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Review

Implant Screw Loosening: A Narrative Review of Medium- and Long-Term Clinical Evidence

by
Socratis Thomaidis
,
Sofia Diamantopoulou
and
Efstratios Papazoglou
*
Department of Operative Dentistry, Dental School, National and Kapodistrian University of Athens, 2 Thivon Str., 11527 Athens, Greece
*
Author to whom correspondence should be addressed.
Appl. Sci. 2026, 16(12), 6253; https://doi.org/10.3390/app16126253
Submission received: 25 May 2026 / Revised: 12 June 2026 / Accepted: 13 June 2026 / Published: 22 June 2026
(This article belongs to the Section Applied Dentistry and Oral Sciences)
Versions Notes

Abstract

Implant screw loosening remains among the most frequently reported technical complications in implant-supported prostheses and may compromise prosthetic stability, maintenance requirements, and long-term clinical outcomes. Etiology is multifactorial and involves biomechanical, prosthetic, occlusal, and patient-related factors. This narrative review aimed to synthesize medium- and long-term clinical evidence (>5 years whenever available) regarding mechanisms, prevalence, and risk factors associated with screw loosening in implant-supported restorations. A structured literature search was conducted in PubMed, Web of Science, Cochrane Library, and EBSCOhost to identify clinical studies, randomized controlled trials, systematic reviews, and meta-analyses. Evidence regarding preload, implant–abutment connection design, retention type, implant splinting, framework fit, abutment angulation, implant dimensions, occlusal loading, parafunction, full-arch restorations, and torque protocols was critically interpreted. Current evidence indicates that screw loosening is influenced by inadequate preload, unfavorable occlusal forces, cantilevers, angulated abutments, framework misfit, and parafunctional habits. Single-unit and screw-retained restorations appear to exhibit higher complication rates in several studies, although findings remain inconsistent. Internal connections and splinting may improve mechanical stability; however, superiority has not been conclusively demonstrated. Most screw loosening events occur during the early functional period, emphasizing the importance of preload optimization, occlusal control, maintenance, and follow-up. High-quality long-term comparative studies remain limited.

1. Introduction

Implant-supported rehabilitation has become a predictable treatment modality for replacement of missing teeth, demonstrating high survival rates across single-unit, partial edentulism, and full-arch indications. Although osseointegrated implants provide long-term functional and esthetic benefits, biological and technical complications continue to affect treatment outcomes [1,2,3,4,5].
Biological complications mainly include peri-implant mucositis and peri-implantitis [1,2], whereas technical complications comprise ceramic chipping, decementation of cement-retained restorations, prosthetic or abutment screw loosening, abutment fracture, framework fracture, and implant fracture [3,4,5]. Among these, implant screw loosening remains one of the most frequently reported mechanical complications in implant-supported restorations [4,6].
Although screw loosening rarely results in immediate catastrophic failure, recurrent preload loss may increase micromovement at the implant–abutment interface and contribute to progressive mechanical deterioration. Consequences include implant–abutment instability, abutment fracture, screw fracture [7], microleakage [8], bacterial colonization [9,10], and potential impairment of long-term prosthetic performance [11].
Reported incidence of screw loosening varies considerably among studies because outcomes are influenced by implant–abutment connection geometry [6,7,12], preload magnitude [13,14,15], prosthetic design [16], retention type [16,17], framework fit [18,19], occlusal loading [20,21,22], parafunction [22], and maintenance protocols [23,24,25,26,27,28,29,30].
Existing literature includes biomechanical investigations, retrospective cohorts, randomized clinical trials, systematic reviews, and meta-analyses [6,7,9,10,12,16,17,31,32,33]. However, findings are frequently heterogeneous and occasionally contradictory, limiting translation into evidence-based clinical recommendations.
Therefore, the aim of the present narrative review was to critically summarize medium- and long-term clinical evidence regarding mechanisms, prevalence, and risk factors associated with implant screw loosening, with emphasis on clinically relevant variables influencing occurrence and prevention. Preference was given to evidence derived from studies with follow-up periods exceeding 5 years whenever available.

2. Methods

This narrative review followed a structured approach to improve transparency and reproducibility while allowing critical interpretation of heterogeneous evidence. Literature search was performed in PubMed/MEDLINE, Web of Science, Cochrane Library, and EBSCOhost, consistent with the original methodology of the manuscript. The final search was conducted on 7 March 2026.
Search concepts combined terms related to dental implants and screw loosening. Representative strategies included: (“dental implant*” OR “implant-supported”) AND (“screw loosening” OR “abutment screw loosening” OR “prosthetic complications”). Controlled vocabulary (MeSH) was incorporated when available. Search terms were adapted according to database requirements.
Eligible publications included randomized clinical trials, prospective and retrospective clinical studies, systematic reviews, and meta-analyses investigating implant screw loosening or associated risk factors [6,7,9,10,12,16,17,31,32,33]. Preference was given to studies with ≥5 years follow-up whenever long-term evidence existed [6,9,16]. In vitro studies [34,35,36,37,38], technical reports, isolated case reports, and non-English publications were excluded.
Literature was reviewed and interpreted according to predefined inclusion criteria. Additional references were identified through manual searches of included articles, similar to previously described review methodologies [6,7,12].
Because this manuscript is a narrative review rather than a systematic review, formal risk-of-bias assessment was not performed. This should be considered a limitation when interpreting conclusions. Nevertheless, evidence was interpreted according to study hierarchy, giving greater weight to systematic reviews and meta-analyses [6,9,17,26,29,39,40,41] and randomized clinical trials [42,43,44] than retrospective observational studies [39,45,46,47].
Findings were synthesized thematically under biomechanical mechanisms, prosthetic variables, implant design factors, occlusal influences, and maintenance-related variables. The review aimed to emphasize clinically meaningful patterns and long-term implications rather than pooled quantitative estimates.
Studies consisting of in vitro investigations, short-term clinical studies with <5 years follow-up, irrelevant content, case reports or technical reports, insufficient clinical outcome data, or non-English publications were excluded. Consequently, 67 publications were included in the final qualitative synthesis, comprising clinical studies, randomized controlled trials, systematic reviews, and meta-analyses.

