Edentulism is a world wide pandemic. It is estimated that the Western world has more than 35 millions edentulous individuals, while the Eastern world has more than 250 millions (estimates from the World Health Organization, WHO).
In USA the percentage of edentulous patients (by age group) is declining 10% each decade (Slade et al 2014); however this reduction is more than off-set by the aging on the baby boomers and the increase of life expectancy. Therefore an increased marked in the total number of the edentulous patient is foreseen for the near future. Dentists are on the forefront for the treatment of these patients that not only address an obvious limitation in function (ability to chew) but also other important factors such as: esthetic, speech, nutrition, social interaction and more in general well-being and quality of life. The conventional approach to the rehabilitation of the edentulous patient is the removable complete denture (CD). Nowadays improvement in surgical protocols and dental technology allows the clinician to offer their patient predictable and reliable implant-based treatment options (Adell et al 1990) offering different level of function and comfort, each one with its own indications and contraindications.
Several factors play a role in the decision of the treatment option that best suits the patient, such as: anatomy, phonetics, esthetic, interocclusal space, neuromuscular functions, cost and patient compliance (i.e. oral hygiene). Moreover, the maxilla and mandible present different anatomical, and functional challenges related to different arch morphology, resorptive pattern, quantity and quality of the bone, presence of anatomical structure, and biomechanics considerations (Wicks 1994). Successful outcomes of the treatment of the edentulous patient depend on proper treatment planning and execution (Lekholm & Zarb 1985; Roblee 1994).
Meticulous diagnosis and treatment planning is of critical importance to obtain a predictable outcome. When a clinician is planning the rehabilitation of an edentulous patient, he/she should consider the edentulous maxilla and mandible pose different challenges. The maxilla is mostly affected by a vertical- and horizontal- type of bone resorption (Tallgreen, 1966) that it is likely to require support of the upper lip, in order to properly restore the esthetic. The mandible present a more functional challenge, with reduce bone support and an important role of the neuromuscular control of the tongue, in order to have a successful restoration.
Medical and Dental History
Dental implants can be safely used to rehabilitate the vast majority of the patients (Brånemark et al, 1977), including the one presenting chronic debilitating diseases such as heart disease and diabetes. As long as the medical condition is well controlled and the patient is compliant to the recommended medical therapy, the surgical placement of dental implants is not contraindicated. Some reversible contraindications although exists, and must be evaluated with the patient’s treating physician to avoid intra-operative and post-operative complications. Such reversible contraindications include: uncontrolled diseases (i.e. uncontrolled diabetes, recent myocardial infarction, disease requiring chronic intake of steroid anti-inflammatory medications, etc), anticoagulant therapy and cancer treatment (i.e. patient receiving IV bisphosphonates, or under current or with history of radiation treatment) (Marx et al, 2005). For patients presenting these reversible contraindications, a consultation with the treating physician is required, and might result in modification of the pharmacological therapy (i.e. reduction of the dosage, or drug “holiday”) or the delay of the treatment until the aforementioned condition have been brought back under control.
From a dental point of view, there is no absolute contraindication to the use of dental implants, however the clinician should consider several factors that might steer him towards the selection of a more adequate treatment option based on the patient dental history. For instance a patient with reduced or absent salivary flow, will benefit from an implant-supported type of restoration versus an implant retained over denture. In fact, in this case, despite implants offering a considerable increase in retention of the denture, the lack of saliva and therefore its humectant function, will still result in mucositis, due to the repetitive pounding of the removal denture. Moreover, while caries or history of periodontal disease are not a contraindication to the use of dental implants, a patient with poor plaque control, that can not be motivated to improve his/her oral hygiene, would be a better candidate for a removable type of prosthesis (i.e. complete denture, implant-retained complete denture, or implant-supported removable denture) versus a fixed one. A removable kind of prosthetic solution would facilitate the domiciliary hygiene routine of the patient. Patients with reduced dexterity, or reduced neuromuscular control of the extremities (i.e. fine motor skills of hands and fingers), will benefit from an easy to maintain implant-retained overdenture versus more sophisticated implant prosthesis that require more challenging oral hygiene protocols.
Parafunctional habits such as bruxism and/or clenching are not a contraindication to dental implant therapy, and do not seem to reduce implant success rate. However, these habits considerably increase the incidence of prosthetic complications (i.e porcelain chipping, fracture of framework, fracture of denture base and denture teeth) and increase the need for professional maintenance and chair-side adjustments. While all kind of implant prostheses (fixed or removable) are prone to develop technical failure and require some sort of maintenance, removable prostheses are easier to retrieve and to repair. Also, fixed prostheses that are multi-layers, either metal-acrylic (metal bar, acrylic resin and denture teeth) or metal-ceramic (metal framework and veneering porcelain) offer more failing points and seems to present higher incidence of failure than monolithic prostheses (zirconia all ceramic fixed prosthesis with glazed anterior teeth).
