The rehabilitation of edentulous maxillary bones can be obtained through different treatment options such as complete dentures, removable implant-retained or fixed implant-supported dental prostheses.
When assessing patient-centered treatment outcomes, implant-retained or fixed implant-supported prostheses provide the highest satisfaction in comparison with removable ones.1,2 3 1,2,4 5
This clinical modality is not new. According to authors' knowledge, a first description was published in 1977.6 7–13
In brief, to use the available bone, avoiding bone grafting procedures, and to prevent damage to anatomical structures (inferior alveolar nerve and maxillary sinus), at least 4 angled or axial endo-osseous implants are positioned.14,15 16,17 18 19
Various studies and reviews confirmed the clinical success of this approach that it seems not related to a specific implant surface, design, system, or clinical procedure preferences.1,2,4,20–28 29 3
In a recent review, Kwon et al2 30,31 31 32
The purpose of this study was to analyze the differences in terms of complications and failures of final full-arch implant rehabilitations made in resin versus those made in fully veneered zirconia. Fixed complete-arch prostheses supported by 4 to 6 implants and used to rehabilitate edentulous maxillae and mandibles in a private practice set were retrospectively followed up for 5 years.
Materials and Methods
This article was written according to STROBE guidelines.33
Patient Selection
From January to December, 2012, 125 patients (57 men and 68 women; mean age, 65 years; age range, 42–90 years) were selected from a private clinic (Clinica SST, Milano, Italy) from dental hygiene clinical recall appointments. The general inclusion criteria consisted of having received one or two four to six implant-supported full-arch rehabilitations in resin or zirconia, from the same private practice between May, 2005 and December, 2012.
Overall, the study inclusion criteria were as follows:
white patients of both genders, older than 18 years, with impaired esthetic and stomatognathic functionality, at patient-level consideration;
completely edentulous maxilla/mandible (144 arches, 71 maxilla and 73 mandible) or presence of teeth with an unfavorable long-term prognosis (87 arches, 41 maxilla and 46 mandible); in particular, this means encountering one or a combination of the following points:
a. tooth mobility Modified Miller Index ≥ 334
b. residual coronal tooth structure to have a tooth restorability index value ≤ 1, that is, no residual coronal dentine for restoration35
c. less than 2 vital abutments teeth for each hemiarch, available to support a 12-teeth cross-arch fixed dental prosthesis (FDP) able to incorporate no more than one-end cantilever, in the absence of healthy periodontal tissue
d. esthetic impossibilities to incorporate a supra-gingival location of the crown margins36
e. low motivation to maintenance program
adequate bone height in the region between the premolar teeth for the placement of at least 10-mm long and 4-mm wide implants; and
last but not least, patients who were seeking minimization of financial intervention and consequently refused any kind of bone augmentation procedures.
Patients self-reported good general health without severe medical or psychological conditions. Specifically, they were able to undergo surgical treatment and restorative procedures.37 38
The exclusion criteria were as follows:
presence of acute infection at the future implant site;
ASA ≥ 3;
poor motivation to accept a customized oral hygiene maintenance program; and
irradiation of the head or neck region or chemotherapy within the past 60 months.
Preliminary screening was performed using clinical examination, panoramic orthopantomographs, computerized tomographic scans, study mouth models, blood tests, electrocardiography, and anesthesiological and cardiological examinations. All included patients were scheduled for oral hygiene maintenance programs. Patients were informed about the study, and agreed to be a part of the investigation.
Out of the original 125 patients, 12 patients (10%), representing 79 implants and 21 prostheses, withdrew from the study. Lost patients were older than followed-up patients, but there were no differences in gender, prosthesis materials, and number of rehabilitated arches (Table 1 ). Table 2 shows drop out reasons.
Table 1: Comparison Between Lost Patients and Followed-Up Patients
Table 2: Drop Out Reasons
All patients were visited by an independent operator who was not involved in original prosthetic procedures.
Clinical Procedures for 4 to 6 Implants Treatment Concept
Four dental implants (Milde Implants; Titanmed, Bergamo, Italy) were placed by a surgeon (G.M.T.) in the maxilla/mandible of the patients using a one-stage surgical procedure with the aid of a surgical guide. In particular, implant insertion followed standard procedures,6
In case of immediate extraction in the absence of acute infection, the sockets were cleaned and the bone shaped to avoid sharp sockets. The implant neck was positioned at bone level. Primary stability, with an implant stability quotient39
Axial or angulated abutments (3dObjects, Taverne, Switzerland) were placed. The abutment angulations were chosen so that the prosthetic screw-access holes were in the most appropriate position for both location and easy access.
