Two reviews also studied mucositis and fistula as biological complications.^[5,19] Although no meta-analysis was performed, the evidence from experimental studies suggested no difference between different abutment materials.

Among the five studies reviewing mechanical complications,^{[5,15,20,22,23]} two studies had done a meta-analysis.^[15,22] The results of the meta-analyses showed no statistical difference between Zr and Ti with risk ratios of 0.87^[22] and 0.52, respectively.^[15] The prevalence of mechanical complications was reported to be <7.9%.^[22]

There were five reviews elaborating survival rates of Zr and Ti abutments.^{[17,18,21-23]} Three meta-analyses^[17,21,22] and one network meta-analysis^[18] showed that there was no statistically significant difference between the groups. However, Totou et al.^[23] stated the superiority of Ti abutments in terms of abutment fracture. Two meta-analyses^[21,22] reported the survival rates of Zr and Ti abutments as follows: 98.6%, 98.8% and 98.62%, 99.4%, respectively.

This umbrella review analyzed the available evidence by summarizing and critically appraising systematic reviews and meta-analyses evaluating the peri-implant tissue indices of Zr abutments. The examined factors can be divided into three subgroups: biological, esthetic, and mechanical outcomes. Biological indices included MBL, PPD, BOP, PA, and REC. Esthetic parameters contained PES, WES, CIS, ICAI, PI, gingival discoloration, and mechanical outcomes mainly related to prosthetic complications.

Zr abutment had preferred or resembled effects as other abutment materials in terms of biologic parameters.^[15,17-22] However, some randomized controlled trials (RCTs), including Sanz-Sánchez et al.^[22] and Linkevicius and Vaitelis,^[19] showed different outcomes. Nevertheless, these opposing results were usually limited to one or two main studies in a review and were seldom statistically significant. Table 6 demonstrates these inconsistencies.

The marginal bone level is an indicator of implant crestal support normally evaluated by periapical radiography. Compared to sequential follow-up sessions, the alterations in the bone level, which is mainly seen as bone loss, can be assessed. The MBL indicated the clinical situation known as peri-implantitis. The most common reasons for MBL are inflammation due to microbial adhesion and disproportionate masticatory forces. Ti abutments are more susceptible to bacterial colonization than Zr abutments, with a more polished surface impeding microbial biofilm formation.^[11]

In one of the systematic reviews, different surface decontamination methods were used.^[21] It was shown that only when plasma argon cleansing, a robust decontamination technique, was used, the Ti abutment surfaces showed enhanced marginal bone levels. One review demonstrated that MBL around Zr abutments was lower when implants with single crowns were evaluated and not fixed dental prostheses.^[18] In another review, a subgroup analysis showed an even more prominent superiority for Zr in abutments bearing overdentures than single crowns.^[17] This was explained by the fact that larger occlusal forces are applied to implants beneath the overdentures. As the higher elastic modulus of implant superstructures leads to a more uniform distribution of occlusal loads in the framework, a more reliable load transfer to the osseointegrated implants is seen in Zr abutments.^[14] Therefore, the protection effect of Zr is more apparent when overdentures are of concern.

Another major factor that influences the results of studies on MBL is the length of the follow-ups. It has been shown that, on average, it takes 3 years of function to see the signs of peri-implantitis onset.^[22] However, most studies on MBL do not have adequate follow-up length. This also explains the results of those studies that despite having better soft tissue outcomes such as BOP in Zr groups, they fail to demonstrate any significant difference in terms of MBL.^[22]

Finally, the limitations of measurement techniques should be considered. MBL is usually evaluated through repeated mesial, distal, or average crestal bone level radiographic measurements. The typical limitations of such dental radiology, such as distortion, elongation, superimposition, and different X-ray angles, all introduce errors in the measurement technique. The results of studies become unreliable when the difference between groups (even when supported by a significant P value) is smaller than the measurement error.^[18,22]

