Ensuring the preservation of bone quantity is crucial for determining the long-term functionality of dental implants [1]. Unlike natural teeth, which are supported by the periodontal ligament, osseointegrated dental implants exhibit lower resilience and shock absorption capacity [2, 3]. Under loading, the stress experienced by the dental implant is directly transmitted to the surrounding bone, leading to remodeling of peri-implant bone [4]. Consequently, unfavorable stress distribution of implant components can result in mechanical and biological issues during rehabilitation of totally or partially edentulous patientss [5].
For completely edentulous jaws, the 'All-on four' technique has gained widespread adoption among clinicians due to its lower demand for anatomical conditions and intraoral abutment preparation. The multi-unit abutment (MUA) is a specialized screw-retained component specifically designed for use in 'All-on four' scenarios [6, 7]. The popularity of MUA in full arch implant restoration can be attributed to its ability to completely eliminate the need for cement and elevate the restoration's edge in cases of height disparities, thus facilitating regular monitoring procedures [8, 9]. Additionally, the angulations of the MUA can assist individuals with compromised intra-oral topography, such as those with teeth misalignments or uneven jaw bone, by offering 3–4 angle correction options ranging from 0° (straight) to 45° [10].
Ideally, implants should be parallel to each other and adjacent teeth [11]. However, achieving parallelism can be challenging due to inevitable deficiencies in bone anatomy. Placing implants with an angled trajectory could offer more flexibility and avoid the need for additional therapy such as GBR [12]. Regarding the influence of straight and angled abutments on the bone, opinions vary among published papers. Clelland and Gilat assessed the effect of increasing abutment angulation (0°, 15°, 25°, 30°, and 35°) on stress transfer to bone [13]. They found that compressive forces were positively correlated with abutment angulation, while Saba et al. noted that deformation of the angled abutment mainly occurred in cancellous bone, which could stimulate mineralization [14].
In addition, few studies have provided a detailed analysis of stress distribution on the components of MUA, particularly the abutment screw and prosthesis screw, which are among the most fragile parts of the restoration [15]. Since there is no compensation provided by the cement space, MUA is challenged by a higher possibility of mechanical failure, especially in cases with significant divergent axes [16]. Compared to the abutment and implant components, Armentia et al. [17] observed that the prosthesis screw was more vulnerable to mechanical problems, and Pjetursson et al. [18] reported a 10.8% abutment screw loosening rate after a 5-year follow-up. For MUA, there are essentially two pieces of screw components (an abutment screw and a relatively smaller prosthesis screw) to tighten the transmucosal abutment and dental restoration. Notably, researchers have pointed out that the prosthesis screw of MUA is quite small, which further increases the risk of loosening or breakage [5].
Implant-supported prostheses are continually exposed to multi-axial loading, including vertical, horizontal, and oblique occlusal directions. Richter et al. compared horizontal and vertical forces applied to the axis of dental implants to measure bending moments [19]. They found that horizontal force could generate higher stress on the bone-implant interface, particularly on the neck of the implant. Another study conducted by Cozzolino et al. also found that horizontal loads caused greater deformations of cortical bone than vertical biting forces [20]. However, there is limited information available on the impact of horizontal force on MUA.
In this study, our objective was to investigate the differences in stress patterns among MUA with different angulations (0°, 17°, and 30°) using finite element analysis. Additionally, we compared the stress distribution of the implant and each component of the MUA-implant complex (including the implant, abutment screw, and prosthesis screw) under a 200 N static force along the prosthesis axis, with varying horizontal forces (0 N, 30 N, and 100 N).