Abutment screw torque changes with straight and angled screw-access channels

A method of removing the stripped abutment screw from the angled screw channel: Technical details and a clinical report

An angled screw channel (ASC) avoids a facial screw hole by correcting the pathway of the screwdriver. However, the structure of the specially designed screw is prone to mechanical complications, including screw-head stripping. Removing an angled screw is challenging because regular screw-removal tools cannot access the ASC. A safe and convenient method for the retrieval of the stripped screw is reported.

Angled Screw Channel-Retained vs. Cement-Retained Implant Crowns in Nonmolar Sites: A Systematic Review and Meta-Analysis

OBJECTIVES

To evaluate the clinical, radiographic, and aesthetic outcomes of angled screw channel (ASC) retained implant crowns to cement-retained implant crowns in nonmolar sites.

METHODS

Randomized and nonrandomized trials comparing ASC-retained and cement-retained implant crowns in single nonmolar locations were found by searching electronic databases (COCHRANE, EMBASE, and MEDLINE) up to January 2025. Changes in marginal bone level were the primary outcomes, whereas periodontal parameters, aesthetic outcomes, and technical complications were the secondary outcomes. Random-effects meta-analysis was used to calculate pooled effect sizes.

RESULTS

A total of 973 studies were identified, of which four studies with 167 single nonmolar implant crowns were included. Overall meta-analysis showed that the difference in marginal bone level was in favor of the ASC group, but the difference was not statistically significant (MD -0.03; 95% CI -0.12 to 0.06; p = 0.57). The ASC group had more positive changes in pink aesthetic score than the cemented group; however, the difference between the two groups was not statistically significant (MD -0.18; 95% CI -0.88 to 0.51; p = 0.61).

CONCLUSIONS

ASC-retained implant crowns in nonmolar sites have comparable short-term clinical, radiographic, and aesthetic outcomes to cement-retained implant crowns, with less bleeding on probing expected with ASC-retained crowns.

CLINICAL SIGNIFICANCE

The superiority of ASC-retained implant crowns over cement-retained implant crowns in nonmolars was not proven. There were no significant differences between ASC-retained and cement-retained implant crowns in terms of mean changes in marginal bone levels, probing pocket depths, aesthetic outcomes, technical complications, and implant failure.

Assessment of the internal connection of an angled screw channel system

STATEMENT OF PROBLEM

The angled screw channel (ASC) design has been well accepted for implant prostheses. However, investigation into the behavior of the ASC connection is sparse.

PURPOSE

The purpose of this in vitro study was to assess the effect of cyclic loading on the internal connection of an ASC system compared with straight access systems by measuring reverse torque values (RTVs) and using microcomputed tomography (µCT) imaging.

MATERIAL AND METHODS

Thirty dental implants (SLActive; Institut Straumann AG) were embedded in autopolymerizing acrylic resin and divided into 3 groups: Group V0, prefabricated abutment with a straight access channel; Group V25, prefabricated abutment with a 25-degree access channel; and Group C, cemented custom abutment with straight access. All specimens were initially secured with 10 Ncm using a digital torque gauge. A subset of specimens from each group (n=4) were randomly selected and assigned for µCT imaging examination and then reimaged after tightening to 35 Ncm. All specimens were subjected to cyclic loading for 5000 cycles with a loading force of 155 ±5 N at 1.2 Hz frequency in a 37 °C water bath using a fatigue machine. Repeat µCT imaging was done after cyclic loading for the representative specimens. The changes to the internal connection were measured on µCT images using a software program, and the RTV measurement of each specimen after cyclic loading treatment was recorded. The ANOVA test was used to analyze the statistical significance of the RTVs among groups (α=.05). The paired t test was used to conduct within-group comparisons of the measurement data.

RESULTS

Mean ±standard deviation of RTVs varied from 29.05 ±2.99 Ncm to 29.75 ±2.61 Ncm, with no statistically significant differences among the 3 groups (P=.820). The distances from the base of the screw head to the apex of the screw-abutment-implant (SAI) system and internal air space volume within the groups V0 and V25 decreased significantly (P<.05) between the 10-Ncm and 35-Ncm torque applications. Off-axis loading resulted in rotational movement of the abutment that might have created a tensional force on the implant screw.

CONCLUSIONS

Results for the prefabricated abutment groups (V0, V25) indicated that the RTVs were not affected by the amount of angle correction. A counterclockwise rotational movement of the abutment and screw within the SAI system would be formed during off-axial loading.

Mechanical and biological complications of angled versus straight screw channel implant-supported prostheses: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Scientific evidence to determine the clinical performance of angled screw channel (ASC) versus straight screw channel (SC) implant-supported prostheses is lacking.

PURPOSE

This systematic review and meta-analysis investigated the mechanical and biological complications of ASC compared with those of SC implant-supported prostheses.

