|Year : 2021 | Volume
| Issue : 2 | Page : 89-96
Hybrid implants in the dental rehabilitation of posterior maxilla: A prospective clinical evaluation
Rajkumar Krishnaprabhu, R Arunkumar Shadamarshan, Sanjay Kumar Roy Chowdhury
Department of Dental Surgery and Oral Health Sciences, Division of Oral and Maxillofacial Surgery, Armed Forces Medical College, Pune, Maharashtra, India
|Date of Submission||29-Aug-2021|
|Date of Decision||15-Oct-2021|
|Date of Acceptance||06-Nov-2021|
|Date of Web Publication||14-Dec-2021|
Dr. R Arunkumar Shadamarshan
Graded Specialist (Oral and Maxillofacial Surgery), Dental Centre, Wangchuck Lo Dzong Military Hospital, Haa Dzong, Bhutan, Pin -15001
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Context: Implant-retained prosthodontic rehabilitation of the posterior maxilla poses a unique challenge due to deficiency in bony characteristics in many cases, thereby requiring elaborate adjunctive surgical procedures to aid in implant placement.
Aims: We aimed to evaluate the efficacy of hybrid implants in the prosthodontic rehabilitation of edentulous posterior maxilla.
Subjects and Methods: Prospective clinical evaluation of 27 patients (30 implants) rehabilitated using hybrid implants at 1 and 4 weeks after implant placement and 3, 6, and 12 months after functional loading was conducted.
Statistical Analysis Used: Descriptive statistics were used for statistical analysis.
Results: The average pain score on the Visual Analog Scale was 4.53 and 0.76 at the end of 1st week and 4 weeks. Four implants (13.33%) were found unstable by 4 weeks. Two implants (6.67%) had exposure by 12 months. Less than 1 mm of mobility was seen in one implant (3.33%) by 3 months, four implants (13.33%) by 6 months, and five implants (16.67%) by 12 months. One implant (3.33%) developed mobility up to 2 mm by 12 months. Seven implants (23.33%) showed a probing depth of ≥5 mm but none more than 6 mm. Gingival recession of 2 and 3 mm was seen in two implants (6.67%) and one implant (3.33%), respectively, at the end of 12 months. The average bone loss was 0.17, 0.31, and 0.46 mm by 3, 6, and 12 months. The average rate of bone loss was 0.02 mm per month.
Conclusions: Hybrid implant is an excellent alternative in patients with inadequate bone in the posterior maxilla precluding the requirement of maxillary sinus lift and grafting.
Keywords: Hybrid implant, maxillary sinus lift, posterior maxilla
|How to cite this article:|
Krishnaprabhu R, Shadamarshan R A, Roy Chowdhury SK. Hybrid implants in the dental rehabilitation of posterior maxilla: A prospective clinical evaluation. J Dent Implant 2021;11:89-96
|How to cite this URL:|
Krishnaprabhu R, Shadamarshan R A, Roy Chowdhury SK. Hybrid implants in the dental rehabilitation of posterior maxilla: A prospective clinical evaluation. J Dent Implant [serial online] 2021 [cited 2022 Jan 20];11:89-96. Available from: https://www.jdionline.org/text.asp?2021/11/2/89/332470
| Introduction|| |
Dental rehabilitation of partially edentulous or completely edentulous patients with dental implants has become a common practice in recent decades and has produced reliable long-term results., Current trends and demands have revealed the need for faster restoration of the dental function using implants, which have led to the introduction of early and immediate loading protocols. In the maxillary region, especially in the premolar and molar regions, the approximation of the maxillary sinus floor due to postextraction alveolar crest resorption and the pneumatization of the maxillary sinus poses a unique problem. Rehabilitation of the posterior maxilla using implant-supported prosthesis is often compromised due to the poor quality of the alveolar bone, resorption of the buccal alveolus with the palatal placement of the ridge crest, reduced buccolingual and mesiodistal dimensions of the alveolus, and reduced vertical dimension of the alveolar ridge. The major drawbacks of endosseous implants include the need for good quality alveolar bone with at least 1 mm of bone around the implant. For replacement of the teeth with conventional implants, the recipient site has to satisfy specific criteria regarding the bone quality and quantity of the alveolus. The length of the endosseous implants is often 8 mm upward to obtain primary stability. If the length of the implant is decreased due to inadequate bone height, the diameter of the implants has to be increased to achieve adequate bone–implant interface. In the maxillary sinus area, the vertical height of the bone is often <10 mm. When the bone volume is inadequate, additional surgical procedures, namely sinus lifts and bone grafting, are required for bone regeneration. However, these procedures are technique sensitive and expensive and require a more extended postoperative period before the placement of the implant. The success rate of such procedures depends on proper technique and use of expensive equipment.
