REGISTRO DOI: 10.5281/zenodo.8342858
Dra. Andresa Rodrigues Lopes1
SUMMARY
Oral rehabilitation on dental implants depends on a series of factors related to the osseointegration process and the connection of the prosthetic implant abutment to the crown. The osseo integrable implant platform (cervical region of the implant) is considered critical and receives the placement of the prosthetic component, influencing the transmission of occlusal forces to the bone. Lack of adaptation between the prosthetic component and the implant platform can lead to treatment failure, primarily due to the induction of tensions and infiltration of microorganisms. The success of this type of implant prosthesis is directly linked to the health of the surrounding tissues, as well as the precision and adaptation of the prosthetic components. The geometry of these components can play a decisive role in this success, as they may have the ability to better distribute forces, thus reducing the load on the adjacent bone. Along with these factors, an unbalanced occlusion, meaning extreme occlusal force, especially during chewing, can lead to high levels of stress on the bone and abutment screw, potentially causing complications in the prosthetic/implant system, such as bone resorption, screw loosening, implant fractures, and/or prosthetic component fractures (abutment screw). The occurrence of screw loosening connecting the prostheses to the implants is common, with single implants, especially in the posterior region, having more complicated biomechanics due to higher occlusal forces in these regions, which can lead to elevated levels of stress at the implant-prosthesis interface. The most frequently used implants have external hexagonal (EH), internal hexagonal (IH), and internal conical or Morse taper (CM) connections. Although external hex implants have been the most marketed for years and offer a wide variety of prosthetic components, facilitating choice, they have limitations regarding the height of the hexagon to ensure the final aesthetics of the implant prosthesis. Thus, it seems essential to establish a comparison between internal hex, external hex, and Morse taper implants.
Keywords: Osseo integrated implants, external hex, internal hex, Prostheses, prosthetic connection
1.0 LITERATURE REVIEW
Tooth loss is a problem that is leading to aesthetic, functional, and often psychological consequences in the global population. Significant advances in Dentistry have shown results in new techniques and materials for the development of prostheses that are increasingly similar to natural teeth, both aesthetically and biomechanically. Initially, only fully edentulous patients were rehabilitated with implant-supported prostheses through the Bränemark system. However, a significant number of longitudinal studies have been described in the literature, demonstrating a high success rate in this type of rehabilitation, exceeding 90% (Motta, 2002). This success led dentists to also create single-tooth prostheses supported by Osseointegrated Implants.
Despite the high success rate mentioned above, longitudinal studies have also shown a high incidence of mechanical complications. For example, in 2000, ECKERT et al. reported that the failure of screws in single-tooth prostheses occurred with an incidence of approximately 7.1%. Clinical factors are reported to be possible causes of screw loosening, resulting in the loss of preload, which translates into the tension generated in the screw during initial tightening. Therefore, controlling excessive occlusal loads on implants and achieving harmony in the biomechanics of the prosthesis-implant-bone system is essential (Binon, 2000-b). Other associated causes may include geometric design and the precision of adaptation in the union of components. From a biomechanical standpoint, the main difference between implant systems is the shape of the hexagon. There are different types of prosthetic connections, but the two major categories are external and internal connections, which can be further subdivided into various types. However, the majority of studies focus on external hexagonal connections (Ferreira et al., 2007). The small size of this type of hexagon makes the stability of the connection dependent on the retention screw, explaining the high rates of loosening in longitudinal studies. In internal connections, it is possible to create a deeper connection with greater contact between the abutment walls and the internal walls of the implant, reducing the possibility of micromovements during loading and not overloading the retention screw (Maeda, 2006). In vitro studies have demonstrated the superiority of internal connections in terms of stability (Khaisat et al., 2002). External hexagon connections were initially used for fully edentulous patients. In partial and single-tooth prostheses, this interface and its screw are more exposed to various types of loads, with lever arms and lateral forces being the most damaging. In these cases, the frictional retention power of the internal hexagon, which is approximately four times greater, prevents screw loosening. To address some of these inherent problems, solutions such as torque wrenches, screw surface technology, platform size, and materials have been investigated to achieve a fixed preload and increase retention forces (Binon, 2000-
a). Mc Glumply et al. (1998) mentioned that optimal adaptation tolerance, minimal rotational freedom, improved physical properties, and appropriate torque application are crucial for good biomechanics of the assembly. However, once disadapted, the component also leads to a decrease in preload and consequently higher stresses on the screw and the bone around the implant, leading to fractures and microfractures and even bone loss (Misch, 2000). The choice of components, retention level of prostheses to abutments, and prosthesis design are also fundamental in the analysis of the biomechanics of these implant-supported prostheses and, consequently, in their prognosis. Schawarz (2000) commented that the nature of such complications can be directly linked to the loss of preload. This preload is the only force that resists functional/occlusal forces to prevent the abutment from loosening from the implant. However, these forces seem to play an important role in loosening screws in hexagonal implants. If this preload is exceeded by occlusal force, the screw tends to loosen. Gratton et al. (2001) state that preload is influenced by the initial torque established, component adaptation, geometric design of the prosthetic connection, cyclic fatigue, and chewing load on the system. Martin et al. (2001) stated that this preload is related to the type of material used in the production of components and screws, showing that the physical properties of each material lead to a greater bonding power between the involved parts. Guichet et al. (2002) commented that splinting, despite not having adequate passivity in most cases, helps dissipate forces around the implant.
