolysis, and degradation in the oral cavity, as well as follow-up concerning the individuals’ oral hygiene and compliance.
CONCLUSION In patients with good oral hygiene, direct and semidirect fabricated FRC FDPs provide a successful treatment alternative to the replacement of single teeth in the anterior area. The FRC FDPs observed in this study showed an overall survival rate of 72.6% and a functional survival rate of 85.6% after 4.5 years. Sixteen FRC FDPs were classified as successful, 3 FRC FDPs were lost (failure), and the FRC FDPs of 5 patients could be subsequently repaired and were classified as survival. The main failure modes of the FRC FDPs were delamination, debonding, and fracture of the veneering composite. The evaluation of the quality of esthetic, functional, and biological outcomes of the FRC FDPs after 4.5 years were clinically excellent or good. The use of preimpregnated FRC materials with a higher fiber fraction might improve the clinical fabrication of FRC FDPs, although it did not influence long-term clinical survival in the anterior area.
REFERENCES 1. Pjetursson BE, Tan WC, Tan K, Bragger U, Zwahlen M, Lang NP. A systematic review of the survival and complication rates of resinbonded bridges after an observation period of at least 5 years. Clin Oral Implants Res 2008;19:131-41. 2. Kern M, Sasse M. Ten-year survival of anterior all-ceramic resin-bonded fixed dental prostheses. J Adhes Dent 2011;13:407-10. 3. Vallittu PK, Sevelius C. Resin-bonded, glass fiber-reinforced composite fixed partial dentures: a clinical study. J Prosthet Dent 2000;84:413-8. 4. van Heumen CC, Tanner J, van Dijken JW, Pikaar R, Lassila LV, Creugers NH, et al. Fiveyear survival of 3-unit fiber-reinforced composite fixed partial dentures in the posterior area. Dent Mater 2010;26:954-60. 5. Bruckmann H, Huttinger KJ. Carbon, a promising material in endoprosthetics. Part 1: the carbon materials and their mechanical properties. Biomaterials 1980;1:67-72. 6. Freilich MA, Karmaker AC, Burstone CJ, Goldberg AJ. Development and clinical applications of a light-polymerized fiberreinforced composite. J Prosthet Dent 1998;80:311-8. 7. Freilich MA, Goldberg AJ. The use of a preimpregnated, fiber-reinforced composite in the fabrication of a periodontal splint: a preliminary report. Pract Periodontics Aesthet Dent 1997;9:873-4. 6.
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8. Freilich MA, Meiers JC, Duncan JP, Eckrote KA, Goldberg AJ. Clinical evaluation of fiber-reinforced fixed bridges. J Am Dent Assoc 2002;133:1524-34. 9. Mannocci F, Machmouridou E, Watson TF, Sauro S, Sherriff M, Pilecki P, et al. Microtensile bond strength of resin-post interfaces created with interpenetrating polymer network posts or cross-linked posts. Med Oral Patol Oral Cir Bucal 2008;13:E745-52. 10. Mannocci F, Sherriff M, Watson TF, Vallittu PK. Penetration of bonding resins into fibre-reinforced composite posts: a confocal microscopic study. Int Endod J 2005;38:46-51. 11. Narva KK, Vallittu PK, Helenius H, Yli-Urpo A. Clinical survey of acrylic resin removable denture repairs with glass-fiber reinforcement. Int J Prosthodont 2001;14: 219-24. 12. Vallittu PK. Glass fiber reinforcement in repaired acrylic resin removable dentures: preliminary results of a clinical study. Quintessence Int 1997;28:39-44. 13. Vallittu PK. Survival rates of resin-bonded, glass fiber-reinforced composite fixed partial dentures with a mean follow-up of 42 months: a pilot study. J Prosthet Dent 2004;91:241-6. 14. van Heumen CC, Kreulen CM, Creugers NH. Clinical studies of fiber-reinforced resinbonded fixed partial dentures: a systematic review. Eur J Oral Sci 2009;117:1-6. 