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. 2023 Dec 7;34(2):167–173. doi: 10.1111/jopr.13806

Effects of screw channel angulation on the fracture resistance of one‐piece zirconia crown with titanium base

Chen‐xuan Wei 1,✉, Berna Saglik 1, Hsiao Hsin Sung Hsieh 2, Paramvir Singh J Guram 3
PMCID: PMC11795339  PMID: 37986128

Abstract

Purpose

The design of the angulated screw channel in implant restorations allows the possibility to correct angulation discrepancies, especially in the anterior maxilla. However, the effects of varied screw channel angulations on fracture resistances and fracture patterns of the implant restorations are still uncertain, and thus the aim of this study.

Materials and Methods

Angulated screw channel monolithic zirconia crowns (Nobel Biocare) with three different angulation groups—straight (ASC1), 15° (ASC15), and 25° (ASC25)—were digitally designed from a left central incisor prototype scan. Following fabrication, 10 samples of each group were individually mounted onto implant replicas embedded in standardized type V stone gypsum cylinder jigs (25 mm × 25 mm). All screws were manually torqued to 35 Ncm according to the manufacturer's recommendation, and screw access openings were subsequently sealed with resin composite. To mimic the off‐axis loading of the central incisor, the specimens were then loaded at a cephalometric interincisal relationship of 135° between the long axis of the crown and the Instron force applicator, with crosshead speed set at 0.5 mm/min. Fractured abutment surfaces were examined, and selected specimens were further evaluated by scanning electron microscopy. Screw torque values were also measured after the catastrophic loading. One‐way ANOVA was used to compare load‐to‐fracture values between groups, with the statistical significance set at 0.05 (p values).

Results

The mean load‐to‐fracture values in descending order were 331.24N (±34.00N) in ASC15, 325.22N (±35.50N) in ASC25, and 302.04N (±45.10N) in ASC1, with no statistically significant differences between groups. Considerable screw torque losses were found in all groups after catastrophically loading. The average torque loss was 84% in ASC1, 86% in ASC15, and 94% in ASC25. 16 out of 30 specimens experienced screw loosening; one ASC1 screw underwent slight deformation. Crowns of all tested groups exhibited cohesive fracture patterns at the screw‐metallic‐zirconia interfaces.

Conclusions

Within the limitations of this in vitro study, one‐piece monolithic zirconia implant crowns with varied screw channel angulations shared similar fracture‐strength and fracture‐mode characteristics. The zirconia‐titanium base junctions exhibited the weakest link of all restorations.

Keywords: fracture pattern, fracture strength, implant crown, load to fracture, zirconia fracture

Maxillary anterior single implant restorations have always been clinically challenging due mostly to their less‐than‐ideal angulations and positions. 1 To guarantee a maximum esthetic outcome, without compromising the anatomical features of the overlying crowns, dentists often choose cement‐retained implant restorative designs with custom zirconia (Zr) abutments to correct implant angulation. Nonetheless, the cement‐retained restorations have their inherent drawbacks, such as residual cement and lack of retrievability. 2 The alternative to the cement‐retained prosthesis is the screw‐retained restorative design, which commonly involves the triadic complex of implant, metal adaptor, and Zr‐crown. 1 This restorative design offers more predictable retrievability without incurring any complex technical management or concerns about residual cements. 2 However, conventional screw‐retained restoration designs lack the versatility to accommodate intra‐arch spaces and angulation corrections. The one‐piece angulated screw channel (ASC) implant crown is a relatively new screw‐retained prosthesis design characterized by a Zr superstructure and titanium (Ti) insert/base. This design better accommodates implant angulation, allowing screw access placement with an angle of up to 25° from the implant axis within a 360° radius. 3 , 4 , 5 , 6 , 7 The disadvantage is that, in order to allow a proper screwdriver engagement at the lingual aspect of the crown, the ASC system requires a screw channel widening for the required angulation design. This results in Zr thinning of some lingual and cervical areas, undermining the overall prosthesis strength. 8

Nothdurft et al. compared the fracture strengths of straight and angulated zirconia implant abutments and found that restorations with angulated abutments exhibit higher mean fracture loads than those with straight abutments. Another study revealed that a lingually tipped implant axis significantly reduces fracture strength of angle‐corrected Zr abutments. 5 Our previous work compared the ASC abutment fracture‐resistance group with different screw angulations and demonstrated that angulated screw channel Zr abutments with 25° have significantly lower fractural resistance than straight channel angulation. 9 This is why, in order to limit mechanical complications, manufacturers have restricted indications for Zr abutments with a maximum angulation of 15° to 20° for stock abutments, and 30° for custom abutments. 10 Despite these indications, Zr abutment fractures can still happen and are regularly reported.