Age is not a deciding factor in planning the rehabilitation of an edentulous patient, and while with increased age there is also an increase in the incidence of the aforementioned determining factors, age per se should not play a major role in the choice of the treatment modality.
Lip Support & Lip Line
Lip support is one of the most important criteria in the selection of a fixed versus removable implant prosthesis. Lip support is determined by the shape of the alveolar ridge (supporting the portion of the lip closer to the base of the nose, the columella) and by the buccal aspect of the incisors and canine teeth (supporting the vermillion border of the lip). The maxillary alveolar process presents a resorption pattern that proceeds cranially and medially (Tallgren, 1966), resulting in a loss of vertical dimension and lip support. In fact when evaluating the position of the anterior teeth in a complete denture, they are often anterior to the alveolar ridge. Depending on the severity of the bone resorption there can be a considerable discrepancy between the position of the anterior teeth and the alveolar bone. The patient’s existing maxillary denture (or a newly fabricated diagnostic denture) can provide valuable information regarding the need for compensating this discrepancy with an acrylic flange. When the acrylic flange is needed for a full support of the upper lip, a fixed implant might not be possible, unless the patient undergoes extensive bone grafting procedures. The amount of alveolar ridge shown during smiling is also an important factor in the selection of the treatment option to rehabilitate an edentulous patient (Tjan et al, 1984). If the alveolar ridge is displayed during smiling the esthetic of a fixed implant supported restoration, can be very challenging as the junction between the restoration and the gingiva (transition line) will be visible.
This can be corrected with a surgical alveoloplasty at the time of implant placement. The amount of alveolar ridge shown while smiling is measured preoperatively and a corresponding amount of bone is removed during the surgical procedure. This results in a lower smile line (the alveolar ridge has been surgically reduced and moved cranially) and in a transition line that is more easily camouflaged. The illusion of natural looking interproximal papillae can be recreated prosthetically using pink porcelain.
Bone Quality, Quantity and Location
The presence of adequate bone volume is of critical importance for the placement of dental implants. While contemporary surgical techniques allow to predictably increase the amount of bone available in the resorbed mandibular and maxillary alveolar ridgs, that might not always be a feasible option. Therefore, it is important to understand the challenges that progressive bone resorption sets for the clinician planning the rehabilitation of the edentulous patient. Lekholm and Zarb (1985) complied a classification of bone resorption and quality that is still widely adopted.
This classification system considered the residual amount of alveolar ridge and basal bone. Class A is a perfectly preserved alveolar ridge, that does not show any vertical or horizontal resorption, while Class E is completely resorbed alveolar ridge, with moderate to advanced resorption of the basal bone. This classification also included the quality of bone (class 1 to 4) based on the ratio of cortical/medullar bone. For ease of understanding we will refer to a three level bone classification, so that we can discuss advantages and disadvantages of each treatment modality related to the quality of available bone. This 3-level classification is extremely simplified and the clinical reality might present many more variations based on bone availability and location.
Class I is the well preserved alveolar ridge presenting a vertical and horizontal bone resorption varying from none to mild. A patient with class I bone presents a good lip support even without a flange. This can be assessed using the patient complete denture or during the fabrication of a diagnostic denture.
