For partially or completely edentulous individuals, screw-retained implant restorations offer high long-term clinical success, with survival rates exceeding 90% (Addy, 2024; Hamed et al., 2020). Screw-retained restorations offer retrievability, enabling easy evaluation, maintenance, and repair without damaging the implant component (Carpentieri et al., 2019; Fiorillo et al., 2024; Kraus et al., 2022; Samosir et al., 2021). Furthermore, it requires only 4–5 mm of vertical space with no risk of excess cement (Carpentieri et al., 2019; Samosir et al., 2021; Altinbas et al., 2025). Screw retained restorations comprise a screw access channel, which can result in esthetic compromise in addition to increasing cost (Carpentieri et al., 2019; Fiorillo et al., 2024; Samosir et al., 2021). Moreover, the construction of screw-retained reconstructions has technical challenges and clinical complications, including loss of screw torque, veneering ceramic fracture, misfit, screw fracture, and abutment fracture (Samosir et al., 2021; Seloto et al., 2020; Wittneben et al., 2017).

Applying torque to an implant screw creates stress along its shank and threads, generating preload that is essential for maintaining component connection. If external forces acting on the implant exceed the preload-induced clamping force, the screw may loosen (Anniwaer et al., 2025; Siamos et al., 2002; Szajek and Wierszycki, 2023). The clamping force from the screw’s elastic recovery maintains the mechanical stability of the implant assembly (Siamos et al., 2002; Szajek and Wierszycki, 2023; Son et al., 2019). Several mechanical factors can lead to screw loosening: insufficient preload, the finish of the abutment surface, misalignment of implant components, embedment relaxation (settling effect), high occlusal loads, and repeated loosening and tightening of screws (Son et al., 2019; Benjaboonyazit et al., 2019; Lang et al., 2003; Srivastava, 2025). At the same time, increased surface friction reduces adequate preload (Winkler et al., 2003a). Both the settling effect and the coefficient of friction are influenced by the micro-roughness of implant components (Burguete et al., 1994; Sawase et al., 2000). For this reason, regulating the surface roughness at the implant-abutment interface is critical for long-term screw stability (Fernández et al., 2014; Kano et al., 2006; Quirynen et al., 1994). This roughness is primarily determined by the fabrication and finishing processes used to produce the components (Fernández et al., 2014; Kano et al., 2006; Quirynen et al., 1994; Guda et al., 2008).

To prevent screw fatigue or breakage, the applied preload should be between 60% and 75% of the screw’s elastic limit (Winkler et al., 2003a; Burguete et al., 1994; Sawase et al., 2000; Fernández et al., 2014; Kano et al., 2006; Quirynen et al., 1994; Guda et al., 2008; Dixon et al., 1995). However, part of this preload is lost through the settling effect, in which uneven contact points on the mating surfaces are smoothened over time (Burguete et al., 1994; Klongbunjit et al., 2021). Siamos et al. (Siamos et al., 2002) proposed a protocol to counteract the settling effect, recommending that the screw should be retightened 10 min after the initial load is applied. Retightening the screw after initial settling can partially restore the lost preload (Burguete et al., 1994; Barucca et al., 2015; Kim et al., 2011). In addition, increasing the coefficient of friction minimizes preload (Winkler et al., 2003a). Micro-roughness of the implant apparatus creates the settling effect and coefficient of friction (Burguete et al., 1994; Sawase et al., 2000; Winkler et al., 2003b). The implant-abutment interface and screw stability can therefore be obtained by regulating the surface roughness of the mating surfaces. Furthermore, the surface roughness of implant components is primarily influenced by restorative fabrication techniques and surface finishing methods (Fernández et al., 2014; Kano et al., 2006; Quirynen et al., 1994).

Cobalt-chromium (Co-Cr) is a widely used base-metal alloy in dental prostheses (Barazanchi et al., 2017). Base metals have proven their ability to meet physical requirements for use in high-demand scenarios, such as long-span bridges and implant frameworks (Ucar et al., 2009). Traditional methods for casting Co-Cr frameworks present significant manufacturing challenges. These challenges are primarily due to metal oxidation during casting, the wide melting temperature range of the alloys, diminished ductility, and increased hardness (Choi et al., 2014; Jemt et al., 2017). Moreover, as the length of the multiple-unit fixed dental prosthesis (FDP) increases, the distortion of the implant superstructure during the lost-wax process also becomes more pronounced (Kan et al., 1999; Li et al., 2015).