Current available investigations towards fracture resistance of angulated Zr abutment and angulated screw channel Zr abutment have been mostly focused on two‐piece cement retained Zr restorations with limited sample size and angulations. To our knowledge, the current literature fails to provide adequate information on the mechanical characteristics of a one‐piece Zr crown with a Ti base connection, especially with the adoption of angulated screw channels. 11 This lack of knowledge limits clinical applications. Hence, the purpose of this study was to investigate comprehensively the fracture resistances and fracture patterns of ASC Zr‐crowns and to provide clinical references for ASC restorative design applications. The null hypothesis of this study was that one‐piece ASC Zr‐crowns should exhibit similar force resistances and fracture patterns, as a result of different angulations and opening locations of screw channels.

MATERIALS AND METHODS

The test specimens in this in vitro study consisted of an implant replica—Ti‐base—Zr crown assembly. A maxillary left central incisor was waxed up, based on the anatomic average, 12 on temporary implant abutment (Procera Conical Connection RP; Nobel Biocare, Kloten, Switzerland), and subsequently scanned with a desktop scanner (Kavo, LS 3 Scanner, Biberach, Germany). One‐piece zirconia implant crowns with screw channels angled to 1° (ASC1), 15° (ASC15), and 25° (ASC25) lingual to implant long axis (Figure 1) were then designed and fabricated with a computer‐aided design and computer‐aided manufacturing (CAD‐CAM) system (DTX ImplantStudio, Nobel Biocare AG, Kloten, Switzerland) (Table 1). To ensure the fabrication repeatability, the emergence profile of the prosthesis was automatically generated with the default settings of the software. In this way, all specimens were fabricated in proximally identical dimensions, with proximally 0.4 mm width of circumferential chamfer, proximally 9 mm of incisor‐gingival height, and proximally 8 mm mesiodistal width on the buccal surface. The identical three‐dimensional wall thickness in each group was also confirmed with a digital caliper (500‐196‐30, Mitutoyo, Kawasaki, Japan) in the same fashion described in the previous study. 13

FIGURE 1.

FIGURE 1

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Digital design captured during the designing of the ASC screw angulations. (Left) 1 degree design for ASC1 group, (middle) 15 degree design for ASC15, (Right) 25 degree design for ASC25.

TABLE 1.

Composition of zirconia used in this study. (Copied from Nobel Biocare. MSDS No. 78790).

Element Compositional limits % by weight
Zirconia + Hafnium oxide + Yttrium oxide ZrO2 + HfO2 + Y2O3 >99.0
Yttrium oxide Y2O3 >4.5 to <6.0 (percent mass fraction)
Hafnium oxide HfO3 <5 (percent mass fraction)
Aluminum oxide AI2O3 <0.5
Other oxides Other oxides <0.5
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A sample size calculation was performed to determine how many samples per group would be needed to detect a statistically significant difference at α = 0.05 for the incidence of material fractures if a power of 80% was intended. Software nQuery Advisor (Version 7.0, Statistical Solutions Ltd., Cork, Ireland) was used for power analysis, and demonstrated a sample size of 10 specimens per group (n = 10) was sufficient to detect a statistically significant difference between three groups with adequate power. Thirty implant replicas (Conical Connection RP 36698; Nobel Biocare, Kloten, Switzerland) were randomly selected for specimen assemblies. Each replica was held in position with a guide pin (Implant Level Conical Connection RP 30 mm; Nobel Biocare, Kloten, Switzerland) attached to a laboratory surveyor (Ney Surveyor Parallometer System; Dentsply Neytech) The surveyor acted as a device to standardize the mounting position of the implant replicas. Each implant replica was then embedded in Type V dental stone (Jade Stone; Whip Mix Corp, Louisville, Ky) to form standard cylinder jigs (25 mm × 35 mm), which were adjusted at 30° relative to the vertical arm of the surveyor. Subsequently, zirconia crowns were engaged onto the implant replicas manually with the Omnigrip screws and screwdriver (Omnigrip, Nobel Biocare, Kloten, Switzerland). Each of the zirconia abutment/crown assemblies was torqued to 35Ncm according to the manufacturer's recommendations.

