Because overloading of implant overdentures (OVDs) can lead to bone loss at the implant-bone interface,1,2 it is important to evaluate the influence of several variables on load distribution in bone and around the implants. As it has been proposed that bone-anchored prostheses could be sustained in the oral environment for a lifetime,3 it is important to know the effect of different factors on bone resorption. It was emphasized that even the proposed rate of bone loss of <0.2 mm/y may be too liberal for young implant patients who could lose up to 8 mm of bone over the ensuing 40 years,4 so every millimeter would seem to impart a key role in the long-term prognosis of an implant. Compared with conventional removable dentures, implant overdentures (IODs) help obtaining better retention, thus a more comfortable function.5
Depending on the clinician’s preference, several types of OVD attachments can be used. There are controversial opinions about design and indications for different attachment systems for IODs.6–8 Several clinical studies reported that retention mechanisms have comparable effects on marginal bone reactions.9–11 In a recent clinical study, no statistically significant differences between single standing (retentive anchor) and splinted (bar) attachment types had been found, although the bone loss rate observed in those with retentive anchor attachments tended to be lower.12 On the other hand, it was reported that less pronounced bone loss due to distribution of the loads by splinting of the implants can be expected because splinted implants act together as a group.13
The number of implants necessary for the support of mandibular OVDs is rather not well documented. The retention, stability, and support of IODs are provided by both the mucosa and implants. It is expected that an increasing number of implants will provide a better support.14 However, more implants may not necessarily translate to improved prosthesis retention or stability. A randomized study was conducted to evaluate 2- versus 4-implant bar attachments, where the authors had found a higher rate of complication for the 4-implant design, but in terms of treatment efficiency, both methodologies were the same after 5 years of service.15 Some authors have indicated that successful IOD treatment outcome can be achieved regardless of the number of implants used.16,17 Two implants have been considered the minimum necessary for mandibular IOD treatment16,18 and can be used either with independent nonsplinted attachments or splinted together using a cast metal bar and a bar-clip attachment.19 The difficulty for the clinician lies in the selection of the correct number of supporting implants and the attachment type suitable for optimal stress distribution. No consensus has yet been reached.14,20 The question is whether the number of supporting implants influences the stress arising during mastication. The use of fewer implants would be less expensive, but would it adversely influence the long-term success?
This in vitro study was launched to get insight into the influence of the number of implants and type of attachment on stress distribution in mandibular IODs.
Materials and Methods
The mandible of a formalin-fixed human cadaver was involved in this in vitro strain gauge (SG) measurement study. After removal of the mandible from the cadaver, the remaining teeth were extracted and the alveolar ridge was flattened.
The obtained mandible was scanned by dental volumetric tomography (Newtom, Elmsford, NY) for planning of the implant positions and fabrication of surgical guides. Five 4.1-mm-wide, 12-mm-long Standard Plus Straumann Dental Implants (Institute Straumann, Basel, Switzerland) were placed symmetrically in the following positions using the surgical stent: midline first premolar positions bilaterally and lateral incisor positions bilaterally. The implant positions were determined in a manner that allowed adequate interimplant distances for conform bar fabrication for the 2-, 3-, and 4-implant situations. During implant placement, special attention was given to succeed cervicoocclusal leveling of the implant shoulders.
The soft tissue was simulated with a silicone elastomer of 1.5 mm thickness, which was fabricated with a previously described method21 and placed on the mandible.
An implant-level impression was made of the transmucosal parts of the implants using the open impression technique with individual acrylic tray and polyether impression material (Impregum; 3M Espe, St. Paul, MN). Upon removal of the impression, 5 implant analogues (Straumann) were placed and poured with type IV dental stone (GC Fujirock EP, type 4 dental stone; GC Dental, Tokyo, Japan) to obtain a working cast. Two-, 3-, and 4-implant retained bars were fabricated on “synOcta” abutments and burnout plastic copings (Straumann). After adjustment of the bar castings, a mandibular full denture was fabricated and processed according to the principles and guidelines established for processing removable dentures.22 Then, 6 mandibular dentures were duplicated from the original with a previously described method23 to have 6 identical dentures. Each denture was connected to either one of the bars or the retentive anchors (Straumann) chair side with self-cure acrylic resin (Meliodent; Heraeus Kulzer, Dormagen, Germany), and 6 different IODs were obtained: 2-implant, 3-implant (Fig. 1), or 4-implant–supported bar IODs and IODs retained by 2, 3, or 4 retentive anchors.
