Subperiosteal Dental Implants: Past or Future? A Critical Review on Clinical Trials/Case Reports and Future Directions

Dantas TA, Vaz PCS and Silva FS

Published on: 2022-01-12

Abstract

Subperiosteal implants were first introduced in 1942 in Sweden and were subsequently used worldwide for the treatment of mandibular and maxillary arches with advanced bone atrophy. However, due to the high complication rates and unsuccessful outcomes, this therapy fell in disuse. Advances in digital technology have led to a new interest in subperiosteal implants therapy and investigators are looking for innovative and effective techniques for dental rehabilitation. In this review, a critical analysis on the performance and characteristics of subperiosteal implants was carried out. Parameters such as materials and surface coatings, the design and manufacturing techniques, methods of implantation, complementary strategies and the main clinical outcomes were carefully assessed. Also, a comparison with current dental implants and a proposal for an optimized solution are presented

Keywords

Dental Rehabilitation; Subperiosteal Implants; Endosseous Implants; Clinical Trials; Customized Solutions; Minimally Invasive

Introduction

Dental implants are a very common practice for tooth rehabilitation. Throughout the years many efforts have been carried out aiming to improve their clinical performance and success. Dental implants may be classified accordingly to their materials, their reaction with bone, the treatment options and, their placement within the tissues [1]. As far as the placement within the tissues is concerned, dental implants are classified as endosseous (most commonly used nowadays), subperiosteal and transosteal implants. Subperiosteal implants were first described in the late 1940s and different types of this design evolved up to the 1980s [2].  Subperiosteal implants are custom-fabricated structures designed to rest on top of mandibular bone, under the periosteum [3]. However, due to inappropriate or non-rigid fixation, difficulty in positioning them, and the high complication rates, this type of dental restoration fell into disuse [4]. In fact, in 15-year follow up periods, survival rates of only 50-60% are reported [5]. In this sense, endosseous implants became a better solution for the replacement of a missing tooth, solving several issues associated with subperiosteal implants [4]. Despite being a worldwide practice, endosseous implants are still not well accepted by patients because of their high cost, time of treatment, postoperative complications, and because of the invasive surgeries that are associated with this type of rehabilitation. Digital technology has led to a new era in the dentistry field. The development of acquisition methods with considerably reduced energy given to the patient, such as the cone-beam computed tomography (CBCT), as well as the developments in the manufacturing techniques and materials, has simplified and improved the procedures related to the placement of dental implants [4]. With the technology that is available nowadays, it is possible to design and fabricate custom-made implants, perfectly adapted to the patient’s specific anatomy [6]. These constant technological developments allow the opportunity to revisit some ancient concepts such as the subperiosteal implants, by adapting and improving them, based on a new technological context [7]. In this sense, the aim of this critical review was to collect and analyse clinical trials performed in subperiosteal implants in order to better understand their weaknesses. Additionally, a comparison with conventional endosseous implants was carried out, aiming to purpose an alternative solution, that encompasses features of both subperiosteal and endosseous implants, for the design of an optimal solution for dental rehabilitation. The authors believe that minimally invasive procedures together with customized solutions are the future of implantology.   

Methods

This review was carried out according to the PRISMA Statement- Preferred Reporting Items for Systematic Reviews and Meta-Analyses. In this section of the work, the authors describe the search strategy as well as the criteria for the study selection.

Search Strategy

An electronic database search was performed using five databases: PubMed, Google Scholar, Medline, Science Direct, and Scopus. To conduct the search, Boolean operators such as “AND” and “OR” were used to correlate the keywords. The following keywords were explored and inserted with the field tag [Title/Abstract/Keywords]: “subperiosteal” and “dental implants”. In addition, the references of review articles were manually searched in order to include all the relevant articles available in the literature.

Study Selection

A screening process was conducted over the titles and abstracts retrieved by the databases search, in order to select the articles for full-text reading. From this screening process and after duplicates removal, 95 articles were selected for full-text reading.  In order to assess their eligibility to be included in this systematic review, the following inclusion criteria were applied: (1) articles written in English, (2) articles reporting clinical trials or case reports, (3) articles regarding subperiosteal implants. Articles not meeting the inclusion criteria were excluded from the review. It is important to mention that the full text of some articles found on the databases were not available online (for being too old) and, therefore, were not included in this analysis. Moreover, review articles and studies performed in animals were also excluded. Articles that met the inclusion criteria were then entirely read and analyzed considering the aim of this systematic review.

Data Collection and Extraction

The information extracted from each article was divided into five main groups: the materials and surface coatings; the implant design and manufacturing techniques; the methods of implantation; complementary strategies; and the main clinical outcomes. In this sense, for each of these topics, the authors summarized the information available in the selected literature and the extracted information can be seen in the next section of this work.