Ten specimens per group, based on the screw channel angulations, were subjected to a load‐to‐fracture test with a straight group (ASC1) as control. Each specimen assembly was individually mounted on the metallic platform, previously positioned under the surveyor, and adjusted at 30° relative to the vertical arm of the surveyor and the mechanical indenter of the universal testing machine (Model 5566; Instron Co., Canton, MA). This was done to recreate the appropriate off‐axis loading between the central incisor crown supported by the implant and the universal testing machine applicator simulating the mandibular incisor. The specimens were then subjected to loading until failure with the crosshead speed set at 0.5 mm/min for the universal testing machine. The indenter contacted the center of the mesiodistal lingual surface and approximately 1.5 mm below the incisal edge, with a contact width of approximately 1 mm. The universal testing machine was controlled with a software system (Bluehill 2 Software, Instron Co., Canton, MA). The same software also registered the stress‐strain diagram and breaking loads. Screw torque values were also measured after the catastrophic loading with a Tohnichi BTG‐6 torque gauge (Tohnichi American Corporation, Northbrook, IL).

Fractured abutment surfaces were photographed with a digital camera (Nikon D550, Tokyo, Japan) and a high magnification macro lens (Laowa Ultra Macro, Hefei, China), with selected specimens further evaluated by scanning electron microscopy (SEM‐JEOL‐JSM‐7800FLV, Japan), with secondary electron imaging and backscattered electron imaging. Digital images of representative fracture regions were recorded at various magnifications to evaluate the fracture pattern and to determine the mode of failure.

Descriptive statistics including means, standard deviations, and minimum and maximum values with 95% confidence interval were reported for each group. One‐way ANOVA was used to compare variance between control and material groups. The level of statistical significance (p‐values) was set at 0.05.

RESULTS

The mean load‐to‐fracture values in descending order were 331.24N (±34.00 N) in ASC15, 325.22N (±35.50N) in ASC25, and 302.04N (±45.10 N) in ASC1, with no statistically significant difference between groups. Hence, the null hypothesis was accepted.

Crowns of all test groups exhibited cohesive fracture patterns at the screw‐metallic‐zirconia interfaces. All samples were fractured at the palatal‐cervical side of Zr crown—Ti‐base junctions (Figure 2), with one catastrophic failure (complete displacement of the crown from Ti base) each in the ASC1 and ASC15 group, and two in the ASC25 group. Typical fracture of zirconia abutment propagating from the palatal side of the hexagon (thinnest portion of abutment). Note the Ti‐base is apparently intact and still loosely attached to the implant replica by Omnigrip screws. Some metal smudge from the screw head was visible at chipped threads along the body of the implant crowns (Figure 3). The most common fracture pattern observed was oblique fracture lines at the implant shoulder extended incisally till the screw opening, and the end of the fracture lines failed to be traced even under higher magnification (Figure 4).

FIGURE 2.

FIGURE 2

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Representative image of all specimen groups after loading and removal from mounting jigs. From the top to the bottom rows: ASC1, ASC15, ASC25.

FIGURE 3.

FIGURE 3

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Representative image of fractured specimens and fracture surface.

FIGURE 4.

FIGURE 4

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Digital photo of a representative fracture sample with oblique crack line.