The hybrid implant system is a combination of both subperiosteal and endosseous implants. The implant consists of a long malleable titanium plate having a length of 45 mm, a thickness of 1 mm, and a breadth of 3–5 mm with screw holes and a stump called abutment, projecting from the flat surface of the plate. The implant's malleable laminar plate has a buccal vestibular anchoring part with three screw holes and a lingual or palate anchoring part with two screw holes for fixing using titanium screws. The screws on the buccal vestibular and lingual or palatal plate are positioned in such a way that the screw ends do not meet each other. Hybrid implants are a novel implant system that provides a welcome solution to overcome the problems encountered in the compromised maxillary posterior area due to the subperiosteal placement of implants and being fixed to the cortical bone using screws. The difficulty in removing titanium plates and screws, which were used for open reduction and internal fixation of craniofacial fractures, inspired the design of such an implant.
Furthermore, the hybrid implant derives its support from the strong cortical basal bone providing an additional advantage of firm anchorage. The implant system helps to avoid sinus lifting and bone grafting procedures and reduces the functional stresses on the compromised bone due to the greater surface area of the implant. Moreover, the system is less technique sensitive and more economical and requires minimum armamentarium for implant placement. This study evaluates clinically and radiographically the efficacy of hybrid dental implants for early loading in edentulous posterior maxilla for implant-supported functional prosthetic rehabilitation.
| Subjects and Methods|| |
Institutional Ethical Committee approval was obtained for the study designed according to Helsinki Declaration. Patients who reported the replacement of missing maxillary dentition were randomly selected for the study with the following criteria.
- Residual well-healed residual maxillary posterior ridge, which has remained edentulous at least for 6 months
- A residual alveolar bone height of ≤10 mm and at least 5 mm of residual buccolingual bone width and at least 7 mm of interocclusal distance measured from the crest of the gingiva to the central groove of the opposing mandibular teeth
- No localized pathology, maxillary sinus pathology, or scarring after previous surgery
- Natural teeth adjacent to the tooth to be restored should have a complete occlusal surface and be free of any periodontal or endodontic infection.
Patients with active infection, poor oral hygiene, periodontally compromised dentition, parafunctional habits, systemic diseases that may impede wound healing, pregnant women or lactating mothers, therapy with bisphosphonates, and smokers were excluded.
Steps of the conduct of study
Patients were selected randomly based on the inclusion and exclusion criteria. Written informed consent was obtained from the patients. All patients underwent a thorough preoperative evaluation by the surgeon and the restorative dentist, including wax-up and fabrication of surgical template. Hybrid implant placement was carried out using the standard protocol. Early functional loading of the implant was undertaken after the initial soft-tissue healing phase of 3–4 weeks.
The hybrid implant consists of a thin long malleable plate made of titanium alloy (Ti6 Al4 V) with length varying from 30 to 45 mm, a thickness of 0.9 mm, a width of 5 mm with screw holes, and a stump called abutment. The implant is an elongated laminar plate with 2–3 screw holes on the buccal side and 2 screw holes on the palatal side for anchoring the screws to the most favorable site available [Figure 1]. At least two screws are fixed on the buccal side and one screw on the palatal side. The screws used are of 2 mm in diameter and vary in length from 4 to 12 mm. The diameter of the abutment is variable, with an external diameter of 3–4 mm and length of 4–8 mm.
