A systematic review and meta-analysis on the clinical outcome of zirconia implant–restoration complex

Abstract

Purpose

This systematic review evaluates the clinical outcome of zirconia implant-associated survival and success rates, marginal bone loss, and implant–restoration complex integrity.

Study selection

Using the preferred reporting items for systematic reviews and meta-analysis (PRISMA) guidelines, studies including ≥ 10 patients restored with zirconia implants supporting single crowns (SCs) or fixed dental prostheses (FDPs) prior to January 2017 were identified. Primary outcomes were survival rates and marginal bone loss around one and two-piece zirconia implants and the associated implant–restoration complex integrity.

Results

1349 studies were selected; after duplicate removal and title screening, 36 remained for full-text screening. 17 studies met the inclusion criteria: 2 randomized controlled clinical studies, 11 prospective clinical studies and 4 retrospective studies. In total, 1704 implants from 1002 patients were evaluated, including 1521 one-piece and 183 two-piece zirconia implants with follow-up between 1 and 7 years.

The mean survival rate was 95 % (95 % CI 91–97 %). The overall mean marginal bone loss was 0.98 mm (95 % CI 0.79–1.18); the mean marginal bone loss after 1 year was 0.89 mm (95 % CI 0.60–1.18). No meta-analysis regarding prosthetic outcomes was possible.

Conclusions

Survival and marginal bone loss values after one year for one-piece zirconia implants are acceptable, but long-term studies are required to support their clinical use. No particular restoration material can be recommended; this decision is apparently based on clinicians’ preferences.

Results from two-piece implants do not provide sufficient data to support their clinical use and no abutment or cementing materials for two-piece zirconia implants can be recommended.

Introduction

The use of dental implants for the replacement of missing teeth is a well-documented treatment modality. The procedure is facilitated via postoperative osseointegration, defined as a direct functional and structural connection between living bone and the surface of a load-bearing implant [1].

For more than 40 years, medical-grade commercially pure titanium or titanium alloy has been the gold standard material for the fabrication of dental implants. Studies investigating the clinical outcome of titanium implants showed survival rates of 97.2 % after 5 years for single crown-supporting implants, 96.4 % after 5 years for implants supporting fixed dental prostheses (FDPs) [2] and 96.86 % after 10 years for implants supporting FDPs in edentulous jaws [3].

Despite the clinical success of implants, biological complications, namely peri-implant diseases [4], [5], [6] implant loss, can and do occur. In addition to these complications, a number of studies reported that titanium can cause hypersensitivity reactions [7], [8], [9], [10], [11], [12]. Hypersensitivity to biomaterials in the oral cavity typically manifests as facial eczema, non-keratinized edematous hyperplastic gingiva, vague pain, skin rashes, or in some cases, implant loss [8], [11]. A clinical study involving 1500 patients documented the prevalence of titanium allergy as low as 0.6 % [13]. Despite growing concerns about its biocompatibility, the current evidence for hypersensitivity or allergy to titanium remains very weak: in fact, reports relate the observed hypersensitivity to impurities in the implant material and not to the titanium itself [14]. Interestingly, the majority of patients hypersensitive to titanium implants also exhibited hypersensitivity to other materials, namely chromium and nickel [13]. Therefore, the potential contraindication of titanium implants due to the possibility of hypersensitivity seems to be limited to a very small group of patients [10].

In addition to biological complications, technical complications in titanium implants, such as veneering material fracture, screw loosening and fracture, and implant fracture, have been reported. In a systematic review, the most frequent complications over a 5-year observation period of titanium implants were fractures of the veneering material (13.5 %), loss of access hole restoration (5.4 %), abutment or screw loosening (5.3 %), and retention loss of cemented FDPs (4.7 %) [5]. In addition, esthetic complications are also present in titanium implants. For example, the presence of a thin gingival biotype around a titanium implant may lead to tissue discoloration due to the visible, dull grayish background color of the titanium [15]. In the anterior visible region, especially with a high lip line, such a discoloration is considered a disadvantage of titanium that jeopardizes esthetics if no interventions are performed to mask it.

The biological and technical concerns for titanium implants, along with patient-specific requests for metal-free treatment, have pushed the search for alternative implant materials. Among these, zirconia has been undergoing extensive experimental and clinical research to evaluate its feasibility as an alternative implant material that guarantees a successful long-term outcome.

Zirconia has a number of favorable characteristics, such as high flexural strength (900–1200 MPa) and hardness [16]. Also, several animal investigations have demonstrated its biocompatibility as an implant material [17], [18]. Zirconia also presents a significantly reduced plaque affinity, which leads to a reduced risk of inflammatory reactions around soft tissues [17], [19]. For these reasons, a growing number of (mostly European) implant manufacturers are introducing zirconia implants. As previously described in part I of this review, zirconia implants differ in their material composition, surface treatment, and implant and component design. Two zirconia-based materials commonly evaluated in clinical studies are yttria-stabilized tetragonal zirconia (YTZ) and alumina-stabilized tetragonal zirconia (ATZ) [16], [20], [21].

In addition to planning and selection of the implant material, surface treatments are often performed to obtain a rough surface, which has been shown to improve the bone-to-implant contact and thus the osseointegration [22]. The most frequently used commercially available surface modifications on zirconia implants are sandblasting, acid-etching, and laser peening [22]. In certain instances, surface treatments such as sandblasting and acid etching are combined in order to further improve the bone-to-implant contact capacity (e.g. Zeramex, Dentalpoint, Spreitenbach and Pure, Straumann, Basel).

As a final variable potentially affecting clinical outcome, zirconia implant design can be classified into one of two types: one-piece implants, which consist of an implant and abutment as a single unit and two-piece implants, which do not include the abutment on the same unit. The latter offers the possibility to screw or cement the abutment in place. Previously published clinical studies include only cemented abutments and report lower implant strength and more frequent technical complications than one-piece implants [23].

Currently, more data is available for one-piece implants, which show improved mechanical properties relative to two-piece implants [24].

Regarding the material and restorative option (e.g., single crowns (SCs) or fixed dental prostheses (FDPs)) used for zirconia implants, no specific recommendations based on observed survival rates have been proposed. These remain open questions to be clarified by clinicians.

As discussed in part I of this review, preclinical and animal studies on zirconia implants have demonstrated comparable results to those for titanium implants regarding biocompatibility, osseointegration capacity and soft tissue response [25]. Hence, zirconia implants may be considered a potential alternative to titanium implants. In order to fully evaluate clinical performance and therefore, the feasibility of recommending zirconia implants. Clinical studies are the ideal tool to provide clinicians with important information when considering zirconia implants [6], [26]. Therefore, the aim of this systematic review was to evaluate the clinical outcome of zirconia implants and their associated implant–restoration complexes.