Scanning electron microscope (SEM) images of the fractured zirconia crown at low magnification (Figure 5) showed that the crown fracture occurred at the junction of the external hex and the shoulder of the Ti base. The fractured surface is figured by a twisted crack region and transitions to a relatively smooth fractured surface on one side and deflected and propagated down in a spiral manner on the other side and arrested at the junction of the hex and the shoulder of the implant (Figure 6). With closer examination, both digital and SEM images of the fracture surfaces demonstrated a typical fracture pattern with a relatively smooth main fracture region, commonly known as “irror”. 14 Inside the main fracture region, there were crack lines (hackles) deflect, branch, and run parallel to the direction of the crack propagation. The deflection hackle lines typically originated on the concave side of the mirror, and clearly defined arrest lines could be observed (Figures 6, 7).

FIGURE 5.

FIGURE 5

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SEM image demonstrated the general fracture pattern. SEM, scanning electron microscopy.

FIGURE 6.

FIGURE 6

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Magnified (5×) digital image demonstrated a representative main fracture origin at the junction of the intaglio surface of the Zr crown and Ti‐base.

FIGURE 7.

FIGURE 7

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Representative SEM image of close‐up fractured surfaces showing a twisted crack region. SEM, scanning electron microscopy.

Considerable screw torque losses were observed in all groups after catastrophically loading. The average torque loss was 84% in ASC1, 86% in ASC15, and 94% in ASC25. A total of 16 specimens (out of 30) revealed screw loosening, with one screw of ASC1 revealing slight deformation. Crowns of all test groups exhibited cohesive fracture patterns at the screw‐metallic‐zirconia interface.

DISCUSSION

In the present study, the fracture loads for all specimens ranged from 248.42N to 418.49N, with no statistically significant differences between different angulation groups. Previous similar studies of anterior single unit Zr implant restoration revealed fracture strength ranging from 215N to 534N. 9 , 15 Another study on one‐piece Zr restoration on maxillary single units, with different implant‐abutment connections and loading angles (30° to 60°), showed load‐bearing capacities ranging from 215N to 793N. 9 , 16 , 17 , 18 , 19 , 20 Moreover, studies on healthy young adults’ maximum central incisor bite forces ranged from 90N to 743N, with predominant measurements between 120N and 370N. This broad range was due to the elevated bite forces of occlusal parafunctional subjects. 21 , 22 , 23 , 24 , 25 All these suggest that Zr crown designs may not be able to bear occlusal forces in parafunctional cases, Therefore, these cases may require a stronger type of Zr material or may need post‐delivery bite splint for their standard of care.

In this study, all the screw accesses were designed lingualized to incisal edge. Therefore, during occlusal loading when the Zr crown lingual surfaces were subjected to tension and the buccal sides were subjected to compression, this lingual design was critical to the overall strength of the crown. 5 The results failed to reveal fracture‐strength differences between different angulated screw channel designs. Similar results were observed by Thulasidas, et al., who reported that screw channel location affects the material bulk thickness at the crown lingual aspect that may subsequently reduce its fracture strength, yet it is not a controlling factor.

Many other factors may also influence the fracture strength of Zr superstructure, such as restorative design, implant diameter, and material composition. A previous study that assesses the relationship between implant‐Zr abutment connection type and Zr fracture strength reveals that different connection geometry types, such as morphology and dimension (internal or external hex), significantly influence the Zr junctional thickness and the subsequent fracture strength. Internal connections exhibit higher bending moments than external. 26 Also, added Ti insert/base connection to Zr superstructures proved to decrease ceramic‐to‐implant interface frictions and to increase the fracture resistances. 27 , 28 Additionally, implant diameters are also reported to influence Zr fracture‐strength as seen in narrow diameter implants with a relatively thinner Zr material bulk. 29 Given all factors, direct comparisons between studies are challenging. Because study designs vary, further in vitro investigations are still needed to verify comprehensively these results.