3. Mechanism of Implant Screw Loosening

The long-term stability of implant-supported restorations depends largely on the integrity of the implant–abutment interface, where the abutment screw plays a critical biomechanical role. During tightening, manufacturer-recommended torque induces elastic elongation of the screw below its yield strength, generating tensile force commonly referred to as preload [13,14]. Based on mechanical principles and in vitro studies, Preload may also be defined as the axial force created between the screw threads and the internal surface of the implant following tightening [13,14]. Functionally, the screw behaves as a tensioned spring that generates a clamping force between implant and abutment components, maintaining joint stability and resisting separation during function [48].
The magnitude of preload is studied in laboratory studies, and is considered one of the most important determinants of screw stability because it influences the ability of the implant–abutment complex to withstand functional loading [14,15]. Optimal preload should generally remain below approximately 60–75% of the yield strength of the screw material to avoid plastic deformation or fracture [14]. Insufficient preload may fail to resist functional forces adequately, increasing the risk of micromovement and loosening, whereas excessive preload may induce material fatigue or screw fracture [14,15]. Consequently, achieving an appropriate preload is essential for maximizing fatigue resistance and reducing mechanical complications [12].
Importantly, the relationship between tightening torque and preload is not linear. According to in vitro data, only a relatively small proportion of applied torque (approximately 10%) is converted into preload, whereas the majority is dissipated in overcoming friction at the contacting interfaces, including screw threads, the implant–abutment interface, and the screw head [19]. Therefore, factors affecting friction—such as thread geometry, material properties, lubrication, and surface coatings—may significantly alter preload generation [14,15]. Experimental studies have suggested that reduced friction through coatings or lubrication can increase preload and improve joint stability [15].
Screw loosening develops when forces acting to separate the implant and abutment exceed residual preload within the joint [49]. Under functional conditions, repetitive loading may produce micromovement between contacting components, progressively reducing clamping force and compromising anti-rotational mechanisms [40,49,50]. Cyclic loading, vibration, non-axial forces, and parafunctional activity may accelerate this process by promoting micro-slippage at the thread interface [40]. Once frictional resistance is overcome, gradual rotational displacement may occur, leading to further preload reduction and eventual clinical screw loosening [40,50].
Based on in vitro data, an additional contributor to early preload loss is the settling effect, also known as embedment relaxation [35,51,52]. This phenomenon results from flattening of microscopic irregularities at contacting surfaces following tightening, leading to reduction in preload even in the absence of external loading [15,51]. Studies have suggested that preload loss associated with settling may reach approximately 2–10% shortly after torque application [15,49]. To compensate for this effect, retightening protocols have frequently been recommended. Retightening after approximately 10 min may partially restore preload and reduce torque loss [13,15,35]. Although commonly advocated, evidence supporting standardized retightening intervals remains limited.
Repeated tightening and loosening cycles may further influence mechanical behavior of the implant–abutment complex. Several investigations reported progressive reductions in removal torque and preload following repeated tightening procedures [36,37]. Such reductions may decrease fatigue resistance and increase susceptibility to loosening over time. Nevertheless, conflicting findings exist, with some studies suggesting improved preload following repeated tightening due to changes in frictional characteristics [36]. These inconsistencies may reflect differences in implant systems, materials, and experimental methodologies.
Material properties of implant components may also affect screw stability. Variations in elastic modulus, surface characteristics, and thermal expansion coefficients between materials can alter stress distribution during cyclic loading and thermomechanical aging [15,40]. Consequently, screw loosening should not be considered an isolated mechanical event but rather the cumulative result of interactions among preload magnitude, implant geometry, prosthetic design, material characteristics, and patient-related loading conditions.
Overall, current evidence suggests that implant screw loosening represents a multifactorial biomechanical complication, which may partly explain the substantial heterogeneity observed in long-term clinical studies evaluating incidence and risk factors [12,15,40,49,50]. Understanding these mechanisms is essential for interpreting clinical evidence and developing preventive strategies aimed at improving long-term prosthetic stability.