Locations on the exterior surface of the mandible where the SG rosettes (350W, C2A-XX-062WW-350; Vishay Micro-Measurements, Raleigh, NC) be applied were initially cleaned roughly and then using pure alcohol. The surface smoothing was achieved by 2-stage application of sandpaper. The SGs were oriented, and bonding material (M-Bond; Vishay Micro-Measurements) was applied to the bone and the gauges were finger pressure glued to the bone. All connections have then been soldered. After completing the assembly process, all single elements of the rosettes have been circuit balanced and shunt calibrated electronically in situ. Each SG was named with an abbreviation (Fig. 2): R1V (right first premolar vestibular), R2V (right second incisor vestibular), SV (symphysis midline vestibular), L1V (left second incisor vestibular), L2V (left first premolar vestibular), R1L (right first premolar lingual), R2L (right second incisor lingual), SL (symphysis midline lingual), L1L (left second incisor lingual), L2L (left first premolar lingual), RRK (right retromolar pad), LRK (left retromolar pad), RYC (right buccal pouch), LYC (left buccal pouch), RMK (right mylohyoid ridge), and LMK (left mylohyoid ridge).
The mandible was mounted via an acrylic junction to the modified semiadjustable articulator (Major AVM 100M Prodotti Dentari, SPA, Torino, Italy) in such a manner that the intercondylar distance was adapted to the test mandible (Fig. 3). The maxillary denture was mounted on the opposing part of the articulator after it was brought to maximal intercuspidation with one of the lower dentures. Later, all lower dentures were adapted too. The mandible was supported by 2 Teflon cylinders contacting at both angulus mandibulae corners (Fig. 4) mimicking the insertion of the masseter muscles. Then, the maxillary denture was removed together with the upper articulator part.
The mandible that had been provided with SGs was placed under the testing machine (MTS Bionix 858-II; MTS). For strain data acquisition, a multichannel, multipurpose, universal data acquisition system (PROSIG, Inc, Model 8048, 128 Channel Configuration) was used. The strains were collected in synchro with the axial force values that were the output from the MTS Controller Unit. A 40-kg load cell (ESIT, Istanbul, Turkey) was used for achieving optimum force measurement accuracy during the tests. Quasi static tests have been conducted consequently 6 times per implant and attachment configuration at each of the simulated “biting” situations: centric occlusion, left or right chew, left and right chew, and anterior chew (6 different loading conditions). All tests have been conducted under force-controlled scenarios; thus, effects of deformations of the mandible were fully considered. The proportional-integral-differential tuning of the MTS Controller with respect to the whole system has been done before the tests were started. The loading and unloading rate was chosen to be 7 N/s, constant. Each test has been recorded separately, with measurement bias being removed at the beginning of each cycle. A preload of 5 N was applied to the system as the reference state to eliminate possible minor misalignment errors due to assembly and production tolerances. Each member of the strain rosette has been formed a quarter bridge; thus, a total of 48 channels of data have been collected. Thermal drift has not been observed during any test course. All static tests have been conducted up to 105 N (thus, 100 N net difference is observed), and strain values have been continuously recorded. The xy-resolved strain components, principal strain components, xy-resolved stress components, principal stress components, and the Von Mises stress have been calculated for each gauge in each test and every cycle.
Statistical analyses were used in this study to analyze the measured strains from 16 SGs from 3 different loading situations (Figs. 5 and 6) of 6 different mandibular OVDs. For the statistical analysis of the results, the NCSS 2007 and PASS 2008 Statistical Software (Kaysville, UT) was used. The comparison of quantitative data was accomplished with the Mann-Whitney U test for the comparison of groups without normal distribution. For the comparison of 3 groups, the Kruskal-Wallis test was used. The results were assessed at 95% confidence interval, at a significance level of 0.05.
The calculated Von Mises values from measured strains in all measurement sites and loading conditions for nonsplinted attachments (retentive anchor) were higher than splinted (bar) attachments. The rise in the implant number resulted in a decrease of Von Mises values at all sites.