Results

Study Selection

Automatic and manual database searches resulted in a total of 949 records. After duplicate removal, 462 titles and abstracts were evaluated and a total of 95 full texts were selected to assess their eligibility to be included in this systematic review. Of these 95 publications, 62 could not be included in the final analysis. These articles were excluded for ≥1 of the following reasons: 1) they did not present any clinical trial nor case report; 2) studies were performed in animals; 3) they were review articles; 4) the articles were not written in English. The application of the exclusion criteria resulted, therefore, in 33 articles to be evaluated and analysed in the scope of this work. In Figure 1 it is possible to observe the PRISMA flowchart used in this selection process. 

Figure 1: Search strategy flowchart, adapted from [8].

This study analyses relevant articles published in the literature on the clinical performance of subperiosteal dental implants. The selected articles include case reports or clinical trials, the implants’ design and manufacturing techniques, the implantation process and report the main clinical outcomes. In Table 1 it is possible to observe the selected articles’ author, year of publication and title. Additionally, the number of tested implants and the implant type are also presented. As can easily be seen, articles found in literature and included in this review are dated from 1959 to 2020.

Table 1: Summary of the selected articles.

Authors/ Year of publication

Title

Nº of implants

Implant type

Obwegeser, 1959 [9]

Experiences with subperiosteal implants

35

23 complete and 2 partial mandibular subperiosteal implants 8 complete and 2 partial subperiosteal maxillary implants

Weber, 1968 [10]

Complete bilateral subperiosteal implants for partially edentulous mandibles

1

Complete bilateral subperiosteal implant.

Kratochvil and Boyne, 1972 [11]

Combined use of subperiosteal implant and bone-marrow graft in deficient edentulous mandibles: A preliminary report

1

Combination of subperiosteal implant and bone-marrow graft

Boyne, 1974 [12]

Restoration of deficient edentulous ridges by bone grafting and the use of subperiosteal metal implants

12

Combination of subperiosteal implants and bone-marrow graft

Bodine, 1974 [13]

Evaluation of 27 mandibular subperiosteal implant dentures after 15 to 22 years

27

Complete mandibular subperiosteal implants

Bloomquist, 1982 [14]

Long-term Results of Subperiosteal
Implants Combined with Cancellous
Bone Grafts

19

Combination of subperiosteal implants and cancellous bone graft

Hess, 1982 [15]

Two cases of incompatibility to
carbon-coated subperiosteal implants

2

Complete mandibular subperiosteal implants

Kreutz and Carr, 1986 [16]

Bilateral oronasal fistulas secondary to an infected maxillary subperiosteal implant

1

Complete maxillary
subperiosteal implant

Kay, 1987 [17]

Hydroxyapatite-coated subperiosteal dental implants: Design rationale and clinical experience

339

66 complete maxillary; 165 complete mandibular; 11 unilateral mandibular; 97 unilateral maxillary

Truitt, 1988a [18]

Use of computer tomography in subperiosteal implant therapy

41

3 submerged full subperiosteal implants; 33 exposed full subperiosteal implants; 3 circumferential subperiosteal implants;
2 unilateral subperiosteal implants

Truitt, 1988b [19]

Morphologic replication of the mandible using computerized tomography for the fabrication of a subperiosteal implant

10

Mandibular subperiosteal implant

Cranin, 1988 [20]

Reconstruction of the Edentulous Mandible with a Lower Border Graft and Subperiosteal implant

15

Combination of subperiosteal implant and lower border bone graft

Falomo, 1988 [21]

A retrospective survey of patients treated with subperiosteal and endosseous implants

16

2 maxillary subperiosteal implants; 14 mandibular subperiosteal implants

Bailey, 1988 [22]

The mandibular subperiosteal implant denture: A fourteen-year study

74

Mandibular subperiosteal implants

Fischer, 1993 [23]

CAD/CAM subperiosteal implants in Australia. Case report

1

Mandibular subperiosteal implant

Yanase, 1994 [24]

The mandibular subperiosteal implant denture: A prospective survival study

81

Mandibular subperiosteal implants

Bodine, 1996 [25]

Forty years of experience with subperiosteal implant dentures in 41 edentulous patients

41

Mandibular subperiosteal implants

Perry, 1998 [26]

Reconstruction of advanced mandibular resorption with both subperiosteal and root-form implants

3

Circumferential mandibular subperiosteal implants

Mansueto, 1999 [27]

Replacement of a mandibular subperiosteal implant

1

Mandibular subperiosteal implant

Fish and Misch, 2000 [28]

Mandibular bone growth induced by a Hydroxyapatite-coated subperiosteal implant: a case report

1

Mandibular subperiosteal implant

Sirbu, 2003 [29]

Subperiosteal implant technology: Report from Rumania

2

Mandibular and maxillary subperiosteal implants

Minichetti, 2003 [30]

Analysis of ha-coated subperiosteal implants

22

6 full maxillary 1 full mandibular 6 unilateral maxillary 9 unilateral mandibular

Moore and Hansen, 2004 [31]

A descriptive 18-year retrospective review of subperiosteal implants for patients with severely atrophied edentulous mandibles

40

Mandibular subperiosteal implants

Lozada, 2004 [32]

Immediate functional load of mandibular implant overdentures: A surgical and prosthodontic rationale of 2 implant modalities

5

Mandibular subperiosteal implants

Kusek, 2009 [3]

The use of laser technology (er;cr:ysgg) and stereolithography to aid in the placement of a subperiosteal implant: Case study

1

Mandibular subperiosteal implant

Loperfido, 2014 [33]

Severe mandibular atrophy treated with a subperiosteal implant and simultaneous graft with rhBMP-2 and mineralized allograft: a case report.