4. Clinical Studies

Clinical investigations consistently identify screw loosening as one of the most prevalent technical complications associated with implant-supported restorations. Nevertheless, reported incidence varies substantially across studies due to differences in follow-up duration, implant systems, prosthetic design, maintenance protocols, and definitions of complications. Variables repeatedly associated with screw loosening include the number of implants supporting the restoration, retention type (screw-retained versus cement-retained), implant–abutment connection geometry, screw material, prosthetic fit, abutment angulation, occlusal loading, and parafunctional habits. Consequently, interpretation of prevalence requires consideration of both prosthetic and patient-related factors.
Systematic reviews and meta-analyses provide the most robust evidence regarding long-term incidence. Pjetursson et al. [6] reported cumulative 5-year screw loosening incidences of 1.2% (95% CI: 0.6–2.3%) for implant-supported single crowns (SCs) and 2.0% (95% CI: 0.7–5.4%) for fixed dental prostheses (FDPs). Higher complication rates were associated with metal abutments, anterior regions, and external implant–abutment connections. In a subsequent meta-analysis, Pjetursson et al. [31] observed an annual screw loosening rate of 0.44% in all-ceramic single crowns, whereas monolithic zirconia restorations demonstrated substantially higher rates (2.25%). Similarly, Jung et al. [9] identified screw loosening as the most common technical complication in implant-supported single crowns, reporting cumulative incidences of 12.7% after 5 years, decreasing to 5.8% after exclusion of an outlier study [53].
Comparable findings were reported by Sailer et al. [16,54], who consistently identified screw loosening as the predominant technical complication in implant-supported single crowns. Estimated incidence ranged between 0 and 1.36 events per 100 abutment-years for ceramic abutments and 0–10.32 events for metal abutments, although differences were not statistically significant. In a later systematic review, Sailer et al. [3] reported a 4.1% incidence of screw loosening in metal–ceramic multi-unit FDPs, whereas zirconia-based FDPs demonstrated no cases of loosening. These findings suggest that restoration material alone is unlikely to explain complication rates and may interact with prosthetic design and connection type.
Retrospective cohorts further highlight variability. Lee et al. [39], evaluating 1928 implants over 6 years, reported a 7.2% incidence of screw loosening, with most events occurring during the first 6 months of function and 22.3% demonstrating recurrence. Loosening occurred more frequently in molars (8.5%) than premolars (3.8%) and was more common in external connections (8.9%) than internal systems (5.4%). Importantly, approximately 78% of cases occurred only once, suggesting that recurrence, although clinically relevant, is not inevitable [39]. Lang et al. [55] reported lower incidence (5.3%) in a cohort of 1282 implants, whereas Pozzi et al. [56] observed only 2.7% over up to 12 years. Olander and Stenport [57] similarly identified screw loosening as one of the most common technical complications over 10 years, with one screw loosening in 29 zirconia abutments and one in 58 titanium abutments.
Randomized clinical trials provide equally heterogeneous findings. Scherrer et al. [58] observed six cases of screw loosening among 90 screw-retained crowns after 5 years, two in PFM, two in Lithium disilicate and two in PICN, whereas Pirc et al. [39] reported technical complications in 39.5% of patients after 10 years, with screw loosening accounting for 43.4% of all complications. Conversely, Hosseini et al. [59] reported only one case in 89 restorations after 5 years, and Mangano et al. [41] identified a single case in Morse taper implants, with no significant influence of crown-to-implant ratio. Collectively, these findings illustrate the multifactorial nature of screw loosening and suggest that prosthetic design and loading conditions may be more influential than the implant system alone (Table 1).

4.1. Number of Implants—Splinting of Implants

Whether splinting reduces screw loosening remains controversial. Several long-term studies suggest a lower incidence of splinted restorations compared with single-unit crowns, likely due to improved force distribution across implants. Sarzynski et al. [45], in a retrospective analysis with up to 30 years of follow-up, reported screw loosening rates of 12.2% in single-unit restorations and 10.7% in splinted prostheses. Similarly, Lemos et al. [64] observed screw loosening exclusively in monolithic ceramic single crowns and not in fixed partial dentures, accounting for 27.5% of complications within the single-crown group.
Ju et al. [46] reported screw loosening as the most frequent complication (46.4%) in a 6-year retrospective study, with significantly higher incidence in single-unit restorations. Interestingly, prostheses delivered by more experienced prosthodontists exhibited lower complication rates, emphasizing the importance of operator-dependent factors. In another long-term investigation, screw loosening occurred only in freestanding narrow-diameter implant restorations, whereas no cases were observed in splinted prostheses [41].
However, evidence remains inconsistent. Li et al. [32], in a systematic review and meta-analysis, found no statistically significant difference between splinted and non-splinted short implants regarding screw loosening. Therefore, although splinting may theoretically reduce stress concentration and improve load distribution, current evidence does not definitively support its universal protective effect [16,32] (Table 2).

4.2. Screw Retained vs. Cemented

Retention type has been extensively investigated as a potential determinant of implant screw loosening. Most clinical studies suggest that screw-retained restorations exhibit a greater incidence of screw-related complications compared with cement-retained alternatives, although findings remain inconsistent across the literature.
In a 6-year retrospective study, Lee et al. [39] reported higher rates of screw loosening in screw-retained restorations (10.1%) than in cement-retained prostheses. Similarly, a large retrospective analysis of 5491 implant-supported restorations demonstrated greater frequency of screw-related complications among screw-retained crowns, with lateral screw loosening occurring at a rate of 1.06 events per 100 prostheses annually, whereas decementation in cement-retained restorations occurred at a lower rate of 0.57 events [66]. These findings suggest that screw-retained prostheses may be more susceptible to mechanical complications, particularly during the early period of functional loading [67].
Comparable observations were reported by Nissan et al. [43], who, in a split-mouth randomized clinical trial, demonstrated significantly higher incidence of abutment screw loosening in screw-retained restorations. Systematic reviews and meta-analyses generally support this trend, reporting increased prevalence of screw loosening in screw-retained restorations for both single crowns and fixed dental prostheses [6,16]. Similar conclusions have been proposed in narrative reviews evaluating long-term technical complications [47,48,68].
However, conflicting evidence exists. Kraus et al. [44], in a 7.5-year randomized clinical trial, reported screw loosening in 10.5% of cement-retained crowns and none in screw-retained restorations, although the difference was not statistically significant. Likewise, Sherif et al. [17], in a systematic review, identified screw loosening as a frequent technical complication (3.66 events per 100 implant-years) but observed no significant influence of retention type, findings further supported by Hamed et al. [69].
The heterogeneity among studies suggests that retention type alone is unlikely to determine screw loosening risk. Rather, the incidence of loosening appears to be influenced by interactions among prosthetic design, occlusal loading, implant position, connection geometry, clinician technique, and maintenance protocols. Importantly, although screw-retained restorations may exhibit greater susceptibility to screw-related complications in some studies, they offer substantial advantages regarding retrievability, maintenance access, and management of biological or prosthetic complications.
Consequently, current evidence does not conclusively support the superiority of either retention strategy in preventing screw loosening. Selection between screw-retained and cement-retained restorations should therefore be individualized according to prosthetic design, esthetic requirements, maintenance considerations, and patient-specific risk factors.

