Rawan Alsaif1
Mohammed Alrabiah- 1Department of Prosthetic Dental Science, College of Dentistry, King Saud University, Riyadh, Saudi Arabia
- 2Division of Prosthodontics, Department of Restorative Dentistry, School of Dentistry, University of Minnesota, Minneapolis, MN, United States
- 3Department of Prosthetic Dental Sciences, College of Dentistry, Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia
- 4Department of Clinical Dental Sciences, College of Dentistry, Princess Nourah University, Riyadh, Saudi Arabia
Aim: This study evaluated the effects of selective laser melting (SLM), milling, and casting fabrication techniques on surface roughness and screw preload loss of three-unit screw-retained implant-supported Cobalt-Chromium (Co-Cr) frameworks.
Methods: Ten frame-works were fabricated by each fabrication method (n = 10), and the surface roughness (Ra) was measured using a 3D optical profilometer. The frameworks were torqued to 30 Ncm, retightened after 15 minutes, and subjected to 500,000 cycles in a dual-axis chewing simulator. Pre- and post-load reverse torque values (RTV) were recorded for molar and premolar, and the reverse torque difference (RTD) was calculated. Data were analyzed using ANOVA and Tukey-Kramer tests (α = 0.01). Correlation analyses were performed to determine any relation between surface roughness and preload (RTV and RTD) across all groups.
Results: Significant differences were found in surface roughness and preload loss among groups (p < 0.01). In molars, the milled group showed the lowest Ra (0.30 µm) and highest post-load RTV (25.2 Ncm), followed by the SLM group (1.73 µm, 20.2 Ncm) and cast group (1.65 µm, 16.9 Ncm). In premolars, the milled group again showed the lowest Ra (0.24 µm) and highest RTV (25.2 Ncm), followed by the SLM group (1.66 µm, 19.4 Ncm) and the cast group (2.57 µm, 17.4 Ncm). There was no significant correlation between roughness and preload loss.
Conclusion: Different manufacturing technique significantly affects surface topography and screw preload. Milled frameworks showed the least surface roughness and the most torque stability.
1 Introduction
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.
2 Materials and methods
2.1 Sample size
Using the means and SD from a prior study, the power calculation was used to determine the sample size for the frameworks (Elsayed et al., 2017). The study’s mean and SD [SLM group (6.9 ± 2.1), milling group (41.3 ± 15.3), and casting group (41.3 ± 24.6) were calculated (GraphPad Instat program, San Diego, CA, United States) at 90% power. Based on the power analysis, a minimum of 8 samples per group was required. Previous studies on similar topics also used 10 samples per group (Elsayed et al., 2017; Bulaqi et al., 2015). Accordingly, a total sample size of 24, with eight specimens per group, was considered adequate. However, to minimize the risk of errors or sample exclusion during evaluation, 30 frameworks were fabricated, with 10 specimens per group.
2.2 Specimen preparation
A cylindrical mold was created and stored as a standard tessellation language (STL) File (Ceramill Mind/D-Flow software, Amann Girrbach). A 5-axis milling machine (Ceramill Motion 2; Amann Girrbach AG, Germany) was used to mill a PMMA cylindrical block (Ceramill A-Splint; Amann Girrbach AG, Germany). Using the milled cylindrical block, a silicone mold (Wirosil duplicating silicone, Bego, United States) was prepared. A dental surveyor (Ney surveyor; DENTSPLY International Inc., York, PA, United States) was used to mount the implant fixtures in parallel using a pour of clear acrylic resin (Major Ortho™, Torino, Italy) inside the block, with 15 mm separating the centers of each implant. The surveyor’s metal rod was used to fix the implant’s location and angulations to avoid inaccurate implant positioning. A total of 60 internal connection implants measuring 4.0 mm in platform diameter and 10 mm in length (Bone level implant, Superline™, Dentium Co., Seoul, Korea) were installed. In accordance with ISO 14801 guidelines for dynamic fatigue testing of dental implants, implants were positioned 3 mm parallel to the thread exposure (Presotto et al., 2019; Albaijan et al., 2025; Elsayed et al., 2018; Hjerppe et al., 2022).
