Continuing Education (CE)
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Dr. Vasileios Soumpasis looks at the evidence base for complete oral rehabilitation using implant-retained prostheses
Rehabilitation of complete maxillary arches is often complicated by bone that is not only of poor quality but also scarce (Mozzati, et al., 2012). Corrections can be made, in part, through constructive surgery and bone grafting.
These more complex techniques, however, can cause discomfort and disturbance to the patient and often are poorly accepted because they take longer and involve more surgical procedures compared to less invasive techniques (Mozzati, et al., 2012).
Educational aims and objectives
This clinical article aims to present a literature review on the existing evidence base for full arch implant-supported oral rehabilitation.
Implant Practice US subscribers can answer the CE questions to earn 2 hours of CE from reading this article. Correctly answering the questions will demonstrate the reader can:
- Recognize tilted implants as a viable alternative in certain circumstances.
- Realize some benefits of immediate implant placement.
- Identify some accepted parameters for assessing oral implant success.
- Identify four risk factors for implant failure.
- Identify biomechanics of full arch implant-supported prostheses.
Continuing Education Process
A number of studies have shown that treatment with tilted implants (that is, implants placed at positions off the vertical axis) may be a viable alternative in the distal parts of the mouth. Tilting allows for the use of a longer implant that is able to reach better-quality bone, which can lead to a final prosthesis built with the aim of reducing or eliminating cantilevers (Malo, et al., 2003, 2005, 2011; Testori, et al., 2008).
Agliardi proposed tilted implants that do not interfere with the maxillary sinus floor for the rehabilitation and immediate loading of complete maxillary arches with atrophic posterior sections, which can lead to high degrees of patient satisfaction (Agliardi, et al., 2010; Capelli, et al., 2007; Szmukler-Moncler, et al., 2000). Moreover, this procedure eliminates the need for the patient to wear provisional removable dentures during the periods before and after healing, which is a typical requirement of the traditional protocol (Drago, Lazzara, 2006).
The benefits of this protocol are not limited to time reductions but also include enhanced tissue stimulation induced by the immediate loading, which has a beneficial effect on both the healing process and the time required before good-quality tissue is visible (Davies, 1998; Duyck, 2006). According to Del Fabbro and Ceresoli (2014), tilting of the implants does not induce significant alteration to crestal bone level as compared to conventional axial placement after 1 year of function.
This trend seems to be unchanged over time even though the amount of long-term data is still scarce. The use of tilted implants to support fixed partial and full arch prostheses for the rehabilitation of edentulous jaws can be considered to be a predictable technique with an excellent prognosis in both the short- and mid-term.
Further long-term trials, possibly randomized, are needed to determine the efficacy of this surgical approach and the remodeling pattern of marginal bone in the long term (Del Fabbro, Ceresoli, 2014).
The use of tilted implants with angled abutments to reduce cantilever length could be considered a viable therapeutic option from a biomechanical point of view (Francetti, et al., 2015). Tilted implants should be placed mesially or in direct contact with the mesial walls of the maxillary sinus without invasion or rupture of the Schneiderian membrane (Jensen, et al., 2010).
The reported advantages of immediate implant placement include a reduction in the number of surgical interventions, shortened rehabilitation time, and higher patient satisfaction compared with late implant placement (Gokcen-Rohlig, et al., 2010; Penarrocha-Diago, et al., 2011; Soydan, et al., 2013).
Another advantage of implant placement in the extraction socket is the counter-acting of the hard tissue resorption that occurs following tooth extraction (Bhola, et al., 2008). Altintas, et al. (2016), stated the success rate of immediately placed implants is 97.8%. With thorough patient evaluation, the extraction of all residual teeth and implant placement in a single surgical procedure are a safe and predictable treatment modality for successfully rehabilitating the edentulous patient with a fixed prosthesis.
Implant placement in fresh extraction sockets is technique sensitive because primary implant stability is critical (Altintas, et al., 2016). Implants placed immediately into fresh extraction sites engage precisely prepared bony walls only in their apex, whereas the coronal space is filled by the end of the healing phase (Polizzi, et al., 2000).
