Results indicate that the overdenture with locater attachments show minimal stress concentration compared with IRPD for the two implants models; however, in the locater attachments, fringes were observed only around the loaded side implants because of non-split attachment-type of implant design. For IRPD, bridge design with precious attachments demonstrate minimum stress compared with the crown design and clasp attachments-type designs for both the two and four implants models.
In many studies, stress patterns in the photoelastic model have been reported to be very similar to those of natural structures.15 This is because the stress distribution can be observed directly in the model and there is a similarity among the shape of stress, the area where it is formed, and natural structures. In photoelastic stress analysis models, cortical and cancellous bone are composed of a single resin. Therefore, even if the amount of stress changes, the areas where stress occurs will not; therefore, this method is useful for comparing stresses occurring in different prosthetic designs.16 Maximum biting force in individuals with natural teeth differ individually in different areas of the jaw. The largest biting force has been previously measured in premolar region at 415 N for females and 475 N for males. Average values of bite forces in premolar region for females 345 N and 401 N for males. 17 Mericske-Stern and Zarb evaluated occlusal forces in a group of partial edentulous patients treated with an implant-supported fixed partial prosthesis and determined the maximum occlusal force to be 200 N for the first premolars and 300 N for the second premolars.18 In our study, a 300 N 18, 19 vertical force was applied to the right and left first premolars. The reason for the selection of this loading site being that food is chewed between premolars and molars in anterior loading applications.14 In a previous study, as a result of the application of laterotrusive load, a low level of stress was observed on the non-working side of the implant in the bar-ERA attachment, and the opposite arch interaction was determined to be low by providing balanced side contact.20 Furthermore, the highest stresses were formed in the posterior implants on the loading side.20,21 In our study, similar to the results of previous studies, the highest stress value in all models occurred in the posterior implant on the loading side. In addition, similar to the results of Ochiai et al.,12 the lowest stress level was observed in locator attachments where the implants were not splinted. In the study of Celik and Uludag, the fact that there were higher stress values in the locator attachments, in contrast to the present findings, might be due to the different retention forces of the retentive rubbers used.22 In our study, a blue-colored retentive rubber with a retention force of 1.5 lbs = 66.7 N was used. This rubber was selected because the incoming forces are transmitted between the retentive rubber and the locator abutment, and a lower load is transferred to the implants. In this case, resilient design provides appropriate stress distribution characteristics. 22
In addition, the fact that a lower stress concentration was observed around the implant in the model with locator attachment may be due to the location of the implant further away from the loading site in the locator attachment. The increased mucosa support may also be due to the fact that a portion of the force is transmitted to the alveolar crest on the loading point. 23,24 While very low stress was observed in the implant on the non-loading side in the locator attachment, a moderate level was observed in the middle implants in the bar attachment and a low stress was observed in the opposing implants. This can be explained by the fact that the implants are connected to the bars and the stress is transferred to the other implants via the bar and the locator attachment, a resilience attachment preventing implant overloading25. However, the lack of stress distribution to the contralateral implant in the model with a locator attachment may be risky in terms of both bone loss monitoring in the implants on the loading side and, in particular, the deformation of locator attachment rubbers due to single side loading. Also in the present study, more stress fringes was observed in the overdenture prosthesis with bar attachment IO than the locator attachment. However, stress concentrations occurring not only on the loading side implants but also on the opposite side implants (1.22-1.08 N) indicated that the load was distributed equally to all implants by splinting. Fringes formed in this type of prosthesis with reduced tissue support were less sparse than those in the edentulous areas.