A scan marker (Ø 4.8 mm, Superline Dentium System; ZBAD4834, ZirconZahn, An der Ahr, Gais, Italy) was attached to each model and coated with scan spray to ensure an accurate optical impression. The models were scanned using a Zirkonzahn optical scanner (S600 ARTI, ZirconZahn, An der Ahr, Gais, Italy) to capture the exact position and angulation of the implants. The scanned data were merged in CAD software (Zirkonzahn Modellier, ZirconZahn S.r.l.) to design a three-unit screw-retained framework. The abutment hex dimensions were selected from the implant’s digital library, and the final framework design was exported as an STL File. All implant models across the three study groups were scanned, and the corresponding screw-retained implant-supported frameworks were digitally designed for each. The STL files were then saved for subsequent manufacturing.
The STL file was transferred to the Concept Laser Machine (m-lab metal laser melting system; GE Additive, Boston, United States) using the recommended parameters. Co-Cr alloy (Starbond Easy Powder 30; Scheftner GmbH, Mainz, Germany- Co 61%, Cr 27.5%, W 8.5%, Si 1.6%, C, Fe, and Mn < 1%; grain size, +10/-30 μm). The 3D model was vertically positioned, and the printing process was carried out in an atmosphere of nitrogen and argon. The thickness of the powder layer was 20 μm. This procedure continued to produce ten three-unit screw-retained frameworks.
The same STL file was also imported to a 5-axis milling machine (Arum 5X-450, Daejeon; Arum Dentistry) equipped with editable software. The frameworks were milled from Co-Cr alloy blocks (Magnum Solare, Mesa, Italy), composed of 66% cobalt, 27% chromium, 6% molybdenum, and 1% silicon and manganese. The alloy exhibited 11% elongation at break, with a melting point of 1,417 °C and a solidus-liquidus range spanning from 1,307 °C to 1,417 °C. Its thermal expansion coefficient was measured at 14.3 × 10-6 K−1 between 25 °C and 500 °C, and 14.5 × 10-6 K−1 from 25 °C to 600 °C. A total of ten CAD/CAM frameworks were produced.
Ten three-unit screw-retained implant frameworks were made from cast Co-Cr alloy. The process was initiated by transferring the STL file to a milling machine (Zirkonzahn M5, Zirkonzahn; An der Ahr, Gais, Italy) to produce wax patterns (Dental Wax Blank, MedNet GmbH; Münster, Germany). Each wax pattern was sprued individually and invested using a phosphate-bonded investment material (Fast Fire 15; Whip Mix, Louisville, KY) with a 16 mL to 60 g liquid-to-powder ratio. Casting was done with a Co-Cr alloy (Wirebond RC; BEGO, Bremen, Germany) in a casting machine (FORNAX 35 ER, BEGO, Bremen, Germany) at 1,500 °C. The molten Co-Cr alloy was injected into the mold under vacuum pressure in accordance with the manufacturer’s instructions. The alloy composition was 63.3% cobalt, 24.8% chromium, 5.3% tungsten, 5.1% molybdenum, and 1.0% silicon. After casting, the investment cylinders were allowed to cool for at least 2 h. Once cooled, the frameworks were divested. Sprue formers and small nodules were then removed using sharp tungsten carbide burs at approximately 30,000 rpm under ×10 magnification. Each framework was subsequently examined under a stereomicroscope, and any frameworks with significant distortion, large voids, or porosities due to casting inaccuracies were excluded and replaced with new frameworks. No additional polishing or finishing was applied to the cast frameworks. Figure 1 presents the specimens fabricated using different techniques.
Figure 1. Specimens fabricated using three different techniques.
2.3 Surface roughness (Ra) measurement
Each abutment’s surface was evaluated using an optical non-contact profilometer (Contour GT-K 3D Optical Microscope, Bruker, Tucson, Arizona, United States), which operates on the principle of white light interferometry to capture detailed 2D surface profiles and surface roughness (Ra). Frameworks were positioned horizontally on the platform using a custom-made polyvinyl siloxane mold such that the measurement plane was perpendicular to the optical light beam. The platform was manually adjusted to ensure optimal image quality. Surface roughness was measured on the intaglio implant–abutment mating surface, focusing on the flat horizontal platform area just above the implant hex. This area is the main contact zone responsible for friction, settling, and preload behavior. The scanning was performed using a 1 μm laser light beam and the measurement length was 1.261 mm, 0.946 mm and 500 μm in X, Y and Z-axis, respectively. Measurements were taken at six predefined points above the hex on the inclined surfaces, and the average Ra was calculated in micrometers (μm). The images were captured at ×2.5 optical zoom at a resolution of 0.01 mm. The vision 64 Control and Analysis Software (Bruker) was used to ensure accuracy and consistency of the surface roughness data.