15. van Heumen CC, van Dijken JW, Tanner J, Pikaar R, Lassila LV, Creugers NH, et al. Fiveyear survival of 3-unit fiber-reinforced composite fixed partial dentures in the anterior area. Dent Mater 2009;25:820-7. 16. Wolff D, Schach C, Kraus T, Ding P, Pritsch M, Mente J, et al. Fiber-reinforced composite fixed dental prostheses: a retrospective clinical examination. J Adhes Dent 2011;13:187-94. 17. Abdulmajeed AA, Narhi TO, Vallittu PK, Lassila LV. The effect of high fiber fraction on some mechanical properties of unidirectional glass fiber-reinforced composite. Dent Mater 2011;27:313-21. 18. Dyer SR, Lassila LV, Jokinen M, Vallittu PK. Effect of fiber position and orientation on fracture load of fiber-reinforced composite. Dent Mater 2004;20:947-55. 19. Dyer SR, Lassila LV, Jokinen M, Vallittu PK. Effect of cross-sectional design on the modulus of elasticity and toughness of fiberreinforced composite materials. J Prosthet Dent 2005;94:219-26. 20. van Heumen CC, Kreulen CM, Bronkhorst EM, Lesaffre E, Creugers NH. Fiber-reinforced dental composites in beam testing. Dent Mater 2008;24: 1435-43. 21. Basaran EG, Ayna E, Vallittu PK, Lassila LV. Load bearing capacity of fiber-reinforced and unreinforced composite resin CAD/CAMfabricated fixed dental prostheses. J Prosthet Dent 2013;109:88-94. 22. Ryge G, Snyder M. Evaluating the clinical quality of restorations. J Am Dent Assoc 1973;87:369-77.
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23. Hickel R, Peschke A, Tyas M, Mjor I, Bayne S, Peters M, et al. FDI World Dental Federationclinical criteria for the evaluation of direct and indirect restorations. Update and clinical examples. J Adhes Dent 2010;12:259-72. 24. Hickel R, Roulet JF, Bayne S, Heintze SD, Mjor IA, Peters M, et al. Recommendations for conducting controlled clinical studies of dental restorative materials. Science Committee Project 2/98-FDI World Dental Federation study design (Part I) and criteria for evaluation (Part II) of direct and indirect restorations including onlays and partial crowns. J Adhes Dent 2007;9(suppl 1):121-47. 25. Yokoyama D, Shinya A, Lassila LV, Gomi H, Nakasone Y, Vallittu PK. Framework design of an anterior fiber-reinforced hybrid composite fixed partial denture: a 3D finite element study. Int J Prosthodont 2009;22:405-12. 26. Mühlemann HR, Son S. Gingival sulcus bleeding-a leading symptom in initial gingivitis. Helv Odontol Acta 1971;15:107-13. 27. Turesky S, Gilmore ND, Glickman I. Reduced plaque formation by the chloromethyl analogue of vitamine C. J Periodontol 1970;41:41-3.
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7 28. Kaplan EL, Meier P. Nonparametric estimation from incomplete observations. J Am Stat Assoc 1958;53:457-81. 29. Unlu N, Belli S. Three-year clinical evaluation of fiber-reinforced composite fixed partial dentures using prefabricated pontics. J Adhes Dent 2006;8:183-8. 30. Jung RE, Pjetursson BE, Glauser R, Zembic A, Zwahlen M, Lang NP. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19:119-30. 31. Lassila LV, Vallittu PK. The effect of fiber position and polymerization condition on the flexural properties of fiber-reinforced composite. J Contemp Dent Pract 2004;5:14-26. 32. Lassila LV, Nohrstrom T, Vallittu PK. The influence of short-term water storage on the flexural properties of unidirectional glass fiber-reinforced composites. Biomaterials 2002;23:2221-9. 33. Göhring TN, Gallo L, Luthy H. Effect of water storage, thermocycling, the incorporation and site of placement of glass-fibers on the flexural strength of veneering composite. Dent Mater 2005;21:761-72.
