The results of the present study provide in-vitro evidence of the significant impact of peri-implant defects on the mechanical stability of dental implants. Statistically significant differences were observed between the control groups and the groups with simulated bone defects, demonstrating that even modest peri-implant bone loss can compromise the mechanical anchorage of implants. Specifically, the reduction in both IT and RT values in the defect groups underscores the detrimental influence of bone loss on implant retention. These findings are consistent with previous studies indicating that primary stability, commonly assessed by IT, is a key determinant for the predictability of osseointegration and long-term implant success [27, 28].

IT is widely recognized as an important indicator of primary implant stability, reflecting the mechanical engagement between the implant surface and the surrounding bone at the time of placement [29, 30]. Higher IT values generally indicate greater resistance of the surrounding bone to implant rotation and are therefore associated with improved mechanical anchorage [31]. In the present study, implants with larger diameters consistently exhibited higher IT and RT values across all simulated bone densities. These findings are consistent with previous reports demonstrating that implant diameter and bone density are major determinants of primary stability, as larger implants provide increased bone–implant contact area and more favorable load distribution [32, 33].

The positive relationship between implant diameter and mechanical stability has been well documented in the literature [34, 35]. The biomechanical advantage of wider implants is primarily attributed to their greater surface area in contact with bone, which enhances load distribution and reduced micromotion at the bone–implant interface, thereby promoting a more favorable environment for osseointegration. From a clinical perspective, these findings provide biomechanical insight into how implant diameter and peri-implant bone support may influence primary stability. While direct clinical extrapolation should be made with caution due to the in vitro nature of the model, the results suggest that implant dimensions and bone conditions are likely to play an important role in mechanical anchorage at the time of placement. Therefore, these observations may help inform clinical decision-making, particularly in scenarios involving compromised bone support.

A key finding of the present study was the significant reduction in IT and RT values in the presence of circumferential peri-implant defects. This reduction was consistent across all bone densities and implant diameters, indicating that loss of peri-implant bone support directly compromises mechanical anchorage. As bone–implant contact decreases, resistance to mechanical forces is reduced, increasing the likelihood of micromotion and jeopardizing implant stability. These observations are consistent with clinical and experimental evidence showing that progressive peri-implant bone loss is associated with impaired stability and an increased risk of implant failure [36,37,38].

Supporting these results, Pérez-Pevida et al. [34] reported similar outcomes using an in vitro model with a standardized 2-mm circumferential peri-implant defect. Their study revealed an average reduction of 9 points in implant stability quotient (ISQ) values without significant changes in IT. This observation suggests that secondary stability, reflected by ISQ values, may be more sensitive to early bone loss than primary stability parameters such as IT. Importantly, resonance frequency analysis (RFA) enables the early detection of such changes [31], offering clinicians a valuable tool for monitoring osseointegration and identifying at-risk implants before clinical failure occurs [34, 35].

The analysis of peri-implant defect size further revealed a direct, proportional relationship between the extent of bone loss and the reduction in mechanical stability parameters. Larger bone defects were consistently associated with lower IT and RT values, indicating a higher susceptibility to early implant failure when bone support is extensively compromised. These observations align with clinical reports demonstrating that progressive peri-implant bone loss not only jeopardizes the initial retention of the implant but also accelerates the risk of late complications, including peri-implantitis progression and eventual implant loss [7]. Such findings emphasize the need for timely diagnosis and rigorous preventive protocols to halt bone destruction and preserve the longevity of implant-supported rehabilitations. The circumferential defect configuration adopted in this study most closely represents advanced peri-implantitis lesions characterized by circumferential vertical bone loss, while also partially simulating revision implant placement scenarios in sites with reduced bone support.

In addition to anatomical factors, surgical protocol also plays a relevant role in implant stability. Rosas-Díaz et al. [35] demonstrated that under-preparation of the implant site, using drills with diameters smaller than the implant, can significantly modulate IT values. In their study, under-preparation levels of 0.2 mm, 0.5 mm, and 0.8 mm resulted in progressively higher torque values, with the 0.8 mm protocol producing the highest levels of primary stability. Interestingly, ISQ values remained unchanged across groups, suggesting that IT and ISQ capture different aspects of implant stability. While torque values primarily reflect resistance to rotational forces during placement, ISQ values are more closely related to the lateral stiffness of the bone–implant interface. This distinction is consistent with the present findings, where torque measurements were more sensitive to variations in bone conditions, reinforcing the need for complementary assessment methods.