Computer-aided design and computer-aided manufacturing (CAD/CAM) technology has significantly enhanced framework production by reducing manual errors and increasing precision, thereby offering a viable alternative to traditional casting techniques (Heiba et al., 2025). Furthermore, CAD/CAM technique provides enhanced control over the micro- and macrostructures of Co-Cr frameworks. CAD-CAM systems are categorized as subtractive manufacturing (SM), also referred to as milling, and additive manufacturing (AM) also known as 3D printing (Berman, 2012). In SM, prostheses are fabricated from block-shaped materials using diamond rotary instruments (Li et al., 2015). Frameworks produced through SM demonstrate marginal fit and stress levels comparable to those achieved with casting techniques. While it offers benefits such as precision, rapid operation, and ease of use, it also has several drawbacks, including material waste with up to 90% of the original material discarded, wear and tear on milling tools, and the formation of microcracks in the milled specimens (Abduo et al., 2014; Dehurtevent et al., 2017; van Noort, 2012; Vulović et al., 2022). Additive manufacturing technology (AMT) offers significant advantages for fabricating complex geometries by enabling layer-by-layer construction of intricate designs, such as undercuts (Dehurtevent et al., 2017; Fiedor and Ortyl, 2020; Rodriguez and Garcia, 2018; Ventola, 2014). This ability makes AMT particularly suitable for high-precision prosthodontics. Unlike milling and casting methods, which are limited by the dimensions of the block or investment chamber, AMT can produce single-unit prostheses (Park et al., 2017). It promises to enhance the precision of indirect restorations while minimizing material waste with advanced technology (Koutsoukis et al., 2015). A recent systematic review has concluded that AMT with direct 3D printing achieves superior marginal and internal fit accuracy, greater cost-effectiveness and shorter fabrication times compared to subtractive or traditional methods (Conceição et al., 2023).

Powder bed fusion (PBF) is the primary additive metal fabrication technology used in dentistry. Its three methods—selective laser melting (SLM), selective laser sintering (SLS) and electron beam melting (EBM)—differ in their energy sources and binding mechanisms (Konieczny et al., 2020). Within the PBF category, SLM has advanced through the integration of advanced materials, laser, and computer technologies, creating significant potential for future development (Ansari et al., 2019). Dental implants produced using SLM exhibit increased density, improved strength, and precise dimensional accuracy, reflecting the benefits inherent to SLM processes (Ansari et al., 2019; Presotto et al., 2019; Alqutaibi et al., 2024). During the SLM process, a layer of metal powder is first spread onto a build platform. A high-intensity laser beam then selectively melts and fuses the powder according to the cross-section defined by a CAD model. After each layer is completed, the build platform lowers incrementally, and a new layer of powder is spread (Huang et al., 2016). This step-wise process of spreading, melting, and lowering is repeated until the entire 3D component is formed. Accordingly, SLM enables the fabrication of intricate, high-precision metal parts including complex porous geometries that are challenging to produce with conventional techniques (Ansari et al., 2019). Furthermore, unused metal powder can be recycled for subsequent manufacturing, minimizing material waste (Presotto et al., 2019).

In CAD/CAM applications for implant restorations, materials like Co-Cr alloy, zirconia oxide, and titanium alloy are commonly used (Vohra et al., 2024; Yilmaz et al., 2018). The effectiveness of implant-supported prostheses depends on the stability of the implant-abutment connection, which relies on preload during insertion and its maintenance (Bertl et al., 2021). Abutment fabrication techniques have certain drawbacks that may result in less effective torque retention than preformed, machined abutments. According to a previous study, the original abutments demonstrated a smaller reduction in torque than the three duplicate abutments (Bertl et al., 2021). The outcome might be attributed to the constraints in manufacturing precision and the accuracy of milling machines. The existing literature provides limited evidence on the reduction in torque and strength of SLM Co-Cr three-unit implant-supported frameworks compared with conventional casting or milled frameworks. Therefore, the present study aimed to investigate the effect of different manufacturing techniques, including SLM, milling and lost-wax casting on the micro-roughness and screw preload of a three-unit screw-retained implant framework and their correlation. The null hypothesis was that there is no difference in micro-roughness or screw preload between specimens produced by different manufacturing methods.