34. Vallittu PK. Effect of 10 years of in vitro aging on the flexural properties of fiber-reinforced resin composites. Int J Prosthodont 2007;20:43-5. Corresponding author: Dr Cornelia Frese School of Dental Medicine University Hospital Heidelberg Department of Conservative Dentistry Im Neuenheimer Feld 400 69120 Heidelberg GERMANY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications
Fiber-reinforced fixed dental prostheses (FRC FDPs) made of fully preimpregnated FRC materials showed promising clinical survival data. They may provide a reliable treatment option for the restoration of single missing teeth in the anterior area.
The development of metal-free restorations and prostheses for the replacement of missing teeth continues to interest both dentists and patients. In the anterior area, the use of a resin-
bonded fixed dental prosthesis (FDP) contributes to a conservative treatment but also requires a choice of material. During the past decades, cast metal FDPs have been successfully used in
clinical dentistry and are reported to have a 5-year survival rate of 87.7%.1 In the early 1990s, glass-infiltrated alumina ceramics were introduced and proposed as metal-free alternatives for
This study was supported by the German Association of Dental and Maxillofacial Medicine (DGZMK). a
Senior Dentist, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. Statistician, Institute of Medical Biometry and Informatics, Ruprecht Karls University Heidelberg. c Department Head, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. d Assistant Professor, Department of Conservative Dentistry, School of Dental Medicine, University Hospital Heidelberg. b
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Volume FDPs in the anterior dentition. The 10year survival rate was reported to be 73.9% for 2-retainer FDPs and 94.4% for single-retainer FDPs.2 Frequent technical complications such as loss of retention or fracture of veneering or framework material are caused by imbalances between biomechanical stress in the oral cavity and material properties.1 Consequently, a framework material with a lower modulus of elasticity than that of cast metal alloys or ceramic restorations might be useful to reduce the chipping of veneering material from a comparatively rigid framework. Furthermore, because of their lower modulus of elasticity, fiberreinforced composites (FRCs) should minimize the stress level at the toothrestoration interface and enable an adjustment of the framework’s mechanical properties by inserting adequate fiber volumes.3,4 The incorporation of glass fibers into composite resin material was an earlier variation of today’s FRC and was considered promising in diverse fields.5,6 After 30 years of experience, FRCs have been successfully applied in clinical dentistry.4,7-16 Various studies have shown that framework design, flexural and fatigue properties, and load-bearing capacities of FRCs have improved.17-21 Direct and semidirect fabricated FRC FDPs now compete successfully with conventional cast metal resinbonded FDPs. Having shown that the short-term survival and quality outcomes of FRC FDPs are promising,16 studies should now focus on exploring outcome after longer follow-up periods. However, midterm and long-term survival data with regard to the clinical outcome and number of patients suitable for such observations are limited. The aging of composite resin and the disintegration of veneering composite and fiber material leading to fatigue fractures might affect long-term survival and occur only after several years of clinical use. Therefore, the purpose of this study was to reevaluate a cohort of patients who had received FRC FDPs in the anterior area between 2002 and 2007.
The clinical quality rating of esthetic, biological, and functional parameters of composite resin restorations was mostly obtained by the original Ryge and Snyder methods.22 Because the veneering material of FRC FDPs consists entirely of composite resin, the clinical performance of these prostheses was rated with a quality index similar to that used for composite resin restorations. The standard protocol recently recommended by Hickel et al23,24 (modified United States Public Health Services [USPHS]/Ryge or FDI World Dental Federation [FDI] clinical criteria) was used. The purpose of this study was to evaluate the survival of FRC FDPs in the anterior area and to report on the quality of their outcomes. The null hypothesis of this investigation was that FRC FDPs are not a recommendable treatment alternative for the anterior area after a longer follow-up period.