If the buccal acrylic flange of the denture is removed, the columella (the part of the lip close to the base of the nose) it is still well supported and does not sink in. The artificial teeth are well position on the residual alveolar ridge with a minimal buccal cantilever. The discrepancy between the cervical portion of the teeth and the surface of the underlying alveolar mucosa is minimal (within 1 to 2 mm), allowing for the fabrication of artificial teeth of natural or slightly larger than natural length without the need of a pink (porcelain or acrylic) transition line. Several treatment options are available for patients with class I bone. An implant-retained option would most commonly require the use of four implants in the maxilla (canine and premolar areas) and two implants in the mandible (intraforaminal area, most commonly canine or first premolar areas). If the treatment plan should include an implant-supported fixed restoration several options are available: a full arch implant prosthesis cemented on custom abutments (requiring 6 or more implants), or a screw-retained full arch implant prosthesis (requiring 4 or more implants, i.e. All-on-4), are the most commonly chosen ones. The former is a prosthetic solution commonly based on metal ceramic techniques similar to classic crown and bridge dental work. Custom abutments (titanium, all-ceramic or combination) are used to provide the support needed for the metal ceramic or all ceramic prosthesis. Commonly, these prostheses are fabricated in sections, including single crowns, 3-or more unit bridges, but a one-piece solution can also be used. The well preserved alveolar ridge allows for the fabrication of a prosthesis that only replaces the missing teeth, without need for lip support through the use of a flange or an artificial transition line. At time pink porcelain (or pink acrylic) is used to recreate an illusion of interproximal papilla. Ovate pontic design might be used to achieve an even more natural look of the prosthetic solution. The position of the implants varies according to different prosthetic plans for the maxilla and mandible. In the maxilla (8 implants) the most commonly used positions are the first molar, first premolar, canine and central incisors area, bilaterally. This allows the fabrication of 4 separate 3-unit fixed partial dentures (3-unit bridges), reducing the technical challenges of this type of restoration and increasing retrievability. In the mandible (six implants) the most commonly used positions are the first molar, first premolar and canine area, bilaterally. This allows for the fabrication of two 3-unit fixed partial dentures (first molar to first premolar) and one 4-unit fixed partial denture (canine to canine). While the aforementioned are the most commonly used “design” of implant-supported fixed prostheses, the position and number of dental implants can vary according to several factor, including: bone quality and quantity, biomechanics, parafunctional habits, ease of fabrication, clinical experience, and school of thought.
The latter (screw-retained full arch implant prosthesis) requires the use of less implants (4 or more) that are spaced out to obtain the maximum anterior-posterior spread (Jemt 1994). Most commonly in the maxilla the implants are placed in the premaxillary area, anteriorly to the maxillary sinus. The use of tilted implants (Krekmanov et al 2000), that follow the slope of the anterior wall of the maxillary sinus, greatly increase the anterior posterior spread, and eliminates the need for sinus augmentation surgery.
In the mandible the implant are commonly placed between the mental foramina, however if bone is available distal to the mental foramen, the placement of a dental implant in the second premolar or first molar area might be more advantageous compared to using a tilted implant in the mental foramen area. Screw-retained full arch implant prostheses are always one piece, and can either be fabricated using a titanium bar veneered by acrylic and denture teeth, or with monolithic zirconium oxide.
Class II is the alveolar ridge that underwent moderate to advanced resorption. The anterior maxillary alveolar bone had a considerable vertical resorption and an horizontal resorption of such extent that the support to the upper lip is not sufficient. The posterior maxillary alveolar ridge presents a reduced vertical height and the placement of dental implants is not possible without additional bone surgery (i.e. sinus augmentation surgery) (Kent & Block 1989). In the mandible the bone resorption does not allow for the placement of dental implants distal to the mental foramen.
The maxillary arch of a patient presenting class II bone can be rehabilitated with either a fixed or removable implant prosthesis. One of the most important selection criteria is the need for support of the upper lip (Jemt et al 1992). If the horizontal bone resorption of the maxillary alveolar ridge affected the support of the columella, leaving the patient with a completely unsupported upper lip (Chice & Leriche 1998), the only prosthetic options that will properly restore the esthetic providing sufficient support to the upper lip, are the ones that allow for the fabrication of an acrylic flange. Specifically an implant-supported removable complete denture, or an implant-retained removable complete denture (Hutton et al 1995).
The difference between these two type of prostheses consists in the distribution of the masticatory forces. In an implant-supported complete denture, the implants will receive 100% of the masticatory forces and will provide retention for the denture. The number and positioning of the dental implant needed for this type of prosthesis is similar to what has been discussed for patients with class I bone rehabilitated with a fixed implant-supported prosthesis. This is done through the use of six or more implants, and it frequently requires bone grafting of the maxillary sinuses. The implant-retained complete denture, distributes forces in a different way (Naert et al 1988; Palmqvist et al 1994). During mastication, forces are distributed on the alveolar mucosa and alveolar ridge, similar to a complete denture, as well as on the implants. Moreover the implants will offer additional retention to vertical dislodging forces. This option requires the placement of a minimum of four dental implants, and it might require grafting of the maxillary sinuses. Unfortunately, the use of tilted implants, with the intention of avoiding sinus grafting, is still not commonly adopted, as it increases the technical difficulties of fabricating the implant-retained prosthesis. The recent introduction of angled prefabricated denture attachments (i.e. angled Locator attachment) might prove helpful for this application, however it has been only recently introduced and lacks of long-term results. In summary, if the support of the columella is lacking, the indicated type of rehabilitation for the patient is a removable (either implant-supported or implant-retained) prosthesis. So that we can achieve adequate lip support and easy access for oral hygiene.