Results for “Bilateral Loading”
Although the highest stress values were obtained in the 2-implant retentive anchor situation at the R2V site (2.82 MPa), the lowest strains were observed at the L2L site (1.28 MPa) in the 4-implant bar situation (Fig. 7, Table 1). The highest stress value except the peri-implant sites has been observed in the 2-implant retentive anchor OVD at the RRK site (3.88 MPa), whereas the lowest stress was at the LMK site (2.58 MPa) in the 3-implant bar situation. Considering the SG measurement sites, the comparison of nonsplinted (retentive anchor) and splinted (bar) attachments revealed higher Von Mises values.
Results for “Unilateral Loading on the Right Side”
Although the highest stress values were obtained in the 2-implant retentive anchor situation at the R2V site (3.60 MPa), the lowest stress values were observed at the L1L site (0.98 MPa) in the 4-implant bar situation (Fig. 8, Table 2). The highest stress value except the peri-implant sites has been seen in the 2-implant retentive anchor OVD at the RRK site (4.10 MPa), whereas the lowest stress value was at the LMK site (2.15 MPa) in the 4-implant retentive anchor situation. Considering the SG measurement sites, the comparison of nonsplinted (retentive anchor) and splinted (bar) attachments revealed higher Von Mises values.
Results for “Anterior Loading”
Although the highest stress values were obtained in the 2-implant retentive anchor situation at the R2V site (4.16 MPa), the lowest stress values were observed at the R1L site (2.07 MPa) in the 4-implant bar situation (Table 3). The highest stress value except the peri-implant sites has been seen in the 2-implant retentive anchor OVD at the RYC site (4.45 MPa), whereas the lowest stress value was at the LMK site (3.17 MPa) in the 4-implant bar situation. Considering the SG measurement sites, the comparison of nonsplinted (retentive anchor) and splinted (bar) attachments revealed higher Von Mises values.
The aim of this study was to give clinicians a reorientation, if the results showed equivalent measured strain and/or stress distribution; the clinical treatment modalities for mandibular IODs could be modified to define more economic and still safe solutions for patients. If the null hypothesis, which claimed that the 2 single implant treatment option would provide similar treatment outcomes, was supported, it would imply that it might be preferred to more costly and complex treatment involving additional implants and bar splinting.
Because the knowledge about functional loads on implants is essential to achieve long-term implant success, correct qualification and quantification of forces on implants is crucial to understand their biomechanical characterization.6,7,24,25
Strain gauge analysis is a commonly used method in dentistry for biomechanical evaluation of stress distribution in vivo26,27 as well as in vitro.28–30 The validation of biomechanical strain measurements and calculated stress values is important to gain reliable results. The information obtained from SGs under differing experimental conditions should be regarded rather skeptically before clinical interpretation and prediction, because these measurements are limited to the area where the gauge is bonded. The levels of force as measured at different sites in this study were comparable to those measured in vivo in recent studies with SG abutments on implants in humans.31,32
During the patients' function with an IOD, loads are transmitted to the alveolar bone surrounding the implants, as well as to the abutments and residual ridges; thus, implant-supported OVDs depend on support from both the implants and oral mucosa.25 It is important not to cause unfavorable loads on the implant abutments that house the attachments, as these loads can be detrimental to the osseointegrated implants. The mucosal thickness of the alveolar ridge of the mandible was observed to be 2.0 mm on average,33 which was used in the present in vitro study too.
The question to be answered is whether implants need to be splinted together to better withstand the loads associated with supporting an OVD or whether freestanding implants alone can withstand the loads. The influence of the attachment type on stress distribution was investigated in numerous studies,34–39 and controversies could not be eliminated. Although an experimental study concluded that the spherical abutments transferred less bone stress than the bar-clip type attachments when the model was subjected to posterior vertical loading,40 a longitudinal clinical study showed a higher rate of loss of implants with retainers of the spherical type (38.8%) compared with standard bar-clip retainers (20.6%).41 In a very recently published article reporting of the results of a randomized, prospective, clinical study, it was emphasized that bar attachments may warrant better retention but patients had been more satisfied with retentive anchor attachments and preferred the nonsplinted attachments.42 Wismeijer et al43 studied 110 mandibular IOD patients, who received 2 implants with a retentive anchor attachment, 2 implants with an interconnecting bar, or 4 implants splinted with 3 bars. Treatment outcome had been measured using patient questionnaires. Nearly all subjects were satisfied with treatment after 16 months, and no statistical difference was found among the 3 treatment strategies. The authors concluded that the 2-implant retentive anchor attachment treatment was a good choice, but the need for additional clinical trials was emphasized. The main advantage of the bar comes to light in cases of severe ridge resorption because the bar then provides an additional plane of stability,7 but on the other hand, single attachments generally require less space in the denture base and are easier to clean.42 It should not be forgotten that a pointed jaw, which leaves insufficient tongue space for a bar construction, indicates single attachments. Nonetheless, the 2 single implants used for IOD retention offer a simpler and less expensive treatment. In a very recent systematic review and meta-analysis of the literature where 4200 implants had been identified, there was no difference in marginal bone loss around implants retaining/supporting mandibular IODs relative to attachment designs.44 Because there are opposite opinions about maintenance requirements and patient satisfaction, and additionally from a stress distribution standpoint, there was no significant difference between the tested bar and retentive anchor attachments in the present study, it seems that the choice of attachment can be left to the clinician’s experience and predilection. The slightly lower stress values for the bar attachment could be important in cases with diminished implant size and/or low bone quality.