Hu et al evaluated the stress transfer of different overdenture attachment systems loaded directly over the implants, second premolar, and second molar regions. The authors stated that stress transfer to the edentulous crest region occurred and the stresses in the implants decreased as they approached the posterior area. At the same time, they observed that the difference between the stress transfer of overdenture attachment systems was also reduced as the load applied shifted to the posterior 24. In our study, the low and moderate levels of stress values observed instead of high stress values, and the stress lines occurring in the region of the edentulous crest, might be due to the application of force from the premolar region. However, the stress lines observed in parts of the edentulous crest were greater in the two implant-supported models than in the four implant-supported models. This finding is comparable to those of a 10-year clinical follow-up by De Jong et al. in which the residual crest resorption of mandibular overdentures with two and four implants was compared.26 De Jong et al. concluded that a significantly greater atrophy was observed in the cases with two implants than in the cases with four implants. This result was observed to be independent of peri-implant bone loss, and this atrophy was believed to be formed due to the movement of the posterior prosthetic base towards the tissue under occlusal forces.26,27
There are many case reports in which researchers have used removable partial dentures combined with implant-supported fixed prostheses.6,8–11,27-29 Clinicians usually experience difficulty in providing effective and affordable implant restorations for patients with resorbed edentulous jaws. The resorbed residual alveolar crest and other anatomical limitations can prevent the placement of implants in the desired positions, especially in the posterior regions of the mandible.30,31 Implant placement in posterior areas of the jaws with anatomical limitations may be difficult or may require advanced surgical techniques. Despite these anatomical limitations, implant placement in the anterior regions of both jaws is generally reliable.30-32 The effective use of implants with a minimal number of removable dentures provides a more economical and comfortable treatment option for edentulous patients.31-33 Some of the total edentulous patients prefer implant-supported fixed restorations in the anterior region and removable partial dentures without requiring advanced surgical methods in the posterior regions while treatment options are being discussed.9
The present study was evaluated the different IRPD designs, also. This study is the first report evaluating the different type of IRPD designs. Accordin to our results, The stress concentration of crown-support and clasp attachment prosthesis was higher in the apical regions of the implants than in the cervical regions of the implants. However, when compared with other precious attachment models, the dentures with two implants with crown-support and clasp attachment generated higher stresses in the neck region, which can lead to marginal bone loss. It is thought that the use of precision attachments in IRPD prevents bone resorption in the implant and its surrounding.32-33
Bilhan et al evaluated the stress transfer of two-interforaminal implant-retained bridge and a removable partial denture with strain gauge method.25 They reported that, according to retentive anchors or bar attachments, the lowest strains occurred in the new design IRPD. This supports the results of our study. In the models with both two and four implants, the precision attachment of removable partial dentures generated less stress concentration then clasp attachment designs. Also splitting the anterior implants decreased the stress observation around the implants for this new treatment protocol. Crown design IRPD with clasp attachment was observed the highest stress values around the implants.
As mentioned in the case reports in the literature, the use of IRPD in total edentulous jaws enables more comfortable patient movement in terms of chewing, esthetics, and speech.6,10,30 When an overdenture is compared with full dentures, the pressure forces in the residual crest are also less common in IO prostheses.33 At the same time, chewing movements become more balanced and regular in patients using IRPD.31-33 Bilhan et al also mentioned that the new design implant supported anterior bridge retained removable dentures with precission attachments eliminates two problems of conventional systems ease of hygiene with regard to bar attachment prosthesis and increased stability in severe resorbe mandible according to single attachments.25
As shown in a clinical study comparing removable partial dentures and conventional total prostheses; while the retention and comfort satisfaction of patients using removable partial dentures for mandibular prostheses was higher, a greater dissatisfaction in total prosthesis patients with mandibular prostheses was recorded.34 This suggests that the use of IRPD in total edentulous mandibular jaws is more appropriate. Collapse is observed in all physiological functions with old age; therefore, implant overdentures in elderly individuals should be designed to show satisfactory retention, be easily placed and removed, easy to clean, and cover a minimal surface area. In addition, it is important for younger total edentulous patients to feel comfortable in social settings with increased retention and to meet their esthetic expectations.27 IRPD and implant-supported fixed prosthetic restorations in the anterior region can have esthetic and psychologically positive effects so increase the self-confidence for young patients. In addition, prosthesis parts requiring less space for older patients can offer less complex retentive structures, and prosthetic components that will cause less plaque accumulation and are easy to maintain. Besides,in a recent study, wear of the matrices of unsplintted attachments are more remarkable especially the implants inserted not parallelly 35, that cause more adjustment and maintenance in a short span of time. It was mentioned that splinting the angulated implants is an suitable solution and IRPD may also be a good alternative in this situtaions.
The present study is in vitro and presents limitations.
- in vivo studies and studies with other methodologies, such as finite element and strain gauge analysis, are necessary to complement the results found.
-stress transmission evaluated with static occlusal loads that limits the generalizability of our results. In fact, natural chewing is a dynamic movement and involves oblique forces. It would be beneficial for future studies to evaluate the stress transfer under different load directions.
-In this study, implant supported removable prosthetic designs were compared but for edentulous mandibles implant supported fixed prosthetic design such as hybrid prosthesis are commonly used in clinically. Therefore, for future studies both fixed and removable types can be investigated to extend the comparision.