The same day of surgery, all the implants were immediately loaded with a 10-teeth acrylic screw-retained interim prosthesis. Loading was distributed all along the occlusal surface, with full contact on every tooth in static condition without interferences in lateral excursion. This treatment was applied to all patients, independently from their features (dietary habits, muscle activity, or face morphotype).
The patients were instructed to remain on a soft diet for the subsequent 2 weeks. After 2 months, all implants showed good osseointegration at clinical, instrumental (resonance frequency analysis), and radiographic tests, and final prosthesis material was selected. All the immediate loaded prostheses were replaced with final prostheses within the subsequent 4 months using computer numerical control milled-framework veneered techniques. Two different materials were used: polymethyl methacrylate (PMMA) veneered with resin, and zirconia veneered with ceramic. Technological opportunities (Computer Aided Manufacturing [CAM]/Computer Aided Drafting [CAD] systems) to fabricate implant frameworks were used trying to minimize certain degrees of inaccuracies shown with handcrafted frameworks.40
Dichotomy of selection between PMMA frameworks veneered with resin versus zirconia frameworks veneered with ceramic for the final prosthesis were based on the following:
occurred fracture of the interim prosthesis (16 patients) and
low rate of esthetic final judgement at patient-level consideration (32 patients).
If at least 1 of the above mentioned points were covered, patients were addressed to a prosthesis with a zirconia framework veneered with ceramic (48 patients).
Using a pick-up technique, polyether complete-arch impression (Impregum/Permadyne; 3M ESPE AG, Seefeld, Germany) was performed in a customized open tray (Apex trays; Megadenta Dentalprodukte, Radeberg, Germany). A plaster model (Esthetic-base gold; Dentona AG, Dormund, Germany) was made and used to obtain an anatomical contour wax-up. Soft tissue was reproduced in the impression using vinylpolysiloxane (Gingifast Rigid; Zhermack, Rovigo, Italy). The maxillary relation was taken with a postural facebow.41
The framework core was designed considering the veneering material and with additional caution for zirconia. Specifically, a maximum of 2 mm of unsupported porcelain was allowed and the connectors within the crowns were designed with a 10-mm2 area at minimum, as measured by the software used for the CAD technique. A CAM system was used to mill the framework material. Zirconia was milled in the presintered state (Zirite, Keramo, Tavernerio, Como, Italy), then sintered (3dObjects), and covered by feldspathic porcelain (CZR Noritake Kizai Co. Ltd, Nagoya, Japan). Porcelain fusion was made with zirconium oxide margins by a single master ceramist (SST Dental Clinic, Milano, Italy), following a slow cooling protocol.42,43
The PMMA prosthesis was heated in accordance with Doğan et al44
The passive fit of the fixed detachable prostheses on the abutments was evaluated in 3 ways. First, pressure was applied on one end abutment and then on the other one45 46 47 48
By using 8-mm wide, 8-mm thick shim stocks (Roeko; Hanel, Langenau, Germany), occlusal contacts were tested in maximum intercuspation without interferences in lateral excursions, and adjusted as necessary. All prostheses were polished and lustered before final insertion by using a pearl surface paste (Noritake Kizai Co. Ldt, Nagoya, Japan). According to a previously standardized protocol,49 50,51
Success Outcomes Criteria
In case of implants, the possibility to restore the placed artificial root in absence of pain, suppuration, swelling, mobility, discomfort, ongoing pathologic process, signs of peri-implantitis, neuropathies, or persistent paresthesia were considered outcomes of success.52
For prosthesis success, outcomes were defined as prostheses in function, stable, without pain, and with good esthetic results at the patient consideration level. Specifically,
complications were classified as problems related to prosthodontic repair necessity like reparable fractures, abutment-prosthesis screws and access holes restoration loosening, retightening, and all the grades (grades 1–3) of the scale of porcelain chipping fractures for zirconia prostheses53; and
failures were classified as problems that necessitated prosthodontic replacement like nonreparable fractures (zirconia frameworks) and implant loosening.
Survival rate was defined as surviving FDPs minus altered FDPs based on complications.
Statistical Analysis
Chi-square test or Fisher exact test when necessary, and unpaired t test were used to compare, respectively, categorical and continuous data between lost and in-study patients.
