This manuscript conforms to the STROBE reporting guidelines.

This study was designed as a multicenter, prospective, single-cohort, post-market clinical follow-up (PMCF) study testing a two-piece bone-level-tapered (BLT) implant with a diameter of 2.9 mm (Straumann® Bone-Level-Tapered ∅ 2.9 mm SC, Roxolid®, SLActive, Institut Straumann AG, Basel, Switzerland) in the position of the lateral incisor of the maxilla or in any incisor position in the mandible with a follow-up time of 1 year (NCT02699866). The participating sites were all located in Germany and included oral and maxillofacial surgeons in the University Hospital of Mainz (coordinating site), University Hospital of Münster as well as private practices in Mainz, Düsseldorf, and Zwickau. The study protocol was developed in collaboration with all sites and was led by the coordinating site. All participating sites agreed on the final study protocol, including the statistical analysis. The local ethical committees from each state accepted the protocol.

The study population consisted of primary male and female patients sampled from those who came to the clinic for regular check-ups (convenience sampling). From October 2015 to December 2019, adults requiring a single tooth replacement with a dental implant in central and lateral incisors in the mandible and lateral incisors in the maxilla were enrolled and included in the study according to the following inclusion criteria: minimum age of at least 18 years, missing tooth in the Federation Dentaire Internationale (FDI) regions 12, 22, 32–42 for at least 4 weeks with natural adjacent teeth or implants (single tooth gap) with a complete soft tissue coverage of the socket.

The following exclusion criteria were applied: inadequate bone volume or quality, local root remnants, inadequate wound healing capacity, incomplete mandibular or maxillary growth, serious internal medical problems, uncontrolled bleeding problems, psychoses, prolonged therapy-resistant functional disorders, xerostomia, weakened immune system, illness requiring periodic steroid use, uncontrolled endocrine disorders, poor general health, drug or alcohol abuse, allergies or hypersensitivity to chemical ingredients of titanium–zirconium alloy, pregnancy or a plan to conceive during the study period. Prior to surgery only one implant per patient was defined as the study implant.

In total seven visits per patient were scheduled during the study. Screening visits were conducted up to 2 months before implant placement. After confirming eligibility, the ∅ 2.9 mm BLT implant was placed according to the manufacturer’s recommendations. Implant lengths used in the study were 10, 12, and 14 mm. Bone augmentation was performed when required. The bone quality (types I–IV), potential bone augmentations (contour, vertical or lateral augmentation), and type of bone augmentation material (autogenous bone graft, xenograft, allograft, synthetic, and others) were recorded. The implant healing procedure was either subgingival (Straumann SC Closure Cap; Institut Straumann AG, Basel, Switzerland) or transgingival (conical Straumann SC Healing Abutment; Institut Straumann AG, Basel, Switzerland). After implant placement, sutures were removed 7–14 days later, and the provisional crown, bonded to a Straumann SC Temporary Crown (Institut Straumann AG, Basel, Switzerland), was placed approximately 6 weeks after surgery. The patients were then referred to a prosthodontist to take the final impression (not a scheduled visit) to finalize the crown. The final crown was bonded to Straumann SC Variobase® abutments (Institute Straumann AG, Basel, Switzerland) and then delivered ~ 4 months after implant surgery and was placed according to each prosthodontist’s routine. A 6-month and a 1-year follow-up were performed, as shown in Fig. 1.

Follow-up visits and treatment protocol. Implants were placed during visit two (day 0 = baseline). A 6-month and 12-month follow-up was conducted adhering to the study protocol

In addition to demographic data, dental (reasons for tooth loss, socket preservation history) and medical (current clinically relevant conditions and concomitant medications) history were documented. Smoking history was recorded, and participants were classified as non-smokers, past smokers, and current smokers with up to ten or more than ten cigarettes per day. Time since tooth extraction or loss at the planned study implant site, and type of bone-fill material (autogenous bone, xenograft, allograft, synthetic, and others) were also noted as a part of the pre-operative planning.

The primary analysis was the survival of the implant at month 12. Implant success was defined as the presence of an implant at month 12 and absence of persisting subjective complaints (pain, foreign body sensation, and/or dysesthesia), absence of recurrent peri-implant infection with suppuration, absence of tactile implant mobility, and absence of continuous radiolucency around the implant.

