The design of the study is a retrospective analysis of all patients treated with a single implant-supported two-unit cantilever FPD in the posterior region of the maxilla or mandible between the period January 1, 2004, and January 1, 2018, in a private referral practice in Apeldoorn, The Netherlands. Inclusion criteria for the study were:
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One dental implant restored with a crown with one cantilever unit positioned mesially or distally, in the posterior region of the maxilla or mandible
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Presence of antagonistic teeth
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Follow-up period at least 1 year after placement of the restoration
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Presence of a radiograph taken directly after placement of the implant
Patients fulfilling all the inclusion criteria were informed verbally and in writing about the study and signed the informed consent form. Two-unit cantilever fixed partial denture implant treatment in the posterior region was judged as being standard care in daily dental practice. The Medical Ethical Committee of the University Medical Center Groningen considered this retrospective case series study not to be subject to the Medical Research Involving Human Subjects Act (METc RR-Number 201800656).
Surgical and prosthetic protocol
All patients were treated following the same treatment protocol by one surgeon (FLG). Implant surgery was performed using the standard Astra Tech Implant System protocol (document 79254-usx-1002 Astra Tech) to a final drill diameter. One hour preoperative antibiotic prophylaxis (3 g amoxicillin or, if allergic to penicillin, 600 mg clindamycin) was administered. Postoperative treatment included a chlorhexidine rinse twice daily starting 1 day before the operation and ending 10 days later. No further anti-microbial therapies were used. The surgical procedure was performed under local anesthesia. A crestal incision and performance of buccal and palatal flaps were made. Dependent on the height of the bone, an 8- to 13-mm implant (Astra OsseoSpeed™, Dentsply Implants, Mölndal, Sweden) was placed. The diameter of all implants was 4.0 mm. Maximum torque used during implant installation was set according to Astra Tech’s surgical manual, and primary implant stability was estimated manually. A cover screw was placed and the wound was closed with slowly resorbable sutures (Vincyl & Johnson Health Care, Piscataway, New Jersey). During the 12-week healing period, the implants were left in a submucosal position. One week after implant placement, a follow-up visit was scheduled for suture removal and review of the healing process. Twelve weeks after implant placement, second stage surgery was performed and a healing abutment was placed. Implant stability was manually examined.
An impression at implant level was made 2 weeks after second stage surgery for fabrication of the FPD. Two weeks thereafter, either a titanium individual abutment (Atlantis Abutment, Dentsply Implants, Mölndal, Sweden) was placed (25Ncm torque) and a metal- or zirconia-based porcelain FPD with cantilever was cemented (GC Fuji 1, GC Europe NV, Leuven, Belgium) or a metal- or zirconia-based porcelain FPD with cantilever was directly screw-retained to the implant (25 Ncm torque) (Fig. 1).
Outcome measures
Survival rate
Survival rate was defined as the percentage functional implants at the follow-up evaluation. The criteria for successful osseointegration according to Smith and Zarb were adopted [7]. Patients not examined at the follow-up evaluation were counted as having functional implants, unless their medical record revealed otherwise.
Radiographic assessments
To calculate changes in marginal bone level, a digital peri-apical radiograph was taken using a paralleling technique with an x-ray holder, immediately following implant placement (baseline) and at the follow-up evaluation.
Radiographs were analyzed using the known implant length as a reference. The interface of the implant and the abutment was used as a reference line, from which all distances were measured with the designated software (DicomWorks, Biomedical Engineering, University Medical Center Groningen, The Netherlands). The error of this method was reported 0.13 ± 0.01 mm [8]. The following linear measurements were assessed to the nearest 0.01 mm: the vertical distance between the reference line and the first bone to implant level, measured at the mesial and distal side of the implant (Fig. 2). Measurements were performed by one examiner (HS).
Clinical assessments
The following clinical variables were assessed at the follow-up evaluation:
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Plaque: assessed per implant using the modified plaque index [9]
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Presence of calculus
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Bleeding: assessed per implant using the modified sulcus bleeding index [9]
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Gingival health: assessed per implant using the gingival index [10]
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Probing depth: assessed at four sites per implant using a manual standardized pressure periodontal probe (Click-Probe®, Kerr, Bioggio, Switzerland) measuring to the nearest 1 mm
All clinical data were retrieved by one examiner (HS).
Complications
Peri-implant mucositis and peri-implantitis were calculated at the implant level. As definition for peri-implant mucositis and peri-implantitis, the consensus reached at the Seventh European Workshop on Periodontology was used [11], being peri-implant mucositis (radiographic bone loss less than 2 mm), bleeding on probing and/or suppuration and peri-implantitis, and bleeding on probing and/or suppuration in combination with marginal bone loss of 2 mm or more.
All technical complications (e.g., restoration failure, cement loosening, screw loosening, fracture of veneering ceramics) throughout the period from restoration placement until the follow-up evaluation were noted at the follow-up evaluation and collected from the patients’ medical record.
Patients’ satisfaction
Patient satisfaction was measured using a self-administered questionnaire on the specific treatment outcome with a visual analogue scale (VAS; five questions) and the validated oral health-related quality of life questionnaire (Dutch-validated version of the Oral Health Impact Profile questionnaire: OHIP-NL49; 49 questions in 7 domains) [12, 13].
Statistical analysis
All analyses were performed at implant level and restoration level, except for patients’ satisfaction. Descriptive statistics were applied to describe the means (with standard deviations) and medians (with interquartile ranges) of variables used in current study. For the bone level, in case of a normal distribution of data, a “one sample t-test” was used, if not, a non-parametric tests was used: the “Wilcoxon’s signed Rank test.”
Possible correlations between peri-implant bone level change and position of the implant (premolar/molar), jaw (maxilla/mandible), and method of attachment of the restoration (screw-retained/cement-retained) were calculated either with the “independent samples t-test” (sufficiently normal distribution) or with the “Mann-Whitney U test” (insufficiently normal distribution). Data were analyzed using the Statistical Package for Social Sciences (version 24.0, SPSS Inc., Chicago, Illinois).