4.3. Screw Material, Implant–Abutment Fit and Component-Related Variables

Evidence regarding the influence of component-related factors on implant screw loosening remains limited and heterogeneous. Among these variables, screw material, framework fit, and implant–abutment compatibility may contribute to preload maintenance and long-term mechanical stability (Table 3).
In a 14-year retrospective study evaluating mechanical complications in internal conical connection implants, Yi et al. [47] reported screw loosening in 46.4% of implants and 56.8% of patients. Gold screws demonstrated a lower risk of loosening compared with titanium screws; however, this advantage was accompanied by a greater susceptibility to screw fracture. These findings suggest a potential trade-off between improved preload maintenance and long-term mechanical durability, although evidence remains insufficient to support recommendations favoring a specific screw material. Consequently, screw composition alone is unlikely to represent an independent determinant of screw loosening risk.
The accuracy of implant–abutment fit may exert a more substantial influence on screw stability than material properties themselves. Experimental studies have shown that horizontal misfit at the implant–abutment interface can reduce preload and increase torque loss, thereby predisposing screws to loosening [38]. Clinical investigations, although scarce, generally support this association.
Kallus and Bessing [18], in an early long-term evaluation of full-arch osseointegrated prostheses, associated screw loosening with inaccuracies in framework fit and operator-related variables. Similarly, Jokstad and Shokati [19] demonstrated that greater framework misfit in implant-supported fixed prostheses was associated with increased incidence of screw-related complications, including loosening and fracture, over more than a decade of follow-up.
Biomechanically, non-passive superstructures may lead to uneven preload distribution despite identical tightening torque. When frameworks exhibit misfit, tightening may generate irregular elongation and localized stress concentrations within the screw, increasing the likelihood of preload loss and subsequent mechanical complications [19]. These observations emphasize the importance of passive fit as a prerequisite for long-term prosthetic stability.
The use of compatible or non-original abutments represents another clinically relevant but poorly investigated variable. Although non-original components are increasingly used because of lower cost and wider availability, long-term clinical evidence remains extremely limited. Most published studies have evaluated original components under controlled university-based conditions. To date, no clinical investigations with follow-up periods exceeding 5 years have directly compared original and non-original abutments across different implant–abutment connection types. Consequently, the influence of compatible abutments on screw loosening remains uncertain, and definitive clinical recommendations cannot currently be made.
Overall, current evidence suggests that component-related factors—particularly framework fit and implant–abutment accuracy—may influence screw stability more strongly than screw material itself. However, substantial gaps remain in the literature, especially regarding non-original components and long-term clinical performance.

4.4. Abutment Angulation and Implant–Abutment Connection

Abutment angulation and implant–abutment connection geometry have both been proposed as important determinants of screw stability because they may alter force distribution, preload maintenance, and resistance to rotational displacement. However, clinical evidence remains heterogeneous, and the independent contribution of these variables is difficult to isolate.
Recent studies suggest that abutment angulation may increase the risk of screw loosening, particularly in restorations exposed to non-axial loading. De Beus et al. [70], in a 5-year prospective study of full-zirconia single molar restorations with angulated screw channels, reported screw loosening in 9 abutments, accounting for 30% of the cases. Three of those 9 instances of screw loosening were observed in the same patient. Likewise, Ju et al. [46] identified screw loosening as the most common complication (46.4%) and demonstrated that buccolingual implant–abutment angulation significantly increased loosening risk by approximately 6% per degree. Cai et al. [71] further reported screw loosening in 13% of angulated screw-retained restorations compared with 5% loss of retention in cement-retained prostheses, supporting a potential interaction between abutment angulation and prosthetic design.
Conversely, substantially lower incidences have been reported in Morse taper systems. Yang et al. [72] observed screw loosening in only 1.27% of implants after up to 9 years of follow-up, although complications remained more common in restorations utilizing angulated abutments. A systematic review by Omori et al. [73] similarly identified screw loosening and abutment loosening among the most frequent complications associated with angulated abutments, indicating a potential adverse influence of angulation on long-term screw stability. Nevertheless, angulation frequently coexists with additional risk factors—including cantilevers, implant position, and occlusal overload—making causality difficult to establish.
The influence of implant–abutment connection type remains equally controversial. External connections, traditionally characterized by external hexagonal designs, have been progressively replaced by internal geometries (internal hexagon, octagon, Morse taper), which are presumed to improve anti-rotational stability and reduce micromovement. Several investigations support this assumption. Pjetursson et al. [6] reported greater incidence of screw loosening in external connections (up to 5%) compared with internal systems (approximately 1–1.3%), findings supported by Lee et al. [39], who observed higher rates in external implants (8.9%). Similar trends favoring internal connections were reported by Gracis et al. [12] and Sailer et al. [53].
However, conflicting evidence exists. Vigolo et al. [74] found no significant differences between internal and external systems, while Theoharidou et al. [10], in a systematic review, reported nearly identical 3-year screw-loosening rates for external (≈2.7%) and internal connections (≈2.4%). Furthermore, some retrospective evidence indicates higher rates of screw loosening and component fracture in internal connections, whereas external systems may be associated more frequently with biological complications.
Lemos et al. [33], in a systematic review including 11 studies, 530 patients, and 1089 implants, reported numerically higher overall complication rates in external connection implants; however, differences were not statistically significant. These findings suggest that although internal connections are often assumed to improve mechanical stability, current evidence does not conclusively demonstrate their superiority in preventing screw loosening.
Collectively, available evidence indicates that abutment angulation and connection design may influence screw stability, but neither variable appears to act independently (Table 4). Instead, their effects possibly interact with prosthetic design, occlusal loading, implant position, retention type, and maintenance protocols.