On the other side, if the columella is well supported, despite the moderate to advanced bone resorption, a fixed-type of restoration might be indicated. In fact there will be no need of an acrylic flange to support the upper lip and the options mentioned for the class I bone resorption can all be pursed. In this specific situation the use of tilted dental implants proves to be very valuable (i.e. All-on-4). In fact, as mentioned above, the class II bone resorption frequently presents bone volumes that are not sufficient for implant placement in the posterior maxilla. However, the anterior maxilla is most commonly well preserved, therefore the use of tilted dental implants that follow the slope of the anterior wall of the maxillary sinuses allow for a good anterior-posterior spread of the dental implants without need for sinus augmentation surgery. Moreover the one piece design of the All-on-4 prosthesis allows for the use of acrylic material to mask the transition line, providing an excellent esthetic outcome, unless the patient presents a high lip line (i.e. due to short upper lip, or hyper mobile lip) and could result in a not pleasant esthetic outcome due to the showing of both the transition line and the alveolar mucosa, resulting in a visible discrepancy in color. This should be intercepted during the diagnostic phase, and can be corrected with an alveoloplasty at the time of implant surgery. If the implant are placed without alveoloplasty and the transition line is visible during patient full smile, the only solution is to recur to a removable prosthesis, with the possibility of disappointing the patient considerably.
The class II mandible is a good candidate for different treatment options, ranging from the implant-retained complete denture (using 2 or more implants) or fixed implant-supported solutions (using 4 or more implants). The esthetic and functional challenges of the class II mandible can be reliably overcame with both type of prostheses (removable or fixed). Esthetic and lip support does not play a major factor as it does in the maxilla, therefore the final decision of the type of prosthesis used for the rehabilitation of the class II mandible mostly relies on other factors such as: patient preference, finances, need for additional surgery or bone grafting.
Class III is the severely resorbed alveolar ridge. The majority, if not all, the alveolar process has resorbed, leaving only basal bone. In the maxilla this results in a complete loss of the support of the upper lip and is accompanied by extreme pneumatization of the maxillary sinuses leaving a minimum amount of bone in the posterior maxilla. In the mandible the amount of bone in the intraforaminal area is minimal and posterior to the mental foramina there is no residual alveolar ridge. The implant-based treatment options for patients with class III bone resorption are very limited, unless the patient undergoes extensive bone grafting procedure (Wood et al 1988). In the maxillary arch the use of 2 zygomatic dental implants (Balshi et al 1999) combined with 2 implants in the premaxillary area, could be adopted to provide the patient with a fixed implant supported restoration. The amount of anterior cantilever (the position of the teeth will be very far anterior compared to the position of the implants) will complicate the oral hygiene procedure, therefore the patient should be seen frequently for motivation and maintenance. In the mandible based on the amount of residual bone the options might include an implant-retained overdenture or a fixed implant-supported prosthesis.
Technical factors and materials
It is important for the clinician to be aware of the relevance that technological factors play in the planning of an implant-based prosthesis, so that complications such as unexpectedly high laboratory charges, or last minute changes in the design of the prosthesis can be avoided. Some of these factors include: interocclusal space requirement, metal ceramic vs all ceramic, anterior-posterior spread, attachment vs bar, etc.
Interocclusal space requirement.
Implant-supported restorations require a minimum amount of interocclual (or interarch) space, in order to provide a esthetically acceptable result and long-term function with reduced incidence of complications. In the edentulous patient the interocclusal space is bound by the alveolar mucosa and the occlusal plane. The minimum space for the fabrication of an implant-retained over denture, is 9mm when locator-type attachment are used and 14mm when a bar is used. Implant-supported prosthesis have different space requirement, fixed implant-supported prostheses on custom abutments require a minimum of 7mm of vertical height. However, the average height of a tooth is 10mm, therefore realistically the clinician should consider this (10mm) to be the minimum space requirement as anything less it is likely to look unattractive. An implant-supported overdenture can require up to 16mm of vertical space (Chee 1992), depending on the design of the milled-bar and the respective female counterpart, as well as the type of attachment used. Latch-type connection require less vertical space than locator-type attachment soldered on the bar, however are more cumbersome to use and require patient full dexterity. The clinician should keep in mind that dexterity mind be reduced with time, specially after stroke or other ischemic phenomena. The screw-retained implant supported prosthesis requires a minimum of 12mm of vertical space to accommodate all the components. When limited interocclusal space is diagnosed before the placement of the dental implants, it can be easily corrected with an alveoloplasty or by increasing vertical dimension.