1

Mandibular subperiosteal implant

Nazarian, 2014 [34]

Placement of a modified subperiosteal implant:
A clinical solution to help those with no bone

1

Maxillary subperiosteal implant

Mapkar and Syed, 2015 [35]

Revisiting the maxillary subperiosteal implant prosthesis: A case study

1

Maxillary subperiosteal implant

Peev and Sabeva, 2016 [36]

Subperiosteal Implants in Treatment of Total and Partial Edentulism - A Long Term Follow Up

93

Partial and total subperiosteal implants

Gellrich, 2017 [5]

A customised digitally engineered solution for fixed dental rehabilitation in severe bone deficiency: A new innovative line extension in implant dentistry

3

2 mandibular subperiosteal implants 1 maxillary subperiosteal implant

Nguyen, 2018 [37]

A subperiosteal maxillary implant causing severe osteolysis

1

Maxillary subperiosteal implant

Cerea and Dolcini, 2018 [38]

Custom-Made Direct Metal Laser Sintering Titanium Subperiosteal Implants: A Retrospective Clinical Study on 70 Patients

70

Maxillary and mandibular subperiosteal implants

Mangano, 2020 [4]

Custom-made 3D printed subperiosteal titanium implants for the prosthetic restoration of the atrophic posterior mandible of elderly patients: a case series

10

Mandibular subperiosteal implants

After a careful analysis of the thirty-three articles, authors were able to extract a lot of information to be analyzed and compared. In this sense, and as previously mentioned, results of this study were divided into five main groups: the materials and surface coatings; the implant design and manufacturing techniques; the methods of implantation; complementary strategies; and the main clinical outcomes.

Materials and Surface Coatings

The materials applied for the fabrication of subperiosteal implants have changed throughout the years.  The subperiosteal implants first described in literature were made of different biomaterials. Chromium-cobalt-molybdenum alloys (particularly Vitallium) and tantalum are the most reported ones [5,9]. In fact, the first subperiosteal implant, placed in 1948 by Gershkoff and Goldberg [39], was made of Vitallium. Their electric inertness, mechanical strength, hardness, insolubility in body fluids, resistance to corrosion and also their biocompatible nature made these alloys appropriate materials for such applications [17]. Later, titanium and its alloys have also been indicated as good candidates for subperiosteal implants [33]. A controversy regarding the side effects of the metallic materials in the body environment, namely the ions release to the surrounding tissues has led to the development of alternative solutions. In a first attempt, it was thought that the subperiosteal implants would be better tolerated if coated with carbon due to its supposed outstanding biocompatibility [40]. This thin coat over the metallic substrates was thought to minimize the formation of a connective tissue capsule around the implant frame. However, this approach was not much developed since the purported biocompatibility of the carbon-tissue interface was not effectively confirmed.  A study performed by Hess [15] reported two cases of incompatibility to carbon-coated subperiosteal implants. Authors believe that the implants failure may be related to the fragmentation of the carbon coating on the implant surface and demonstrated the histopathologic effects that this type of implant material can induce in human tissues that would eventually lead to the implant failure.  Later, in the 1980s, another strategy was implemented and evaluated in this regard - the creation of hydroxyapatite (HAP) coatings over the metallic structures [17,34]. By then it was already known that this ceramic material has a high similarity in composition to the bone mineral, is bioactive and osteoconductive and has a high ability to form a strong interface with bone. The incorporation of HAP over metallic substrates had already been found to combine the excellent mechanical properties of the metallic materials with the biocompatibility and bone bonding characteristics of the ceramic. When comparing coated to uncoated implants, during some experiments in dogs, results revealed that the coated metallic implant induced a stronger implant-bone adherence [17]. Since then, several studies have reported clinical results on the use of HAP-coated subperiosteal implants. Golec [41] performed a study on the performance of 241 HAP-coated mandibular subperiosteal implants and, in a follow-up period of 7 years, a survival rate of 98% was achieved [31,41]. In another study [24], survival rates of 79% were achieved in a follow-up period of 10 years, and 60% after 15 years. A more recent study [30] evaluated the performance of HAP-coated subperiosteal implants over 10 years and a success rate of 91% was achieved. However, 36% of those implants needed a corrective intervention. Despite these rates not being so high as desirable, the use of HAP coatings over metallic substrates is still recommended since it has been proved to improve the interactions between the implant and bone, to decrease strut dehiscence and to improve the soft tissue environment [34].