4.5. Implant Dimensions and Crown-to-Implant Ratio

The influence of implant dimensions on screw loosening remains controversial, with available evidence demonstrating inconsistent associations between implant diameter, implant length, and mechanical complications. Overall, current findings suggest that implant dimensions alone are unlikely to be major determinants of screw stability.
Regarding implant diameter, early evidence by Cho et al. [75] indicated a lower incidence of screw loosening in wide-diameter implants (5.8%) compared with standard-diameter implants (14.5%) when screws were tightened manually. However, after retightening with a torque driver, no additional cases of loosening occurred, emphasizing the potential importance of preload maintenance rather than implant diameter itself.
Conversely, Lee et al. [39] reported the highest incidence of screw loosening in implants with diameters ≥ 5 mm (14.2%) and in molar restorations (8.5%), with complications occurring most frequently in implant-supported single crowns (14.0%). Similarly, a 6-year retrospective observational study [76] identified screw loosening as the most common technical complication (16.2%), demonstrating greater incidence in implants with diameters ranging from 3.5 to 4.8 mm compared with narrower or wider implants.
In contrast, other investigations found little or no influence of implant diameter. An 8-year retrospective study evaluating 450 single Ankylos implants reported screw loosening in only 2.2% of cases, without significant differences among implant diameters. Likewise, Yeh et al. [77], in a systematic review and meta-analysis, reported incidences of 1.73%, 4.08%, and 12.45% for narrow, regular, and wide platform implants, respectively, but observed no statistically significant differences in calculated risk ratios among groups. Collectively, these findings suggest that although numerical variations exist, implant diameter does not consistently predict screw loosening risk.
Evidence regarding implant length is more consistent, generally indicating minimal influence on screw loosening. Badaro et al. [78], in a systematic review evaluating extra-short implants (≤6 mm), reported screw loosening in 14.73% of restorations without significant differences compared with conventional implants. Similarly, Gulje et al. [79], in a 5-year prospective study comparing 6 mm and 11 mm implants supporting single crowns in the posterior maxilla, found no significant differences in prosthetic complications between groups.
The crown-to-implant ratio (CIR) has also been proposed as a potential biomechanical risk factor because increased leverage may theoretically elevate stress at the implant–abutment interface. Nevertheless, clinical evidence remains inconclusive. In a 5-year prospective study evaluating extra-short implants, restorations with a crown-to-implant ratio ≥ 2 demonstrated numerically greater prosthetic complications, including screw loosening (12.5%), compared with restorations exhibiting a ratio < 2 (6%); however, differences were not statistically significant [41].
Overall, available evidence suggests that implant diameter, implant length, and crown-to-implant ratio exert, at most, a modest influence on screw loosening when considered independently (Table 5). Occlusal loading, prosthetic design, parafunctional activity, and maintenance protocols may play substantially greater roles in determining long-term screw stability.

4.6. Occlusal Loading, Parafunction and Cantilevers

Functional loading conditions, parafunctional habits, and prosthetic design appear to exert a substantial influence on implant screw stability. Among the various risk factors investigated, bruxism, cantilever extensions, and excessive occlusal loading demonstrate some of the strongest associations with screw loosening, suggesting that patient-related and biomechanical variables may be more influential than implant dimensions or connection type alone.
The adverse effect of cantilever loading has been repeatedly reported. Vieira et al. [20], in a systematic review and meta-analysis of posterior implant-supported cantilever fixed dental prostheses, identified loss of retention as a frequent complication, reporting pooled rates of 12% for cement-retained restorations and 20% for screw-retained prostheses, with screw loosening representing a major mechanical complication. Similarly, Da Silva Bezerra et al. [21], in a scoping review, observed increased incidence of screw loosening in prostheses with one or two cantilevers compared with restorations lacking cantilever extensions. These findings support the hypothesis that non-axial loading and increased bending moments may accelerate preload loss and compromise screw stability.
Parafunctional activity has also emerged as a significant risk factor. Vidal et al. [22], in a systematic review, reported substantially greater incidence of screw loosening in patients with bruxism (28.25%) compared with non-bruxers (17.96%), corresponding to an approximately 3.4-fold increase in calculated risk ratio. Long-term retrospective evidence reinforces these observations. Chrcanovic et al. [80], in a 30-year retrospective analysis, documented 292 screw loosening events and identified younger age and bruxism as significant risk indicators.
Clinical studies similarly emphasize the importance of occlusal overload. Kumari et al. [81], evaluating screw-retained cantilever prostheses in the anterior mandible, reported screw loosening in 15 of 64 patients after only 2.5 years of follow-up. Ju et al. [46] identified generalized attrition as one of the strongest predictors of screw loosening, increasing risk approximately 15-fold. Likewise, Malo et al. [82], in a retrospective cohort with follow-up up to 10 years, reported prosthetic and abutment screw loosening rates of 6.5% and 8%, respectively, primarily associated with parafunctional habits, occlusal factors, and cantilever presence.
Collectively, current evidence indicates that occlusal overload and parafunctional activity may represent among the most clinically relevant and potentially modifiable predictors of screw loosening (Table 6). Unlike implant dimensions or connection design, these factors may be addressed through occlusal adjustment, prosthetic design optimization, reduction of cantilever length, and protective interventions such as night guards in patients with bruxism.