However when the limited interocclusal space is not diagnosed and the implant are placed, the fabrication of the final prosthesis might involved higher laboratory cost than anticipated, or might even be impossible, requiring removal of the dental implants.
Metal ceramic vs All ceramic.
Metal ceramic has traditionally been the material of choice for implant-supported prostheses on custom abutment, however delamination of the veneering porcelain has been reported. In order to overcome this limitation, monolithic materials such as zirconium oxide have also been used. The adoption of a one piece zirconium oxide structure, reduces the number of interfaces that could undergo failure as well as reduce space requirement. Traditionally zirconium oxide was avoided for the anterior area, due to the lack of translucency, however contemporary manufacturing and glazing technique have greatly improved the esthetic of zirconium oxide prostheses. Zirconia is quickly becoming the material of choice also for the screw-retained type of prosthesis as reduces the incidence of fracture of cantilever and eliminates chipping of denture teeth, that can affect up to 50% of the patients at 5 years.
Attachments vs Bar.
The maxillary implant-retained overdenture is most commonly restored with a tissue bar (i.e. dolder bar) to overcome the bucco-lingual angulation of the implants (due to angulation of the alveolar process) that would quickly wear nylon retention-based attachment (ie locator-type attachment). The mandibular implant-retained overdenture is most commonly restored using prefabricated attachment (i.e. locator attachments) as the implants can be placed with an almost absolute parallelism (Henry 2002). This solution provides an excellent retention as well as low initial and maintenance costs. In cases in which the implants are not parallel (<10 degrees of divergence), special attachment can be used to increase retention and longevity of the retentive components. If the divergence is more than 10 degrees, the fabrication of a tissue bar (ie dolder bar) is required. Recently, angled prefabricated attachments have been introduced to the market. These new components could be used to solved implant angulation problems, and reduce the need for fabrication of tissue bars, however their clinical application lacks of long-term follow up and has not been extensively tested as of yet.
The anterior-posterior spread is the distance from the most anterior portion of the most anterior implant to the most posterior portion of the most posterior implant. This distance is commonly used to assess how many posterior teeth can be replaced distal to the last implant.This distal extension is commonly referred to as cantilever. Cantilevers are commonly used in implant-supported fixed prostheses, however given the unfavorable nature of the lever created by this design (class 3 lever), it produces a considerable stress and strain on the restorative materials, specially the metal framework. Therefore a good anterior posterior spread of the dental implants is recommended. Traditionally this was achieved with placement of the implant in the first molar area, that might not always be possible (in the severely resorbed mandible) or resulted in the need for sinus augmentation surgery (in the posterior maxilla).
In certain clinical situations (Class II maxilla and mandible, Class III maxilla) the use of tilted implants results in an increased anterior-posterior spread with consequent improved biomechanics of the distal extensions. In other situations (Class I maxilla and mandible, Class III mandible) the advantage of using tilted implants is limited and might not overcome the increase cost of components required to parallelize the tilted implants.
It is important to correctly estimate the laboratory cost of the different type of implant complete dentures. An implant-retained complete denture with prefabricated attachment (i.e. Locator attachment) has a laboratory cost comparable to the one of a conventional denture. The attachments are purchased separately, and most commonly picked up with a chair-side procedure.
However, an implant complete denture, retained by a tissue bar with clips (i.e. dolder bar) could add one to two thousand dollars to the lab bill (approximate cost, variable by location). Implant-supported prostheses have a considerable higher cost that can range from a few thousand dollars for the fabrication of a bar with acrylic flange and denture teeth, to the tens of thousand of dollar for the fabrication of spark erosion bar with custom made latch connection and porcelain teeth.
The clinician is advised to contact his/her usual dental technologist to discussed about the laboratory fees for the different type of implant prosthesis, so that an accurate estimate can be provided to the patient, and disappointment for both the clinician and the patient is avoided.
A variety of treatment options is available for the edentulous patient, all offering reliable, long-term, comfortable solutions. Several factors play a role in the selection of the most appropriate treatment for each patient, including but not limited to: lip support, quality and quantity of bone, patient desire and expectation, financial reasons, etc. The treatment should be customized to meet the patient need and the clinician should keep in mind that lip support plays a major role in the esthetic outcome of rehabilitation of the edentulous patient. Recently introduced treatment modalities that rely on the use of tilted dental implants (i.e. All-on-4) have reduced the need for bone augmentation surgery, reducing cost, time and complexity of the dental treatment.