Implant Planning Principles for the Edentulous Maxilla
A guide for managing diverse case presentations
Edentulism is a disabling condition affecting tens of millions of individuals in the United States and many more worldwide.1 A recent evaluation of the condition of edentulous elderly2 revealed that this group experiences significant social exclusion compared with dentate individuals. The esthetic value of dentures and their social significance was regarded as important. The treatment of the edentulous population affects their socialization through self-recognition of appearance.
The value of the maxillary rehabilitation is tightly linked to esthetics. These findings differ from rehabilitation of the edentulous mandible, for which comfort and stability appear more tightly linked to self-reported satisfaction. The challenge for clinicians faced with rehabilitation of the edentulous maxilla using fixed implant prostheses begins with reconciling patients’ primary motive of appearance with the biologic and anatomic challenges presented by the edentulous maxilla.
Alveolar bone resorption following tooth extraction significantly reduces the vertical and horizontal dimension of the residual alveolar ridges. Tallgren3 observed that residual ridge resorption during protracted periods of denture wearing was slight compared to the rapid reduction during the year following tooth extraction, and this was particularly obvious in the edentulous maxilla. Importantly, Tallgren noted that there was great individual variation in resorption rates and patterns.
Clayman4 summarized the attendant clinical complications associated with maxillary residual ridge resorption to include progressive narrowing and shortening of the anterior maxilla over time, diminished quality of bone, vertical resorption that both reduces bone volume and increases interarch space, alteration in the intermaxillary relationships leading to pseudoprognathism with resultant midface soft tissue changes, and muscle changes that impair both function and appearance. Bedrossian and colleagues5 define and illustrate a “composite defect” as alveolar resorption with resultant vertical and horizontal space between the cervical portion of the planned prosthesis and the edentulous ridge. Treatment planning requires proper diagnosis of the existing deficiency and examination of the defect with regard to the planned prosthesis.
The variability among individuals challenges treatment planning of the edentulous maxilla. The vertical heights of maxillary and mandibular alveolar bone and localization of anatomic landmarks are important factors for planning of the implant-supported prosthesis. Panoramic radiography has been used to explore the variation in both alveolar bone architecture and demonstrated the important variability in the location of the most inferior border of the maxillary sinuses (molar vs. premolar regions).6
The goal of this report is to provide a conceptual framework for managing the diversity of clinical presentation of the edentulous maxilla using four general topical areas: display of anterior teeth, density of bone, dimension of bone, and distribution of implants. Reproducible excellence in implant therapy for the edentulous maxilla can be achieved by addressing these four topical areas in planning prior to implant placement.
Display of Anterior Teeth
An attractive smile dependent on a prosthetic dentition requires that the appropriate amount of teeth and gingiva is displayed. In addition to tooth and gingival display and display of the prosthesis, alveolar junction comprises the third concern when addressing display issues. Tooth location in three dimensions aids in development of lip posture and speech as well as phonetics. Further, the location of teeth cannot compromise the vertical dimension of occlusion. Fortunately, there are some general guidelines to aid in location of teeth prior to implant planning (Table 17-12). Realization of this planning is provided by denture tooth arrangement that is carried forward in planning by integration into CBCT images (preferably by a dual scan method, Figure 1).
Anthropomorphic measurement of the relationship of tooth incisal edges to the vestibule offers a starting point for determining the vertical tooth position. A general rule is that maxillary incisal edges are located 20 mm inferior to the vestibule and 8 to 10 mm anterior to the incisive papilla. While these values may not characterize an individual’s ideal tooth position, they offer an opportunity to approximate an acceptable position for tooth display and occlusal plane orientation. It is critically important that planning of maxillary implants does not interfere with anterior tooth position and related esthetics and phonetics. The display of soft tissue (papilla, gingival contours) is a second important aspect of the smile. Excessive gingival display, although a subjective measure, may be unacceptable when exceeding 3.0 mm.13
While tooth display is essential to the creation of an attractive smile, the prosthesis/tissue junctions must be hidden. The Bedrossian pretreatment screening protocol considers the visibility of the residual soft tissue crest as one of three guideposts in planning implant prosthesis for the edentulous maxilla.14 The junction of the prosthesis with the alveolar ridge should not be visible. In some cases, implant placement without alveolar reduction will create esthetic complications, require ridge lap prostheses that promote biofilm accumulation and peri-implantitis, and/or reduce prosthesis height leading weakness and potential failure. Orientation of the planned prosthesis to existing bone, as well as to extraoral landmarks, is key to successful planning of maxillary implant prostheses (Figure 2 and Figure 3).
