Patients without all posterior teeth are mostly treated with removable partial dentures (RPDs). It is important to understand the load transfer characteristics of various RPD designs (1-5). Treatment possibilities for the mandibular Kennedy Class (6) patients without all posterior teeth include different options: clasp retained RPDs (C-RPDs), precision/semi-precision attachment-retained RPDs (A-RPDs), implant retained RPD (IR-RPD), implant assisted RPD (IA-RPD) or fixed partial dentures on dental implants (I-FPD). The rehabilitation with the A-RPD usually requires the remaining anterior teeth preparation and a construction of a fixed partial denture (FPD) with incorporated attachments prior to the manufacture of a RPD.
Osseointegrated implants can be incorporated into RPD designs in an effort to overcome adverse effects of dislodging forces upon a RPD and adverse effects of strain forces to abutment teeth. Most design systems include standard size dental implants (SSI) in previous molar sites (7, 8). Implant-assisted RPDs have healing abutments on distal implants as vertical stops, providing only support to a RPD, thus disabling denture saddle subsidence in posterior alveolar ridge sites (9, 10). Implant-retained RPDs have abutments with an attachment system for retention of a RPD (7, 8, 11), thus providing both, support and retention to a RPD. It was reported that patients were more satisfied with implant-retained RPDs (12), but there was also a higher rate of late implant failure and maintenance issues (12, 13). However, in majority of patients standard size implants cannot be inserted in previous molar sites due to anatomical restrictions (horizontal or vertical alveolar bone atrophy) without performing alveolar augmentation procedures (14, 15). Due to the alveolar ridge atrophy, the mandibular canal can sometimes be located only few millimeters below the crest of the ridge. Therefore, some designs place standard size implants in premolar sites to assist or retain a RPD. More mesial implant placement still retains the Kennedy Class I status, while implants in molar sites change the Kennedy Class I to the Kennedy Class III. However, no difference between masticatory outcomes, periimplant bone loss, or other clinical/radiographic parameters in relation to implant position (molar or premolar) has been reported (16-18). Standard size implants in the first premolar sites require minimum alveolar ridge width of 5.5 mm in the buccal-lingual direction, which is often not available in long-term RPD wearers due to bone atrophy in buccal-lingual direction.
Mini dental implants (MDI) are slim one-piece implants recommended to retain mandibular complete dentures in patients with thin/narrow alveolar ridges. Usually four MDIs of at least 10 mm length are recommended to be inserted in the intraforaminal region (19-22). However, no studies have been conducted on whether MDIs can be used for better retention and stability of mandibular long saddle RPDs and what short and/or long-term effects would such treatment provide.
The aim of this study was to assess the effect size of a treatment in patients without posterior teeth (Kennedy Class I) treated by RPDs retained on 2 MDIs in the mandible. The aim was also to compare the esthetic and masticatory outcomes of such treatment between the MDI-retained RPDs and the clasp-retained RPDs (C-RPD) and to follow-up such patients for the period of first 6 months.
Materials and Methods
Design and eligibility criteria
A convenience sample of patients of Kennedy Class I without all posterior teeth (only frontal teeth left -maximum 6, minimum 4), providing only linear support to a denture in the mandible were selected from patients who had been referred for a treatment with a new mandibular RPD. Full medical history was taken and clinical examination was performed. Panoramic radiographs and/or CBCT before treatment were also obtained. For inclusion into the MDI-RPD group the buccolingual alveolar ridge width needed to be less than 4.5 mm in the first premolar or canine region (measured with a caliper after a topic anesthesia of oral mucosa and/or after obtaining a CBCT). The bone length needed to be at least 10 mm and the time elapsed from any tooth removal had to be at least 6 months. The patients with flabby alveolar ridges were excluded.
The patients who met such criteria were attempted to be distributed randomly into the 2 treatment groups (MDI-RPD or C-RPD) with the 1:1 allocation ratio. However, those patients who were not willing to receive MDIs (free of any charge) or patients with medical contraindication for MDI insertion were relocated into the C-RPD group. Full ethical approval was obtained from the Institutional Ethic Committee and each participant signed the informed consent.
The exclusion criteria for the MDI insertion were: poor general health (Class III-IV according to the classification of the American Society of Anesthesiology (23) severe renal/or liver disease, history of a radiotherapy in the head and neck region, chemotherapy at the time of surgical procedure, non-compensated diabetes mellitus, HIV), ongoing intravenous bisphosphonate therapy, mental disorders (anamnestic), drug abuse (anamnestic). In 2 patients one of two inserted MDIs failed before loading and they were also excluded from the study.