Design and Manufacturing Techniques

The design of the subperiosteal implants must allow the transfer of load from the denture to the post, and from the post to the strut structure, with neglectable stress concentrations and static or cyclic fatigue mechanisms [17]. The technique of designing this type of implants was first described by Boyne and Kratochvi [11]. This technique used to start with a direct impression of the bone where the implant would lay, leading to the need of a two-stage surgery, as will later be explained, in the Methods of implantation section. In this method, a previously prepared plastic tray used to be fitted over the exposed bone and over the remaining anterior teeth, followed by some corrections to ensure a good fit between the implant and bone [10]. Then, the tray was filled with elastic materials, namely rubber adhesive. After approximately 10 minutes, the impressions were removed, and the occlusal registration was made [10,25]. It is important to mention that some more recent studies performed this step without the need of using the prefabricated trays, by simply making bone impressions with materials such as polysulfides, silicones, and polyethers [42]. During this procedure, the patient was occluded onto a prepared baseplate and occlusion rim, which was seated on the exposed mandible. The base of the occlusion rim was lined with soft wax to ensure that it would seat directly onto the bone. This record was later used to determine the interocclusal distance and to serve as a guide to the height of the implant posts [10]. Following the impression step, a bone model was made in dental stone and mounted at the proper vertical dimension for the implant fabrication [30]. This method was characterized for inducing a significant postoperative discomfort to the patients due to the excessive bone exposure [14].  In this sense, aiming to reduce the patient discomfort and avoid a two-stage surgery, in 1985 Truitt [43] developed a noninvasive technique for the design of mandibular subperiosteal implants, based on the computerized tomography (CT) scanning technique. The main advantage of this method is that the clinician is able to obtain the bone model by making use of a CT and a computer-generated model. The CT is performed prior to any surgical intervention, and only one surgery is needed to insert the implant, making the overall process much less invasive [30]. Since the introduction of this new technique, some studies can be found in literature with quite satisfactory results [4,5,18,19,23,34,38]. Apart from its noninvasive nature, this technique also avoids the potential toxic effects of foreign bodies of the impression material, and does not require the use of anesthesia [19]. After CT acquisition, stereolithography, has been used to fabricate very precise anatomical models of the patients’ jaws anatomy. This highly accurate technology uses the data from the 3D computer model to, layer by layer, fabricate a 3D model of the patient anatomy. The model is then delivered to a dental laboratory to ultimately fabricate the cast framework [3,33]. Some surface treatments such as polishing, sandblasting, acid etching, HAP coating and sterilization are also reported. Direct metal laser sintering (DMLS) is an additive manufacturing technique characterized by its ability to produce custom-made grids and implants, perfectly adaptable to specific anatomical requirements. In this sense, in the past few years, DMLS has emerged as a potential manufacturing technique for the production of subperiosteal implants. One study performed by Cerea [38] evaluated the clinical performance of 70 custom-made DMLS titanium subperiosteal implants and a satisfactory survival rate of 95.8% was reported in a two-year follow up. Briefly, in this technique, the implant was fabricated on a moveable platform by applying layers of grade 5 titanium micro-powders. For each layer, the machine lays down a thin film of the metallic powder with a specific thickness. The laser melts selected areas and the platform then moves down by the pre-stablished layer thickness, a fresh film of metal powder is poured and the next layer is melted via exposure to the laser source [38]. This process is repeated, layer by layer, until de implant is complete. In this specific case, the implant was then polished by electroerosion and finally sterilized. Even more recently, Mangano [4] evaluated the clinical outcomes of ten subperiosteal implants fabricated by DMLS. Despite the fit of two of the implants not being satisfactory, at the one-year follow-up no implants were lost, leading to a 100% survival rate. This approach of fabricating subperiosteal implants by DMLS is a novel technique that still requires further clinical evidence to corroborate these positive preliminary clinical outcomes. Clinical studies on a larger number of patients and a longer follow-up periods are needed.