MATERIAL AND METHODS

A systematic search was conducted by following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guideline in the PubMed, Embase, and Web of Science databases and was supplemented with a manual search for clinical studies reporting the mechanical and biological complications of ASC compared with those of SC implant-supported prostheses. The search was focused on porcelain fracture, screw loosening or fracture, marginal bone loss (MBL), and pink esthetic score (PES). The data were extracted from selected articles and compounded to estimate the complications with a 95% confidence interval (CI) using a random effects meta-analysis. The publication bias was assessed using the Cochrane Risk of Bias and Newcastle-Ottawa Scale (α=.05).

RESULTS

A total of 4217 records were identified, and 14 studies were selected for quantitative synthesis of 629 participants with 658 ASC and 166 SC implant-supported prostheses. The meta-analyses of comparative studies showed no statistically significant difference in mechanical complications between ASC and SC prostheses, with odds ratio (OR) of 1.75 (95% CI=0.71-4.34, P=.224). Porcelain fracture and screw loosening were the most common complications with ASC prostheses. In addition, no statistically significant difference was found between ASC and SC prostheses in the MBL (mean difference=-0.07, 95% CI=-0.15-0.01, P=.077) and PES (mean difference=-0.19, 95% CI=-0.90-0.52, P=.593).

CONCLUSIONS

The clinical performance of ASC may be comparable with that of SC implant-supported prostheses in terms of mechanical and biological complications. However, the moderate level of evidence necessitates further research to validate these findings.

Screw loosening in angulation-correcting single implant restorations: A systematic review of in vitro studies

STATEMENT OF PROBLEM

Various options are available to allow angle correction for screw-retained restorations for malaligned implants, including angled abutments, angled screw channel abutments, and angled implants. However, the effect these angle correction components have on prosthetic screw loosening is unclear.

PURPOSE

The purpose of this systematic review was to assess the effect of angled abutments, angled screw channel abutments, and angled implants on prosthetic screw loosening.

MATERIAL AND METHODS

This manuscript followed the Preferred Reporting Items for the Systematic Review and Meta-Analyses (PRISMA) statement. Searches were performed through 31 December 2021 in PubMed/MEDLINE, EMBASE, and Web of Science with no year limit targeting in vitro studies evaluating the effect of angulation correction components on screw loosening.

RESULTS

A total of 460 articles were identified. After removing duplicates, 306 titles were screened. Nine of the remaining 36 articles selected for full-text analysis met the selection criteria. The qualitative analysis used data from 394 implants. All studies featured straight abutments as the control group, and, in 8 studies, the angle correction component was included on the level of the abutment. Only in 1 study was the angle correction component at the level of the implant. The angle correction of the prosthetic component varied from 0 degrees in all studies up to a maximum correction of 30 degrees. Statistically significant increases in screw loosening with increasing abutment angle correction were reported by multiple authors (P<.05). However, other articles reported nonsignificant differences in screw loosening because of angulation after cyclic loading (P>.05). The only study investigating angle correction at the implant level found significantly less screw loosening (P<.05) in the angled implant group compared with the nonangled implant group.

CONCLUSIONS

Several options are available to correct discrepancies between the surgical axis and the ideal prosthetic axis for rehabilitations supported by dental implants. The current evidence does not clearly indicate the superiority of any single solution for minimizing screw loosening.

Evaluation of different tightening protocols on angled screw channel abutments after cyclic loading: An in vitro study

STATEMENT OF PROBLEM

Screw loosening remains one of the most common complications associated with implant-supported restorations. The available literature is scarce regarding the most efficient method of tightening definitive abutment screws for angled screw channel (ASC) abutments.

PURPOSE

The purpose of this in vitro study was to identify the optimal screw tightening protocol for ASC abutments by evaluating the influence of different protocols on reverse tightening values (RTVs).

MATERIAL AND METHODS

Fifty implants were randomly assigned to 5 groups (n=10). Implants were mounted and received a cover screw and impression coping to simulate the clinical scenario. Each group was allocated to one of the following protocols: tightened once (1×), tightened twice (2×), tightened twice at a 10-minute interval (2× [10 min]), tightened, countertightened, then tightened again (TCT) and tightened, countertightened, tightened, countertightened, then tightened (TCTCT). A monolithic zirconia central incisor implant crown was fabricated by using computer-aided design and computer-aided manufacturing (CAD-CAM) and cemented onto the ASC abutment. The crown-abutment assemblies were secured onto the implants with the Gold-Tite abutment screw by using their assigned tightening protocols. Each screw was tightened to the recommended value of 20 Ncm with a calibrated digital torque measuring device. Specimens were subjected to 250 000 cycles of cyclic loading simulating 3 months of function with a universal testing machine. RTV was measured for each specimen.

RESULTS

The mean RTV for all the groups ranged from 13.5 Ncm to 15.9 Ncm. The highest mean RTV was found with the (1×) protocol. However, the difference was not statistically significant among any of the groups (P>.05).

CONCLUSIONS

Multiple retightening and time intervals between tightening events of these ASC abutment screws did not have a significant influence on RTV under simulated function of 3 months. The one time (1×) tightening protocol can be recommended as a suitable method for ASC abutments, thus avoiding unnecessary retightening.