2.4 Implant abutment displacement
The assessment of implant abutment displacement was based on changes in preload and post-load reverse torque values (RTV). Both the preload and post-load RTV were measured in newton centimeters (Ncm). To prevent stress on the screw from prior tightening and loosening, a new screw was used for each framework. All frameworks were securely fastened to their respective implants using a torque of 30 Ncm, in strict accordance with the manufacturer’s recommendations (Att et al., 2006). The abutment was secured in a customized holder (Figure 2a) and torqued using a Tohnichi BTGE digital torque gauge (Tohnichi Mfg, Tokyo, Japan) (Figure 2b) after calibration using a calibration tool (BTGCL Calibration Kit, Tohnichi). After the initial torque was applied and a 15-min interval had passed, the screws were tightened again to reduce any preload loss due to embedment relaxation (Szajek and Wierszycki, 2023; Burguete et al., 1994). All measurements were performed by a single trained prosthodontist (RA).
Figure 2. (a) Specimen secured in a customized holder. (b) Specimens torqued using a digital gauge.
The implant block was placed in a customized metal holder designed for use in the chewing simulator machine. Cyclic loading was conducted using a four-station multimodal dual-axis chewing simulator (Robota, AD-TECH Technology CO., Germany). The device enabled simultaneous simulation of vertical and horizontal movements. The chewing simulator contained four chambers, each consisting of an upper Jacob’s chuck as antagonist holder (Ø 5 mm steel indenter with a spherical tip) and a lower sample holder. All samples were tested under standard conditions following the ISO 14801:2016 standards of the dynamic loading test (Eser et al., 2018; Zarone et al., 2020). For all assemblies, preload was assessed using 500,000 loading cycles, simulating 2 years of function. A loading force of 49 N was chosen to replicate a load that fell within the clinical range (Presotto et al., 2019; Albaijan et al., 2025; Breeding et al., 1993). The specimens were tilted by 30° and the load was applied with the indenter positioned in the central fossa of the premolar pontic, contacting the lingual incline of the buccal cusp and the buccal incline of the lingual cusp (Figure 3). The chewing simulation parameters were a 1.6 Hz cycle frequency, 90 mm/s forward ascending speed, 40 mm/s of descending speed, 3 mm vertical movement, and 1 mm horizontal movement (Altamimi et al., 2025). Post-load RTVs were assessed immediately after sample removal from the chewing simulator. The difference in RTVs between pre- and post-load values was calculated and referred to as the RT difference (RTD). Correlation analyses were performed to determine if any possible relation exists between surface roughness and preload (RTV and RTD) across all groups.
Figure 3. Cyclic loading was employed using a four-station multimodal dual-axis chewing simulator.
3 Results
3.1 Surface roughness
The mean Ra values for the study groups are presented in Table 1. In molars, the SLM group exhibited the highest mean surface roughness (1.73 ± 0.16 µm), followed by the cast group (1.65 ± 0.12 µm). The milled group showed the lowest values (0.30 ± 0.006 µm), consistent with the smoother surface finish typically associated with subtractive CAD/CAM processes. The comparison between the groups showed a significant difference in Ra (p < 0.01). Among premolars, the milled group again demonstrated the lowest Ra (0.24 ± 0.01 μm), while cast samples displayed the highest values (2.57 ± 0.08 μm), significantly exceeding those of SLM (1.66 ± 0.05 μm) and milled groups (p < 0.01). As with molars, the comparison between groups revealed a significant difference in Ra (p < 0.01). These findings confirmed that both SLM and casting produce rougher internal surfaces than milled parts.
Table 1. Mean surface roughness (Ra, in μm) among study groups.