|Figure 1: Structure of a hybrid implant. The two-hole portion is adapted on the palatal aspect and the three holes are adapted on the buccal aspect|
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Standard draping and extraoral painting were carried out. An intraoral chlorhexidine rinse was given, the operative site was wiped with gauze, and local anesthesia was achieved by administering 2% lignocaine hydrochloride with 1:80,000. A crestal incision followed by an anterior vertical release was given. A mucoperiosteal flap was elevated, and the alveolar bone was exposed. The implant was molded to the contour of the exposed alveolar bone in such a way that the abutment was projecting into the oral cavity in the direction of the tooth to be replaced. The three-hole side of the plate was adapted to the buccal aspect and the two-hole side, to the lingual/palatal aspect. A small crestal portion of the alveolar bone was shaved to make the surface of the alveolus flat. This was done to aid in the submerging of the hybrid implant plate into the alveolar bone. After proper adaptation, the implant was fixed to the alveolar bone using titanium screws of size 2 mm × 6 mm [Figure 2]. Three screws were placed in the buccal aspect and two on the lingual/palatal aspect. Before closure of the mucoperiosteal flap, a horizontal incision was placed only the periosteal part of the flap to aid in the proper closure of the flap. The closure was done with 3-0 silk suture. The abutment was the exposed part of the implant projecting into the oral cavity in the direction of the missing tooth. A course of empirical antibiotics and analgesics was given for 5 days. The patient was recalled after 7 days for suture removal. The implant was loaded after 3 months.
|Figure 2: Placement of hybrid implants. Note the three-hole portion is on the buccal side|
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After a soft-tissue healing period of 3–4 weeks [Figure 3], the stability of the implant was verified clinically using a probe. A closed tray abutment level impression technique would be followed for fabricating the prosthesis. An appropriately sized stock tray was selected for impression making. A light body silicone impression material was flown around the abutment. The putty impression was loaded in the tray and placed over the light body material to obtain an accurate impression. A model was poured, a wax-up was done, and a metal-ceramic restoration was fabricated. The crown was cemented on the final abutment in full functional occlusion [Figure 4].
|Figure 3: Healed surgical sites with abutments visible. This representative case shows four implants placed for the rehabilitation of partially edentulous posterior maxilla|
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|Figure 4: Occlusal and buccal views of the cemented metal-ceramic crowns in the implants in Figure 3|
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All patients were evaluated using the Visual Analog Scale (VAS) for the degree of pain and clinically detectable mobility at the end of the 1st week and 4 weeks. Implant exposure, postrestorative mobility, gingival probing depth, and gingival recession were recorded at 3, 6, and 12 months postoperatively following prosthetic rehabilitation. A radiographic positioner was attached to the X-ray machine to obtain radiographs in a standardized manner. A grid calibrated in millimeters was be superimposed on the radiograph to obtain the preoperative measurements [Figure 5]. Prerestorative distance between the implant shoulder and the point of the implant to bone contact at the mesial and distal to the implant on periapical radiographs taken in a standardized manner was recorded at 3, 6, and 12 months. Cone-beam computed tomography (CBCT) was not used due to the scattering effect of the implant which makes it difficult to estimate bone loss. Data on all parameters were compiled and statistically analyzed. The following criteria were applied in evaluating the success of each implant:
|Figure 5: Intraoral periapical radiograph with grid to assess the position postplacement|
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- No complaint of pain at the site of treatment
- No clinically detectable mobility when tested with opposing instrument pressure
- No recurrent or persistent peri-implant infection
- No complaint of screw loosening and implant exposure at the end of 1 year
- Crestal bone loss not exceeding 1.5 mm by the end of the 1st year of functional loading.
| Results|| |
A total of 27 subjects (30 implants) who required rehabilitation of the missing maxillary posterior teeth were undertaken for the study. All the patients received hybrid implants for the missing maxillary posterior teeth. The study comprised 17 male and 10 female patients with an average age of 44.43 years (range: 34–56 years, standard deviation [SD] = 6.70). None of the cases were lost to follow-up [Table 1].
The average pain score on VAS at the end of 1st week was 4.53 (range: 3–7, SD = 1.43). The average pain score on VAS at the end of 4 weeks was 0.76 (range: 0–2, SD = 0.72) [Table 2].
All implants were found to be stable by the end of 1st week. By the end of 4 weeks, four (4/30) implants (13.3%) were found to be unstable [Table 3].
None of the implants were exposed by the end of the first 3 months. One each had exposure by the end of 6 and 12 months, respectively. Hence, a total of two implants (6.67%) had exposure by the completion of 12 months [Table 3].