The mechanical complications and behavior of angulated dental implant abutment systems versus conventional abutments, a narrative review

Background

Angulated screw channel (ASC) abutment allows off-axis dental implants to be used in dental restorations without the need for cementation. As this is a relatively new system, research on its clinical performance is limited.

Objectives

To summarize the available in-vitro and in-vivo studies on the mechanical and technical issues associated with the ASC system and compare its clinical performance with that of conventional implant-supported abutments.

Methods

A comprehensive literature search in PubMed, Web of Science, and ScienceDirect databases was performed, focusing on articles about angulated (angled) screw channel (ASC) systems published in English between January 2015 and November 2023. Only in-vitro and in-vivo studies were included.

Results

After analyzing the recorded articles, 26 studies (11 in vivo and 15 in vitro) were included in the final discussion and review.

Conclusion

Although the ASC system is still relatively new, and is presently outperformed by conventional abutment systems in terms of technical and mechanical properties, in short- and medium-term in-vivo studies, it was shown reliable for retaining single or multiple-unit implant restorations in both posterior and anterior zones. Still, further long-term clinical research is needed to fully elucidate the risk factors associated with ASC failures.

Comparison of Aesthetic, Mechanical Outcome, and Bone Loss in Angulated Screw Channels (ASCs) and Cement-Retained Implant-Supported Prosthesis: A Case-Control Study

Angulated-screw channels (ASCs) allow the clinician to employ screw-retained restorations in almost all cases, as the access hole can be moved away from the vestibular portion of the crown, where it would jeopardize the final esthetic result. The objective of this study was to compare screw-retained restorations employing ASCs with restorations cemented on angled abutments. In this study, 30 subjects, equally divided into two groups: group 1 (cemented restorations on angulated abutments) and group 2 (screw-retained restorations adopting ASCs), were treated and retrospectively compared after 2 years using the pink esthetic score (PES) and the white esthetic score (WES). All restorations were in use at the last follow-up, with a survival rate of 100%. Three mechanical complications were observed (2 chippings and 1 crown came loose), with a success rate of 93% in group 1 and 87% in group 2 (p > 0.05). No statistically significant differences were reported regarding the esthetic outcome; the marginal bone loss (MBL) showed better results for the screw-retained restorations, both at the distal aspect (group 1 = 0.98 mm ± 0.16; group 2 = 0.45 mm ± 0.06; p = 0.006) and at the mesial aspect (group 1 = 1.04 ± 0.27; group 2 = 0.45 ± 0.005; p < 0.001). From an esthetical perspective, screw-retained restorations with ASCs and cemented restorations on angulated abutments are both effective means of restoring implants; both have excellent esthetic outcomes, but screw-retained restorations have reduced bone loss when compared to cemented ones but are more prone to mechanical complications. Still, our results must be cautiously observed given the reduced dimension of our sample. Larger studies are needed to confirm our findings.

Influence of screw channel angulation on reverse torque value and fracture resistance in monolithic zirconia restorations after thermomechanical cycling: an in-vitro study

BACKGROUND

While the concept of angled screw channels has gained popularity, there remains a scarcity of research concerning the torque loss and fracture strength of monolithic zirconia restorations with various screw channel angulations when exposed to thermomechanical cycling. This in-vitro study aimed to evaluate the reverse torque value and fracture resistance of one-piece screw-retained hybrid monolithic zirconia restorations with angulated screw channels after thermomechanical cycling.

METHODS

One-piece monolithic zirconia restorations, with angulated screw channels set at 0°, 15°, and 25° (n = 6 per angulation) were fabricated and bonded to titanium inserts using a dual-cure adhesive resin cement. These assemblies were then screwed to implant fixtures embedded in acrylic resin using an omnigrip screwdriver, and reverse torque values were recorded before and after thermomechanical cycles. Additionally, fracture modes were assessed subsequent to the application of compressive load. One-way ANOVA and Bonferroni post hoc test were used to compare the groups (α = 0.05).

RESULTS

The study groups were significantly different regarding the fracture resistance (P = 0.0015), but only insignificantly different in the mean percentage torque loss (P = 0.4400). Specifically, the fracture resistance of the 15° group was insignificantly higher compared to the 0° group (P = 0.9037), but significantly higher compared to the 25° group (P = 0.0051). Furthermore, the fracture resistance of the 0° group was significantly higher than that of the 25° group (P = 0.0114).

CONCLUSIONS

One-piece hybrid monolithic zirconia restorations with angulated screw channels can be considered an acceptable choice for angulated implants in esthetic areas, providing satisfactory fracture strength and torque loss.

Clinical effect of Nobel Biocare angulated screw channel crown compared with cement crown in the aesthetic area: A retrospective cohort study with a mean 32 months follow-up (range 12 to 70 months)

OBJECTIVE

The objective of this study is to compare the clinical efficacy of angulated screw channel abutment applied in the anterior area with regular cemented crowns.