The present results are consistent with previous biomechanical studies investigating factors influencing primary implant stability [25, 27, 28, 39, 40]. Several experimental studies have demonstrated that implant diameter and bone density are major determinants of IT and mechanical anchorage. For example, Di Stefano et al. [27] reported that higher-density polyurethane substrates significantly increase IT values, reflecting improved mechanical engagement between implant threads and the surrounding material. Similarly, Comuzzi et al. [25] observed that implant macrogeometry and diameter directly influence primary stability parameters, with wider implants providing greater resistance to rotational forces during placement. Previous biomechanical investigations have demonstrated that implant macrogeometry, surface treatment, and substrate density are key determinants of primary implant stability. In an in vitro study evaluating Conexão® implants inserted into polyurethane blocks of varying densities (15, 20, and 40 PCF), higher-density substrates significantly increased IT and pullout force values, confirming the strong influence of bone density on mechanical anchorage [41]. Cylindrical implants with surface treatment showed superior mechanical performance, particularly when placed in the highest-density substrate (40 PCF), while differences among implant designs were less pronounced in lower-density materials. These findings reinforce the concept that both implant design characteristics and surrounding bone quality critically affect primary stability, with denser substrates and treated implant surfaces promoting stronger mechanical engagement at the implant–bone interface [42, 43]. These observations align with the present results, where implants with a 4.3-mm diameter consistently exhibited higher IT and RT values compared with 3.5-mm implants across all simulated bone densities.

The detrimental effect of peri-implant bone defects observed in this study is well supported by previous experimental evidence. Reduction in peri-implant bone support decreases the bone–implant contact area and consequently reduces the mechanical interlocking between implant threads and surrounding bone. In vitro studies simulating peri-implant defects have consistently demonstrated reductions in implant stability parameters under these conditions [34]. Moreover, reduced bone support has been associated with increased micromotion at the bone–implant interface, which may impair osseointegration and compromise long-term stability [33, 40, 43, 44]. These findings corroborate the present results, which showed a marked decrease in IT and RT values following the introduction of circumferential defects, particularly in low-density substrates.

The results of the present study should be interpreted in the context of existing clinical evidence. Previous clinical studies have reported that insertion torque values below 20 Ncm and ISQ values below 60 are associated with an increased risk of early implant failure [45]. Similarly, insertion torque values above approximately 30–35 Ncm have been suggested as favorable thresholds for immediate or early loading protocols. It is important to emphasize that these thresholds are derived from clinical literature and are not outcomes directly established by the present in vitro experimental model. In this study, these reference values are used to provide clinical context for the observed biomechanical differences in implant stability across experimental conditions [45, 46]. These findings further highlight the biomechanical relevance of achieving adequate primary stability at the time of implant placement. However, given the limitations inherent to in vitro experimental models, these observations should be interpreted as supportive evidence instead of direct clinical guidance. They may contribute to a better understanding of factors influencing implant stability and assist clinicians in considering potential risks associated with compromised bone conditions.

The findings of this study also emphasize the importance of individualized treatment planning. Implant diameter selection should be carefully tailored to the patient’s bone quality and the presence or risk of peri-implant defects. Larger-diameter implants may offer mechanical advantages in compromised sites, providing greater initial stability and potentially reducing the risk of early implant loss [27]. At the same time, preventive strategies and early interventions for peri-implantitis, such as regular clinical monitoring, nonsurgical therapy, and, when indicated, regenerative surgical techniques, are essential to preserve peri-implant bone and maintain long-term implant function [2].

It is important to recognize that peri-implantitis is a multifactorial condition influenced not only by bone morphology but also by soft tissue health, host immune responses, and the composition of the oral microbiota [47,48,49]. In clinical conditions, microbial biofilms and inflammatory processes drive progressive peri-implant bone loss, while peri-implant soft tissue integrity plays a key role in protecting the underlying bone from bacterial invasion [50, 51]. These biological factors cannot be reproduced in artificial experimental models such as polyurethane blocks. However, the primary objective of the present study was to investigate the biomechanical consequences of peri-implant bone defects on implant stability under controlled conditions and not to reproduce the pathogenesis of peri-implantitis. By isolating the mechanical component of bone loss, this experimental model allows the specific effect of reduced bone support on implant stability to be evaluated without the confounding influence of biological variability. Nevertheless, future studies are necessary to further explore the interaction between biomechanical and biological factors in peri-implant disease progression.