Goodacre et al. (2003), through a systematic literature review, described the most common causes of clinical complications related to implants and implantsupported prostheses. Ten studies reported the incidence of fistulas at the implant-abutment connection level. The average incidence from combined data was 1%. The loss of abutment screws was detected in 6% of prostheses, with 45% of these losses occurring in single crowns. The average loss of abutment screws in single implants using old screw designs was 25%; however, when analyzing more recent studies, this average drops to 8%, indicating a substantial improvement with the creation of new screw designs. Sixteen studies showed fractures of abutment screws with an average incidence of 2%. The average incidence of implant fractures was 1%. The biomechanics of implant-supported prostheses are a complex system. Their high success rates are documented by numerous clinical research studies, but this type of prosthesis is not free from mechanical complications. Among the most frequently cited complications are loosening or fracture of intermediate screws, especially in single-tooth prostheses. Keating (2004) provided an engineering perspective on methods of connecting prosthetic intermediates to implants. In his work, he emphasized the direction of chewing forces and their effect on the implant assembly, showing that chewing forces can be vertical, inclined, lateral, and torsional. Rosen et al. (2004) showed that another important clinical aspect to consider regarding biomechanics is the type of prostheses to be fabricated: cemented or screw-retained, and single or splinted. Biomechanically, screw-retained prostheses have disadvantages because, besides not allowing any alteration in the abutment in the laboratory, under the risk of increased transmission of tensions and consequently loss of preload, these prostheses have a perforation in the occlusal region of the crown to receive the fixation screw, thus hindering the achievement of occlusal contacts that direct forces in the axial direction along the long axis of the implant. Kim et al. (2005) described that to minimize prosthetic complications on implants in daily clinical practice, fundamental biomechanical criteria and conditions were established. These include the reduction or absence of cantilevers, reduction of occlusal tables, slight cuspal inclinations, centralization of occlusal contacts, the use of splints in patients with functional habits, in other words, protective occlusion for the implant (OPI), which is essential for the long-term success of these rehabilitations.
2.0 COMPARISON BETWEEN PLATFORMS
The characteristics of the platform can affect the success of the implant. In an implant with an internal hexagon (HI), the anti-rotational device of the prosthetic abutment is designed within the body of the implant. The anti-rotational device is typically deeper within the HI body compared to external hexagon (HE) implants. However, since the anti-rotational device is wider than the prosthetic abutment screw, the wider diameter of the body in the platform region is reduced (BERNARDES et al., 2006). Consequently, the threads on the external part of the implant cannot be designed in the region or above the anti-rotational device of the implants. Moreover, larger smooth surfaces and shear forces are observed above the first thread of the implant compared to HE implants. Threads can progress more coronally with HE implants because the diameter of the prosthetic abutment screw is narrower, and the outer wall of the body is thicker. For this reason, threads can reach the most coronal region of the implant (MISCH, 2008). Regarding aesthetics, there is no difference when an internal or external hexagon system is used, and this condition is impossible to verify unless the prosthesis is removed (STEVÃO, 2005). Partial and single-tooth prostheses with HE connections have an interface and their screw more exposed to various types of loads, with lever arms and lateral forces being the most detrimental. In these cases, the frictional retention power of the internal hexagon is approximately four times greater, preventing this problem from occurring, thus avoiding screw loosening (BINON, 2000). Internal hexagon and Morse taper systems have been compared. For this study, a simulated metal-ceramic crown was created on intermediates of each system, and a force of 100N was applied to the vestibular cusp. The resulting stress was measured in the prosthesis, intermediate, implant, and adjacent bone. The internal hexagon system caused greater stress in the alveolar bone and the prosthesis but less in the prosthetic intermediate. On the other hand, the Morse taper system resulted in higher stress in the intermediate but lower stress in the alveolar bone and the prosthesis. The authors suggested that the Morse taper system could lead to less bone resorption than the internal hexagon, believing that the shape of its prosthetic intermediate dissipates forces generated in the prosthesis more evenly (QUARESMA, 2008). Meanwhile, when comparing the external connection system with the Morse taper system using cyclic loading simulation of 380 N in the long axis of the implant at 15° and 30°, in all situations, the Morse taper connection proved to be more effective in distributing forces to the implants, while the external connection concentrated much more forces on the threads of the prosthetic screw (HIMMLOVA et al., 2003). According to the authors, this could be indicative of the high number of screw failures in external connection systems (PIMENTEL, 2009). Several studies have demonstrated and confirmed the biomechanical superiority of internal connections (TAVAREZ, 2003), (MAEDA et al., 2007). In turn, clinical evaluation also suggests a lower rate of mechanical complications in the internal hexagon system, with a 16% loosening rate, while 84% occurred in the external hexagon system (GONÇALVES et al., 2010).
3.0 FINAL CONSIDERATIONS
Implants are the most favorable option in terms of oral rehabilitation. However, the distribution of stresses in these implants is not yet clearly defined in the literature. Regarding connection systems, it can be said that Morse taper systems are more favorable in terms of biomechanics and microbiology. However, the external hexagon (HE) is more versatile and can be used in any situation, provided it is properly planned, and the use of a narrower abutment seems to transfer the area of occlusal forces absorption to the center of the implant. Despite the significant contradictions found in the literature, it seems clear that all implant systems are susceptible to bacterial infiltration. Linking bone loss to implant types cannot yet be conclusive. Nevertheless, it is suggestive that Morse taper-type implants cause less bone loss. The explanation for these results is not yet fully understood. However, it is believed that a combination of favorable factors for this system, both mechanical and biological, may be involved.
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1Dr. Andressa Rodrigues Lopes is a renowned specialist in dental implants and orofacial harmonization, graduated in 2004 from the FEDERAL UNIVERSITY OF ESPÍRITO SANTO – UFES (ES). She honed her skills through a Specialization in Dental Implants and Oral Rehabilitation at the São Leopoldo Mandic College (SP) in 2007.