The technique of probing around implant restorations is used mainly similarly to probing natural teeth to measure periodontal pockets. However, there are limitations in assessing peri-implant health using traditional probing methods. The restorations’ design, contour, and splinting can restrict the probe’s access to the depth of the pocket, leading to an underestimation of PPD values.^[21] To form the attachment of mucosa around the suprarenal components of the implant, epithelial cells should first adhere to the abutment. It is shown that the roughness of the abutment surface is crucial in how cells behave. As the epithelial cell adhesion is much better to smooth surfaces, it is suggested that the polished Zr abutment surfaces should provide a better adhesion medium.^[14,19] Second, Zr abutments have chemically stable surfaces and are corrosion resistant. Hence, the epithelial cells can grow better around such surfaces, producing better surface adhesion.^[25]

Nevertheless, the results of only one review complied with this theory, while most showed comparable results for Zr and Ti. This contradiction can be partly explained by the fact that many experimental studies had a subgingival position of the cementation margin, meaning that the peri-implant mucosa was usually in contact with the feldspathic ceramic used on the restoration rather than the material of the abutment. In such situations, the subgingival portion of the restoration greatly influences PPD, making the comparison of the abutment material inconclusive.^[19] Only one review showed better results for Ti, which contained a wide range of studies such as nonrandomized trials and case series included in their analysis and the randomized controlled clinical trials.^[19] Hence, the different results might be due to a higher risk of bias in the included studies.

The incidence of the peri-implant REC appears to be influenced by several essential parameters, including 3D implant position, attached mucosa, and microbial activity in peri-implant mucosa.^[19] The REC of peri-implant mucosa is usually assessed by measuring the distance from the mucosal level to a particular reference point on the restoration (e.g. the incisal edge). One approach to maintaining soft tissue integrity is to reduce bacterial adhesion and subsequent plaque formation on the surfaces of the implant restoration. The arrangement of biofilm around implants varies depending on the type of abutment material used. Both Ti and Zr provide hydrophobic surfaces where a thick peptidoglycan layer can form. This layer instantly attracts Gram-positive bacteria when repelling Gram-negative ones. However, the formation of a Ti dioxide layer can alter the surface properties of Ti structures toward a semiconductor medium, which can justify the debated results reported in the systematic reviews.^[26]

All five reviews on REC showed similar results for Zr and Ti abutments.^{[5,14,19,21,22]} The review by Mokhtar et al.^[14] compared REC of Zr and Ti in different follow-up times from 6 months to 5 years. Similarly, no difference was found between the two materials. Linkevicius and Vaitelis^[19] compared Zr and Ti in customized CAD/CAM and stock abutments. Once again, there were no different results for Zr and Ti. When comparing mesial and distal sites multiple times, they found an average increase of 0.2 mm of soft tissue in the mesial area. In contrast, the distal area had an average REC of 0.3–0.4 mm for both the Zr and Ti groups. However, no further explanation was presented for such a difference. Another review focused on the evidence of experimental concave-shaped abutments, suggesting more predictable results for peri-implant mucosa stability for both Zr and Ti abutments.^[5]

The quality of the peri-implant soft tissue was assessed in two reviews by evaluating the width and thickness of the keratinized mucosa.^[21,22] The Zr and Ti abutments were satisfactory and were not influenced by the choice of abutment material.

The peri-implant soft tissue health is paramount to implant long-term success. Diagnosing primary alterations in soft tissue is challenging; however, BOP is a well-established method to identify inflammation in peri-implant mucosa.^[21] Plague accumulation around the exposed parts of implant restoration provides a medium for microbial colonization. Consequently, the inflammatory processes start in the irritated mucosa, with the most prominent sign being a tendency to bleed easily. The restoration material may not directly affect PA, yet lower surface energy and higher stability in the restoration can decrease PA.^[27,28]

The included reviews for PA had a small number of studies evaluating the parameter and failed to show any difference between Zr and Ti.^[15,21,22] Theoretically, based on in vitro studies comparing Ti and Zr discs showing more significant PA for Ti, it is expected to observe more PA around Ti abutments.^[19] However, many other factors influence PA and possibly reduce the effect of the choice of abutment material. These factors include oral hygiene and brushing effectiveness, the misfit between the prosthesis and the platform of the implant due to screw loosening or cement excess, and the contour and roughness of the prosthesis.^[15,19,21]