MATERIAL AND METHODS Patients who had received a FRC FDP in the anterior area in the Department of Conservative Dentistry of the University Hospital Heidelberg between 2002 and 2007 were contacted. Twenty-four of them met the inclusion criteria and could be included in the study. Survival analysis was carried out by the Kaplan-Meier method after success, survival, or failure of the prostheses was recorded. Additionally, 17 of those FRC FDPs underwent quality rating according to modified USPHS/Ryge or FDI clinical criteria. The study was approved by the local ethics committee of the Medical Faculty of the University of Heidelberg (S-011/ 2009). All participants who had received an FRC FDP in the anterior area in the Department of Conservative Dentistry of the University Hospital Heidelberg between 2002 and 2007 and who met the inclusion criteria provided written informed consent. The 8 inclusion criteria are described as follows: participant had received an FRC FDP in the anterior area between January 1, 2002 and December 31,
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2007 in the Department of Conservative Dentistry, University Hospital Heidelberg, Germany; participant was not a member of dental staff or a dental student; participant was not physically restricted in practicing oral hygiene; participant was between 18 and 85 years old; participant was not pregnant or nursing; participant had not received antibiotic therapy within the last 30 days; participant did not require antibiotic prophylaxis before dental treatment; and participant was in good general health. Names and telephone numbers were extracted from the records, and the participants were contacted by telephone. Twenty-four participants were scheduled for inclusion in the analysis of survival rates. Seventeen participants were able to take part in the clinical examination at University Hospital Heidelberg. The clinical investigation of the 17 scheduled participants followed a previously published protocol.16 Seven individuals could not take part because they had either moved to another city or were no longer willing to participate. To obtain at least the information necessary for survival analysis, these 7 individuals were contacted by telephone. The investigator asked the following 4 questions: Is the FRC FDP still in place? When did the failure event take place? Was the FRC FDP repaired? Did you lose the FRC FDP? By including these 7 individuals in the analysis of survival rates, as much information as possible was obtained to minimize bias. However, because of the small study size and limited access to complete clinical data, a biased outcome (nonexamined restorations not representative of group as a whole) cannot be excluded.
Fabrication of FRC FDPs The FRC FDPs had been fabricated by dentists experienced in restorative dentistry and were either surface retained or embedded in minimally invasive cavity preparations of the abutment teeth. They replaced 1 incisor or 1 canine. Retention on the abutment
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teeth was either of fixed-fixed design (3-unit FRC FDP, Fig. 1) or cantilever design (2-unit FRC FDP, Fig. 2). The fabrication and insertion processes were standardized following direct or semidirect technique protocols.16 If necessary, small inlay-cavity preparations were carried out on abutment teeth. For semidirect fabrication, an impression with irreversible hydrocolloid (HS-Alginate; Henri Schein Dental Depot) was made. A cast was prepared with a fast-setting A-silicone material (KwikkModel nature; R-dental Dentalerzeugnisse), and the fabrication of the FRC FDP was done chairside. Fiber frameworks were made of fully impregnated unidirectional FRC (everStick C&B, diameter 1.2 mm, total number of fibers: 4000; Stick Tech) and subsequently covered with viscous composite resin. The pontics were made with a multilayer technique from commercially available hybrid composite resin (Herculite XRV; KerrHawe SA; Enamel HFO plus; Tetric Evo Ceram; Ivoclar Vivadent). The FRC FDPs were then bonded with flowable composite resin (Tetric Flow; Ivoclar Vivadent) after enamel etching. Following a direct technique, the complete fabrication process was done intraorally with the same materials. The abutment teeth were isolated with rubber dam (Hygienic Dental Dam; Coltène/Whaledent). After enamel etching and the application of adhesive resin (Optibond FL; KerrHawe), fiber frameworks were inserted with flowable composite resin (Tetric Flow; Ivoclar Vivadent). The pontics were then formed with a multilayer technique as described above. Following the manufacturer’s instructions, single-tooth replacement was done with 1 strand of unidirectional E-glass FRC material (everStick C&B, diameter 1.2 mm, total number of fibers: 4000; Stick Tech). With regard to the recommendations for framework design in the anterior area, the fibers were placed within the incisal third of the abutment teeth curving buccally within the gap to offer maximal resistance to forces (Figs. 1 and 2).25
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1 Schematic drawing for anterior FRC FDP, fixed-fixed design.