This study used a controlled in vitro design to assess the effect of three fabrication techniques—SLM, milling, and lost-wax casting—on surface roughness and screw preload loss in three-unit screw-retained implant-supported frameworks. Furthermore, the study also examined the correlation between surface roughness and preload loss under cyclic loading. Based on the findings, the null hypothesis was rejected, as significant differences were identified in both surface roughness and post-load RTV and RTD. The milled frameworks exhibited the lowest roughness and the highest preload retention, SLM frameworks showed inferior performance compared to milled, while cast frameworks demonstrated the greatest roughness and preload loss. Testing three-unit prostheses rather than single units reflects clinical needs in posterior regions, where higher forces challenge the mechanical integrity of the prosthesis-implant connection (Eser et al., 2018; Zarone et al., 2020). Multi-unit frameworks create complex stress distributions and a higher potential for deformation under cyclic loading (Alshehri et al., 2022).

The present study aimed to control for confounding variables by standardizing implant positions, torque application, and cyclic loading conditions, in line with established protocols (Elsayed et al., 2017; Albaijan et al., 2025). To simulate clinical conditions, implants were placed in PMMA resin with 3 mm of threads exposed, using a dental surveyor for consistent alignment, in line with ISO 14801 standards for dynamic fatigue testing (Bertl et al., 2021; Elsayed et al., 2017; Elsayed et al., 2018; Hjerppe et al., 2022). The research investigated preload and micro-roughness in premolar and molar samples separately. The study refrained from applying any post-fabrication finishing or polishing to the samples in all groups to maintain surface integrity and assess variations caused by the fabrication process (Fernández et al., 2014; Vohra et al., 2024; Alshehri et al., 2022). To eliminate measurement bias and ensure objectivity, all sample measurements for surface roughness and preload were conducted by a single calibrated operator.

Surface roughness was assessed using a 3D optical non-contact surface microscope, selected for its high precision and ability to provide non-destructive quantitative data. The roughness measurements in this research were above the widely accepted limit of 0.2 μm, which is suggested to reduce bacterial retention (Bollen et al., 1996). Milled frameworks demonstrated the lowest roughness (molars: 0.30 μm, premolars: 0.24 μm), reflecting the precision of the subtractive process, which produces more uniform finishes (Fernández et al., 2014). SLM specimens showed higher roughness values (molars: 1.73 μm, premolars: 1.66 μm) than milled frameworks, primarily due to layer-by-layer fusion defects and residual unmelted powder particles (Fernández et al., 2014). Cast frameworks exhibited the highest roughness (molars: 1.65, premolars: 2.57 μm), a result attributed to oxidation, porosity, and solidification flaws inherent to the lost-wax casting technique (Fernández et al., 2014). These outcomes align with those of Albaijan et al. (2024), who found that SLM generally produces smoother surfaces than casting. For both SLM and cast frameworks, post-fabrication polishing could reduce microbial risk, balancing the design freedom of additive manufacturing with the need for optimal surface quality.

The results of the present study reveal that the RTVs after cyclic loading were considerably lower than the applied torque in all groups. This indicates that loss of screw preload at the implant-abutment connection occurs after cyclic loading, regardless of the manufacturing technique. Benjaboonyazit et al. (2019) evaluated single implants by comparing RTVs before and after cyclic loading ranging from 50,000 to 2,000,000 cycles. Their results showed that, in all groups, post-cyclic RTVs were significantly lower than the pre-cyclic values. The decrease in RTV may be linked to the long-span and complex framework design, as well as to the settling effect, with associated preload loss from friction or micromovements at the implant–abutment interface during functional loading (Benjaboonyazit et al., 2019; Tiossi et al., 2017) have reported that when a screw is tightened to the manufacturer’s recommended torque, the resulting RTV is typically 7%–10% lower than the applied value. To our knowledge, there is no literature comparing the loss of preload in Co-Cr implant-supported three-unit screw retained frameworks manufactured using different manufacturing techniques. The present study demonstrated significant differences in preload retention among the three fabrication techniques, with milled frameworks exhibiting the highest post-load RTV and the lowest difference in reverse torque values, indicating superior screw stability under cyclic loading.