In survival analysis, prostheses were censored if they had not experienced the end-point of interest at the end of the follow-up. Kaplan-Meier estimates of free complications survival time and free failure complications survival time were calculated at 12, 36, and 60 months for all prostheses. Survival at 36 and 60 months were calculated for subgroups. To take into account multiple data for the same patient (some patients have prostheses both in maxillary and mandibular arch), Cox regression model with frailty variance (patients are the cluster variable) was calculated for each independent variable (gender, material, position and number of implants, age, smoker, drinker, and occluding teeth). Hazard ratios (HRs) and respective 95% confidence intervals (CIs) were calculated. Model assumption of proportional hazards was assessed through graphical inspection and statistical tests. Multivariable models were fitted to evaluate adjusted effect of the variables that were strictly addressed as above described.
A subanalysis was made separately for zirconia and resin prostheses. STATA 13.1 was used for all analysis. A P value <0.05 was considered statistically significant.
Results
The analysis was performed on 113 patients (53 men and 60 women; mean age, 65 years; age range, 42–90 years) treated with 214 immediately loaded full-arch prostheses (105 maxillary and 109 mandibular) each supported by 4 to 6 implants. A total of 1058 implants were initially placed in 105 maxillae (on average, 5.2 per arch) and 109 in mandibles (on average, 4.7 per arch).
Follow-up period ranged between 2 and 60 months. Overall, a total of 64 implants in 25 patients failed during the reported follow-up interval (6.04%). Nine (15% of total failures ) implant failures occurred within 2 months and implants were replaced for prosthodontic reasons. Additionally, 37 implants (57%) failed within 12 months of surgical placement, 9 (15%) failed between 12 and 24 months, 4 (7%) between 24 and 36 months, and 6 (4%) after 36 months (range 37–60 months). The observations of prosthesis mobility and patient reported discomfort during habitual oral functions (mastication, swallowing, and speaking) were the causes of implants removal. Neither of the delayed (>2 months) implant failures compromised prostheses survival. Failures were found in 22 maxillary arches and 3 mandibular arches. Eighteen out of 22 patients who had a failure in the maxillary arch had experienced teeth extraction and implant positioning in the same surgical intervention.
One hundred one patients were treated with both maxillary and mandibular prostheses. Sixteen patients (14%) were drinkers and 18 (16%) were smokers. About opposing dentitions, 25 patients had implant-supported zirconia prostheses, 79 patients resin prostheses, and a combination of both or of prostheses and natural teeth were observed in 9 patients.
Complications
Thirty-seven patients had a complication in 1 arch at least. Considering all interested arches, 39 complications were found and consequently, the prosthesis annual complication rate was 6.6%, free complications survival was 90.5%, 81.6%, and 75.5%, respectively, at 12, 36, and 60 months (Fig. 1 ).
Fig. 1: Kaplan-Meier survival probability estimates for complications in the total group of prostheses. The x axis shows follow-up time, the y axis the probability of being complication-free along time. The number of prostheses at risk is also reported for each year of follow-up.
Neither age (HR 0.98; 95% CI 0.9–1.0; P = 0.479) nor number of implants (HR 1.1; 95% CI 0.9–1.3; P = 0.309) influenced the complication risk. Men had a higher risk of complications in 5 years compared with women, whereas prosthesis materials did not influence the risk. Considering that used material and gender were related, with a higher proportion of zirconia prostheses in women (29% vs 14%, P = 0.009), a multiple Cox regression was performed. Adjusted HR confirmed a higher risk for men (HR 4.6; 95% CI 2.5–9.5; P < 0.001), and no significant differences for material (HR 1.5; 95% CI 0.7–3.3; P = 0.267), as shown in Table 3 . To evaluate possible differences, a sub-analysis was performed separately for zirconia and resin prostheses. For both resin (166 prostheses, 96 patients) and zirconia (48 prostheses, 32 patients), men had a higher risk compared with women, but statistical significance was reached only in the resin group (Table 4 ).
Table 3: Complications Survival Rate and Hazard Ratio
Table 4: Complications HR in Resin and Zirconia Prostheses
Failures Twenty-four patients had a failure in 1 arch at least. Considering all interested arches, 26 failures were found; prosthesis annual failure rate was 4.6%, free survival was 90.9%, 86.8%, and 85.5%, respectively, at 12, 36, and 60 months (Fig. 2 ).