Polyurethane blocks are widely used in implant biomechanics research because they provide homogeneous and reproducible mechanical properties that allow controlled comparison between experimental conditions [25, 27, 28]. Unlike human cadaveric bone, which presents considerable variability in trabecular architecture, cortical thickness, and mineral density, polyurethane materials offer standardized densities that can simulate different bone qualities encountered in clinical practice. In particular, polyurethane blocks with varying pounds per cubic foot (PCF) densities have been shown to approximate the mechanical behavior of different bone types described in implant dentistry [52]. For example, lower-density blocks can simulate trabecular bone conditions typically observed in posterior maxillary regions, whereas higher-density blocks resemble denser mandibular bone. Although these materials cannot replicate the biological properties of living bone, they provide a reliable platform for evaluating the mechanical aspects of implant stability under controlled conditions, which is particularly relevant for studies investigating IT and implant anchorage [25, 27, 28, 52].

However, despite the advantages of using polyurethane blocks for evaluating implant biomechanics, some limitations inherent to in vitro experimental designs must be taken into account when interpreting the findings of this study [25, 27]. Synthetic bone substitutes cannot fully replicate the complex anatomical and biological characteristics of human bone. In clinical conditions, implant stability is influenced by multiple factors including trabecular architecture, cortical thickness variability, bone remodeling dynamics, and the viscoelastic behavior of living tissues [44]. Additionally, the present model does not account for biological lubrication provided by blood and extracellular fluids during implant placement, which may influence frictional forces and IT measurements. Thermal conductivity is also different between polyurethane materials and natural bone, meaning that the thermal effects generated during drilling may not accurately reproduce those occurring in vivo [53]. Furthermore, the mechanical loading environment in the oral cavity involves dynamic and cyclic forces that cannot be completely simulated in static laboratory conditions [44]. Additionally, peri-implant diseases are influenced by microbial biofilms and soft tissue conditions that were not reproduced in this experimental setup. Therefore, while the present findings provide valuable biomechanical insights under controlled conditions, caution should be implemented when extrapolating these results directly to clinical scenarios. Future investigations incorporating animal studies, or clinical trials are warranted to further validate the biomechanical implications observed in this study.

The present findings may also have important clinical implications for implant treatment planning and management of peri-implant defects. The marked reduction in IT and RT observed in the presence of circumferential peri-implant defects suggests that loss of surrounding bone support can significantly compromise the mechanical stability of implants [1, 4, 33]. Clinically, reduced implant stability may increase the risk of micromotion at the bone–implant interface and potentially impair osseointegration or long-term implant maintenance [33]. These findings highlight the importance of early diagnosis and management of peri-implant bone loss in order to preserve mechanical support around implants. In addition, the results suggest that implant diameter and bone density should be carefully considered when planning implant placement in compromised bone conditions. In situations with reduced bone support, clinicians may consider strategies such as selecting wider implants, performing bone augmentation procedures, or modifying loading protocols to enhance implant stability and improve long-term outcomes.

From a clinical standpoint, these findings may be particularly relevant when considering implant selection strategies in different anatomical regions. In the posterior maxilla, where bone quality is often characterized by lower density, the marked reduction in stability observed under defect conditions suggests a potential increased risk of insufficient primary stability. In such scenarios, the use of larger-diameter implants, when anatomically feasible, or adjunctive approaches may be considered. Conversely, in dense mandibular bone, the relative impact of peri-implant defects may be less pronounced, although still relevant in advanced bone loss conditions. It is important to clarify, that these interpretations should be viewed as biomechanical considerations that may inform, but not dictate, clinical decision-making due to the nature of our study design.

Future studies should expand upon these findings by exploring advanced treatment strategies to enhance implant stability in challenging clinical scenarios. Investigating the combined use of biomaterials, growth factors, and novel implant surface modifications may offer promising avenues for improving osseointegration in areas affected by bone loss [54,55,56,57]. Furthermore, integrating diagnostic tools such as resonance frequency analysis into routine clinical practice could facilitate real-time monitoring of implant stability throughout the osseointegration period, enabling earlier detection of complications and more timely interventions.