2 Schematic drawing for anterior FRC FDP, cantilever design.
Clinical examination Each examination followed a distinct protocol, including medical history, oral examination, standardized intraoral photography, clinical quality rating according to the modified USPHS or FDI criteria, gingival bleeding index (Mühlemann and Son),26 Turesky plaque index,27 professional tooth cleaning, and oral hygiene instructions. The clinical parameters were evaluated by a single examiner with a conventional diagnostic optical light (Sirolux F; Sirona Dental Systems GmbH), diagnostic probes, and binocular loupes (magnification 2.5). To obtain as much validity as possible, the examiner completed the calibration modules on the basis of the rating of intraoral photographs of composite resin restorations on the recommended Web site (http://www.e-calib.info).23,24 When an FRC FDP was completely lost, making repair impossible, it was registered as a failure. When less severe unfavorable events occurred, such as unilateral or bilateral debonding, chipping fractures, or delamination, the restorations were repaired at a subsequent recall appointment. Unfavorable events were documented in the patient’s records. These restorations were defined as survival. Restorations with no failure or with unfavorable events were classified as success.
As a result of the descriptive character of this study, categorical data were described by absolute and relative frequencies. Continuous data were described by arithmetic mean and standard deviation. The median observation time was calculated on the basis of the observation times of all participants. The analysis of survival rates of the prostheses was performed by the Kaplan-Meier method.28 Asymptotic pointwise 95% confidence intervals were provided for the median observation time of 54 months. Two survival times were defined. Functional survival was the time from treatment to complete failure (that is, clinical events that could be repaired were not regarded as failure in this analysis), and overall survival was the time from treatment to any failure (that is, complete failure or clinical event with repair [defined as survival], whichever came first). Observations with no event in the respective analysis were censored at the end of the individual observation period. Statistical analysis was performed by SAS statistical software (v9.2; SAS Institute Inc).
RESULTS In the group of 24 participants, there were 12 men and 12 women with a mean (standard deviation) age
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Survival data Data are shown in Figures 3 and 4. The median follow-up time, calculated on the basis of the observation time of patients, was 54 months (lower to upper quartile: 42.4-75.1). During the follow-up period, 3 FRC FDPs were severely damaged and were lost; they were therefore considered as failures. A total of 16 FRC FDPs required no intervention (repair) and were classified as successful. Five FRC FDPs had unfavorable events (n¼8), required intervention, and were classified as survival. Two FRC FDPs from the survival category had more than 1 unfavorable event. After the first unfavorable event, 2 more failure events took place. In each of them, a successful repair could be carried out. Details of FRC FDP performance can be found in an explicit life-cycle diagram according to van Heumen et al4,15 (Fig. 3). The overall survival rate for the FRC FDPs was 72.6% after 54 months (95% confidence interval 53.2 to 91.9) (Fig. 4). The functional survival rate for the FRC FDPs was 85.6% after 54 months (95% confidence interval 70.1 to 100) (Fig. 4). The null hypothesis tested in this investigation could be rejected.