MATERIALS AND METHODS

Forty-eight patients were included and divided into two groups: the angulated screw channel group (ASC) and regular cemented group (RC) in this retrospective cohort study. The evaluation criteria included implant/restoration survival rate, keratinized mucosa width (KMW), bleeding on probing rate (BOP%), probing depth (PD), pink aesthetic score (PES), mechanical/biological complications, emergence angle (EA), the site of implant axis penetrate (SA), marginal bone loss (MBL), and buccal bone thickness (BBT) at 0 mm, 1 mm, 3 mm, 5 mm below the implant shoulder were evaluated in immediate postoperative (T0) and follow-up period (T1).

RESULTS

This retrospective cohort study included a total of 48 patients, with a mean 32 months follow-up period range from 12 months to 70 months. The study did not find any cases of implant failure or restoration failure. EA was significantly wider in the RC group than ASC group (RC: 33.53° ± 8.36° vs ASC: 27.43° ± 8.08°, p = 0.016*). While the BOP% was statistically significant higher in the RC group than ASC group (RC: 28.35% ± 22.92% vs ASC: 13.18% ± 20.00%, p = 0.027*). No significant differences were observed in the other measurements of comparison.

CONCLUSION

Within the limitations of the study, angulated screw channel (Nobel Biocare) crowns might allow the implant axis aim at incisal edge to reduce the emergence angle in the anterior area and benefit the soft-tissue during the 12-70 months follow-up period.

Effect of internal connection type and screw channel angle on the screw stability of anterior implant-supported zirconia crowns

OBJECTIVES

To investigate the effect of implant-abutment connection and screw channel angle on screw stability by comparing a newly introduced and an established connection, before and after cyclic loading.

MATERIALS AND METHODS

Implants (N = 44) with Torcfit (TF) or Crossfit (CF) connection were divided to be restored with a straight (CFS and TFS) or an angled screw access channel (CFA and TFA) titanium-base abutment (n = 11). CFA and TFA received screw-retained crowns, whereas CFS and TFS received hybrid zirconia abutments and cement-retained crowns. The initial torque value (ITV) of each complex (ITVI ) and removal torque value (RTV) after 24 h (RTVI ) were measured. Screws were replaced with new ones, ITVs were recorded again (ITVF ), and crowns were cyclically loaded (2.4 million cycles, 98 N) to measure RTVs again (RTVF ). Percentage torque loss was calculated. Data were analyzed (α = 0.05).

RESULTS

ITVs were similar among groups (p ≥ .089). CF led to higher RTVs (p ≤ .002), while CFS had higher RTVI than CFA (p = .023). After 24 h, CFS had lower percentage torque loss than TF, while CFA had lower percentage torque loss than TFA (p ≤ .011). After cyclic loading, CF led to lower percentage torque (p < .001).

CONCLUSION

The implant-abutment connection affected the removal torque values. However, no screw loosening occurred during cyclic loading, which indicated a stable connection for all groups. Screw access channel angle did not affect screw stability after cyclic loading.

Effects of cyclic loading on loss of abutment screw torque of angled screw channel single implant crowns on narrow diameter implants

STATEMENT OF PROBLEM

Screw access channels that emerge on the facial aspect of anterior screw-retained implant crowns can compromise esthetics. Recently, angled screw channels (ASCs) have been developed that can alter screw access channel angulations to improve esthetics. While ASCs can be used on narrow-diameter implants, information is limited on the loss of abutment screw torque on narrow-diameter implants with ASCs.

PURPOSE

The purpose of this in vitro study was to compare the loss of abutment screw torque after thermocycling and the cyclic loading of ASCs from 3 different companies connected to narrow-diameter implants.

MATERIAL AND METHODS

A total of 40 narrow-diameter implants (NobelReplace Conical Connection 3.5×11.5 mm) were mounted individually in acrylic resin blocks and divided equally into 4 groups. The first group, NB-0 (Control), consisted of screw-retained zirconia crowns fabricated on ASCs at 0-degree angulation (n=10). The other 3 groups consisted of a total of 30 screw-retained zirconia crowns fabricated on ASCs at 20-degree angulation: NB-20 (Angulated Screw Channel Solutions), ATL-20 (Atlantis Custom Base Solution with angulated screw access), and DA-20 (Dynamic TiBase). Each crown was secured on the mounted implant with its corresponding titanium base insert and screw and then tightened to the manufacturer's recommended torque with a digital torque gauge. The initial reverse torque value (RTVI) was obtained and recorded at baseline. Subsequently, a new set of screws were tightened to recommended values, and each specimen underwent thermocycling and then cyclic loading at 0 to 100 N at 10 Hz for 1 million cycles to simulate 1year of functional loading. After cyclic loading, the final reverse torque values (RTVF) were recorded and compared with the RTVI to evaluate the percentage torque loss (PTLF). Statistical analysis was performed using the Kruskal-Wallis analysis, Rank base analysis of covariance (ANCOVA), and the Tukey HSD post hoc comparisons (α=.05).

