## 4 Discussion  
This study was conducted to analyze the geometric characteristics of the implant-abutment connection, their effects on the long-term stability of the connection, and the marginal bone loss of the conical interface joint. In the first instance, the major limitation of the present review is represented by the risk of bias of the articles included which, in most cases, described a non-randomized study model design. This aspect could affect and significantly indicate the strength of a study's effectiveness. Long-term peri-implant stability under loading represents a critical factor when evaluating implants under functional conditions. This aspect is determinant, especially considering the epidemiological consistency of peri-implant-related disease, which represents the most frequent non-disease-free survival condition, with a prevalence of \(>\)50% for mucositis and peri-implantitis [151]. The microleakage of the implant-abutment interface occurs in all implant systems, with variability between systems [152]. Cone more joints with an \(8^{\circ}\) internal angle in implant dentistry were first proposed by the ITI group [93] in order to provide a more stable mechanical coupling of the implant-abutment interface [153]. Today, other manufacturers use cone morse designs with different cone angles. However, comparative evaluations of the clinical performance of implants with different conical angles are rare in the literature. The NMA approach could represent an optional approach able to overcome these limits offering the possibility to evaluate the marginal bone loss obtained from different studies. Considering the wide range of variability of the study designs and methodologies, the present analysis considered only one experimental time 6 months after the loading in order to avoid the risk of indirectness bias. The main advantage of the increased mechanical stability of the implant/abutment coupling is the reduction in the micro-gap and microleakage at the interface [63]. Morse taper connections have proven to be more stable from a biomechanical point of view [154; 155]. The main advantage of the conical interface with or without a geometrical index is determined by the cone-in-cone principle, where the joint stability is consistently increased by the abutment's lateral contact with the internal chamber walls. Different types of implant connections were evaluated in this review, considering a cone angle cut off \(>\)8\({}^{\circ}\) to be a concentric joint and a cone morse with a joint angle \(<\)8\({}^{\circ}\) to have an external and internal hexagon. The purpose of this review was to investigate marginal bone loss and the mechanical complications related to dynamic function [93]. The main findings were that conical connections seem to provide a better reduction in  
Figure 7: Forest plot summarizing the direct comparative performance between the groups. mechanical complications and a lower incidence of marginal bone loss compared to internal and external implant-abutment designs. On the other hand, no significant difference in marginal bone loss has been reported when comparing both of the conical implant joints. The higher mechanical stability of conical joint profile seems to support the hypothesis of its influence on the maintenance of peri-implant health. Several studies reported that the formation of micro-gaps could be correlated with the micromovements generated during masticatory loading, with the forces producing possible biological and mechanical sequelae [152]. Biologically, the bacteria infiltration could represent a critical factor, and the cone morse design seems to reduce the risk of interface penetration at the level of the joint interface [154; 155]. The biological response associated with the two types of connections was also evaluated, and it turns out that the biological response is the same, although differences may occur when evaluating the mechanical part.