4.7. Complete-Arch Implant-Supported Prostheses

Long-term evidence indicates that screw-related complications remain common in complete-arch implant-supported rehabilitations, despite improvements in implant systems, restorative materials, and prosthetic protocols. Reported incidence varies considerably, often influenced by prosthetic design, attachment type, cantilever length, and implant positioning.
Saponaro et al. [60], in a retrospective study evaluating zirconia implant-supported prostheses on Ti-base abutments, reported prosthetic screw loosening in 4.57% of cases, making it the second most frequent complication after veneering fracture. Importantly, complication rates differed according to prosthetic design, with complete-arch restorations demonstrating greater incidence than single-unit prostheses.
The influence of attachment design was highlighted in a randomized clinical trial, where screw loosening occurred most frequently in restorations supported by telescopic attachments compared with bar and stud systems [61]. Notably, most complications developed within the first year of function, suggesting that early loading periods may represent a critical phase for the development of screw-related complications.
Additional evidence suggests that prosthetic geometry significantly affects long-term screw stability. In a clinical study evaluating maxillary full-arch restorations, prosthetic and abutment screw loosening occurred in 16.6% and 8.1% of cases, respectively. Higher incidence was associated with 30° abutments, increased cantilever length, and distal implant positioning [62], reinforcing the importance of load distribution and prosthetic planning.
Broader evidence from an umbrella review including studies with ≥5 years of follow-up reported screw loosening rates ranging between 5% and 15% in full-arch implant-supported restorations [63]. These findings indicate that screw-related complications remain prevalent over time, irrespective of prosthetic material or implant configuration.
Collectively, current evidence suggests that complete-arch prostheses represent a higher-risk restorative category for screw loosening, particularly when combined with cantilevers, angulated abutments, distal implant positioning, or complex attachment systems. Consequently, careful prosthetic planning and long-term maintenance protocols appear essential for minimizing complications in these restorations. The variables associated with screw-loosening in complete-arch restorations are summarized in Table 7.

4.8. Torque-Limiting Devices and Tightening Protocols

Accurate torque application during implant restoration is considered essential for achieving optimal preload and maintaining long-term screw stability. Consequently, torque-limiting devices are routinely employed during implant prosthetic procedures. However, their effectiveness may be influenced by operator experience, calibration frequency, sterilization procedures, and variability among device types, potentially affecting the torque delivered to the screw [23].
Surveys among clinicians demonstrate considerable heterogeneity in tightening protocols, calibration practices, and overall understanding of preload concepts [24]. Despite widespread use of torque devices, awareness regarding factors influencing preload maintenance appears limited, suggesting variability in clinical implementation.
Experimental and meta-analytic investigations have reported inconsistent findings regarding the accuracy and reliability of different torque wrench systems. Variability has been observed among mechanical and electronic devices, between manufacturers, and following repeated clinical use or sterilization cycles [25,26,27,28,29,30]. Such findings indicate that deviations between intended and delivered torque may occur in routine clinical practice.
Nevertheless, the clinical implications of these differences remain uncertain. Although inaccuracies in torque delivery may theoretically influence preload and screw stability, most clinicians report a relatively low incidence of screw loosening despite heterogeneous tightening protocols [24]. Moreover, current evidence has not demonstrated a consistent association between specific torque wrench types, brands, or tightening values and the long-term occurrence of screw loosening.
Therefore, available evidence suggests that appropriate torque application is important for achieving preload, yet no specific torque device or tightening protocol has been conclusively shown to reduce screw loosening risk clinically (Table 8). This discrepancy highlights the need for long-term prospective studies investigating the relationship between torque delivery, preload maintenance, and mechanical complications.

5. Clinical Recommendations for Minimizing Implant Screw Loosening

Based on current biomechanical and clinical evidence, prevention of screw loosening in implant-supported restorations requires a multifactorial approach integrating optimal preload generation, prosthetic design, occlusal management, and long-term maintenance. Because screw loosening arises from interactions among mechanical, prosthetic, and patient-related factors, no single preventive measure appears sufficient.
Achieving and maintaining adequate preload remains fundamental. Clinicians should adhere strictly to manufacturer-recommended torque values and use calibrated torque-limiting devices during prosthetic procedures. Given the documented variability in torque delivery associated with device type, sterilization, and operator technique [23,24,25,26,27,28,29,30], periodic calibration and proper handling are essential. Retightening the abutment screw after a short interval (approximately 10 min) may compensate for embedment relaxation (settling effect) and help reduce early preload loss [13,15,35].
Prosthetic design plays a major role in screw stability. When clinically feasible, splinting implants—particularly in regions subjected to elevated occlusal forces—may improve load distribution and reduce stress concentration at the implant–abutment interface. Conversely, single-unit restorations require careful planning because several studies suggest greater susceptibility to screw loosening in non-splinted restorations [32,45,46,64,65]. Similarly, minimizing cantilever extensions and optimizing the cantilever-to-anteroposterior spread ratio may reduce bending moments associated with screw-related complications [20,21,82].
The selection and design of abutments should also be considered carefully. Straight abutments may be preferable to angulated alternatives whenever clinically possible, as increased angulation has repeatedly been associated with greater incidence of mechanical complications, including screw loosening [46,70,71,72,73]. In situations requiring angulated abutments, clinicians should recognize the increased biomechanical demands and consider additional strategies to control occlusal loading.
Ensuring an accurate and passive prosthetic fit is equally important. Framework misfit may result in uneven preload distribution, localized stress concentration, and progressive torque loss, increasing susceptibility to screw loosening and fracture [18,19]. Consequently, passive fit should remain a key objective during prosthetic fabrication and delivery.
Occlusal management appears particularly important in long-term screw stability. Careful adjustment to achieve stable, evenly distributed contacts and minimize excessive lateral loading is recommended. Special consideration should be given to patients exhibiting parafunctional habits such as bruxism, as these individuals consistently demonstrate increased risk of screw loosening [22,46,81]. Protective measures—including occlusal splints—and more frequent maintenance intervals may therefore be justified in high-risk patients.
Regarding retention strategy, screw-retained restorations may exhibit greater incidence of screw-related complications in some studies; however, they provide substantial advantages in retrievability, maintenance access, and management of biological or prosthetic complications [6,16,17,43,44,66,67,68,69]. Selection between screw-retained and cement-retained restorations should therefore be individualized according to prosthetic requirements, esthetic considerations, and patient-specific risk factors rather than complication rates alone.
Finally, structured maintenance protocols and follow-up are indispensable. Multiple studies indicate that a considerable proportion of screw loosening events occur during the first year of function [39,61], emphasizing the importance of early monitoring. Periodic clinical and radiographic evaluation facilitates early identification of complications before progression to recurrent loosening, component fracture, or prosthetic failure.
Overall, current evidence suggests that implant screw loosening can be minimized—but not entirely eliminated—through precise torque application, optimized prosthetic design, passive implant–abutment fit, occlusal control, and individualized maintenance strategies (Table 9).