RESULTS

Significant differences were found for the PTLF among all groups after cyclic loading (P<.001). The PTLF in ATL-20 (51.4%) was significantly higher than in NB-0 (22.2%) (P<.001) and NB-20 (29.2%) (P=.010). No significant difference was found in the PTLF among other groups (P>.05).

CONCLUSIONS

The abutment screw torque loss after cyclic loading of the ASCs on narrow diameter implants among the 4 groups did not perform comparably. The largest percentage torque loss was recorded for the ATL-20 group. The NB-20 group demonstrated the lowest percentage torque loss. DA-20 showed percentage torque loss less than ATL-20; however, its RTVF was the lowest.

Influence of straight versus angulated screw channel titanium bases on failure loads of two-piece ceramic and titanium implants restored with screw-retained monolithic crowns: An in-vitro study

OBJECTIVE

To analyze the influence of titanium-base (straight [SSC]/angulated-screw-channel [ASC]) on failure-loads and bending-moments of two-piece ceramic and titanium-zirconium implants restored with monolithic-zirconia crowns after fatigue.

MATERIALS AND METHODS

Thirty-two anterior monolithic-screw-retained zirconia crowns were divided into four groups (n = 8/group) according to the factors: (1) type of implant material: two-piece titanium-zirconium implant (Ti-Zr; control-group) versus two-piece ceramic implant (CI; test-group) and (2) type of titanium-base: SSC (0° angle) versus ASC (25°). An intact implant was used for field emission gun-scanning electronic microscopy (FEG-SEM) characterization and Raman spectroscopy for phase analyses and residual stress quantification. All samples were exposed to fatigue with thermodynamic loading (1.2-million-cycles, 49 N, 1.6 Hz, 5-55°C) at a 30° angle. Surviving specimens were loaded until failure (SLF) and bending moments were recorded. Failed samples were examined using light microscope and SEM. Statistical analyses included ANOVA and Mann-Whitney U-test.

RESULTS

Raman-spectroscopy revealed the presence of residual compressive stresses. FEG-SEM revealed a roughened surface between threads and polished surface at the cervical-collar of the ceramic implant. All samples survived fatigue and were free of complications. Mean bending-moments (±SD) were: Ti-Zr-0: 241 ± 45 N cm, Ti-Zr-25: 303 ± 86 N cm, CI-0: 326 ± 58 N cm, CI-25: 434 ± 71 N cm. Titanium-base and implant-material had significant effects in favor of ASC titanium bases (p = .001) and ceramic-implants (p < .001). Failure analysis after SLF revealed severe fractures in ceramic implants, whereas titanium implants were restricted to plastic deformation.

CONCLUSIONS

Ceramic and titanium implants exhibited high reliability after fatigue, with no failures. From a mechanical perspective, titanium bases with ASC can be recommended for both ceramic and titanium implants and are safe for clinical application.

Frequency of screw-retained angulated screw channel single crown following immediate implant placement and provisionalization in the esthetic zone: A cone beam computed tomography study

OBJECTIVE

The purpose of this cone beam computed tomography (CBCT) study was to determine the frequency percentage of screw-retained crown using angulated screw channel (ASC) abutment for single immediate implant placement and provisionalization (IIPP) in the esthetic zone.

METHODS

The CBCT images of 200 patients without disease and without metal restorations in maxillary anterior teeth were evaluated. The mid-sagittal-sectional CBCT images of maxillary anterior teeth (#6-#11) were created in an implant planning software, screen-captured, and transferred to a presentation program. Template of tapered implants with diameter of 3.5 mm (for central and lateral incisors) and 4.3 mm (for central incisors and canines) and lengths of 13, 15, and 18 mm were applied to the sagittal images to identify the IIPP cases. To qualify for IIPP, the implant must engage >35% bone with at least 1 mm of surrounding bone and no perforations. The IIPP cases were further divided into straight screw channel (IIPPSSC) abutment or 25-degree angulated screw channel abutment (IIPPASC), based upon its restorability. The frequency percentages of possible IIPP, IIPPSSC, and IIPPASC were reported and compared among all maxillary anterior teeth.

RESULTS

A total of 1200 maxillary anterior teeth sagittal images from 200 patients (88 male and 112 female) with a mean age of 51.3 years (range 20-83 years) were evaluated in this study. The overall frequency percentages of IIPP, IIPPSSC, and IIPPASC possibility were 84% (74%-92%), 14% (10%-24%), and 75% (66%-87%), respectively.

CONCLUSION

Within the limitations of this CBCT study, 90% of single IIPP in the esthetic zone can be restored with screw-retained crown when utilizing ASC. In addition, the possibility of using a screw-retained restoration following IIPP increases about five times with ASC abutment compared to the SSC abutment.

Retention of zirconia crowns to titanium bases with straight versus angled screw access channels: an invitro study

BACKGROUND

The aim of this study was to assess the impact of abutments with angled screw access channel on the retention of zirconia crowns.