It has been stated that the procedure should be limited to alveoli with sufficient bone for primary stability, which is generally achieved by exceeding the apex by 3 mm-5 mm or by using implants that are wider than the alveolus (De Rouck, et al., 2008).
To achieve these conditions, a minimum of 4 mm-5 mm of alveolar crest width and a residual bone length no less than 10 mm is recommended (Becker, Goldstein, 2008). It is also suggested that periodontitis-affected tissues may have a negative local influence on the failure rates due to the presence of infra-bony defects, which could increase the gap between bone and implant (Cosyn, et al., 2012) or jeopardize the achievement of primary stability (Ivanoff, et al., 1996) at immediate implant placement. However, it is not known to what extent periodontitis may contribute to the difference in failure rates between immediate and non-immediate implants (Chrcanovic, et al., 2015).
The higher failure rate of immediate implants in relation to non-immediate implants in the maxilla in comparison to the mandible may be attributed to the low density of medullary bone and thin cortical plates (Kourtis, et al., 2004), which may have resulted in significant reduction in insertion torque for implants in the maxilla and fewer implants with primary stability, and further resulted in a lack of resistance to mechanical stresses (Horwitz, et al., 2007).
For that reason, achieving bicortical fixation and splinting of the implants with a rigid metal framework within 10 days after surgical placement is crucial. Raes, et al. (2013), observed that a trend toward bone gain was found following insertion in fresh extraction sockets, which may be explained by the fact that the gap between the original bone and implant diminishes during healing, and the bone-to-implant contact increases in coronal direction during the healing phase. These findings can be related to a coronal bone remodeling around immediate implants and a healing pattern with new bone apposition around the neck of the implants (Covani, et al., 2003).
Most of the studies, if not all, do not reveal how many implants were inserted and survived/lost in several different conditions. The use of grafting in some studies is a confounding risk factor, as well as the placement of implants in different locations, with different healing/loading periods, different prosthetic configurations, varying types of opposing dentition, implant splinting, and the presence of smokers, diabetics, or periodontally compromised patients. Moreover, in these studies, different implant brands and surface treatments were used (Chrcanovic, et al., 2015).
It is not clear whether, in general, one surface modification is better than another (Wennerberg, Albrektsson, 2010). The initial studies on osseointegration were conducted on implants with turned surfaces. Since then, enhanced implant surface technology has been developed to improve the predictability, rate, and degree of osseointegration.
Buser, et al. (1991), showed that bone implant contact increased from 37.5% for titanium plasma-sprayed implants to 55% for those with a sandblasted, large grit, and acid-etched surface. The insertion of dental implants in fresh extraction sockets affects the implant failure rates. However,
Chrcanovic, et al. (2015), found that it does not affect the marginal bone loss or the occurrence of postoperative infection.
A statistically significant difference was not found for implant failures when studies evaluating implants inserted in maxillae or in mandibles were pooled, or when the studies using implants to rehabilitate patients with full arch prostheses were pooled. The difference was statistically significant between the procedures for the studies that rehabilitated patients with implant-supported
After a follow-up of 18 and 24 months, both Mozzati, et al. (2012), and Grandi, et al. (2012), found 100% success rates for immediately placed implants and definitive prostheses for 45 and 47 patients, respectively, suggesting that immediately loaded mandibular cross-arch fixed dental prostheses can be supported by four post-extraction implants.
Survival and success
Fixed-implant prosthetic restorations supported on four implants represent a well-proven treatment modality for rehabilitation of the edentulous mandible (Malo, et al., 2003; Grandi, et al., 2012; Crespi, et al., 2012; Krennmair, et al., 2016). However, recently the main focus in oral implant surgery has shifted from survival to success and to peri-implant infections (Zitzmann, Berglundh, 2008; Klinge, Meyle, 2012; Tomasi, Derks, 2012). Therefore, the most frequently used and accepted parameters for assessing oral implant success are related to peri-implant marginal bone loss, peri-implant soft tissue health, prosthesis stability, and patients’ subjective evaluation (Krennmair, et al., 2016).