Less than 1 mm of mobility was seen one implant (3.33%) by the end of the first 3 months, four implants (13.33%) by the end of the 6th month, and five implants (16.67%) at the end of the 12th month. One implant (3.33%) developed mobility up to 2 mm by the end of the 12th month [Table 4].
|Table 4: Details of postrestoration mobility, probing depth, and gingival recession|
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The average postoperative probing depth remained at 3.4 mm (range: 2–5, SD = 0.92), 3.5 mm (range: 2–5, SD = 0.93), and 3.7 mm (range: 2–6, SD = 1.12) at the end of 3, 6, and 12 months, respectively [Table 4].
The average postoperative gingival recession remained at 0.46 mm (range: 0–1, SD = 0.51), 0.63 mm (range: 0–2, SD = 0.67), and 0.7 mm (range: 0–3, SD = 0.79) at the end of 3, 6, and 12 months, respectively [Table 4].
Amount of bone loss was measured by taking sequential intraoral periapical radiographs using a calibrated grid at the end of the 3rd, 6th, and 12th months postoperatively. Ten patients showed no bone loss by the end of the 3rd month, out of which only six showed no signs of bone loss by the 6th month and only five patients showed no bone loss by the end of 1 year. The average bone loss by the end of 3 months was 0.17 mm (range: 0–0.5, SD = 0.16) by the end of the first 3 months, 0.31 mm (range: 0–1.1, SD = 0.31) by the end of the 6th month, and 0.46 mm (range: 0–1.8, SD = 0.52) by the end of 1 year [Table 2]. The average rate of bone loss was 0.02 mm per month.
| Discussion|| |
Maxillary posterior partial or complete edentulism is one of the most common conditions in dentistry, with approximately 20%–30% of the adult partially edentulous population older than 45 years having missing maxillary posterior teeth in one quadrant and 15% of this age group in both posterior regions. In other words, approximately 40% of adult patients are missing at least some maxillary posterior teeth.
The maxillary posterior edentulous region presents many unique and challenging conditions in implant dentistry. In general, the bone quality is poorest in the posterior maxilla compared with any other intraoral region. A literature review of clinical studies from 1981 to 2001 reveals that the poorest bone density in the posterior maxilla may decrease implant loading survival by an average of 16% and as low as 40%. The cause of these failures is related to several factors. Bone strength is directly related to its density, and the poor-density bone of this region is up to 5–10 times weaker compared with bone found in the anterior mandible. After tooth loss, the maxillary alveolar process undergoes progressive, irreversible resorption that results in a massive loss of substance, both vertically and horizontally, depending on the duration of edentulism in this area.
The maxilla has a thinner cortical plate on the facial side compared with any region of the mandible. The loss of posterior maxillary teeth results in an initial decrease in bone width at the expense of the labial bony plate. The width of the posterior maxilla decreases at a more rapid rate than in any other region of the jaws. The resorption phenomenon is accelerated by the loss of vascularization of the alveolar bone and the existing fine trabecular bone type. However, because the initial residual ridge is so wide in the posterior maxilla, even with a 60% decrease in the width of the ridge, adequate-diameter root form implants usually can be placed.
However, only rarely sufficient is the residual bone height available between the maxillary sinus and the alveolar ridge after long-term edentulism. The progressive resorption of the maxillary alveolar process and the tendency of the alveolar recesses to expand into the alveolar ridge results in progressive pneumatization of the sinus. In most cases, the available host bone does not suffice for anchorage of endosseous implants, especially in the molar region.
Hybrid implant system is a versatile implant system. It comprises both subperiosteal and endosseous components. Hence, it is crucial to address both the subperiosteal and endosseous aspects of the different types of implants in order to analyze the efficacy of the new hybrid implant system.
The first implant system was of the subperiosteal type proposed by Dahl and later on placed by Aaron. It consisted of a metal framework which was placed directly on the alveolus below the soft tissue with an abutment emerging from the surface to carry the denture prosthesis. The procedure was carried out under general anesthesia. A crestal incision was placed, and the mucoperiosteal flap was raised both buccally and lingually followed by a primary impression of the alveolar bone with alginate. A stone cast was poured from the impression. Some clinicians even used cold cure acrylic resin for making the primary impression. The use of substances potentially toxic to the tissues, for making the primary impression on the surgically exposed alveolar bone, should be noted. The surgical site was sutured after the primary impression. A metal framework was constructed on the stone cast in the laboratory. Vitallium alloy was the most preferred for this. After 2 weeks, a second surgery was required for placing the metal framework subperiosteally over the alveolar bone following which primary closure was done. Many authors reported failure to close the flap properly due to the additional framework over the alveolar bone. Moreover, most of the cases reported are of fully edentulous arch rehabilitation. There are little reports in the literature about the use of subperiosteal implants in single edentulous spaces.