METHODS

Seven implant replicas were inserted in epoxy resin blocks. Fourteen zirconia crowns for central incisor tooth were digitally fabricated and cemented to titanium bases (Ti-bases) with resin cement. Titanium bases were categorized into 2 groups (n = 7). Control group (Group STA) included straight screw access channel abutments. Study group (Group ASC) included angled screw access channel abutments. Following aging (5 °C-55 °C, 60 s; 250,000 cycles, 100 N, 1.67 Hz), the pull-off forces (N) were recorded by using retention test (1 mm/min). Types of failure were defined as (Type 1; Adhesive failure when luting agent predominantly remained on the Ti-base surface (> 90%); Type 2; Cohesive failure when luting agent remained on both Ti-base and crown surfaces; and Type 3; Adhesive failure when luting agent predominantly remained on the crown (> 90%). Statistical analysis was conducted by using (IBM SPSS version 28). Normality was checked by using Shapiro Wilk test and Q-Q plots. Independent t-test was then used to analogize the groups.

RESULTS

Mean ± standard deviation of retention force records ranged from 1731.57 (63.68) N (group STA) to 1032.29 (89.82) N (group ASC), and there was a statistically significant variation between the 2 groups (P < .05). Failure modes were Type 2 for group STA and Type 3 for group ASC.

CONCLUSIONS

The retention of zirconia crowns to abutments with a straight screw access channel is significantly higher than abutments with angled screw access channel.

Reverse Torque Value of Angulated Screw Channel Abutment before and after Cyclic Loading: An In Vitro Study

This in vitro experiment aimed to understand the difference in preload acting on an abutment screw under different angles of angulated screw-retained crown and the performance after cyclic loading. In total, thirty implants with angulated screw channel (ASC) abutments were divided into two parts. The first part consisted of three groups: a 0° access channel with a zirconia crown (ASC-0) (n = 5), a 15° access channel with a specially designed zirconia crown (sASC-15) (n = 5), and a 25° access channel with a specially designed zirconia crown (sASC-25) (n = 5). The reverse torque value (RTV) was measured at 0° for each specimen. The second part consisted of three groups: a 0° access channel with a zirconia crown (ASC-0) (n = 5); a 15° access channel with a zirconia crown (ASC-15) (n = 5), and a 25° access channel with a zirconia crown (ASC-25) (n = 5). The manufacturer’s recommended torque was applied to each specimen, and baseline RTV was measured before cyclic loading. Each ASC implant assembly was cyclically loaded at 0 to 40 N with 1 million cycles at 10 Hz. RTV was measured after cyclic loading. Kruskal–Wallis test and Jonckheere–Terpstra test were used for statistical analysis. All specimens were examined under a digital microscope and scanning electron microscope (SEM) to observe the wear of the screw head before and after the whole experiment. A significant difference in the different percentages of straight RTV (sRTV) between the three groups was found (p = 0.027). The angle of ASC to the different percentages of sRTV showed a significant linear trend (p = 0.003). No significant differences were found in RTV difference after cyclic loading among the ASC-0, ASC-15, and ASC-25 groups (p = 0.212). The ASC-25 group had the most serious degree of wear based on a digital microscope and SEM examination. The ASC angle will affect the actual preload acting on a screw: the larger the ASC angle, the smaller the preload. The performance of the angled ASC groups in RTV difference was comparable to that of 0° ASC after cyclic loading.

Numerical and Experimental Evaluation of Biomechanical Behavior of Single Implant-Supported Restorations Based on Angled Abutments with Different Gingival Heights

The effect of angle abutment on the stress distribution of bone tissue around implant is not clear. Using abutments with different gingival height (GH) may cause changes in the stress distribution of the implant and implant-bone interface. This study aims to investigate whether angled abutments with varied GH have a significant effect on stress distribution of surrounding bone and the biomechanical behavior of the implant system. Three implant-supported restoration models were designed by changing the angled abutment GH (1 mm, 3 mm and 5 mm). Force of 200N was applied on the crown surface at 45° to the long axis of the implants. The biomechanical performance of the restorations (including implants and angled abutments) and stress distribution pattern were evaluated by finite element analysis (FEA). Results showed that angled abutments with larger GH resulted in increased stresses on the implant and implant-bone interface.

Clinical and Laboratory Outcomes of Angled Screw Channel Implant Prostheses: A Systematic Review

The purpose of this systematic review was to evaluate the clinical and laboratory outcomes of angled screw channel (ASC) restorations and to summarize the influencing factors. An electronic search of the English language literature was performed in four databases and enriched by manual searches. Retrieved studies were screened against the predefined exclusion and inclusion criteria. Eight clinical and seven laboratory studies were eligible for the analysis. The risk of bias for included observational studies was performed using the Newcastle–Ottawa quality assessment scale. Laboratory studies quality assessment method was adapted from previous published systematic reviews. Two clinical studies focused on technical outcomes and the rest reported the biological outcomes of the ASC restorations. Out of the seven laboratory studies, two studies investigated the fracture resistance of ASC restorations, four studies evaluated the reverse torque value of the nonaxially tightened screws, and one study evaluated both variables. The present review revealed that while the performance of ASC restorations is promising in short-term clinical studies, the evidence of their long-term reliability is still lacking. The laboratory studies indicated comparable fracture resistance results of the ASC restorations with the straight screw channel restorations. In addition, factors, such as initial torque value, configuration of the screw driver, screw design, abutment system, and the angulation of screw channel, were shown to influence the screw resistance to loosening.