The reviews evaluating BOP showed similar^[15,19] or better^[21,22] results for Zr. Apart from the plaque index, a better soft tissue attachment to Zr can improve the BOP scores. In in vitro studies, Zr demonstrated a higher degree of fibroblast proliferation, thereby promoting the quality of soft tissue attachment.^[21,29]

The most common biological complication was the presence of a fistula in buccal mucosa.^[5,19] In cement-retained restorations, the remnant of cement causes a biological reaction in the form of fistula and suppuration. It is more prominent when resin cements are used because the complete removal of the excess cement is more challenging or when the crown margins are placed more than 1 mm submucosally.^[5,19] Even in customized abutments, which are shown to facilitate the cement excess removal, the deep margin of the crowns hinders complete cement remnant removal.^[19] Therefore, this can be attributed to the design of the abutment and the cementing agent rather than the abutment’s material. Fistula formation is not exclusive to cement-retained restorations. In screw-retained restorations, complications were limited to cases with external hex implant platforms where ill-fitting abutments created a gap in the interface of the implant and the abutment penetrated by soft tissue invaginations.^[5] Similarly, the choice of abutment material was not influential.

Esthetics of implant restorations are receiving increasingly more attention as they are becoming more reliable. Soft tissue color in the implant area is a key indicator of a natural appearance.^[16] Based on the current evidence, Zr abutments provide similar^[18,21,22] or better^[16,19] mucosal color than Ti abutments. When compared to the gingival color of natural teeth as a control, spectrophotometry assessments showed mucosal discoloration for both abutment materials.^[30] This discoloration is more visible when there is thin mucosa with a thickness of <2 mm. However, when the thickness of the mucosa is more than 2 mm, the abutment shadow beneath the mucosa becomes undetectable by human eyes. At the same time, the discoloration can still be recorded by spectrophotometry.^[5,19,21] Therefore, the choice of abutment material between Zr and Ti, where there is adequate thickness of mucosa, may not have clinical significance for mucosal color.

Notably, all the included studies were on cases with either no defect or a repaired defect on the buccal site. There is insufficient evidence of the mucosal color of different abutment materials for cases with buccal defects, and our findings may not be applicable to these cases.^[20] A newer technique that may improve the mucosal color in such situations involves coloring the abutment by veneering the Zr or anodizing the Ti.^[20] There are currently very few studies on this technique, so it only remains a suggestion at this point, and much more evidence is needed to draw a concrete conclusion.

Different indices have been developed to evaluate the esthetics of soft tissue and crowns of implants.^[31,32] Seven criteria shape the PES, including the papillae’s shape to the crown’s mesial and distal, contour of soft tissue, contour of alveolar process, level of the margin of soft tissue, and color and texture of soft tissue. The PES/WES index assesses the esthetic aspects of the crown and those estimated in PES. These aspects include the tooth form, the volume, the translucency, the surface texture, and the color of the implant-supported crown.^[20] The ICAI is another index that examines the esthetics of soft tissue and crowns by comparing them to adjacent teeth. The other index for both esthetic aspects is CIS, which assesses the morphology and color matching of the crown along with the soft tissue esthetics. Finally, the PI only focuses on the papillae around the crown of implants.^[22]

None of the included review studies had a meta-analysis on these indices. As these indices are fundamentally different, combining them into one another in a single meta-analysis was impossible. The high heterogeneity and inconsistency in the results, with only a few studies on each index, made doing a meta-analysis for each index separately impractical. In summary, the Pink Esthetic Score (PES) serves as a more focused and appropriate tool for comparing abutment materials, as it specifically evaluates peri-implant soft tissue esthetics, thereby excluding numerous aesthetic factors related to crown fabrication and laboratory procedures that do not directly reflect the influence of the abutment.^[19]

