Among the systematically revised literature, the data seems unequivocal when four or six splinted implant-supported prostheses are analyzed referring to patient satisfaction, in which high scores are reported by either of the groups. The data analysis of the included studies indicated that patients appear to be equally satisfied with MIODs supported by four or six splinted implants. In addition, most of the included studies (11/15) reported a horseshoe design of overdentures in both groups [19,20,21,22,23,24,25, 27,28,29, 32]. Patients usually require an overdenture without palatal coverage in order to increase comfort, taste, phonation, pharyngeal control, salivary flow, and hygiene. Another issue of discussion was the number of dental implants recommended to be installed to support a maxillary overdenture [33]. In the literature, it seems that the minimum favorable number to support a MIOD without palate coverage is four or six splinted implants [4,5,6, 33,34,35,36]. This concept is in line with several included studies in this review, reporting a survival rate of implants greater than or equal to 97%, both for palateless MIOD on 4 splinted implants [19,20,21,22,23, 25, 27,28,29] and palateless MIOD on 6 splinted implants [20, 21, 25, 26, 28, 29, 32]. Slot et al. highlighted that the implant-supported split bar anchorage system has a stronger influence on patient satisfaction than conventional dentures, supporting the splinted design over four or six implants [20, 21, 28, 29]. The reason lies in the concept that the splinted design offers more retention and stability and allows to realize a palateless MIOD, ensuring better predictability of treatment in terms of implant and overdenture survivals and patient satisfaction.
Several systematic reviews have proposed that implants supporting maxillary overdentures should be splinted in order to provide better force distribution on the prostheses, more retention, and stability when subjected to both vertical and oblique forces, and to avoid potential overloading of single implants [4, 5, 9, 31, 37]. In addition, implant-supported overdentures have been able to provide edentulous patients a stable centric occlusion and improved chewing capabilities [26, 34], irrespective of the number of implants placed and the opposing natural or artificial dentition [23, 24]. However, the question arises as to whether the number of splinted implants, 4 or 6, or their location is more important [20, 21, 28]. According to the data analysis of this systematic review, the analyzed studies investigating 4 splinted implants, employing both the anterior region [19, 21,22,23, 27, 28] and the posterior region, including the sinus [29], reported survival rates of implants higher than 97% and 96%, respectively. Similarly, the analyzed studies investigating 6 splinted implants employing both the anterior region [20, 21, 26] and the posterior region, including the sinus [19, 22, 26], reported survival rates of implants higher than or equal to 97% and 99%, respectively. Thus, no statistical difference was detected in the survival rate of implants between two analyzed groups irrespective of the implant installation zone. However, when sufficient bone in the anterior region is available, extensive bone augmentation procedures such as maxillary sinus floor elevation surgery could be prevented, meaning less treatment time, less morbidity, and few treatment costs [5, 13]. In addition, oral hygiene is easier to perform in the anterior region than in the posterior region, and the repaired bone defect after the often more extended augmentation procedures in the posterior region is less stable than in the anterior region [5, 13]. Therefore, whether the placement of 4 implants is chosen, the tendency is to place implants in the regions between the canine and second premolar, avoiding pneumatized maxillary sinuses and poor bone quality zones [5, 13].
As far as survival of the overdenture is concerned, data analysis of the included studies showed no significant differences between using 4 splinted implants or 6 splinted implants for supporting maxilla overdentures. However, apart from the survival of the prosthesis, which is always high and not sufficiently linked to the number of implants and the type of anchorage, it is important to analyze the success of the prosthesis influenced by the mechanical complications related to implant components (loosening or fracture of abutment or screw), and technical complications including issues related to anchorage structure (clip loosening or fracture, or bar fracture or lost) or prostheses (repairs of fractured prostheses or overdenture teeth) [38,39,40,41,42]. Indeed, in the work by Kiener et al., the increased tightening of the inner abutment screws was the most recurrent mechanical complication in bar supported maxillary overdentures [43]. Differences in maintenance reported between milled gold alloy bars and solid titanium bars could be attributed to the physical properties of the materials used [44]. Katsoulis et al. showed fractures of bars or extensions occurred more often with gold bars than titanium bars [28]. This is supported by the findings of Widbom et al. [45], who recommended a harder and more resistant metal alloy for superstructure construction than the gold alloy. Moreover, the design of the prosthesis should provide the optimal passive fit and stress distribution, especially in type III and type IV bone. Thus, from a mechanical point of view, the absence of abutments and the direct screw fixation of the bars at the implant-level could appear to be advantageous [23, 28, 30]. However, most of the included studies investigated bar design with abutment-level, reporting survivals of implants and overdentures greater than 96% for both 4 splinted implants [10, 19,20,21,22,23, 25,26,27,28,29,30,31] and 6 splinted implants [10, 19,20,21,22,23, 28, 32]. Nevertheless, the performance of more randomized clinical trials with low RoB, analyzing prosthodontic complications and comparing MIODs on 4 or 6 splinted implants connected at abutment-level or implant-level, is encouraged.
The clinical evidence found in this systematic review allows us to suggest that the choice between 4 or 6 splinted implants supporting a maxillary overdenture does not seem to be directly related to the clinical parameters detected. In light of these considerations, there was an indicative advantage in the use of 4 implants instead of 6 implants in order to reduce treatment costs, morbidity, and augmentation procedures. However, poor bone quality and quantity, reduced implant length and diameter, and consequently, low primary stability could lead to implant loss in the maxillae. In this context, if an implant is lost, the use of a 6 implant approach could avoid a new surgical intervention and would just need an adaptation of the overdenture. Contrarily, when an implant is lost in the 4 implants approach, a new implant and prosthesis suprastructure are often needed before the overdenture can be adjusted [47] Apart from that, treatment decision-making also deals with the choice of providing an implant-supported overdenture or a full-fixed prosthesis. In this context, 4 implants have also demonstrated to be sufficient for the long-term success of implant-supported full fixed prostheses [48], and achieving high levels of patients’ satisfaction [49]. Nevertheless, fully edentulous patients often present substantial bone and soft tissue deficiencies, which lead to prognathism, deficient facial support, speech disruption, and general esthetic problems that compromise the ubication of smile line and the length of the upper lip; thus, preventing the use of a fixed implant-supported prosthesis [50].
Therefore, the question: “Whether 6 splinted implants supporting a MIOD compared to 4 splinted implants may produce better treatment outcomes?” Still requires further investigation [13, 14, 19, 20]. This study was limited by the lack of prospective randomized clinical trials with a low RoB comparing maxillary overdentures supported by 4 or 6 splinted implants and considering the possibility to overcome previously reported blinding difficulties. In particular, there were five RCTs [10, 19,20,21,22] comparing 4 and 6 splinted group in this systematic review. However, of these five studies [10, 19,20,21,22], four derived from the same authors, and it appears that these studies represent only two studies with data published at 1 and 5 years each [19,20,21,22]. Thus, only three RCTs could be included in the quantitative analysis. Moreover, substantial heterogeneity between the studies and lack of data prevented the performance of quantitative assessment of patients’ satisfaction. In addition, this study is limited to only two treatment options from the universe of therapeutic modalities that comprehends implant-supported maxillary prostheses for fully edentulous patients, such as the use of 8- or more implants, zygomatic implants, and adjunct tissue augmentation procedures.