6. Limitations

This review has several limitations that should be considered when interpreting the findings. First, although a structured search strategy was applied, the present work was designed as a narrative review rather than a systematic review; therefore, study selection and evidence synthesis may be subject to a greater degree of interpretive bias. In addition, no formal risk-of-bias assessment or methodological quality appraisal of the included studies was performed, limiting the ability to evaluate the strength and certainty of the available evidence. The exclusion of non-English publications should be considered as a potential source of selection bias.
The included literature demonstrated substantial heterogeneity regarding implant systems, implant–abutment connection designs, prosthetic materials, retention strategies, loading protocols, and definitions of screw loosening, making direct comparison among studies difficult. Considerable variability was also observed in follow-up duration, ranging from medium-term evaluations to studies exceeding 10 years, which may influence reported complication rates and long-term outcomes.
Furthermore, much of the available evidence originated from retrospective observational studies, while high-quality randomized controlled trials and long-term prospective investigations remain limited. Retrospective designs are inherently susceptible to selection bias, incomplete reporting, and uncontrolled confounding variables. Finally, publication bias cannot be excluded, as studies reporting significant complications or positive findings may be more likely to be published than studies with negative or inconclusive results.
Consequently, the conclusions of this review should be interpreted with caution. Future research should prioritize well-designed prospective clinical trials with standardized reporting criteria, longer follow-up periods, and direct comparison of implant systems, prosthetic variables, and maintenance protocols to strengthen the evidence base regarding implant screw loosening.

7. Conclusions

Implant screw loosening remains one of the most common technical complications associated with implant-supported restorations and represents a multifactorial phenomenon influenced by biomechanical, prosthetic, and patient-related variables. Current evidence suggests that loss of preload, occlusal overload, parafunctional habits, cantilever extensions, prosthetic misfit, and abutment angulation may exert a greater influence on screw stability than isolated implant characteristics such as implant diameter or length. Although internal implant–abutment connections, splinted restorations, and specific prosthetic designs have been proposed to reduce screw loosening, available clinical evidence remains heterogeneous and, in several areas, inconclusive.
Achieving and maintaining adequate preload appears fundamental for long-term screw stability. Accurate torque application, passive prosthetic fit, careful occlusal management, minimization of cantilevers, and structured maintenance protocols may contribute to reducing mechanical complications. Particular attention should be given to patients with parafunctional habits or high occlusal loading, as these individuals consistently demonstrate increased risk of screw loosening.
Importantly, no single preventive strategy has been shown to completely eliminate screw loosening, reinforcing the need for an individualized and comprehensive clinical approach. Furthermore, substantial evidence gaps remain regarding the long-term performance of non-original abutments, torque protocols, and angulated screw channels.

Author Contributions

Conceptualization, S.T. and E.P.; methodology, S.T.; software, S.T.; validation, S.T., S.D. and E.P.; formal analysis, S.T. and S.D. investigation, S.T. and S.D.; resources, S.T.; data curation, S.T.; writing—original draft preparation, S.T.; writing—review and editing, S.T., S.D. and E.P.; visualization, S.T. and S.D.; supervision, E.P.; project administration, E.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SCSingle crown
FDPFixed dental prosthesis
CIsConfidence intervals
RCTRandomized controlled trial
CAD-CAMComputer-assisted design/computer-assisted manufacture
CIRCrown-to-implant ratio
PFMPorcelain fused to metal
PICNPolymer-infiltrated ceramic network