In contrast, cast frameworks showed the most significant preload loss, followed by SLM frameworks. Milled abutments produce more precise and smoother interfaces, thereby minimizing micromovements and settling effects that contribute to preload loss (Altuwaijri et al., 2022). The inferior performance of cast frameworks may be attributed to inherent material irregularities, porosity, and higher surface roughness, which exacerbate embedment relaxation and friction-induced preload loss (Fernández et al., 2014). The three-unit frameworks experienced more substantial preload loss than typically reported for single units, supporting the concept that increased span magnifies micromotion and settling effects (Tiossi et al., 2017). Using a consistent cyclic loading of 500,000 cycles at 49N replicated approximate masticatory forces over 2 years (AlHomidhi and Alqahtani, 2021; Alqahtani and Flinton, 2014). Moreover, by retightening screws after initial torque and before cyclic loading, the settling effect was accounted for (Siamos et al., 2002) and based on the systematic review conducted by Coray et al. (2016) forces applied in most cyclic loading investigations used either a stainless steel or a Co-Cr indenter, specifically a metallic piston with a diameter of 3 mm. However, the dental literature does not provide a controlled, standardized environment for cyclic loading conditions in implant dentistry.

The correlation analyses between surface roughness and RTD revealed no statistically significant relationships across any of the fabrication groups, suggesting that surface roughness alone is not a strong predictor of screw preload behavior under cyclic loading. Although moderate correlations were observed in the SLM group, they did not reach statistical significance, suggesting that other factors may play a more critical role in preload retention. Potential contributing factors include manufacturing precision, internal fit discrepancies, screw thread engagement quality, and material properties such as elasticity and resistance to embedment relaxation (Winkler et al., 2003a; Fernández et al., 2014; Albaijan et al., 2024). The inherent limitations of each fabrication method, such as porosity in cast frameworks, layer adhesion defects in SLM, and tool wear in milling, may introduce variability that overshadows the influence of surface topography alone. Thus, it is hypothesized that while micro-roughness contributes to settling and friction, its impact on preload loss is secondary to the geometric accuracy and structural integrity of the implant-abutment complex.

The present study provides valuable insights into differences in surface topography and torque loss among implant frameworks fabricated using different techniques; however, the outcomes should be interpreted in light of the study’s limitations. Due to its in vitro nature, the research is limited in its direct applicability to clinical settings, as it overlooks biological variables such as saliva flow, temperature variations, and ongoing bone remodeling (Vohra et al., 2024; Albaijan et al., 2024). Additionally, the standardized loading conditions may not fully replicate the complex multidirectional forces encountered during mastication, potentially underestimating the clinical performance of the tested abutments (Altamimi et al., 2025). The sample size, though statistically justified, may not capture the full range of mechanical behavior under long-term functional stress. Therefore, future clinical studies should incorporate larger sample sizes, fatigue testing under more physiologically representative conditions, and long-term clinical evaluations to validate these in vitro findings. Investigations into the effects of surface treatments, biofilm formation, and corrosion on differently manufactured abutments are also warranted to provide a more comprehensive understanding of their clinical applicability.

Based on the findings of this in vitro study, it can be concluded that the manufacturing technique significantly influences the screw preload retention and surface characteristics of three-unit screw-retained implant-supported Co-Cr frameworks. SLM frameworks exhibited lower roughness and better torque stability compared with cast frameworks, though milled frameworks remained superior. The lack of a strong correlation between surface roughness and preload loss suggests that factors such as precision and structural integrity are more critical determinants of long-term screw stability than surface topography alone.