To date, there is no consensus on the influence of the implant number on stress distribution and scientific information is scarce. Usually, 2 implants are considered sufficient to support an IOD,45 and there is no evidence that such IODs fail more often. In a recent review article, it was reported that neither patient satisfaction nor function of the prostheses were influenced by the implant number but the least complication rate was achieved by the use of 2 implants and a bar.46 Using 3-dimensional finite element analysis, Meijer et al47 studied bone stress in the anterior mandible-surrounding implants when either 2 or 4 implants were used for IOD treatment. The authors concluded that when the occlusal load was distributed over increasing numbers of implants, there was no reduction in the principal bone stresses surrounding the individual implants. In the present study, however, there were statistically significant differences in Von Mises values especially between 2 and 4 implant modalities. The use of 4 implants for mandibular IOD support resulted in lower stress levels. The best force distribution was achieved by the bilateral loading of the OVDs; thus, it can be recommended to patients to chew on both sides simultaneously.
Neglecting retention that is one of the most important tasks of an implant-supported mandibular IOD and considering the stress distribution, there is a significant difference in strains occurring in the mandible and around the implants with the use of different numbers of implants and splinted or nonsplinted attachments. The evolving strains in the posterior regions of the mandible, such as the retromolar pad, buccal pouch, and mylohyoid ridge, are shown to be lower with rise in implant number and use of splinted type attachment for OVDs as well. This finding was in agreement with another experimental study, which had reported that the implant strain and bearing area loading values seemed to be conversely related.7 Rigid connectors had the lowest denture-bearing area loading, whereas nonsplinted attachment situation had shown relatively low implant strain and high bearing area load. It should be kept in mind that the results of the present study focused only on forces arising during occlusal loading, and the implants had been placed exactly symmetrical and parallel. As shown in a recent study, the wear of the matrices of single nonsplinted attachments will be more pronounced in case of divergence of the implants,48 resulting in a shorter service time, more often maintenance requirement, and less retention. The ideal solution in case of divergent implants is the splinted type of attachment.49
The present study was limited to quantification of the strains on cortical bone in the implant collar region and different locations of the mandible with rosette SGs and under static load, as it is not possible to quantify bone-implant interface strains with this technique. In this manner, the results of the present study should be considered carefully. Although the in vivo situation was mimicked as ideally as possible, it should kept in mind that the results were derived from in vitro experiments and should be tested in clinical follow-up studies.
It may be concluded from the occurring stresses standpoint that the use of 2 implants does not necessarily create a handicap and can be a good alternative. In this manner, relying on the results of the present in vitro study and only considering the stress distribution in the mandible, it could be assumed that for the retention of mandibular IODs 2 implants are sufficient. By the use of 2 implants, costs can be dramatically reduced and made affordable for more patients. Because the attachment type, splinted or uncoupled, did not play a significant role on strains, the choice of the attachment type may be left to the preferences of the clinicians.
Within the limitations of this in vitro study, it may be concluded that in cases with low quality and quantity of bone, the increase in number of implants and the use of a splinted attachment can be preferred to reduce forces emerging around the implants during function. The use of 2 single attachments in cases with good bone quality and ideal sized implants seems to be a safe and sufficient solution for the treatment of mandibular edentulism with OVDs.
It is important to point out that these are in vitro results; they are qualitative in nature and their clinical significance should be tested in clinical follow-up studies.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
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