Methods of Implantation

As previously mentioned, the first generation of subperiosteal implants used to encompass two surgeries for complete implant insertion. The first surgery was related to the bone impression for the implant design. This surgery was characterized by excessive bone exposure. Accordingly to Sirbu [29] this procedure used to start with the sterilization of the oral cavity followed by the administration of anesthetic blocks. A crestal incision was made in the periosteum, around the entire arch. To facilitate reflection of the mucoperiosteal flaps, a vertical anterior relieving incision was also required. The vital bearing areas were thus exposed after flaps reflection with a sharp periosteal elevator. If bone irregularities or protrusions were observed at this time, corrections were made with bone files or rongeur forceps. Cross tongue dorsum ligatures were used to promote an adequate flaps retraction and the impression tray (mentioned in the designing techniques) was then placed or made over the bone [29,44]. At this time, the bone was exposed and prepared for the impression method described in section 3.3. An impression was considered adequate if it included the external oblique ridge area, the mental nerve region and the mental symphysis area of the buccal side; and the mylohyoid ridges, the genial tubercles of the lingual side [45]. As far as the second surgery is concerned (that corresponds to the unique surgery in the cases where the bone geometry is acquired by CT), it corresponds to the implant placement procedure. As previously mentioned, with the introduction of the CT for the fabrication of subperiosteal implants, in 1985, this became the only needed surgery for the placement of subperiosteal implants. There is no consensus in the literature regarding the time to wait between the two surgeries. Obwegeser [9] mentioned that, while some previous authors waited three to six weeks between the two surgeries, one to three weeks is the most acceptable time to wait between bone impression and implant placement, in order to ensure a better implant fit.  This author refers that waiting three to five weeks for the implant insertion would result in variable degrees of misfit between the implant base and bone and waiting more than five weeks may lead to such a poor fit that implant placement would no longer be recommended [9]. However, a wide range of periods of time can be found in literature as regards the most appropriate time for the second surgical intervention: Kratochvil [11] performed the second surgery ten weeks after the first; Cranin [20] placed the implant 12h after the bone impression procedure; Mansueto. [27] performed the second surgery four weeks after the first one; Sirbu [29] waited twenty-eight to forty-five days to place the implant; Mapkar [35] performed the implant insertion eight weeks after bone impression acquisition, claiming it was enough time for the tissues to re?establish blood supply and avoid the risk of incision line opening; Leake [40] scheduled the second surgery for two or three weeks after the first one; and Truitt [43] inserted the implant twenty days after the first surgical intervention. This second surgery is more rapid and causes less swelling and discomfort to the patient when compared to the first surgery [42]. In this surgical intervention, the patient was again sedated under local anesthesia, after oral cavity sterilization, and the wounds were then reopened, using the same incision lines [29]. After exposure of the bone, the sterilized implant was inserted under the mucoperiosteal flaps and seated on the bone. The position and fit of the implant were adjusted and screws were used to fix the implant [4,10,11,34,35,38]. There are, however, some authors that reported that there was no need to wire or screw the implant to the bone [9,20]. It was believed that with adequate bone preparation and enough implant framework extension, the implant would be stable enough during the initial postoperative period and after ten days it would already be fixed by connective tissue. The procedure was finally completed by a careful suturing to obtain a tension-free, first-intention closure [4,29,38].

Complementary Strategies

Subperiosteal implants used to be placed in mandibles with extreme bone resorption. In this sense, some strategies used to be applied in order to rebuild the bone and, at the same time, give structural support for the implant base. These strategies were also used to reduce the fit mismatch between the implant and bone when placing the implant [11].  The main advantage of such procedure was the ability to manage and reduce the inaccuracies related to the implant design technique, namely the bone impressions or the CT. Autologous marrow and cancellous bone grafts used to be very common in the first subperiosteal implants [11,12,14,18,19]. The bone grafts were commonly harvested from the iliac crest, and patients had to be under general anesthesia [14]. In more recent studies, HAP particles were applied to induce bone augmentation and fill the void between the bone and the implant [17,27,30,34]. Additionally, a combination of autogenous bone and hydroxyapatite for augmenting the bone inferior border has also been reported [20]. In another clinical report, performed by Kusek [3], the implant struts were grafted with the patient’s blood and beta-tricalcium phosphate (β-TCP) and then covered with a membrane. The development of grafts of bone morphogenic proteins (BMPs) has also been reported by Loperfido [33]. BMPs are multifunctional proteins with a wide range of biologic activities. BMP-2, specifically, had already been reported of being capable of driving multipotent cells into an osteoblastic phenotype culture. In this sense, these authors treated a severely atrophic mandible with a HAP-coated titanium subperiosteal implant together with a BMP-2/mineralized allograft. Results revealed that no clinical complications were observed and grafting the mandible with mineralized allograft and BMP-2 may lead to considerable bone formation. However, not many studies are available on the literature regarding this matter and therefore further studies are required to confirm the benefits of bone morphogenic proteins when placing a subperiosteal implant [33]. As mentioned in section 3.2, strategies to improve the biocompatibility of subperiosteal implants have also been reported. In a first attempt, coatings of carbon were used [15,40] and their clinical outcomes assessed. This strategy did not lead to promising results and therefore did not prevail. These coatings were later replaced by HAP coatings and better outcomes are reported [17,28,32,34]. The constant developments in the field of biomaterials enabled doctors and clinicians to enhance the implant’s biocompatibility and rebuild atrophic bones before implant insertion. However, quite invasive surgeries are associated with these procedures, as, for instance, the harvesting of autologous bone from the patient’s hip. Further research should be performed in order to create alternative and effective solutions for dental rehabilitation.

Main Clinical Outcomes

One of the main reasons why dental implants fail is the lack of functional bone adherence. Osseointegration is a very complex process and there are many factors that influence the formation and maintenance of bone at the implant surface [46]. The implant material and surface properties, the congruence between the implant and bone, the surgical procedures, among others, have a huge impact on this biologic response.   In fact, the lack of direct contact between the implant and bone is expected to lead to fibrous integration, rather than osseointegration [47]. In this sense, the clinical reports evaluated in the scope of this review present varying clinical outcomes. Complications such as pain, swelling, and inflammation have been reported. Removing a subperiosteal implant is characterized by an extremely complicated prosthodontic treatment and, as can be seen in Table 1, at least one-third of the analyzed articles reported implant removal during the follow-up period [13,14,38,15,17,21,24,25,27,36,37]. Despite most of the articles not reporting the need for implant removal, complications were commonly observed. Post-operative infection, strut dehiscence, bone resorption, and fibrous encapsulation with subsequent implant movement were some of the reported complications. Only a few articles reported satisfactory results [11,12,35,18–20,26,28,31–33]. These satisfactory results mean that, during the follow-up periods, no major complications were observed, and the implants were still in place, which does not necessarily indicate that the implants had been successful.