Concerning the influence of the prosthetic rehabilitation on the failure rates, Chrcanovic, et al. (2015), found a statistically significant difference between the procedures when studies only evaluating patients with implant-supported single crowns were pooled, the same not happening when full arch prostheses were the only prosthetic rehabilitation performed.
The splinting of the implants in full arch prostheses allows a more even distribution of the occlusal forces, thereby reducing stresses at the bone-implant interface (Wang, et al., 2002) as well as micromotion (Vogl, et al., 2015).
The following variables were assessed: implant success rate; peri-implant soft tissue conditions; biological and prosthetic post-loading complications; radiographic peri-implant marginal bone loss; patient satisfaction; and quality of life following implant therapy.
In their systematic review, Kwon, et al. (2014), stated that implants and full arch fixed-dental hybrid prostheses showed rather high short-term survival rates, but due to limited available literature, their long-term survival rates could not be obtained. Furthermore, selected studies were vulnerable to potential bias sources.
Clinicians should therefore be aware of the aforementioned limitations in existing literature, and apply this treatment concept in clinical practice on carefully selected cases. Although an implant-supported fixed-dental hybrid prosthesis may be a valuable option for a patient with a completely edentulous ridge, the strategic removal of teeth with satisfactory prognosis for the sake of delivering an implant-supported full arch dental hybrid prosthesis should also be voided.
Various risk factors can threaten oral implant treatment success, and four risk categories can be identified:
- Complications during surgery
- Loss or impending loss of implant
- Fracture or wear of supra-structure parts
- Patient dissatisfaction with outcomes (Fischer and Stenberg 2013)
Fischer and Stenberg (2013) found that for patients, prosthodontists, and third-party providers (such as insurance companies, for example), modifications, repairs, or remakes of the initially expensive implant-supported reconstructions can lead to monetary, emotional, and social costs, if information concerning these costs is not explained prior to treatment.
Pterygoid (Balshi, et al., 1999) and tuberosity (Bahat, 1992; Khayat, Nader, 1994; Venturelli, 1996) implants represent other treatment options to restore the edentulous maxilla. Although these techniques may represent viable therapeutic options, because they provide suitable posterior anchorage, they require considerable surgical experience (Galan Gil, et al., 2007; Aparicio, et al., 2008).
In the literature, the available data provided promising results for CAD/CAM-fabricated implant-supported restorations (Patzelt, et al., 2015); nonetheless, current evidence is limited due to the quality of available studies and the paucity of data on long-term clinical outcomes of 5 years or more.
In the sense of an evidence-based dentistry, the authors recommend further studies designed as randomized controlled clinical trials and reported according to the CONSORT statement.
The biomechanics of implant-supported full arch prostheses
Impression procedures should be executed with the mouth half-closed using individual anatomic impression trays, under conditions of muscle relaxation (Fischman, 1990). The reported biomechanical problems resulting from torsional mandibular deformation are more critical in patients showing parafunctional habits, such as bruxism.
In implant-supported fixed prostheses, an optimal biomechanical distribution of stresses at the prosthetic superstructure and implant infrastructure is of paramount importance (Rangert, et al., 1989), being influenced by many different factors such as correct prosthetic design and occlusal scheme (Apicella, et al., 1998), among others.
Several opinions on optimal mandibular cantilever length are found in the literature, including that the length should be no more than 20 mm (Naert, et al., 1992); less than 20 mm; and preferably less than 15 mm — equivalent to two teeth distal to the most posterior abutment (Adell, et al., 1981); the shorter, the better (Jacques, et al., 2009; Greco, et al., 2009); and equivalent to double the diameter of the abutment in the anterior region and to the diameter of the abutment in the posterior region (Apicella, et al., 1998).
Conversely, others believe that bone loss around implants and/or loss of osseointegra-tion are mainly associated with biologic complications such as infection around the implant (Naert, et al., 2012), stating that the evidence is not enough to support the hypothesis that occlusal overload leads to marginal bone loss. However, it bears emphasizing that occlusal overload may lead to mechanical complications, such as screw loosening and/or fracture, prosthesis fracture, and implant fracture (Schwarz, 2000).