All Zr crown fractures in the present study were located at the junction of the crown margin and Ti‐base, without visible plastic deformation of the Ti sleeves, abutment screws, or implant analogs, concurring with findings reported by Zandparsa et al. The most common crack patterns were observed on the lingual‐cervical sites near the implant screws and platforms. These sites were subjected to tension during loading, and they were previously reported to have the highest torque and stress concentration due to levering effects. 5 , 30 , 31 All specimens of the present study were partially chipped or fractured at cervical portions of the Zr crowns, suggesting that the Zr crown‐Ti‐base junctions were the weakest portion of the entire restoration assembly. One most evident reason was the relatively thin material bulk design at the Zr‐metal junction from the default software design. In this study, the thickness of the Zr fractured portion was approximately 0.5 mm. Aboushelib and Salameh reported that Zr fracture strength decreases dramatically when the thickness is less than 0.7 mm, which could be a reason for the consistent fracture pattern at this location of our specimens. Hence, when designing an ASC Zr crown, one must consider slightly increasing the material bulk at the Zr–Ti‐base junction from default software settings.

Dental Zr mechanical failures often originate from structurally weakened areas that usually exhibit as microcracks on micrometer scales beyond visual detections. 32 SEM images demonstrated a typical main fracture region with a relatively smooth surface, known as the mirror, previously defined as a crack initiation site by Osman, et al. Fracture surfaces with smaller mirror sizes and a large number of fragmentations indicate higher fracture stress sites. 14 , 33 Additionally, the specimens in this study were subjected to 30° static loading from the Instron machine, which formed a lever effect at the implant coronal portion. This lever effect unavoidably generated an external bending force, which coupled with internal clamping forces, resulting in twist hackles as shown in SEM imaging (Figure 7). 34

Screw loosening is one of the most common complications in implant restorations and could potentially lead to screw fractures. 35 The frictional forces between different components in an implant restoration often decrease over time as a result of creep and stress relaxation, eventually likely to cause preload and torque loss decreases. 36 This normal occurrence could be anticipated and corrected by retorquing the prosthetic screw to the recommended moment‐force after a period of time. Studies recommend that prosthetic screws be retorqued 10 min after initial placement and periodically thereafter. 37 , 38 In the present study, all specimen groups experienced a considerable amount of torque and screw loosening after catastrophic loading, which concurs with previous statements that excessive loading could result in torque loss and screw loosening. 39

The observations of this study were based on a limited number of in vitro tested samples, and further studies with larger sample sizes, as well as in vivo verifications are still needed. Secondly, the wear behavior at the Zr‐Ti‐base interface under cyclic loading should be taken into consideration in future studies, as it may affect Zr superstructures’ mechanical properties such as fracture resistance, fatigue resistance, fitting, and micro‐movements. 29 Last but not least, the most common fracture location observed in this study was at the Zr‐metal junction of the implant shoulder, supporting previous findings that report Zr at the level of the implant shoulder was the weakest link in terms of fracture resistance. 19 , 40 According to a previous investigation, fracture strength decreased dramatically when the zirconia thickness was below 0.7 mm. 41 In this study, the thickness of the fractured portion of Zr was measured to be proximally 0.5 mm with the default setting of the design software. Hence, one may attempt to increase material bulk at the implant shoulder, however, restoration contour with an emergence angle of more than 30° was indicated previously as a significant risk indicator for peri‐implantitis. 42 Hence, further endeavors should focus on finding the most desirable restoration cervical contour design to achieve the balance of optimal restoration strength and peri‐implant tissue health.

CONCLUSIONS

Within the limitations of this in vitro study, screw channel angulations did not significantly affect the fracture strength of overall Zr crown‐Ti‐base assemblies. However, care should be taken especially in parafunctional patients in order to guarantee prosthesis longevity. Post‐delivery home care instructions as well as occlusal guards are also recommended as routine practice.

CONFLICT OF INTEREST STATEMENT

This work was presented in the American College of Prosthodontists (ACP) Annual Session as a finalist of the John J. Sharry Competition at the JW Marriott in Austin on Nov. 4, 2022. The authors declare that they have no conflict of interest related to this study.

ACKNOWLEDGMENTS

The presented research project is funded by ACP research fellowship fund. The authors also appreciate the materials provided by Nobel Biocare.

Wei C, Saglik B, Sung Hsieh H, Guram P. Effects of screw channel angulation on the fracture resistance of one‐piece zirconia crown with titanium base. J Prosthodont. 2025;34:167–173. 10.1111/jopr.13806

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