The results for PI were inconsistent. Some studies showed a significant increase in PI during the 1^st year of restoration placement with no difference between Zr and Ti abutments. When considering long-term follow-ups, the index remained stable or slightly increased between 3 and 5 years.^[21]

The main mechanical complications reported in the reviewed studies were veneer failure and screw loosening as minor complications and fracture in different parts of the restoration as a major complication.^[5,15,20-22] In the two meta-analyses comparing Zr and Ti abutments, similar mechanical complications were observed.^[15,22]

Veneer failure indicates problems such as chipping, fracture, or debonding in the veneering layer, a typical mechanical complication for Zr abutments in both posterior and anterior regions.^[5,15] Weak bonding between the Zr abutment and the veneering ceramic is the main reason for such a failure. Although veneer failure may not endanger implant survival, it can compromise the restoration’s esthetics and function and the patient’s satisfaction with the treatment. As occlusal interactions are crucial in veneer failure, patients with parafunctional habits or implant-supported restorations/ceramic restorations as antagonists are more prone to this complication because of higher stresses on the restoration from clenching or mastication.^[15] Different strategies have been suggested to reduce veneer failure, including the use of monolithic restorations, an anatomical preparation design for the Zr framework to provide better mechanical support to the veneering layer, the use of Zr-Ti hybrid systems, using press-veneering as a substitute for hand-layer veneering technique, enhancing adhesion methods and finally, the advent of new ceramic materials.^{[15,22,33,34]}

Abutment screw loosening is another common mechanical complication. There are unavoidable micromovements in the implant–abutment interface and between prosthetic components, which induce wear to these components. The wear is higher when the mechanical properties of materials are different, such as that seen in Zr abutments.^[15] Irrespective of the material, the prevalence of screw loosening is higher in external hex implants and implants for single restorations.^[5]

Fracture in prosthetic components is another issue of concern. The most susceptible parts to fracture are thin screw walls in external connection abutments and implant neck for internal connection abutments.^[20] It has been shown that fractures on Zr abutments occur in the screw–abutment interface, which is believed to follow the other complication: screw loosening. Therefore, with proper preload applied to torque the prostheses, the rate of such fractures will decrease.^[15] Another factor that can lower this complication is limiting the angulation of stock Zr abutments to 15°–20° and CAD/CAM Zr abutments to 30°.^[20] While narrow diameters of abutments and implants have been speculated to increase fracture rates, there is currently no convincing evidence on this matter.^[20]

The survival rate of implant-supported prostheses is a good estimate of the longevity of the treatment. In theory, the difference in fracture resistance of Ti and Zr (Ti = 1454 N, Zr = 443.6 N) and their flexural strength (Ti = 2000 MPa, Zr = 900–1200 MPa) gives the expectation of a higher survival rate for Ti abutments. However, the evidence in the literature shows similar survival rates for the two materials.^{[5,17,18,21,22]}

The main challenge for the quantitative synthesis of data from different studies in a meta-analysis is the high heterogeneity of the studies. A broad range of follow-up durations (0.08–13 years) and lack of a life table survival analysis pose significant obstacles to calculating actual survival or cumulative survival rates. Instead, in one review, an unrefined mean survival estimate was used to indicate the survival of Zr and Ti abutments.^[5] Further studies with long follow-ups (5 or 10 years) and the precise number of remaining implants at each evaluation stage can help estimate these abutments’ actual and cumulative survival rates.

As for any review studies, our umbrella review of Zr and Ti abutments had certain limitations. Not all studies conducted meta-analyses on the outcomes of interest, and even among those that did, sufficient data for statistical analyses and forest plot generation were not always provided. Certain outcomes, including objective esthetic indices, some biological complications, and implant success rates, were considered secondary endpoints in the reviews and were not thoroughly addressed. Further research focusing on these outcomes is necessary to draw more definitive conclusions. Additionally, some meta-analyses were based on a limited number of studies, resulting in inadequate sample sizes to achieve the statistical power typically expected in meta-analytical approaches. More exhaustive search strategies enriched with manually searching the gray literature can enhance the quality of such reviews.

Nil.

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.