The Impact of Combining Engaging and Non-Engaging Abutments in Multi-Unit Implant Prosthesis

Dental implant-supported Fixed Prosthesis (FDP) provides reliable long-term restorations for replacing missing teeth in patients with partial or complete edentulism, significantly enhancing oral function, aesthetics and improving overall quality of life [1]. Their long-term clinical success, however, depends not only on osseointegration of the implant fixtures but also on the biomechanical harmony of the prosthetic superstructure [2]. Among the critical determinants of this success, achieving a passive fit remains one of the most discussed and challenging objectives [3]. Passive fit refers to the intimate and strain-free adaptation of the prosthesis to the underlying implants [4]. This precise adaptation is essential in preventing mechanical stress on implant components and the surrounding bone, thereby reducing the risk of biological complications, such as peri-implant bone loss and mechanical complications, including screw loosening, component fracture, or framework deformation [5,6] Although minor misfits up to 150μ are considered clinically acceptable, minimizing misalignment remains crucial for ensuring prosthesis longevity [7,8] The challenge of achieving passive fit increases significantly in multi-unit restorations, especially when implants are not parallel due to anatomical or surgical limitations. In such situations, the Implant-Abutment Interface (IAI) - the junction between the implant fixture and the restorative abutment-plays a crucial role [9,10]. The IAI contributes to the even distribution of occlusal forces, the maintenance of screw preload, the reduction of micromotion and microgaps and the long-term mechanical stability of the prosthesis [9-11]. Over the years, various abutment designs have been introduced to optimize these functions. Among them, the two main types of abutments, concerning their engagement with the implant's anti-rotational features, are Engaging (E) and Non-Engaging (NE). Engaging abutments, characterized by their anti-rotational features, are primarily indicated for single crowns, where rotational stability is crucial. Conversely, Non-Engaging (NE) abutments are often preferred for multi-unit, screw-retained FDPs as they can accommodate minor implant misalignments, facilitating a passive fit [9,10]. While NE abutments simplify the prosthetic procedure in such cases, they transfer the entire stress of resisting rotational and functional forces to the abutment screws, potentially increasing screw stress and the incidence of mechanical complications, such as loosening or fracture [12]. To address the need for both passive fit and anti-rotational stability, the concept of Hemi-Engaging (HE) abutment configurations was proposed. This design incorporates at least one engaging abutment within a multi-unit FDP, while the remaining abutments are non-engaging; the rationale, as described by Schoenbaum and colleagues, is that the engaging abutment provides rotational resistance and stabilizes the prosthesis, while the non-engaging abutments permit the prosthesis to seat passively [13]. This hybrid approach seeks to combine the mechanical advantages of both systems, offering a practical compromise for complex clinical situations [13-15]. Some in vitro investigations and computational analyses have offered some support for the potential benefits of HE configurations. Certain in vitro studies suggest advantages, such as increased fracture resistance in cantilevered FDPs when the engaging component is strategically placed [15]. Finite element analyses have also indicated potentially favorable stress distribution patterns with HE configurations compared to fully non-engaging designs [14]. However, other investigations report no significant differences in outcomes like screw preload maintenance or microgap formation when comparing HE to fully NE systems [16,17]. Given these diverse findings and the increasing clinical interest in optimizing multi-unit implant restorations, this review aims to provide a comprehensive assessment of the available evidence on the mechanical impact of hemi-engaging abutment configurations in multi-unit, screw-retained, implant-supported prostheses compared to fully non-engaging designs, focusing on parameters such as stress distribution, screw stability, micromotion and overall stability.

A comprehensive literature search was conducted to identify studies evaluating the mechanical performance of engaging, non-engaging and hemi-engaging abutments in multi-unit screw-retained implant-supported prostheses. Studies were included if they assessed multi-unit implant prostheses using any of these abutment types, reported at least one biomechanical outcome and were published in English within the last 15 years. Eligible study designs included in vitro experiments, Finite Element Analyses (FEA) and clinical trials. Exclusion criteria were applied to maintain focus on multi-unit prostheses and relevant abutment configurations. Specifically, studies were excluded if they assessed single-unit restorations, cement-retained prostheses, or non-abutment-related interventions. Additionally, case reports, narrative reviews, expert opinions and studies not reporting relevant biomechanical outcomes were excluded. The search was performed across PubMed, Google Scholar and Scopus in June 2025, using keywords engaging, non-engaging, prosthesis type, abutment design and Mechanical parameters such as stress distribution, screw loosening or fracture, micromotion, misfit and prosthesis stability. Additionally, to ensure thorough coverage, reference lists of all included articles were manually screened to identify any additional relevant studies that may not have been retrieved through electronic searching.

Figure 1 provides an overview of the study selection process, including screening, eligibility assessment and final inclusion.

Figure 1: Flowchart Showing the Study Selection from Database Searches

A total of 12 studies fulfilled the inclusion criteria, consisting of nine in vitro investigations and three Finite Element Analysis (FEA) studies. Collectively, these studies explored the mechanical performance of screw-retained multi-unit implant-supported prostheses using different abutment configurations-Engaging (E), Non-Engaging (NE) and Hemi-Engaging (HE) designs. To facilitate interpretation, the findings were divided into three primary categories. The first category, stress and strain distribution (4 studies), examined how different abutment configurations influence the transmission of occlusal loads to the implant-abutment complex and surrounding bone. These studies, predominantly FEA-based, provided insights into localized stress concentrations and potential risk areas for mechanical failure.

The second category, interface integrity (5 studies), focused on the quality of fit between the prosthetic framework and the supporting implants. This included assessments of passive fit, misfit tolerance and microgap formation at the implant-abutment junction.

The third category, screw stability (3 studies), addressed the mechanical reliability of the screw joint, a critical factor for the long-term success of screw-retained prostheses. These studies assessed parameters such as preload maintenance, torque loss after cyclic loading and incidence of screw loosening or fracture. The findings highlighted the relationship between abutment design and the ability of the screw to resist functional stresses over time. The key findings of these studies are summarized in Table 1.