This study reports the short-term postsurgical symptoms as part of a randomized clinical trial comparing the treatment with mandibular overdentures retained by a single (1-IOD group) or two implants (2-IOD group) opposing to a conventional maxillary complete denture. The study protocol was previously registered (NCT03691285) and approved by the local ethical research committee (CAEE 65240617.5.0000.5083). A detailed description of the randomized clinical trial is previously published , which includes the 1-year results of comparative changes in oral health-related quality of life, patient satisfaction, and chewing function. All patient treatments and data collection were conducted at the School of Dentistry of the Federal University of Goias, Brazil.
Participants were fully edentulous subjects needing replacement of old complete dentures and implant intervention for retention of the mandibular denture. Eligibility criteria included favorable general health and sufficient bone dimensions for placement of an implant at least 8-mm length in the potential implant sites (midline and canine regions), assessed using a panoramic radiograph. The exclusion criteria comprised any general or local contra-indication for implant treatment, presence of oral conditions that demand additional treatments, subjects with significant cognitive decline, and those who disagreed to be randomly assigned to one of the two study groups. Subjects who did not meet the inclusion and exclusion criteria were referred for appropriate treatment in other clinical settings of the dental school. The study sample size was calculated within the context of the main study  and comprised 48 participants, 24 in each group.
After complete denture treatment, participants were randomly assigned to the study groups (1-IOD and 2-IOD groups) using block randomization and stratification by gender, aiming to reduce allocation bias and to achieve a balance of participants in the two groups. To avoid selection bias, each participant’s assignment group was labeled and concealed in sealed black envelopes, and the group identification was only revealed at the time of the implant surgery.
A new set of complete dentures was provided to all participants. They were scheduled for implant treatment planning after reporting being adapted to denture use with no major complaints, except the perceived need to improve the mandibular denture retention and stability.
Implant surgery planning included the clinical assessment of the residual alveolar dimensions in the midline and bilateral canine areas and panoramic radiograph for confirmation of sufficient bone height for placement of one or two regular diameter implants with 8-, 10-, or 12-mm length. Preoperative care included verification of vital signs, medication with antibiotic prophylaxis with amoxicillin 2 g or clindamycin 600 mg, paracetamol 750 mg, and dexamethasone 8 mg in cases of alveolar ridge reduction .
At the time of the surgery, the group assignment for each participant was disclosed to the patient and the surgeon. Implant surgery procedures initiated infiltration anesthesia with articaine hydrochloride with 1:100,000 epinephrine, crestal incision allowing a full-thickness flap elevation to expose the implant site extending between canine regions for the 1-IOD group and immediately beyond the mental foramen bilaterally for the 2-IOD group. When needed, alveolar bone reduction was performed for preparation of the implant site with a straight surgical handpiece and a tungsten drill under sterile saline irrigation. In all cases, a surgical guide was obtained by duplication of the mandibular denture, which served as a reference for correct implant positioning.
Tissue level Straumann® Standard Plus SLActive® regular neck implants (Straumann 033.561S/652S/563S, Institute Straumann AG, Basel, Switzerland) were inserted in the mandible midline (1-IOD group) or the lateral incisor-canine area bilaterally (2-IOD group). Drilling sequence and implant insertion were performed according to the manufacturer’s protocol. The final insertion torque was checked with a torque wrench adjusted to 25 N cm. After implant placement and healing abutment connected to the implants, tissues were sutured, and the mandibular denture was relieved and relined with temporary soft relining material (Soft Comfort, Dencril, São Paulo, Brazil). The patient’s postoperative care included paracetamol 750 mg in case of pain (every 6 h for up to 3 days), a soft diet, and 0.12% chlorhexidine mouthwash rinse for 1 week. Sutures were removed after 7 days.
Postoperative pain and discomfort
After the implant surgery, postoperative pain and discomfort were measured by questionnaires using graduated visual analogue scales (VAS) from 0 to 100 mm. Each participant was asked to rate the value that corresponds to their perception of the following aspects: (1) pain in the surgical area, (2) pain when chewing, (3) bleeding in the surgical area, (4) swelling of the surgical area, and (5) perception of the unpleasantness of the surgery . Data were collected at 24 h and 7 and 21 days after implant placement surgery.
Data concerning participants’ demographic (age and sex) and clinical features were collected. Clinical characteristics included smoking habit, classification of the residual ridges (high well-rounded, knife-edge, flat, depressed) , and prognostic classification scores (Prosthodontic Diagnostic Criteria) according to the American College of Prosthodontists (ACP) for edentulous patients .
The mucosal width and thickness at the implant sites were evaluated before the surgical procedures to record the keratinized tissue width (KTW) and vertical mucosa thickness (VMT). The KTW was measured by identifying the mucogingival junction at the buccal, lingual, and crestal sites using a calibrated probe and the rolling technique. Measurements were categorized as wide (> 2 mm) or narrow (≤ 2 mm) . For assessment of the VMT, a #30 K-file (Dentsply Maillefer) was penetrated in the tissue after local anesthesia until touching the bone crest, and the thickness was measured with a ruler to the nearest millimeter . Measurements were also categorized as a thin (≤ 2 mm) or thick (> 2 mm) mucosa.
The surgery duration was recorded in minutes, ranging from the local infiltrative anesthesia until tissue suture. The need for osteotomy for alveolar bone reduction was also recorded.
Descriptive statistics and between-group comparisons were performed to test differences in baseline characteristics of the two groups. Then, we tested the effect of the different follow-up times on the patient’s perceived outcomes after surgery in the two overdenture groups. The Shapiro-Wilk test was used for testing the normality of data. Non-normal distribution measurement data were expressed as median (interquartile range) and analyzed by non-parametric comparison tests—Mann-Whitney test for between-group comparisons, and the Wilcoxon signed rank test for within-group comparisons. The effect size for longitudinal comparisons was calculated (ES = Z/√N).
Since each subject gave multiple responses in a repeated-measures design, the assumption of independence of ratings was violated, due to an idiosyncratic factor that affects all responses from the same subject, thus rendering these different responses inter-dependent (correlated) rather than independent. Therefore, a generalized estimating equations (GEE) regression was performed for each of the postsurgical outcomes, considering the combined subject and time-point variables as the within-subject variable to identify the repeated measures across the dataset. The gamma with Log-link was selected as the distribution-link function combination for the dependent variables due to the skewed distribution of data. Due to the excessive occurrence of zero values in the outcome variable (zero-inflated data), added a positive constant (c = 10) to all observations “Y”, so that Y + c > 0, preventing the exclusion of the corresponding case in the analysis. GEE models were also tested considering the remission of symptoms (score = 0) for outcome measure, using binary logistic regression. The follow-up period, overdenture group, and clinical features were used as fixed predictor variables in the GEE models. The maximum likelihood estimates were used to calculate the regression parameters, and the Wald statistics were used to test the significance of the model effects. The IBM-SPSS was used for data analysis, and the 0.05 level of significance was considered for statistical inferences.