Pink esthetic score (PES) was calculated by assessing seven aspects of the peri-implant soft tissue, such as mesial papilla, distal papilla, soft tissue contours, soft tissue level, alveolar process, soft tissue coloring, and soft tissue texture. If any of the variables of the PES was missing, this variable was assigned a missing value. The soft tissues were evaluated as follows: unnatural–virtually natural–natural, discrepancy: > 2 mm–1–2 mm–< 1 mm, and coloring: clear difference–slight difference–no difference. The alveolar process was evaluated regarding potential resorption and classified as either “clearly resorbed”, “slightly resorbed”, or “no difference”. Each parameter is rated with a 0–1–2 score, where 0 represents the poorest and 2 represents the best score. The highest achievable score is 14.

Radiographs (peri apical X-rays) were taken both at the time of implantation as well as 6 and 12 months after implantation to assess the bone-level changes. The bone level was calculated in millimeters as the mean of the mesial and the distal measurement. Negative bone-level changes represented bone loss between baseline and the respective visit. The relative marginal bone level was assessed as the distance from the implant shoulder to the first bone-to-implant contact (fBIC). For this relative measure of bone change, all subcrestally placed implants had an initial value of fBIC = 0. To include an analysis of initial remodeling, the absolute change from initial marginal bone level to bone level at follow-up was also analyzed. For this absolute measure, subcrestally placed implants had a negative value as measured from implant shoulder to marginal bone level.

Further assessments regarding the course of oral hygiene, plaque index (PI), modified sulcus bleeding index (mSBI), and probing pocket depth (PPD) were undertaken. PI, mSBI, and PPD were assessed on the implant site on the mesial, distal, buccal, and palatal surfaces. PI was scored according to Silness and Loe [27] as follows: Score 0—no plaque, Score 1—a film of plaque adhering to the free gingival margin and adjacent area of the tooth, Score 2—moderate accumulation of soft deposits within the gingival pocket or the tooth and gingival margin, Score 3—abundance of soft matter within the gingival pocket and/or on the tooth and gingival margin. mSBI, was scored according to Mombelli et al. [28] as follows: 0—no bleeding when a periodontal probe is passed along the gingival margin, 1—isolated bleeding spot, 2—confluent red line on margin, and 3—heavy or profuse bleeding.

Data analysis was conducted after all patients completed the 12-month follow-up visit. Two data sets were defined for the analysis: the safety analysis set (SAS) and the full analysis set (FAS). The SAS consists of all patients in the study, who received the study implant. The SAS population was the basis for the safety analysis and provided the baseline characteristics. The FAS consists of all patients in the study, who received the study implant and from whom at least one follow-up measurement after baseline was available. This analysis included patients regardless of any protocol deviations and/or premature termination. Notably, all identified deviations were deemed to have no impact on study integrity, subject’s rights, safety or welfare and none of the deviations were related to an increased risk to the subjects. Hence, no major protocol deviations were identified. The analysis was performed according to the intent-to-treat principle and was applied to primary and secondary endpoints as well as baseline characteristics. To explore the potential impact of bone augmentation on treatment success, all analyses conducted with the FAS or SAS were performed separately for patients with bone augmentation, patients without bone augmentation, and all patients.

A descriptive statistical analysis was performed. Implant survival at 1 year was analyzed using the Kaplan–Meier method (including 95% confidence intervals [CI]). Missing values of the implant survival were imputed based on the available data from previous (if implant survival = “no”) and subsequent visits (if implant survival = “yes”). Missing values of other variables were not imputed. Categorical data were analyzed by presenting frequency tables. Implant success, individual success criteria as well as individual PES items were analyzed using frequency tables (including 95%-CI). If missing values were present in frequency analyses, adjusted relative frequencies were calculated. If the fraction of missing values was comparatively large, non-adjusted relative frequencies were reported. For numerical data, the sample statistics mean, standard deviation (SD), median, minimum, and maximum were calculated. PES, bone level, and bone-level changes were analyzed using sample statistics (including 95%-CI for mean). The safety analysis included identifying the number and percentage of patients with at least one adverse event (AE), adverse device effect (ADE), serious adverse event (SAE), serious adverse device effect (SADE), unanticipated serious adverse device effect (USADE), device deficiency (DD) and at least one DD leading to a (S)ADE. Demography, medical history, study procedures, and other baseline characteristics were summarized using descriptive statistics. All analysis was generated using the SAS-software, version 9.2.