Discussion

Subperiosteal implants were used for several years, however, some drawbacks are associated with this kind of dental rehabilitation. The positioning of the implant was a very complex process, and high complication rates have been reported, as described in the previous sections of this work [4]. In this sense, subperiosteal implants have been replaced by endosseous dental implants, first introduced by Branemark. Endosseous implants overcome some limitations of the subperiosteal implants and have been proved to be a reliable and successful solution for dental restoration [48]. Despite being a worldwide practice, standard endosseous dental implants only provide limited options for implant diameter, length, and thread parameters [49]. Additionally, the lack of congruency between the implant and socket is expected to lead to plaque formation and accumulation, bone loss (and consequent implant exposure leading to aesthetical issues), poor implant stability and, ultimately, its failure [50]. In this sense, in this section of the present critical review, the authors aimed to compare and discuss some aspects (positive and negative) related to both subperiosteal and endosseous implants. Moreover, a new optimal/improved solution for dental rehabilitation is proposed based on the limitations and advantages of the current solutions. In Table 2 it is possible to observe a summary of the main differences/similarities between subperiosteal and endosseous implants, as well as the parameters that an optimal solution should encompass. 

Table 2: Comparing subperiosteal and endosseous implants.

Aspects to compare/discuss

Subperioseal implants

Endosseous implants

Optimal solution

Number of surgeries

one or two

one

one

Bone drilling

No

Yes

No

Type of procedure

Invasive

Extremely invasive

Minimally invasive

Bone loss

Low

High

Low

metallic/ceramic materials

Metallic

ceramic materials

ceramic

Aesthetics

Unsatisfactory (material +size)

Satisfactory

Good

Lifetime bacterial related diseases expectancy

High

Low/High

Low

Cost

Very high

High

Low

Customization

Yes

No

Yes

Number of surgeries and clinical procedure

When placing a dental implant, the number of surgeries that are necessary to complete the process of implantation plays an important role. The patient will always prefer a treatment that is the less invasive possible and, therefore, one surgery will be preferable when comparing to two surgeries. As before mentioned, the first generation of subperiosteal implants used to encompass two surgical procedures. With the constant technological developments, new strategies were implemented, and it became possible to perform this kind of subperiosteal dental restoration in one surgery only. To place an endosseous dental implant, one surgical intervention is usually enough. Despite being possible to place an endosseous implant in only one surgical intervention, very aggressive techniques are associated with this type of dental restoration. Before placing the implant, the dentist needs to prepare and drill the bone, making it an extremely invasive procedure that causes a lot of discomfort to the patient. Additionally, the trauma caused by this type of surgical intervention, together with other factors such as occlusal overload and presence of micro-gaps may induce bone loss around the implant [51]. In its turn, the loss of bone in the peri-implant zone is expected to lead to the implant’s exposure and inherent aesthetical issues and, ultimately, induce the implant’s mobility and consequent failure [52]. To place a subperiosteal implant there is no need to drill the bone but, given the dimensions and geometry of the implant, big incisions have to be performed in the gingiva and high quantity of bone needs to be exposed for the settling of the implant. Furthermore, the strategies used to reduce the fit mismatch between the implant and bone make this procedure even more invasive and literature reports high levels of discomfort to the patient.           

Materials, Aesthetics and Bacterial Affinity

As mentioned in the results section of this critical review, subperiosteal implants are made of metallic materials such as chromium-cobalt-molybdenum and titanium alloys. Authors believe that the use of metallic materials in subperiosteal implants is mainly due to their plastic deformation nature that makes it easier to adapt the implant to the bone surface geometry. Apart from the problems that may arise from the metallic ions release and bacterial adhesion, aesthetic issues are also commonly associated with the implementation of metallic materials in the dentistry field. This is one of the reasons why ceramic materials, such as zirconia, have been emerging as great alternatives in dental applications [53]. In fact, despite titanium and its alloys being currently the most used materials in endosseous dental implants, there are already some brands producing and implementing dental implants totally made of zirconia. Apart from its biocompatibility, low bacterial affinity, high mechanical strength, and excellent wear resistance, aesthetical issues are solved due to its tooth-like color [54].

Cost and Customization

 Another two parameters that are extremely related and worth discussion are the implant cost and the customization. Subperiosteal implants are customized to each patient’s bone geometry and therefore, high costs are associated with this technique. On the other hand, since an endosseous implant is not customized to each patient, quite lower costs are involved. An optimal solution would be the one able to produce completely customized dental implants, at a relatively low cost.

