# Finite Element Analysis (FEA) of Palatal Coverage on Implant Retained Maximly Overdentures
## 4 Discussion  
Since the stress and strain distribution at the bone level is very difficult to evaluate in a clinical setting, FEA is used in dentistry to evaluate and quantify these stresses conveyed from the prosthesis to the implants and to the supporting tissues [44; 45]. There are a few studies in the literature that have attempted to evaluate the stress and strain distribution at the abutment level using in vivo strain gauge analysis [46; 47; 48; 49]. However, these methods still do not provide a complete understanding of the overdenture stress distribution, nor do they provide sufficient basis to draw conclusions about the need for implant splitting [17; 32; 35]. Ideally, the loads in implant supported prostheses should be evenly distributed across all components and supporting structures. Therefore, excessive force concentration in a particular area of the superstructure or prosthesis should be avoided. Bone resorption may occur when excessive stress loads are applied to the supporting bone, and if left unattended implant failure could follow [1; 7; 19]. Also, when extra stresses are applied to the implant fixtures and or the superstructure components, prosthetic complications--such as screw loosening or screw fracture or fracture of the abutments, etc.--could occur [36; 42; 48; 50; 51; 52]. The photoelastic method [15; 40] has also been used in vitro to analyze how stresses are conducted and absorbed from the prosthesis to the implants and supporting tissues. When comparing the quality of the bone in the maxilla with the mandibular bone, cancellous bone is not as dense in the maxilla and cortical bone is thinner than in the mandible [53]. Consequently, the design of maxillary implant overdentures should include palatal coverage to help with the prosthesis support and to alleviate stress concentration on the supporting implants as suggested in the literature [54]. Hence, palatal coverage could help maintaining the bone around the supporting implants. [PERSON] et al. [15] and [PERSON] et al. [29] used photoelastic analysis to study the role of palatal coverage in maxillary overdentures. They both reported that covering the palate seemed beneficial to the load concentration and distribution over the supporting implants and bone. The findings from this study suggest that more stress was seen in the cortical bone compared to the cancellous bone in all models as expected. However, this study found that palatal coverage may not be necessary when applying axial forces in contrast with some of the above studies (Figure 10). Figure 9: Effect of bar on four-implant overdenture models with 45-degree loading. Occlusal forces in patients with implant overdentures could range between 48-258 N with an average of 128 N [40; 41]. Additional studies in implant overdentures analyzing oblique and a horizontal load in combination with axial loads have found axial loads to range from 100-300 N and also established that the lateral forces could be a quarter (25 N) that of the axial load [43; 44]. In this study, a linearly varying load of 187.5 N at the position of the distal implant and 137.5 N at the model center was applied on the three and four-implant models [55]. The applied varying forces were simulated, with bucco-lingual loading angles ranging from 0 to 45 degrees. The three-dimensional finite element analysis in this study indicated that the higher stresses after implant loading were dependent on the direction of the force and the presence of a splinting superstructure. The finite element models showed that the cortical bone absorbed most of the loading forces and, depending on the palatal extension of the prosthesis, varied in location from the alveolar crest to the palate. Furthermore, the finite element models exposed to 45-degree forces exhibited stress concentration greater than those exposed to axial forces. In this study, the implant splinting with the connecting bar showed a decrease in stress except at the anterior implant site (Figure 11).