Management of Buccal Screw Access Hole Positioning for Implant Fixed Complete Dentures: A Report of Two Patients and a Proposed Decision Tree

The present report demonstrated the use of two methods to correct the positioning of buccal screw access holes in both implant- and abutment-level implant-supported fixed complete dentures in two patients. The report suggests that nonaxially tightened abutments (in this report, dynamic abutments), angled multi-unit abutments, and the use of a milled framework with individual crowns aid in masking buccal screw access holes while maintaining the benefits of a screw-retained design. A decision tree is also proposed to facilitate the workflow when faced with such clinical scenarios.

Clinical performance of posterior monolithic zirconia implant-supported fixed dental prostheses with angulated screw channels: A 3-year prospective cohort study

STATEMENT OF PROBLEM

The choice of retention type of an implant-supported fixed dental prosthesis (FDP) becomes critical when the dental implant is not placed in a prosthetically ideal location. In recent years, computer-aided design and computer-aided manufacturing (CAD-CAM) technology has enabled the correction of the location of screw access hole depending on the clinical needs of the patient. However, how FPDs with angulated screw channels (ASCs) perform clinically is unclear.

PURPOSE

The purpose of this prospective clinical study was to evaluate the mechanical complications and crestal bone loss (CBL) when posterior monolithic zirconia implant-supported FDPs with an ASC are used.

MATERIAL AND METHODS

Participants (N=37) with a missing single posterior tooth or multiple teeth, sufficient bone height, and an implant site without infection were included. Each participant received parallel-walled implants (Nobel Parallel CC) of 7-, 8.5-, or 10-mm length by using a 1-stage approach. After 4 months of healing, a conventional impression was made, and a digital workflow was followed. Monolithic zirconia restorations (Katana ML) were milled, stained, and mechanically attached to a titanium base (NobelProcera ASC abutment). CBL was measured from radiographs at 6, 12, 24, and 36 months after the placement of the prosthesis. Implant and prosthetic characteristics including implant diameter, implant length, screw channel angle (angle≤15 degrees versus angle>16 degrees), prosthetic type (single crown versus multiple-unit FDP), and antagonist dentition (natural versus prosthesis) were also recorded. A generalized linear mixed model with a log link was estimated to assess the independent predictors of CBL among the angles of ASC-retained definitive restorations and the clinically relevant variables (α=.05).

RESULTS

Thirty-seven participants received 51 implants, and the median follow-up period was 30 months (interquartile range: 22-36). Two single-crown implants failed within the first 12 months of delivery. The implant and restoration survival rate was 96% at 36 months. Screw loosening was recorded in 2 study participants. After a follow-up of 36 months, the mean ±standard deviation CBL value was 0.15 ±0.14 mm with an increase over time (P<.001). The effect of the angle of ASC, implant diameter, implant length, prosthetic type, and antagonist on the CBL was not statistically significant (P>.05).

CONCLUSIONS

CBL was not associated with the angle of ASC, implant diameter, implant length, prosthetic type, or antagonist when posterior monolithic zirconia implant-supported FDPs with ASCs were used. Screw loosening in 2 situations was the only mechanical complication during the first 3 years of service.

Aesthetic Outcomes and Peri-Implant Health of Angled Screw Retained Implant Restorations Compared with Cement Retained Crowns: Medium Term Follow-Up

Single tooth implant restorations in the aesthetic area are a demanding challenge. If a complete osseointegration is mandatory, the final result has to result in a higher standard of biomimetic and soft tissue health among natural teeth. This outcome is traditionally pursued by cementing crowns over individualized abutments. However, in recent years, the need for controlling peri-implant health and the preference towards a retrievable solution has led to an increase in screw-retained crowns, which is not always applicable when the implant axis is not ideal. In the aesthetic area, the use of a novel technical solution represented by the angled screw channel (ASC) of the abutment has been proposed in order to match the advantages of the screwed solution with the aesthetic demands. The aim of this study was to compare ASC crowns to cemented crowns (CC) in single implant restorations using the white esthetic score (WES) and pink esthetic score (PES) at the crown delivery and at a follow-up of a minimum of 2 years. Peri-implant health and marginal bone loss (MBL) were also evaluated. The mean follow-up was 44.3 months, with a mean MBL of 0.22 mm in the ASC group and 0.29 mm in the CC group. The total WES/PES score was 16.6 for ASC, compared with 17.3 for CC at baseline, and 16.2 and 17.1, respectively, at follow-up. Both of the groups reached a high WES/PES, and this was maintained over time, without signs of peri-implant diseases or bone loss, regardless of the choice of connection. In conclusion, ASC can be adopted in cases where the implant axis is not ideal, with aesthetic and functional results that are comparable to implants restored by cemented crowns.