Figures 4A–F presents the profilometer images of the roughness profile of the study groups. The images revealed a distinct pattern in roughness across the three fabrication methods. Group SLM (Figures 4A,B) showed increased roughness in both molar and premolar groups, which is similar to the roughness profile of the casting group (Figures 4E,F). In contrast, the milled group (Figures 4C,D) showed a lower roughness profile than the other study groups.
Figure 4. Surface roughness micrographs among the study groups. (A) molar-SLM, (B) premolar-SLM, (C) molar-milled, (D) premolar-milled, (E) molar-cast, (F) premolar-cast.
3.2 Screw preload loss
In both molar and premolar groups, no significant differences were observed in pre-load RTVs (p > 0.01) (Table 2). Post-load RTVs showed significant differences among the study groups (p < 0.01). In molars, the highest post-load RTVs were recorded in the milled group (25.22 ± 3.06 Ncm), followed by SLM (20.20 ± 0.94 Ncm), while the cast group presented the lowest values (16.87 ± 3.21 Ncm). A similar pattern was observed in premolars, where milled frameworks again exhibited the highest post-load RTV (25.23 ± 3.10 Ncm), compared with SLM (19.36 ± 1.44 Ncm) and the cast group (17.40 ± 4.47 Ncm). RTDs also differed significantly across groups (p < 0.01). In molars, the highest RTD was found in cast frameworks (13.27 ± 3.25 Ncm), followed by SLM (10.00 ± 0.81 Ncm), and milled frameworks showed the lowest RTD (5.02 ± 3.12 Ncm). In premolars, the cast group again recorded the highest RTD (12.97 ± 4.37 Ncm), followed by SLM (10.93 ± 1.52 Ncm), and the milled group showed the lowest (4.94 ± 3.10 Ncm).
Table 2. Mean and SD of RTV and RTD among the study groups.
Correlation analyses were performed between surface roughness and preload across all groups. The relationships ranged from weak to moderate, and none of the observed correlations reached statistical significance. In the SLM group, moderate positive correlations were found between roughness and RTV for both molars (r = 0.508, p = 0.198) and premolars (r = 0.429, p = 0.288). However, the correlations with RTD were notably weaker (molars: r = 0.248, p = 0.554; premolars: r = 0.137, p = 0.747). The milled group displayed a moderate positive correlation between premolar roughness and post-load RTV (r = 0.544, p = 0.164), while all other relationships were weak and statistically insignificant. The cast group showed negligible correlations across all comparisons, including molar RTV (r = 0.111, p = 0.793) and RTD (r = 0.041, p = 0.923), as well as premolar RTV (r = 0.138, p = 0.745) and a slightly stronger but still non-significant correlation with RTD (r = 0.565, p = 0.144). These findings suggest that although a trend toward increased roughness being associated with greater torque variability may exist, surface topography alone does not strongly predict post-load behavior.
4 Discussion
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.
5 Conclusion
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.
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
RaA: Conceptualization, Data curation, Investigation, Resources, Visualization, Writing – original draft, Writing – review and editing. FV: Formal Analysis, Resources, Software, Supervision, Validation, Writing – original draft, Writing – review and editing. ReA: Conceptualization, Formal Analysis, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. HA: Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Software, Writing – original draft, Writing – review and editing. SA: Conceptualization, Methodology, Project administration, Software, Supervision, Visualization, Writing – original draft, Writing – review and editing. MA: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing.
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Keywords: 3D printing, additive manufacturing, CAD-CAM, implant abutments, selective laser melting
Citation: Alsaif R, Vohra F, Albaijan R, Alsayed H, Altuwaijri S and Alrabiah M (2026) Surface roughness and preload loss of implant-supported screw retained three-unit framework fabricated by different techniques. Front. Mater. 13:1763096. doi: 10.3389/fmats.2026.1763096
Received: 08 December 2025; Accepted: 20 January 2026;
Published: 04 February 2026.
Edited by:
Zena Jehad Wally, University of Kufa, IraqReviewed by:
Gurel Pekkan, Namik Kemal University, TürkiyeClaudio Cirrincione, University of Florence, Italy
Copyright © 2026 Alsaif, Vohra, Albaijan, Alsayed, Altuwaijri and Alrabiah. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Mohammed Alrabiah, bW9oYWxyYWJpYWhAa3N1LmVkdS5zYQ==