In the case of hybrid implant, the procedure can be done under local anesthesia. No harmful chemicals or tissue irritants are in direct contact with the blood or bone. Plate exposure in these implants is an important aspect to be considered. It can be attributed to tight closure of the flap as there is additional metal framework over the alveolar bone. According to Linkow, poor suturing of the mucoperiosteal tissue immediately after the implant is placed, is one of the key factors that result in plate exposure. The closure should not be too tight or too loose, and there should be sufficient exposure of the alveolar bone beyond the borders of the metal framework during flap elevation. Obwegeser noted that the improper fit of the framework of the subperiosteal implant to the alveolar bone resulted in the failure of the prosthesis. He suggested that the fixation of the subperiosteal implant, to the alveolar bone, using wires or screws is not always necessary. Fixation is necessary only in some instances, like those in the maxilla. This indicates that the success of subperiosteal implants depends more on the adaptation of the metal part to the alveolar bone than on the fixation. According to him, healing tendency of the tissues over a subperiosteal implant although it may appear unsatisfactory at first has been excellent in every case, even without antibiotic protection. The most critical step is to close the wound margins perfectly over the implant using horizontal mattress sutures. Obwegeser reported complications like dehiscence of the wound edges which he attributed to insufficient vascularization, inadequate suturing, and postoperative hematoma. This resulted in higher rates of bone resorption and an improper fit of the implant. He also observed the primary exposure of partial maxillary implant as a direct result of this complication. Other complications include pocket formation, which can be avoided by performing a prophylactic gingivectomy. There were also cases of abscess formation with chronic inflammatory infiltration around the abutment resulting in a fistula either along the abutment or from the margin of the implant. It was concluded that after sufficient experience, the complications could be avoided. In our study, a total of two patients (6.67%) had implant exposure by the completion of 12 months. To prevent implant exposure, the closure should not be too tight or too loose and should ensure complete coverage of the implant. The implant abutment may be slightly submerged below the bone level to aid in the primary closure. Benson reported that under ideal conditions, the tissue surrounding the implant would form a tight cuff around the implant post. This epithelial cuff acts as a protective barrier. To ensure this firm tissue attachment, there should be a preoperative consideration to submerge the implant below the level of the alveolar bone.
Bodine and Mohammed reported that the permanent fixation of the subperiosteal implant occurs by dense, collagenous, fibrous tissue encapsulation around the framework. This proves the lack of bony adhesion between the metal framework and the alveolar bone. In the case of the hybrid implant, the implant gains its stability initially by the cortical screws placed and also later by the osseointegration around the plate and screws. It is to be noted that in the study reported by Mani et al., the CBCT showed osseointegration of the hybrid implant. It showed excellent stability and minimum patient discomfort during 1-year postoperative period evaluation.
Mittal et al. conducted a prospective study using hybrid implants for the dental rehabilitation of missing teeth in the entire oral cavity not specifically related to compromised edentulous posterior maxilla. They reported a pain score of 1 in 70% of patients, 2 in 20% of patients, and 3 in 10% patients. This study has reported an average pain score of 0.76 at the end of 4 weeks. They have also reported a mobility of 11.1% of mild nature at the end of 3 months while our study has recorded a mobility of <1 mm of mobility in only 3.33% implants by the end of the first 3 months. However, our study has recorded a total of 20% of implants with mobility at the end of 12 months (magnitude of 1 mm and less in 16.67% and <2 mm in 3.33%). The study did not measure the gingival probing depth, gingival recession, or bone loss. The outcome of these implants has been determined in terms of infections, implant exposure, and dehiscence, all of which have been reported to be satisfactory at the end of 6 months. Although comparison of our study with this study may not be possible due to the changes in the inclusion criteria and the outcome measures, the success of these implants can be conveniently verified.