Optimal/Improved Solution

Given the limitations and advantages of each of these two restorative treatments (subperiosteal and endosseous implants), new approaches and solutions have been developed and implemented, as, for instance, root-analogue implants [49]. The authors of the present work believe that totally customized implants and minimally invasive procedures are the future of implant dentistry. In this sense, an optimal solution would encompass characteristics of both subperiosteal and endosseous implants.

Implant customization: Today it is possible to easily obtain complete 3D information on the patient’s bone geometry with techniques such as the Cone Beam Computed Tomography (CBCT). This technique generates cone-shaped beams and the images are obtained in one rotation resulting in considerably low levels of radiation. It provides small examination time, reduces the image unsharpness caused by the translation of the patient, and, consequently, reduces the image distortion. It is already reported in the literature that CBCT provides higher levels of accuracy when compared with conventional CT measurements. Al-Ekrish, [55] evaluated the accuracy of CBCT and found mean absolute errors of 0.49 mm for the overall data. Another study conducted by Suomalainen, A. [56] found measurement errors in the range of 2.3%-4.7% for CBCT scans. The information acquired from a CBCT scan allows the technicians and dentists to virtually reconstruct the patient’s bone and posteriorly design a totally customized dental implant, tailored to the specific needs of the patient [38]. This approach of completely customizing a dental implant is expected to preserve more soft and hard tissues, as well as reduce the rehabilitation time and avoid the need for a second surgical procedure [57,58].              

Surgical procedure: The fact of not having the need to drill the bone is one of the advantages of the subperiosteal implants. The authors believe that an optimal solution would be the one that makes use of this concept, thus making the procedure less invasive. In this sense, the idea would be to customize the implant accordingly to the patient’s cortical bone geometry at the time of implant design/implantation. This means that the implant would be designed to perfectly accommodate in the bone socket as it is, whether the tooth has been removed recently or months/years before. In the first situation, after tooth removal, the socket would just have to be properly cleaned and the implant would be designed accordingly to the post-extraction socket geometry. In the second situation (when the tooth has been removed a long time before), since the socket would probably already be filled with new bone, the implant would be designed and manufactured accordingly to the cortical bone new geometry. In both situations the implant would be placed above the available cortical bone, thus improving the implant’s stability. This optimal solution would comprise a small surgery area since a good fitting of the implant to the bone is achieved and a relatively small area is enough for good anchorage. Eventually, some micro-screws can be used to fix this new implant. Additionally, both strategies have the advantage of preserving the natural cortical bone, and consequently reduce the bone loss and inherent aesthetical issues due to improved stress distribution in resistant bone (cortical) [49]. In Figure 2 it is possible to observe a schematic representation of the two aforementioned approaches, as well as an endosseous and a traditional subperiosteal implant.

Figure 2: Schematic representation of an endosseous implant, a subperiosteal implant and an improved solution.

Another advantage of placing a dental implant over the cortical bone (subperiosteal implants and optimal/improved solution) is that it will allow its functional use much more quickly than the endosseous implant because the latter is anchored on the trabecular bone and needs time for osseointegration. When the implant is anchored in the cortical bone, it can be loaded much more quickly since the implant mechanical stability will already be ensured.

Osseointegration and anti-bacterial strategies: Implant osseointegration is mandatory to achieve success and may be enhanced by implant-related factors such as material, geometry, and, as previously mentioned, by surface topography, surface treatments, and coatings. Other factors such as the mechanical stability and loading conditions, bone grafting, osteogenic biological coatings and, biophysical stimulation, and pharmacological agents such as simvastatin and bisphosphonates are also reported in the literature as good osseointegration promoters [59]. On the other hand, extreme implant mobility and micromotions, inappropriate porosity, radiation therapy, certain pharmacological agents, and patient’s related factors, such as osteoporosis, rheumatoid arthritis, age, smoking habits, and renal insufficiency are expected to inhibit osseointegration. In this sense, biological phenomena such as osteoinduction, bone growth, cytocompatibility, and bacterial adhesion are extremely dependent on the interactions between the implant and the host bone [60]. Therefore, the implant’s surface characteristics play a crucial role in the implant’s osseointegration, and a proper implant surface is mandatory to promote osseointegration and avoid bacterial adhesion, critical factors for the implant’s long-term success. It is reported in the literature that rougher surfaces induce differentiation, growth, and attachment of bone cells, as well as increase mineralization [61]. Many techniques have been applied in order to create certain levels of roughness on the implant’s surfaces. The most commonly reported in the literature are acid etching, sandblasting, plasma spraying, and the implementation of coatings, mainly hydroxyapatite coatings. The creation of micro and nano topographies has also been applied to induce surface roughness and improve the osseointegration [61]. On the other hand, smooth surfaces have been used to avoid bacterial adhesion. The accumulation of plaque around dental implants is extremely dependent on the implant’s surface roughness [62,63]. Rougher surfaces are expected to induce a higher bacterial adherence due to the presence of grooves and pits since in these surface irregularities bacteria are protected from salivary flow or other biologic fluids and can more easily attach to the surfaces [64]. Given this information, the authors of the present review believe that an optimal implant solution should encompass a smooth surface in the peri-implant mucosa zone (the soft tissue that surrounds the dental implant), and a rough surface in the implant zone that is in direct contact with the bone. Together with the implant roughness, the implant material also influences the bacterial adhesion process. Apart from the previously mentioned advantages of zirconia, this material has also been proved to have the ability to inhibit the level of bacterial adhesion, when compared to other materials used in dental applications, namely titanium [65-67].