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Table 1. Reported incidence of screw loosening according to prosthesis type and representative long-term studies.
Table 1. Reported incidence of screw loosening according to prosthesis type and representative long-term studies.
Prosthesis TypeRepresentative StudiesReported
Incidence		Interpretation
Single crowns[6,9,39]~1–13%Often higher complication rates
FDPs/splinted restorations[3,16,45]Generally lowerPossible protective effect
Monolithic zirconia
restorations		[31]Up to 2.25%
annually		Material/design dependent
Full-arch restorations[60,61,62,63] *~5–16%Influenced by
cantilevers/angulation
* discussed later in Section 4.
Table 2. Influence of splinting on screw loosening.
Table 2. Influence of splinting on screw loosening.
VariableRepresentative
Studies		Evidence
Trend		Clinical Implication
Single-unit restorations[45,46,64]Often higher loosening ratesIncreased stress concentration
Splinted prostheses[45,65]Potentially lower incidenceImproved force distribution
Meta-analysis[32]No significant differenceEvidence remains conflicting
Table 3. Component-related variables influencing screw loosening.
Table 3. Component-related variables influencing screw loosening.
VariableRepresentative
Studies		Evidence
Trend		Clinical Implication
Gold screws[47]Lower loosening riskHigher fracture risk
Titanium screws[47]Greater looseningLower fracture tendency
Framework misfit[18,19]Increased complicationsImprove passive fit
Horizontal implant–abutment misfit[38]Reduced preloadPotential torque loss
Non-original abutments—Insufficient evidenceNo recommendation possible
Table 4. Implant-related variables and screw loosening.
Table 4. Implant-related variables and screw loosening.
VariableRepresentative
Studies		Evidence TrendInterpretation
Angulated abutments[46,70,71,72,73]Often higher looseningUse cautiously
Internal connection[6,12,53]Possibly lower incidenceEvidence conflicting
External connection[10,33,74]Mixed findingsNo definitive disadvantage
Morse taper[41,72]Low incidence reportedLong-term data limited
Table 5. Implant dimensions and screw loosening.
Table 5. Implant dimensions and screw loosening.
VariableRepresentative
Studies		Evidence TrendClinical Implication
Wide implant
diameter		[39,75,77]Conflicting findingsNo definitive protective effect
Implant length[78,79]Minimal influenceExtra-short implants acceptable
Crown-to-implant
Ratio (CIR) ≥ 2		[41]Numerically higher
complications		Monitor high CIR restorations
Retightening
protocols		[75]Reduced subsequent
loosening		May outweigh implant dimensions
Table 6. Occlusal and patient-related risk factors associated with screw loosening.
Table 6. Occlusal and patient-related risk factors associated with screw loosening.
VariableRepresentative
Studies		Evidence TrendClinical Implication
Bruxism[22,46,80]Strong associationConsider occlusal splints/
maintenance
Cantilevers[20,21,80,82]Increased loosening riskMinimize extension length
Generalized attrition[46]~15-fold increased riskCareful occlusal assessment
Younger age[81]Higher incidenceIncreased follow-up
Excessive functional load[20,21,22,80,81,82]Consistent adverse effectProsthetic design optimization
Table 7. Variables associated with screw loosening in complete-arch restorations.
Table 7. Variables associated with screw loosening in complete-arch restorations.
VariableRepresentative
Studies		Evidence TrendClinical Implication
Complete-arch design[60,61,62,63]Higher long-term
complication rates		Increased maintenance needs
Telescopic attachments[61]Greater loosening
incidence		Careful attachment
selection
30° abutments[62]Increased riskLimit angulation when possible
Cantilever length[62]Higher complication ratesReduce extension
Distal implant positioning[62]Increased looseningProsthetic planning critical
Early loading period[61]More complications in first yearFrequent early follow-up
Table 8. Evidence regarding torque-limiting devices and screw loosening.
Table 8. Evidence regarding torque-limiting devices and screw loosening.
VariableRepresentative
Studies		Evidence TrendClinical Implication
Operator technique[23,24]Influences delivered torqueTraining and calibration important
Torque device calibration[24,25,26,27,28,29,30]Variable accuracyPeriodic verification recommended
Sterilization cycles[25,26,27,28,29,30]May affect performanceMonitor device wear
Torque wrench type/brand[25,26,27,28,29,30]Inconsistent findingsNo proven superiority
Tightening protocol[23,24,25,26,27,28,29,30]Limited clinical evidenceStandardization needed
Table 9. Evidence-based clinical recommendations for minimizing screw loosening.
Table 9. Evidence-based clinical recommendations for minimizing screw loosening.
RecommendationSupporting
Evidence		Expected
Effect		Strength of
Evidence
Retighten after settling[13,15,35]Reduce early preload lossModerate
Use calibrated torque devices[23,24,25,26,27,28,29,30]Improve torque consistencyModerate
Minimize cantilevers[20,21,82]Reduce non-axial loadingModerate–high
Splint implants selectively[32,45,46,64,65]Improve force distributionConflicting
Prefer passive framework fit[18,19]Reduce uneven preloadModerate
Control bruxism/occlusal overload[22,46,81]Lower mechanical complicationsHigh
Early follow-up (<1 year)[39,61]Detect complications soonerModerate
These ratings reflect the authors’ narrative judgment and have not been derived from a formal grading system such as GRADE.
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Thomaidis, S.; Diamantopoulou, S.; Papazoglou, E. Implant Screw Loosening: A Narrative Review of Medium- and Long-Term Clinical Evidence. Appl. Sci. 2026, 16, 6253. https://doi.org/10.3390/app16126253

AMA Style

Thomaidis S, Diamantopoulou S, Papazoglou E. Implant Screw Loosening: A Narrative Review of Medium- and Long-Term Clinical Evidence. Applied Sciences. 2026; 16(12):6253. https://doi.org/10.3390/app16126253

Chicago/Turabian Style

Thomaidis, Socratis, Sofia Diamantopoulou, and Efstratios Papazoglou. 2026. "Implant Screw Loosening: A Narrative Review of Medium- and Long-Term Clinical Evidence" Applied Sciences 16, no. 12: 6253. https://doi.org/10.3390/app16126253

APA Style

Thomaidis, S., Diamantopoulou, S., & Papazoglou, E. (2026). Implant Screw Loosening: A Narrative Review of Medium- and Long-Term Clinical Evidence. Applied Sciences, 16(12), 6253. https://doi.org/10.3390/app16126253

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