Since several studies involving hybrid implants do not exist in the literature, comparison with subperiosteal implants happens to be closest. Parel and Thayer described the potential complications when using screws for subperiosteal implants. They stated that unless the screw is in total intimate contact with dense cortical bone, some resorptive processes will occur, the chrome cobalt screws, which were used for fixation, may break, and the screw can penetrate the sinus or the canal. In the case of hybrid implants, the proper adaptation of the framework to the bone can be achieved because the plate component of the implant is highly malleable and also the screws used are titanium screws, and there were no complications due to the screws penetrating the sinus or the canal. They evaluated 27 complete mandibular subperiosteal implants and reported a 5-year implant survival of 96%, a 10-year survival rate of about 67%, and a 15-year survival rate of about 52%. The reasons they pointed for the removal of 13 implants were senility (inability to wear dentures), the presence of radiolucency under abutments, traumatic fractures of the mandible (bent superstructure), and open-heart surgery. The decrease in long-term survival rate can be attributed to the lesser tissue compatibility of the metal framework and poor oral hygiene. However, with the use of much better biocompatible metals, the results have improved.
Apart from the routinely used (Cr-Co) implants, Reisbick and Benson used alumina-coated subperiosteal implants which showed pronounced tissue adhesion. This is an indication that with the use of more tissue-friendly material, the subperiosteal implant can attain success. In the case of hybrid implant, good tissue compatibility was observed in all the cases – due to the biocompatibility of titanium with which the hybrid implant is fabricated, the most biocompatible material of the modern times. A finite element analysis conducted with four hybrid implants on the rehabilitation of completely edentulous maxilla using a hybrid implant all-on-four concept concluded that mechanically the hybrid implants are much better than distal cantilever situation and short implants. The hybrid implants were functionally adequate and would serve as a viable alternative to the conventional system of implants challenging the need for elaborate adjunctive procedures of bone augmentation.
In 1978, Garefis described in detail the surgical procedure for the placement of complete subperiosteal implants. The procedure was a two-phase procedure under general anesthesia. In Phase 1, a mucoperiosteal flap was elevated, and an impression of the alveolar bone was made using heavy silicone putty material. Sutures were removed 5 days later. The author reported giving corticosteroid injection sublingual to reduce swelling. Phase 2 surgical procedure is done after 20 days wherein the fabricated implants are placed over the bone. The author reported placing accessory incision to prevent difficulty in closure. The final impression for the overdenture prosthesis is made after 20 days once the healing of the surgical wound is complete. However, in the case of a hybrid implant, the surgical insertion of the implant can be done quickly under local anesthesia. Patients are recalled after 7 days for suture removal, and in 2 weeks, the operated site was found to be satisfactorily healed with the formation of the healthy gingival cuff around the abutment. The complications and expenses associated with general anesthesia can be avoided, and the postoperative swelling and pain could be adequately controlled with NSAIDs. A course of antibiotics was also given to the patients after the surgical procedure for 5 days.
| Conclusions|| |
A hybrid implant, as seen from our study, can very well manage both vertical and horizontal deficiencies of the alveolar ridge. In the case of knife-edge ridge, the alveolar ridge is flattened, and a groove equivalent to the width and thickness of the plate is made in the crest of the alveolus, thereby ensuring the effective dipping of the plate into the crestal bone. From our study, we have noted that the overall procedure for the placement of a hybrid implant is simple with minimal postoperative and intraoperative complications in the rehabilitation of posterior maxillary edentulism with a deficient alveolar ridge. Hybrid implant is hence an excellent alternative in such patients as it eliminates the need for procedures such as sinus lift and grafting and precludes the associated risks. Another added advantage of these types of implants is that they can be loaded early as compared to conventional endosseous implants. However, certain limitations are associated with these implants like the role of osseointegration needs to be evaluated in such implants and that the success of the procedure depends on the primary stability and close adaptation of the plate to the bony ridge. Further, long-term multicentric studies are needed to evaluate the efficacy of the hybrid implant system for long-term functional outcomes.
We thank Mr. DR Basannar M.Sc., MPS, Scientist “F,” Department of Community Medicine, Armed Forces Medical College, for the statistical help provided for the design, execution, and interpretation of the study.
Financial support and sponsorship
This paper is based on Armed Forces Medical Research Committee Project No. 4722/2016 granted by the office of the Directorate General Armed Forces Medical Services and Defence Research and Development Organisation, Government of India.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4]