The biofunctionalization of the implant’s surface with the addition of different substances has also been reported by its positive effect on osseointegration. As mentioned in the previous section of this review, the addition of biomolecules, such as BMPs, to the implant’s surface had been proved to drive multipotent cells into osteoblastic phenotype cultures. Other types of biomolecules are also referred in the literature: non-BMPs growth factors, peptides, and extracellular matrix biomolecules [68].  Platelet-Rich Fibrin (PRF), a second-generation platelet concentrate, has also been applied in the dentistry field to accelerate tissue healing and bone regeneration [69]. It is reported in the literature that PRF improves tissue regeneration due to its effects on vascularization, capturing the circulating stem cells, immune control, and closure of the epithelium [70].  There are, in fact, some studies that indicate that PRF may increase the amount and rate of new bone formation during the early healing period and, consequently, provide faster implant osseointegration [69,71-73]. One of the main advantages of this procedure is that PRF can easily be prepared from autogenous non anti-coagulated blood when centrifuged [71]. It is harvested from the patient’s own venous blood without any additives and does not induce immune rejection [72].  Given that PRF is associated with a quite simple clinical technique and has been proved to promote faster osseointegration and consequently enhance the implant stability, the authors of this review suggest that implementing such strategy when placing a dental implant would be beneficial for the overall clinical success. Finally, together with the PRF application, other strategies to promote the vascularization around the implant should be implemented. A rapid and efficient formation of a functional blood vasculature is mandatory for the success of wound healing and functional tissue regeneration [74]. Since the implant will be anchored in the cortical bone, poor blood supply is expected to occur. In one study performed by Yu [75], multiple macro-channels were designed in porous β-TCP scaffolds, in a bone augmentation procedure previously to mandibular implant placement, and their effect on vascularization was assessed.  Results revealed that channelled scaffolds significantly induced the new bone formation and increased the height of the mandible, indicating that the macro-channels facilitated the vascularization and bone formation. In another study [76], a review on the influence of the implementation of micro/macro channels on the vascularization was performed, and, although the size and number of channels found in the literature were different, all the studies showed that the channels played a crucial role in vascularization. The authors of this review are already evaluating and developing strategies that aim to induce an effective vascularization and consequently improve the osseointegration, by the implementation of micro-channels in the implant’s surface. In fact, one study is already available in the literature [53]. The authors designed and produced zirconia surfaces with different micro-channels, and after hydrophilicity and capillarity analyses, authors concluded that the creation of inter-connected micro-channels, with 200 µm of width and 100 µm of depth, on the implant surface may be a promising solution for the promotion of the vascularization around the implants. However, the promotion of a proper vascularization is not enough for achieving the implant osseointegration. In fact, the channels’ size should be designed to allow not only the vascularization but also bone in-growth and consequent implant stability. Literature reports that scaffolds with pores with 100 µm of diameter are appropriate for cellular infiltration and consequent oxygen and nutrients supply [77]. However, for bone tissue ingrowth, pore sizes between 200 and 350 µm are preferable. In another study [78] the authors also suggest that the ideal pore size for bone in-growth range between 100 and 400 µm. In this sense, the authors believe that an implant with micro-channels with the aforementioned dimensions (200 µm of width and 100 µm of depth) would be suitable for the promotion of both vascularization and bone in-growth. Nevertheless, further studies, namely in vitro studies, should be performed in order to corroborate these findings, since the optimal pore/channel size for bone regeneration is still not unanimous.

Conclusion

After a careful analysis on the main outcomes of subperiosteal implants and their comparison with endosseous implants, an improved and optimized solution for dental rehabilitation was purposed by the authors. This new approach should encompass characteristics of both subperiosteal and endosseous implants, overcome their limitations, and make use of the new technological advances:               

  • A completely customized process, tailored to each patient condition;
  • Avoiding bone drilling and excessive bone exposure, as well as a single-step surgery, is required for the implementation of a minimally invasive procedure;
  • A smooth surface in the peri-implant mucosa for antibacterial purposes and a rough surface in the implant zone that is in direct contact with the bone should be applied, aiming to promote the implant’s osseointegration;
  • The implant should be completely made in zirconia, avoiding both aesthetical issues and bacterial adherence;
  • During the surgical intervention PRF should be administrated in order to promote a faster osseointegration; Proper vascularization around the implant should be achieved by the incorporation of micro-channels on its surface.

The authors truly believe that a dental implant encompassing the aforementioned features may overcome the limitations of the current strategies and be the future of implant dentistry.

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