Clinical evaluation data The data are shown in Table I. A standard protocol for the clinical
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Success (S) (n=16)
Survival (SR) (n=5, FRC FDPs) n=8, unfavourable events occurred: 2 FRC FDPs had more than one unfavourable event and repair
24 FRC FDPs
3 failed FRC FDPs 8 unfavourable events: 2x debonding 4x chipping 2x delamination
21 FRC FDPs in function
Failure (F) (n=3) 3 Life cycle and failure analysis of FRC FDPs. 1.0 Functional survival
Survival Probability
of 35.5 (20.4) years. Seven of the 24 participants were interviewed by telephone with no clinical examination. Each participant had 1 FRC FDP. Eleven FRC FDPs were located in the maxilla, and 13 FRC FDPs were inserted in the mandible. Retention on the abutment teeth was either surface retention (n¼13) or minimally invasive cavity preparation (n¼11). Twenty prostheses had a fixed-fixed design (3-unit FRC FDP) with extension wings on both adjacent teeth. For 4 prostheses, a cantilever design (2-unit FRC FDP) was used for attachment.
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0.8 Overall survival
0.6 0.4 0.2
Censored 0.0 0
20
40
60
80
Time (months) 4 Survival rates of FRC FDPs. Median observation time was 54 months. Functional survival at 54 months was 85.6%; 95% confidence interval 70.1 to 100; n¼24, 3 failures. Overall survival was 72.6%, 95% confidence interval 53.2 to 91.9, n¼24, 8 unfavorable events.
testing of restorative materials and procedures according to the modified USPHS/Ryge or FDI clinical criteria was used.23,24 Seven individuals did not attend the evaluation appointment and had to be excluded from the assessment of the modified USPHS/Ryge or FDI clinical criteria. Seventeen participants were evaluated. The clinical quality rating (modified USPHS/Ryge or FDI clinical criteria) demonstrated that most of the restorations displayed excellent or good quality. Only a few prostheses were rated clinically sufficient, 1 as clinically unsatisfactory (category surface staining), and none as poor.
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DISCUSSION The null hypothesis tested in this investigation was rejected. Clinical midterm and long-term data on the performance of FRC FDPs are still limited.4,8,13,15,29 This study reports an overall survival rate of 72.6% after 54 months for FRC FDPs in the anterior area (Fig. 4). With regard to the authors’ previous investigation, data from the present study confirm expectations on survival data after a follow-up period of 4.5 years.16 The results of the present study, however, are comparable with the survival rate of 73.4% at 4.5 years reported by van Heumen
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Table I.
5 Modified USPHS/Ryge criteria for evaluated fiber-reinforced fixed dental prostheses (n¼17)
Clinically Clinically Clinically Clinically Clinically Excellent Good Sufficient Unsatisfactory Poor
Parameter
Variable
Esthetic
Surface luster
4 (24%)
11 (65%)
2 (12%)
-
-
Surface staining
7 (41%)
7 (41%)
2 (12%)
1 (6%)
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Color stability and translucency
11 (65%)
4 (24%)
2 (12%)
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-
Anatomic form
14 (82%)
2 (12%)
1 (6%)
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-
12 (71%)
4 (24%)
1 (6%)
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Marginal adaptation (at abutment teeth)
11 (65%)
4 (24%)
2 (12%)
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Wear
11 (65%)
6 (35%)
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-
-
Patient’s opinion
11 (65%)
5 (29%)
1 (6%)
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16 (94%)
1 (6%)
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Recurrence of caries, erosion, abfraction
10 (59%)
6 (35%)
1 (6%)
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-
Tooth integrity (abutment teeth)
12 (71%)
5 (29%)
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-
-
Periodontal response (abutment teeth)
6 (35%)
9 (53%)
2 (12%)
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Adjacent mucosa (pontic)
6 (35%)
8 (47%)
3 (18%)
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Oral and general health
12 (71%)
5 (29%)
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-
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Functional
Biological
Fracture and retention
Postoperative hypersensitivity (abutment teeth)
USPHS, United States Public Health Services.