The clinical success of osseointegration and long-term survival of dental implants depend on several biomechanical factors, and they are influenced by the way the mechanical stresses are transferred from the implant to the surrounding bone (Kregzde, 1993).
Studies related to the biomechanics of implant-supported prostheses (Barbier, et al., 1998; Teixeira, et al., 1998; Chun, et al., 2005) have shown that a major cause of bone resorption is excessive load on the implant once, when submitted to the load application, it transmits the stresses generated directly to the bone (Brånemark, et al., 1977; Skalak, 1983). This may be influenced by the type of loading, the nature of the bone/implant interface; the length, diameter, shape, and surface characteristics of the implant; the type and properties of prosthesis material; and the quantity and quality of surrounding bone (Geng, et al., 2001).
In the classical protocol, Brånemark, et al., recommended acrylic resin as the material of choice for the occlusal surface of implant-supported fixed dentures. The acrylic occlusal surfaces would cushion the masticatory forces due to its resilience, leading to a relatively physiological load on bone-implant interface (Skalak, 1983; Adell, et al., 1981).
Although acrylic resin presents low stress levels in the bone and around the implants (Yalçın, Canay, 2000) when used on the occlusal surface, it shows complications such as wear and tooth fracture clinically (Stegaroiu, et al., 1998; Soumeire, Dejou 1999).
Porcelain is another material option for artificial teeth and presents greater wear resistance and provides more favorable esthetic results than acrylic resin. However, some authors report that porcelain is a more rigid material and does not absorb stress, meaning the forces developed in the occlusal surface are transmitted directly to the prosthesis, implant and bone/implant interface, unless they are interrupted somehow (Geng, et al., 2001; Van Rossen, et al., 1990; Jemt, et al., 1989).
Hence, a combination of a rigid prosthetic superstructure with a resilient esthetic veneering material is mandatory for the success of full arch rehabilitation via implant-supported fixed prostheses. This approach requires site-specific placement to maximize the biomechanical advantage of the All-on-4® distribution (Malo, et al., 2003).
This is best facilitated by bone reduction — not augmentation — to create the All-on-4 shelf, which serves multiple functions for both surgeon and prosthodontist, as follows:
- Establishment of prosthetic restorative space
- Establishment of a level alveolar plane and uniform implant levels
- Establishment of alveolar width for implant diameter selection
- Bone reduction makes basal bone accessible for implant fixation
- Helps establish arch form, implant distribution, and anterior posterior spread
- Identifies optimal implant sites
- Identifies secondary implant sites
- Exposes lingual plate width and lingual concavities
- Facilitates posterior implant placement with respect to the nerve
- Provides bone stock for secondary bone grafting (Jensen, et al., 2011)
Several techniques have been employed to evaluate the biomechanical behavior of an implant-supported prosthesis — for example, photoelasticity (Bernardes, et al., 2009; Karl, et al., 2009), strain gauges (Karl, et al., 2007, 2005) and two- or three-dimensional finite element analysis (FEA) (Lin, 2008; Ding, et al., 2009).
Bone remodeling followed, as anticipated from Wolff’s law: Bone in a healthy person or animal will adapt to the loads under which it is placed (Klineberg, et al., 2012). Bone reacts to strain (through deformation), and where bone strain surrounding implants is in “mild overload” (1,500-3,000 micro-strains), bone apposition (derived from finite element analysis [FEA] and mathematical modeling) appears to be facilitated.
It also appears from FEA modeling that there is a generic threshold of bone strain below, which remodeling does not occur, and predisposes to resorption and peri-implant bone loss (Frost, 2004; Blanes, 2009).
Optimal design of implant superstructures should maximize bone density and bone remodeling, reduce healing time, and increase bone-implant contact Although there is minimal research focusing on this issue, given the preceding data, the clinical recommendations are that occlusal design should follow a narrow occlusal table, with central fossa loading in intercuspal contact and low cusp inclination to minimize lateral loading in function and parafunction.