A total of 52 patients (36 females and 16 males) in the 56-84 year-old age group participated in the C-RPD group and 38 patients (27 females and 11 males) in the 43-81 year-old age group participated in the MDI-RPD group. Two patients were excluded from the MDI-RPD due to surgical MDI failures; therefore a total of 36 patients (26 females and 10 males) participated in the respective group. All patients had complete dentures in the maxilla.
Prior to insertion of the MDIs, all patients were examined clinically, with a detailed analysis of panoramic radiographs and/or CBCT-s. All surgical procedures were performed by residents under supervision of two experienced specialists, one prosthodontist and one oral surgeon. All patients were prescribed antibiotics prior to surgical procedure, i.e., 2 g of Amoxicillin or 600 mg of Clyndamicin, one hour before surgery. The MDIs dimensions were chosen depending on the available bone (Dentium, Seoul, South Korea – 2.0 or 2.5 mm wide and 10, 12, or 14 mm long). They were placed according to the manufacturer’s instructions using the calibrated burs, a physiodispenser (W&H Implantmed, GmbH, Austria) and saline solution for drill cooling, under local anesthesia (Ubistesine forte 4% or Mepivastesin 3%, 3M, Germany). All MDI insertions were performed without reflecting the mucoperiostal flap. All MDIs were placed distally to the last remaining tooth in the mandible or one tooth width posteriorly, at the sites of previous first premolars or canines (Figure 1). After surgical procedure, the patients were prescribed an antiseptic mouth rinse (chlorhexidine gluconate 0.12% twice daily for 5 days) and were advised to take analgesics (non-steroid anti-inflammatory drug – e.g., ibuprofen 400 mg) one hour after surgery and if necessary up to 5 days. Patients were also provided with standard post-surgical instructions (ice packs from the outside, regular and meticulous oral hygiene, and they were advised to avoid eating hot food or drinking hot beverages for the first two days after surgery.
All RPDs were made by prosthodontic residents under supervision of one experienced specialist. The RPDs were reinforced with Co-Cr framework in order to prevent fractures. All RPDs had lingual plate major connectors and raised cingulum on remaining canines (in cases when they were present). Individual impressions in custom trays were obtained from each patient by impression compound (ISO Functional Stick, GC, Tokyo, Japan) for borders and a medium viscosity silicone (DimensionTM VPS Impression Material, 3M ESPE, Seefeld, Germany). For MDI-RPD impression, transfer caps were attached on the MDIs before impression, laboratory analogues were inserted, and o-ball matrices were attached on the MDI analogues. In the C-RPD group, the clasps were placed on distal remaining teeth.
Patient centered outcomes
Patients assessed their orofacial esthetics prior to treatment, after treatment (RPDs delivered and all adjustments finished), and after a period of 6 month. Patients also assessed how confident they were while chewing hard food, how efficiently their food was comminuted and how confident they were with their RPDs. All assessments were made using the 0-10 visual-analogue scale (VAS scale) and higher VAS scores represented better results.
The SPSS for Windows software was used for statistical analysis. It included descriptive statistics, t-test for independent samples, and standardized effect size calculation using the formula: Mean (baseline score – follow-up score) / standard deviation of the baseline score (22).
A total of 38 patients received 2 MDIs each, but in only 36 of them their MDIs were loaded. Two patients were excluded before loading, since each of them lost one MDI, representing the 97.4% of surgical MDI survival. At a 6-month follow-up in 36 patients, all loaded MDIs survived (100%). The mean values and standard deviations of patients’ assessment of their confidence while chewing hard food, their assessment about how efficiently the food was comminuted, the assessment of their orofacial esthetics and of their comfort with denture wearing (their old dentures) prior to treatment are presented in Figure 2a. The mean values and standard deviations of patients’ assessment of their confidence while chewing hard food, their assessment about how efficiently was food comminuted, assessment of their orofacial esthetic appearance and their comfort with new dentures, after treatment and all adjustments had been finished are presented in Figure 2b. The same assessments after a period of 6 month of their new RPD wearing are presented in Figure 2c.