Effect of cyclic loading on reverse torque values of angled screw channel systems

STATEMENT OF PROBLEM

The angled screw channel concept has become popular. However, research is lacking as to how reverse torque values of nonaxially tightened implant crowns compare with axially tightened cement-retained crowns restored on angle-correcting abutments when subjected to long-term cyclic loading.

PURPOSE

The purpose of this in vitro study was to evaluate the ability of different 25-degree angled screw channel hexalobular systems to apply the target torque value on their screws, the effect of cyclic loading on their reverse torque values, and their survival compared with crowns cemented on conventional 0-degree screw channel abutments.

MATERIAL AND METHODS

A total of 28 implants were divided into 4 groups. Twenty-one angled screw channel crowns were fabricated at a 25-degree angle correction by using angled titanium (Ti) bases by 3 manufacturers DY (Dynamic Tibase), DE (AngleBase), and ASC (Angulated Screw Channel) (n=7). The fourth group, UB (Universal Base, Control), had cement-retained crowns with 25-degree custom-milled, angled zirconia abutments that were cemented onto their respective Ti bases (n=7). All implants were embedded in epoxy resin blocks and tightened to manufacturer recommended values: 35 Ncm for ASC, UB, and DE and 25 Ncm for DY. Initial torque values (ITV1) were recorded. After 24 hours, the reverse torque values (24hr-RTV1) were recorded. A new set of screws was then used for each group, and the initial torque values (ITV2) were recorded. Specimens were loaded at 2 Hz for 5 million cycles under a 200-N load, and reverse torque values (RTV2) were recorded. ANOVA (α=.05) was used to compare differences in the means of deviation of initial torque values and means of reverse torque values followed by a Tukey-Kramer post hoc analysis (α=.05). Preload efficiency was calculated for each system (RTV2/ITV2), and a survival analysis was performed by using the Lifetest procedure.

RESULTS

A significant difference in the means of deviation of initial torque values of the groups with 25-degree torque application (DY, DE, and ASC) was found when compared with UB at 0 degrees. ASC and DE had lower initial torque values than UB (P<.001 and P=.003 for ASC ITV1 and ITV2, P<.001 and P=.006 for DE ITV1 and ITV2). A significant difference was found in mean reverse torque values both for after 24 hours and after cyclic loading among all groups (P<.001). A significant difference was found between mean reverse torque values before and after cyclic loading for each group (P<.001). Preload efficiency was 43.8% for DY, 46.8% for DE, 54.2% for ASC, and 48.5% for UB. No significant difference was found in the time-to-failure survival among groups (P>.05).

CONCLUSIONS

The hexalobular system of DY delivered comparable initial torque values to its target value at 25 degrees, similar to how UB (control group) delivered at 0 degrees. ASC and DE scored lower initial torque values than their target value compared with UB. The DY abutment, which had a lower manufacturer recommended torque value, had lower reverse torque values compared with those of other groups. Time-to-failure survival of all groups was similar. Fractures at the zirconia to titanium base connection were seen with ASC crowns.

The ability to screw-retain single implant-supported restorations in the anterior maxilla: A CBCT analysis

STATEMENT OF PROBLEM

Evidence to validate the routine use of angled screw-channel abutments in the anterior maxilla is sparse. If properly planned, they might provide surgical and prosthetic benefits.

PURPOSE

The purpose of this observational study was to determine the prevalence of digitally placed implants in the anterior maxilla that would allow screw-retained implant-supported restorations with either a straight or an angled screw-channel abutment.

MATERIAL AND METHODS

Two hundred cone beam computed tomography (CBCT) scans met the inclusion criteria for retrospective analysis and digital implant planning. Virtual implants were planned for randomly selected anterior maxillary teeth by using the anatomic crown and root position. Virtual abutments of varying angulation were attached to the implants to determine the ability to screw retain a restoration with either a straight or an angled screw-channel abutment.

RESULTS

One hundred fifty-two (76%) sites required an angled screw-channel abutment to enable screw retention. Forty-eight (24%) sites allowed screw retention with a straight abutment. The percentage of implants requiring angled or straight abutments varied significantly among anterior teeth (P<.005). One hundred nine (71.7%) angled screw-channel abutment sites required a 5-degree abutment, 41 (26.9%) required a 10-degree abutment, and 2 (1.3%) required a 15-degree abutment. Among the anterior teeth, lateral incisors presented a greater need for angled screw-channel abutments. None of the implants in the present study needed cement-retained restorations.

CONCLUSIONS

Angled abutments allowed for screw-retained restorations on digitally planned implants in the anterior maxilla. The required angular correction to a screw-retained restoration was ≤15 degrees. Screw-retained restorations were frequently achievable (76%) with the use of angled screw-channel abutments or with straight abutments (24%), and lateral incisors presented a greater need for angled screw-channel abutments.