Clinical recommendations are based on available data: There is justification for specific occlusal design features to include:
- Anterior guidance for protrusive and lateral contacts in function and parafunction
- Posterior occlusal form of low cusp inclines
- Central fossa location of opposing supporting cusps for minimizing lateral loads on teeth and implants
Variables of occlusal design influence bone strain and bone mineral density, and microarchitecture varies within the jaw. This data confirms the anterior mandible as the most suitable bone structure for implant loading (Klineberg, et al., 2012).
The so-called “postoperative remodeling process” may be attributed to several factors such as implant type, implant positioning, and different surgical procedures followed by different prosthodontic procedures (Krennmair, et al., 2016). According to the accepted remodeling process, success criteria established by Albrektsson, et al. (1986), 1 mm remodeling of the bone contour and an additional 0.2 mm of bone loss during the following years are considered as an acceptable healing outcome.
Splinting is important to limit micromovement of implants and ensure osseointegration. The rigid splinting of the implants by a cast metal bar to support a full arch implant-supported prosthesis has shown successful outcomes.
Paniz, et al. (2013), concluded in an in vitro study that:
- Absolute passive fit cannot be achieved, regardless of the type of material and technique used.
- Anatomic cast frameworks showed significantly larger center point deviations compared to milled anatomic frameworks fabricated through digital technology. Anatomic cast framework accuracy is strictly related to adaptation of the framework through cutting and soldering.
- Anatomic milled frameworks fabricated in titanium or cobalt-chrome displayed reduced center point deviation compared to the cast frameworks. No statistically significant differences were present between the two milled materials (Millen, et al., 2015).
Millen, et al. (2015), in a systematic review came to the conclusion that a higher incidence rate of technical and biologic complications was seen with cement-retained prostheses.
Karl and Taylor, in their randomized clinical trial (2016), observed that bone adaptation around statically and dynamically loaded implants occurred, causing a decrease in misfit strain evoked by non-passively fitting prostheses. Hence, for maintaining osseointegration of dental implants, passivity of fit of multi-unit restorations seems not to be as critical as previously thought.
The rehabilitation of the posterior edentulous maxilla with implant-supported prostheses is often challenging because of the poor quality and quantity of residual jawbone, especially in patients with long-term edentulism (Agliardi, et al., 2009). The use of bone grafting and sinus elevation to increase bone volume may be a viable treatment option (Del Fabbro, et al., 2004; Menini, et al., 2012) to allow implant placement in the atrophic maxilla; however, these procedures are associated with more frequent complications, higher morbidity, increased costs, and duration of treatment time (Sorni, et al., 2005).
Grafted sites usually do not attain sufficient primary stability for immediate loading protocols, and delayed loading protocols are needed (Menini, et al., 2012). Patient acceptance of these protocols is low, due to the invasive nature, increased duration, and costs of treatment (Testori, et al., 2008; Del Fabbro, et al., 2004).
Patient outcomes, decision making, and treatment planning
Edentulism can be disabling and has a profound negative impact on the quality of life of patients (Fiske, et al., 1998; Strassburger, et al., 2006). The latter is particularly relevant in the mandible, where conventional dentures more frequently have a negative impact on the patient’s quality of life (Perea, et al., 2013).
The increased request for implant therapy results from a combination of various factors, including age-related tooth loss, anatomic condition of edentulous ridges, psychological needs, decreased performance of removable prostheses, predictable long-term results of implant-supported prostheses, and increased awareness from both clinicians and patients of the benefits of implants (Weinstein, et al., 2012).
Most patients wearing complete dentures complain about progressive loss of stability during phonetics and mastication, and request for a fixed rehabilitation. Furthermore, progressive bone loss in the posterior mandible may lead to a superficialization of the alveolar nerve, which may cause pain to denture wearers during mastication.
All attempts to minimize stress should be done at the treatment planning stage, healing, and provisional stages, and finally, at the delivery of the permanent prosthesis. This can be accomplished by selecting implants of proper size, with width being more important than length (Lum, Osier, 1992; Lum, 1991), proper implant number, proper position, and by having an appropriate occlusal scheme (Gittelson, 2002).