Although old dentures were of various state and quality before treatment, there were no significant differences between patients assigned either to the C-RPD or to the MDI-RPD group (p>0.05). However, after treatment the MDI-RPD wearers achieved significantly higher all post-treatment scores than the C-RPD wearers (p<0.01). The difference between the baseline and the post-treatment data was significantly higher in the MDI-RPD wearers for self-confidence when chewing hard food (t=11.09, DF=86, p<0.01, efficiency of food comminution (t=7.16, DF=86, p<0.01), orofacial esthetics (t=8,8, DF=86, p<0,01) and a comfort of RPD wearing (t=6.9, DF=86, p<0.01). The standardized effect size of the assessment of orofacial esthetics was 2.9 in the C-RPD wearers and 3.38 in the MDI-RPD wearers, being notable in both patient groups (24). The standardized effect size for the self-confidence while chewing hard food amounted to 3.05 in the C-RPD wearers and 6.19 in the MDI-RPD wearers. The standardized effect size for the assessment of food comminution was 2.1 in the C-RPD wearers and 4.43 in the MDI-RPD wearers. The comfort of denture wearing was similarly increased (C-RPD wearers 3.0 and MDI-RPD wearers 4.9). All effect sizes represented high treatment effects (24), but the effect was always higher in the MDI-RPD group.
After the period of 6 month, all MDIs survived. At that point the difference between the C-RPD and the MDI-RPD group was even more pronounced (p<0.01), as in the C-RPD group the scores dropped down slightly, while in the MDI-RPD group the scores further slightly increased.
To overcome the problems with a RPD retention and stability and to prevent the adverse effects of a RPD to abutment teeth and tissues of a denture bearing area, standard size implants (SSIs) have been used successfully in various positions with various suprastructures (7-13, 15-17). However, for SSI insertion, the patient needs to have an adequate bone volume, which is often lacking in a long term RPD denture wearers. The MDIs have been recommended for narrow ridges and have been successfully utilized for retention of mandibular complete dentures by inserting 4, 3, and even only 2 MDIs intraforaminally (19-27). The MDIs have also been successfully used for FPD retention in the anterior area, and sometimes even in posterior regions of the mandible (28-30). However, we could not find any reports on utilization of MDIs for retention and support of RPDs in a review of selected dental literature. Therefore, we aimed to study treatment effects of insertion of two MDIs for retention and support of mandibular RPDs in patients with slim ridges. Patients with abundant bone volume were not included, as they could receive SSIs.
Although we tried to perform a randomization, it was eventually not possible or ethical, since some patients refused the placement of the MDIs due to pain-related fears. We hypothesized that such patients would favor C-RPDs, but the obtained data did not show that. The patients who received the MDI-RPDs had significantly higher scores after treatment and showed larger size effects of the provided treatment.
Orofacial esthetics represents one of the most important components of patient’s satisfaction (31-37). Although clasps are less visible on mandibular frontal teeth than on maxillary frontal teeth, patients obviously rated their orofacial esthetics lower when they had the C-RPDs. Moreover, the MDI-RPD wearers rated higher than the C-RPD wearers the food comminution, the confidence while chewing and their comfort of denture wearing. They also showed larger effect size of the treatment than the C-RPD wearers. The MDIs which were inserted one or two tooth width distally from the last remaining tooth, obviously provided better support and retention of the RPDs. The MDI insertion converted linear into more favorable polygonal support to a RPD and that obviously allowed patients higher self-confidence while chewing, better food comminution and comfort. Initial treatment effects of the MDI-RPDs show better results than of the C-RPDs. A previous study reported the outcomes by comparing the A-RPD and C-RPD (5). Insertion of 2 MDIs is usually even less expensive than construction of any A-RPD, which requires teeth preparation and construction of a FPD with incorporated attachments prior to a fabrication of an RPD. With insertion of MDIs to enhance a RPD’s retention and support, even slightly movable teeth can be left in patient’s mouth, which would probably be extracted for the A-RPD construction. The price of one MDI is almost three times lower than that of one SSI, and lower than the A-RPD. Usually, the MDI insertion elicits less pain than the SSI insertion, especially when placed without reflecting a flap (38), favoring them also for very old patients. Considering the presented findings, it seems that the MDI-RPDs might be even more favorable than the A-RPDs.
Long term follow-up studies are needed to evaluate whether the MDI-RPD will be a viable treatment option in a clinical practice. Considering that mandibular complete denture on 2 MDIs has already shown longitudinal success (30), and that the lingual plate major connector above cingulum of mandibular frontal teeth in the RPD wearers prevents some adverse dislodging forces, we assume that the insertion of 2 MDIs for better retention and support of a RPD may soon be confirmed as a viable long-term treatment option. The provided results represent patient centered outcomes for the first 6 month of RPD wearing. To provide a long-term evaluation of our MDI-RPD patients, the authors plan to perform regular follow-ups with clinical and radiographic examination and report on results in a future study.
The MDI-RPD wearers reported better post-treatment orofacial esthetics, better hard food comminution, higher self-confidence when chewing and better comfort compared to the C-RPD wearers. The MDI-RPD wearers also showed larger effect size of treatment, which stayed consistent through the first 6 month.