Fig. 2: Kaplan-Meier survival probability estimates for failures in the total group of prostheses. The x axis shows follow-up time, the y axis the probability of being failure-free along time. The number of prostheses at risk is also reported for each year of follow-up.
Neither age (HR 0.98; 95% CI 0.9–1.0; P = 0.346) nor number of implants (HR 1.0; 95% CI 0.8–1.3 P = 0.965) influenced the risk of failure. Men had a higher risk of failures within 5 years compared with women, while prosthesis material did not influence the risk. Maxillary arch prostheses had a higher risk than mandibular arch ones (Table 5 ). After adjusting for each other, men (HR 4.8; 95% CI 1.9–12.5; P = 0.001) and maxillary prostheses (HR 4.8; 95% CI 1.8–13.0; P = 0.002) still had higher risks of failure.
Table 5: Failures Survival Rate and Hazard Ratio
Prosthesis material was not related with arch (prevalence of zirconia prostheses was 25% in maxillary and 20% in mandibular arches, P = 0.422). Analyzing zirconia and resin prostheses separately, men and maxillary arches had a higher risk compared with women and mandibular arches but statistical significance was reached only in resin prostheses group (Table 6 ).
Table 6: Failures HR in Resin and Zirconia Prostheses
Discussion
Teeth substitution is the final goal of dental implant treatment. Based on this clinical necessity, the first aim of this study was to evaluate the short-term follow-up outcomes for immediate implant-supported fixed-dental rehabilitations on edentulous or potentially edentulous patients using 2 different types of restorative materials available for mouth rehabilitation.
Indeed, literature does not report sufficient data to support practitioners and patients in addressing the final choice about the most appropriate oral restoration material protocol. Additionally, the available treatment options are still not provided with sufficient quantitative information to screen the opportunities and those that best fit the needs of a specific patient. Research on implants during the last years were most focused on implants themselves than on prosthetic outcomes.
In the current investigation, a total of 214 arches were treated with screw-retained immediate and final restorations supported by axial dental implants solely or by a combination of axial and tilted implants, getting an overall implant survival rate of 94%. This result is in line with similar reports on immediate rehabilitations,43,54 2 55
According to recent reviews,2 23 56 26 failures and complications. Results are supporting the idea that the ability of the stomatognathic apparatus to develop force during functional and parafunctional habits has to be considered as one of the success keys for oral rehabilitation. Unfortunately, no actual force data in a specific patient could be obtained during the diagnostic phase; so, the clinical choice is left to other considerations and clinical experience.
Recent systematic reviews have shown that biological and technical complications are frequently encountered with complete-arch implant-supported FDPs.26,57 54
For the majority of patients, this concerned fracture of the resin teeth for different hypothesized reasons.26 26
Our results confirm that none of the observed implant-related and prosthesis-related problems could be linked to the opposing dentition.26 failures of final full-arch implant rehabilitations made in resin versus those made in zirconia were not detected during the observation period.
In a private practice set, zirconia implant-supported fixed-dental rehabilitations seem to perform like resin ones at least in terms of technical failures . Both materials seem to be an adequate answer to a daily clinical request of cost-effective treatments, decreased treatment times with a lower patient morbidity, and a higher patient quality of life. Indeed, long-term results are important to avoid dissatisfied patients at control visit, aiming to reduce drop-out patients. Complications are important in relation to the total of repair, time, and cost for both clinicians and patients. The current percentages of complications and failures are in accordance with other investigations26 57
Among the limitations of this study, there is lack of specific assessments of patient satisfaction, as recently underlined.2
Randomized controlled trials that incorporate well-defined clinical and radiographic outcome criteria are necessary to evaluate the long-term success of this treatment approach.
Conclusion
In conclusion, immediately loaded, fixed full-arch rehabilitations using a combination of tilted and axial or with axial implants alone supported with screwed resin- or zirconia-based prostheses were clinically successful in a short-term follow-up (5 years). At 60 months, free complications survival was 75.5%, whereas free failures survival was 85.5%. Prosthesis material did not influence complication risk, showing that implant-supported fixed dental rehabilitations made by fully veneered zirconia frameworks seem to perform similar to resin ones. Both materials can be used in clinical practice at least when implant-supported and immediately loaded full-arch rehabilitations are needed.
Disclosure
All authors have no conflict of interest relevant to the content of the submission, they have read and approved the final version submitted.
Approval
The Institutional Review Board (IRB02–2005 Doc. MQ 03 AL 01) approved the procedures.
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