Background (500)
Despite biocompatibility and excellent clinical outcome of titanium implants, a growing interest for an alternative implant material is noticeable. This is mostly due to patient-specific requests for metal-free treatment and concerns about adverse reactions titanium particle corrosion. Therefore, manufacturers are increasingly introducing zirconia implants with different features and designs. While most systems offer one-piece implant design, little is known about the applicability and features of two-piece implants.

Aim of the study (300)
This review aimed to summarize the preclinical knowledge about two-piece zirconia implant systems. Focus was given on design features, mechanical resistance of the implants and of the implant-restoration complex as well as peri-implant tissue response.

Material and Methods (1000)
An electronic search of the literature until March 2017 was performed to identify all articles related to preclinical research on two-piece zirconia implant systems. The search was conducted using MEDLINE (National Library of Medicine), Pubmed without restrictions concerning the date of publication. The search terminology included: zirconia implant, two-piece, mechanical properties, fracture resistance, strength, osseointegration, bone-to-implant contact, soft tissue, histology, histomorphometry (connecting different keywords with AND, OR). Hand-searching of the bibliographies of all full-text articles and related reviews, selected from the electronic search, was also performed. The literature search was limited to only English language articles. References appraised in related systematic reviews were also considered.

Results (1000)
Of the initially-identified 1026 titles, 58 papers were selected for full-text evaluation. After exclusion 53 articles, a total of 6 article were finally selected and analysed. 
The mechanical resistance was evaluated in 4 studies, where a clinical situation of implants placed in the anterior region was simulated. Interestingly, most studies used titanium abutment screws for evaluation. Irrespective of connection type (screw- or cement-retained), implant fractures were mostly reported in the implant-abutment connection, with the weakest component to be abutment screw, abutment and implant, respectively. In general, two-piece zirconia implant systems showed higher failure rates and lower resistance than one-piece zirconia implants as well as titanium implants.
Peri-implant tissues were evaluated in 2 studies. Regardless of implant type (one- or two-piece), a significant marginal bone loss was recorded 6months after placement (0.19-1.11mm). The height of buccal peri-implant mucosa and bone-to-implant contact ranged, respectively, between 2.64-3.03mm and 78.6-87.9% with no statistically significant differences at follow-up evaluations.

Conclusion and Clinical implications (500)
Irrespective of connection type and the design, two-piece zirconia implants show sufficient strength to withstand normal forces applied in the anterior region. Hard and soft tissue response around two-piece zirconia implants seem to be similar to one-piece implants as well as titanium implants. Although they have the potential for clinical application, the limited knowledge about two-piece zirconia implants does not support their clinical application yet.

A systematic review about two-piece zirconia implants- Results of preclinical studies