The study design is a prospective cross-over study.
Twelve completely edentulous patients aged 45–65 years, having acceptable maxillomandibular relationship, sufficient inter-ridge space, and with no previous denture experience, were selected for this study. Complete maxillary and mandibular dentures were fabricated for each patient.
Since the type of opposing occlusion is a critical factor that influences the magnitude of forces transmitted to the implant bone interface, the opposing occlusion was selected to be mucosa-supported complete denture. This was done to standardize and control the amount of occlusal forces applied to the abutments. Complete dentures were proved to exert less amount of force compared to the natural teeth [23].
The selection of the interforaminal area of the mandible, where two implants were placed, was based on the recommendation by Lekholm and Zarb [24] and Hong et al., [25] as the bone in this area is of good quality. Implants are demonstrated to have fewer micro-movements, increased initial stability, and reduced stress concentration in high-quality bone [26]. Furthermore, it has been established that the survival of the root form titanium implants is very high in the anterior mandible and that the incidence of surgical complications is very low [27, 28].
A clear acrylic radiographic/surgical mandibular template including gutta-percha radiopaque indicators allowed implant alignment along planned prosthetic axes during implant surgery and ensured good visual access [29].
The previously fabricated denture was duplicated in clear acrylic resin and used as a radiographic surgical guide using gutta-percha as radiopaque markers. Each patient was evaluated radiographically using cone beam computed tomography (CBCT) before surgical implant placement. A mucoperiosteal flap was reflected exposing the mandibular interforaminal region for optimal implant placement. Two implants (Dentium Superline, Dentium Co. Ltd., Korea) were screwed in position. The length of the implants was 10 mm and the diameter is 3.6 mm.
After a healing period of 3 months, acrylic maxillary and mandibular overdentures were fabricated with bilateral balanced occlusion for all patients. Positioner attachments were used and incorporated into the dentures using direct pickup method. Six patients used their dentures with bilateral balanced occlusion for 4 weeks then were evaluated by EMG. Following evaluation, it was converted into canine guidance occlusion using the same denture. This was achieved clinically by remounting using the semi-adjustable articulator.
Light-cured composite resin was then added in the mandibular canines to provide an interarch disocclusion space of 2 mm during eccentric movements.
For the other six patients, canine guidance occlusion was applied first in the try-in stage through application of light-cure composite resin on the mandibular canines (Fig. 1).
The patients used their dentures for 4 weeks then were evaluated using EMG. After evaluation, canine guidance occlusion was converted into bilateral balanced occlusion by removal of the composite resin from the mandibular canines (Fig. 2). The positioner attachments (positioner abutment and socket set, Dentium Co.Ltd, Korea) were placed on the implants and attached to the dentures by direct pickup method (Fig. 3).
Recording the electromyographic activity
EMG has also been used to assess the masticatory function of mandibular implant-retained overdentures. The electrical output of a muscle, measured by electromyography, is proportional to the energy consumed to produce contractions. The masseter and anterior temporalis muscles on both sides were evaluated because they are the largest and strongest of the masticatory muscles, the most superficial and are accessible to surface EMG examination. The surface EMG recordings provided a safe, easy, and noninvasive method that allowed objective quantification of the energy of the muscle [30].
Standard amounts and sizes of cake and peanuts were used to reduce patient variability. These test foods were an example of soft and hard food, respectively, and they gave an idea about the effect of different types of food on muscle activity during function. Preformed silicon index was used to measure the muscle activity during clenching [31].
Evaluation of muscle activity was performed by measuring activity of the masseter muscles on both sides for both occlusal concepts at the end of 2 weeks using electromyography (Nicolet VikingQuest version 11, USA) with three types of test foods.
During all recordings, the patients were seated with their head unsupported and were asked to maintain a naturally erect position. The masseteric myoelectric activity of both sides (right and left) were recorded by means of disposable bipolar electrodes positioned on the bellies of the muscles parallel to the fiber orientation (Fig. 4).
Electroconductive gel was applied on the electrodes before they contacted the skin. The recording electrodes were positioned approximately 20 mm apart. The patient was grounded by using a grounding electrode. The third electrode was fixed on the palm of the patient’s hand.
Each patient was instructed to clench with a preformed silicon index made by using vinyl polysiloxane material (putty) of standardized size positioned at the premolar region for 30 s to measure the muscle activity during clenching. Then the patients were instructed to chew on one peanut of similar size and diameter, and the EMG was recorded. Then the patients were instructed to chew on a piece of cake of standardized size (2 cm × 2 cm), and the EMG was recorded. The patients chewed the test samples on the right and left sides at 10-s intervals using their arbitrary chewing frequency until they are ready to swallow before the EMG was recorded.
At the end of the record and before removing the surface electrodes, the positions of the electrodes were marked to be used as a guide for accurate reproducibility. The previous tasks were separated by a recovery rest period of 2 min. The computerized data showed the root mean square (RMS) of the EMG signals (Fig. 5).
Statistical analysis
Data were fed to the computer. Quantitative data were described using range (minimum and maximum), mean, standard deviation, and median. Wilcoxon signed ranks test was applied. Significance of the obtained results was judged at the 5 % level. A p value less than 0.05 was considered statistically significant.
Ethical approval
This study protocol was approved by the research ethics committee of the Faculty of Dentistry, Alexandria University, Egypt.
Ethics, consent, and permissions
All the patients signed an informed consent form before participation in this study.
Consent to publish
All the patients who participated in the study provided consent to publish the data obtained from them during the study.