et al14 in their systematic review including 435 individuals with FRC FDPs. The explorative character nature of this investigation must be considered as one of the limitations because preliminary hypotheses could be tested, but data could only be described descriptively. With regard to the preferable esthetic aspects and the minimally invasive approach of FRC FDPs in the anterior area, it seems appropriate to compare this type of restoration with ceramic FDPs, for which Kern and Sasse2 found a 10-year survival rate of 94.4% in the single-retainer group. Implant-supported single-tooth crowns are further treatment alternatives to FRC FDPs and have a 5-year survival rate of 94.5%.30 However, in relation to treatment time and cost, they are hardly comparable. The unfavorable events, such as the delamination, debonding, and chipping fracture of veneering composite found in the restorations, are in accordance with events described in the literature.4,8,13,15 Here, 8 unfavorable events occurred in 5 FRC FDPs and could be repaired subsequently, and the FRC FDPs remained in situ (Fig. 3).
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The authors observed that unilateral debonding often occurs in patients with periodontal attachment loss and increased tooth mobility. In such clinical situations, the choice of a cantilever design for fabricating FRC FDPs (Fig. 2) seems reasonable.16 Further reasons for failure might include faulty framework design. Low-structure frameworks are known to lead to higher failure modes because weak frameworks do not support restorations sufficiently. Another crucial parameter in framework design is positioning the fibers; optimal fiber position augments framework strength and increases the mechanical properties of FRC FDPs.18,19,31 In accordance with reports of anterior FRC FDPs, the authors recommend placing unidirectional FRC strands within the incisal third of abutment teeth curving out buccally within the gap (Figs. 1 and 2). In this way, declining stress and incisal loading can be sufficiently opposed by the FRC framework. Contemporary, fully preimpregnated, unidirectional fiber bundles are known to have higher fiber fractions than partially preimpregnated glass fiber bundles (65% versus 30%).4,13,15 For FRC FDPs made of partially
preimpregnated glass fibers, van Heumen et al showed a survival rate of 64% for anterior prostheses15 and 71% for posterior prostheses4 after 5 years, and Vallittu et al13 showed 75% survival after 55 months’ median follow-up. Data from the present study (72.6% overall survival and 85.6% functional survival after 4.5 years’ median follow-up; Fig. 4) showed similar survival rates after using fully preimpregnated unidirectional FRCs. Consequently, the use of fully preimpregnated fiber bundles with higher fiber fraction might improve the clinical fabrication of FRC FDPs, although it does not influence long-term clinical survival in the anterior area. Although unfavorable events occur even after longer observation times, the mechanical properties of FRC FDPs can be improved by the appropriate choice of materials, proper design, and fabrication. The aging of the FRC material due to biological and mechanical influences in the oral cavity might compromise the restorations’ survival. Lassila et al32 showed that in vitro water sorption of FRC is linearly dependent on the volume fraction of E-glass fibers. Göhring et al33 found a
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Volume decrease in flexural strength in all tested FRC materials after short-term water storage and thermocycling, regardless of filler content or resin matrix composition. The effect of water storage on FRC was described by Vallittu,34 who noted that the effect of short-term water storage was reversible after specimen drying. Long-term water storage, however, caused irreversible degradation and hydrolysis of the siloxane network that bonds the glass fibers to the polymer matrix. After 10 years of in vitro aging, the reduction in flexural strength and modulus of elasticity of E-glass fibers was 25% for autopolymerized and 23% for heatpolymerized FRC. These findings verify that the clinical survival of FRC FDPs depends on adequate fabrication and design and that it is also limited by material characteristics. Because this was a retrospective evaluation, the authors have no baseline data on the clinical quality rating according to the modified USPHS/Ryge or FDI clinical criteria. All FRC FDPs were assumed to have classifications of clinically excellent. After a period of 4.5 years, the functional parameters displayed slightly more clinically excellent ratings than biological or esthetic parameters (Table I). However, the bestrated clinical quality parameter (94% clinically excellent ratings) was postoperative hypersensitivity on abutment teeth within the group of biological parameters. In the surface staining category, 1 FRC FDP was rated clinically unsatisfactory. The findings of the clinical quality rating show that FRC FDPs provide a minimally invasive and comfortable treatment option for patients needing single-tooth replacement. In the case of unfavorable events, or if corrections of esthetic parameters (surface luster, surface staining, or anatomic form) are indicated, an immediate repair in a single treatment session is possible. In addition, the clinical outcome of FRC FDPs is limited by mechanical stresses, hydrolysis, and degradation in the oral cavity, as well as follow-up concerning the individuals’ oral hygiene and compliance.