The existing literature provides high evidence that immediate loading of microtextured dental implants with one-piece fixed interim prostheses in both the edentulous mandible and maxilla is as predictable as early and conventional loading (Gallucci, et al., 2014). It is possible to successfully load dental implants immediately or early after their placement in selected patients, although not all clinicians may be able to achieve optimal results with immediate loading.
A high degree of primary implant stability (high value of insertion torque) seems to be one of the prerequisites for a successful procedure (Esposito, 2007). Primary stability can be improved by using a tapered implant in a slightly underprepared implant site. This may lead to high compression forces and elevated insertion torques. Khayat, et al. (2013), showed that there was no difference in marginal bone loss between implants placed at low torque (mean = 37.1 Ncm) and those placed at a very high experimental torque (mean = 110.6 Ncm). It had previously been postulated that osteonecrosis would result due to disturbance of local microcirculation at high torque values.
Tilted implants significantly improve prosthesis support, while also allowing for longer implants to be placed with improved bone anchorage (Krekmanov, et al., 2000).
Static continuous loads on implants result in increased bone density (Gotfredsen, et al., 2001). The transient loads generated in function trigger bone remodeling (Heitz-Mayfield, et al., 2004). Klineberg, et al. (2012), found that a key element is the primary stability of the implant, which varies with bone density in different regions of the mouth.
Mechanical stress may have a positive and negative consequence on remodeling bone dependent on the magnitude, frequency, and type of loading. There is an optimal functional strain which encourages bone remodeling with increased bone volume and density, which maintains osseointegration and implant stability.
Where stress levels are within the physiological load-bearing capacity of the bone, remodeling occurs. High stress leads to bone remodeling where stress levels are within the load-bearing capacity of the bone.
Immediate loading of implant-supported full arch prostheses for the edentulous mandible and maxilla is today a predictable procedure, associated with high level of satisfaction for the patients in terms of esthetics, phonetics, and functionality (Esposito, 2007; Castellon, et al., 2004; Chiapasco, 2004; Misch, et al., 2004; Ioannidou, Doufexi, 2005; Attard, Zarb, 2005; Del Fabbro, et al., 2006; Jokstad, Carr, 2007).
Immediate loading of the implants not only has a positive impact on the patient’s esthetics and functional comfort, but also improves the outcome of the implants (Collaert, De Bruyn, 2008). Between the two, the mandible is ideal for immediate loading compared with the maxilla because of the better condition of the bone in relation to quality, quantity, and axial loading conditions. Early short-term studies on immediate loading in the completely edentulous maxilla show survival rates between 87.5% and 100% (Collaert, DeBruyn, 2008).
The use of an immediate loading protocol decreases the duration of treatment and the number of visits necessary to complete it, eliminates the discomfort that comes from wearing a removable prosthesis over the surgical site, and yields the patient the opportunity to be under the care of a prosthodontic team even at remote distances (Balshi, Wolfinger, 2002).
Because of new implant designs and surface configurations and better surgical procedures, the time frame between implant placement and functional loading has been shortened (Vervaeke, et al., 2013). Additionally, the rigid splinting of the implants 0-10 days after placement will avoid micromotion during the healing phase. It appears that premature loading per se does not lead to fibrous tissue encapsulation (Chrcanovic, et al., 2015).
There is no consensus on the threshold that cannot be surpassed, but it is believed to range between 50µm and 150µm (Szmukler-Moncler, et al., 2000; Soballe, 1993; Soballe, et al., 1992; Szmukler-Moncler, et al., 1998). Dental implants in periodontally susceptible patients show radiographic bone changes similar to previous reports in the literature regarding patients with and without a history of periodontitis. After the first year, immediately restored implants exhibited bone loss rates similar to those seen around conventionally restored implants (Horwitz, Machtei, 2012).
Immediate loading of dental implants results in a significant reduction in treatment time and morbidity for patients by avoiding a second surgery to uncover the implants. A comparative clinical trial showed that this protocol results in significantly higher patient satisfaction (Schropp, et al., 2004).
Vasileios Soumpasis, DDS, MSc, is a resident clinician at Evodental Heathrow.
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