CONCLUSION In patients with good oral hygiene, direct and semidirect fabricated FRC FDPs provide a successful treatment alternative to the replacement of single teeth in the anterior area. The FRC FDPs observed in this study showed an overall survival rate of 72.6% and a functional survival rate of 85.6% after 4.5 years. Sixteen FRC FDPs were classified as successful, 3 FRC FDPs were lost (failure), and the FRC FDPs of 5 patients could be subsequently repaired and were classified as survival. The main failure modes of the FRC FDPs were delamination, debonding, and fracture of the veneering composite. The evaluation of the quality of esthetic, functional, and biological outcomes of the FRC FDPs after 4.5 years were clinically excellent or good. The use of preimpregnated FRC materials with a higher fiber fraction might improve the clinical fabrication of FRC FDPs, although it did not influence long-term clinical survival in the anterior area.
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23. Hickel R, Peschke A, Tyas M, Mjor I, Bayne S, Peters M, et al. FDI World Dental Federationclinical criteria for the evaluation of direct and indirect restorations. Update and clinical examples. J Adhes Dent 2010;12:259-72. 24. Hickel R, Roulet JF, Bayne S, Heintze SD, Mjor IA, Peters M, et al. Recommendations for conducting controlled clinical studies of dental restorative materials. Science Committee Project 2/98-FDI World Dental Federation study design (Part I) and criteria for evaluation (Part II) of direct and indirect restorations including onlays and partial crowns. J Adhes Dent 2007;9(suppl 1):121-47. 25. Yokoyama D, Shinya A, Lassila LV, Gomi H, Nakasone Y, Vallittu PK. Framework design of an anterior fiber-reinforced hybrid composite fixed partial denture: a 3D finite element study. Int J Prosthodont 2009;22:405-12. 26. Mühlemann HR, Son S. Gingival sulcus bleeding-a leading symptom in initial gingivitis. Helv Odontol Acta 1971;15:107-13. 27. Turesky S, Gilmore ND, Glickman I. Reduced plaque formation by the chloromethyl analogue of vitamine C. J Periodontol 1970;41:41-3.
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7 28. Kaplan EL, Meier P. Nonparametric estimation from incomplete observations. J Am Stat Assoc 1958;53:457-81. 29. Unlu N, Belli S. Three-year clinical evaluation of fiber-reinforced composite fixed partial dentures using prefabricated pontics. J Adhes Dent 2006;8:183-8. 30. Jung RE, Pjetursson BE, Glauser R, Zembic A, Zwahlen M, Lang NP. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19:119-30. 31. Lassila LV, Vallittu PK. The effect of fiber position and polymerization condition on the flexural properties of fiber-reinforced composite. J Contemp Dent Pract 2004;5:14-26. 32. Lassila LV, Nohrstrom T, Vallittu PK. The influence of short-term water storage on the flexural properties of unidirectional glass fiber-reinforced composites. Biomaterials 2002;23:2221-9. 33. Göhring TN, Gallo L, Luthy H. Effect of water storage, thermocycling, the incorporation and site of placement of glass-fibers on the flexural strength of veneering composite. Dent Mater 2005;21:761-72.
34. Vallittu PK. Effect of 10 years of in vitro aging on the flexural properties of fiber-reinforced resin composites. Int J Prosthodont 2007;20:43-5. Corresponding author: Dr Cornelia Frese School of Dental Medicine University Hospital Heidelberg Department of Conservative Dentistry Im Neuenheimer Feld 400 69120 Heidelberg GERMANY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.