Lekholm and Zarb15 introduced during the inception of endosseous implant therapy a classification of patients according to volume and bone quality (density). The density scale (type I to type IV) was widely adopted. Jaffin and Berman16 identified type IV bone as a risk for implant failure due to its thin cortex and low trabecular density. Type IV bone is often encountered in the posterior regions of the edentulous maxilla.17 This is linked to greater implant failure in the posterior maxilla. Turkyilmaz and McGlumphy18 revealed an average density of 645 ± 240 HU for successful implants versus 267 ± 47 HU for failed implants. Moy and colleagues19 reported implant failure of 8.16% for the maxilla versus 4.93% in the mandible. Most recently, Cakarer and colleagues20 reported that survival of implants in the maxilla was lower than for the mandible (P = 0.028) and among risk factors, only location was a factor influencing survival. Tolstunov21 suggested that posterior maxillary implant survival was less than 80%, whereas other zones of the maxilla and mandible approximated 90% or greater implant survival. Most clearly stated, Rohlin and colleagues22 reported that “70 of every 1,000 implants are at risk of failing in the maxilla after 5 years and 17 of every 1,000 patients in the mandible are at risk after 10 years.”
Low HU density measurements (consistent values below 100 HU) in maxillary medullary bone resulted in commensurate low insertion torque values at surgery.23 Assuming that low bone density is a clinical factor frequently encountered in treatment of the edentulous maxilla, the risk to implant survival must be considered. Thus, it is incumbent on the clinical team to address this factor in treatment of the edentulous maxilla. There are several tactics that clinicians may utilize to address low bone density (Table 2).
Implant surface technology influences implant survival in poor quality bone and is particularly relevant to treatment of the edentulous maxilla. The value of moderately rough implant surfaces in attaining rapid and more extensive bone-to-implant contact has been considered in depth at the experimental and preclinical levels.24 The importance in clinical therapy has been strongly suggested.25-27
Drilling protocols are also important. Clinical investigators have demonstrated that alteration of the drilling protocol (site preparation technique) is helpful in achieving higher implant survival.28,29 The value of a bone density–adapted protocol in achieving primary stability has been reiterated.30 Undersized osteotomies in low density bone may increase implant primary stability, leading to enhanced implant survival.
Taking full advantage of the highest density bone of the maxilla is another approach to finding solutions to low bone density of the maxilla. Inferred is the use of native bone versus creating of bone by grafting. Concern for higher success in grafted bone is suggested by some clinical studies. While bone grafting creates important opportunities for implant therapy,4 there are limitations.31 For example, implants placed into augmented sinus bone and bone block grafted areas were associated with significantly higher failures than native bone.32 Hence, various attempts to stabilize implants in native bone have gained interest. Included are pterygoid implants33 and tilted implants that engage the dense maxillary bone of the nasal fossa or maxilla medial to the maxillary sinus.34 Jensen35 suggests that bone loss need not be recovered by grafting when a tilted implant strategy is used. Grafting is a valuable tool for treatment of the atrophic edentulous maxilla; however, full consideration for other alternative approaches to gaining stability using implants with enhanced surface technology should be made in the consideration of grafting decisions (Figure 4 and Figure 5).
Alveolar resorption is an inevitable consequence of exodontia and a sequelae of edentulism.
Three challenges to vertical dimension maxillary alveolar bone include alveolar resorption, sinus pneumatization, and antagonist tooth extrusion. Physiologic alveolar resorption and iatrogenic buccal alveolar bone loss represent causes of horizontal dimensional changes in treatment of the edentulous maxilla. The resulting changes in the relationship of the maxilla to the opposing mandibular occlusal plane adds additional complexity to the clinical scenario and can only be investigated by use of mounted study casts or by representation of models in the three-dimensional (3D) virtual environment by scanning technologies. This comprehensive approach to planning is essential to reproducible success for implant therapy involving the edentulous maxilla (Figure 6 through Figure 10).
Ultimately, the crestal alveolar dimension will be less than that of the tooth root diameter and shorter than pre-extraction dimensions. For example, Pietrokowski and colleagues36 demonstrated that the mean alveolar width for the maxilla was 3, 4 and 5 mm at the incisor, premolar, and molar regions, respectively, with 47% of the incisor ridges displaying a knife-edged morphology. Vertical height reductions in the maxilla are less than for the mandible, but approach 25% (approximately 3-4 mm).37 Bone width measures made at the first and second molar regions averaged 5.7 and 6.6 mm, respectively, and bone height measures averaged were 5.4 mm and 6.6 mm.38 These observations make it clear that a 3D assessment of maxillary osseous architecture is necessary for treatment of the edentulous maxilla. Horizontal deficiencies in the anterior maxilla are common and posterior vertical deficiencies exist and can be exacerbated by the location of the maxillary sinus and its pneumatization.
In addition to horizontal grafting and more challenging grafting in the vertical direction, there are several strategies to overcome limitations of edentulous maxillary bone dimension for implant placement (Table 3). The dimension of alveolar bone is regarded as critical for implant planning. However, a related minimal or ideal implant dimension has not been defined for treatment of the edentulous maxilla. Treatment strategies using implants of diverse dimension are reported. From the information reviewed above, the use of implants longer than 6 to 8 mm in the posterior regions of the maxilla and implants wider than 3.5 to 4 mm in the anterior regions of the maxilla require consideration of grafting prior to or augmentation at the time of implant placement.
At the present time, it is not possible to identify a minimal implant dimension. However, given that existing data demonstrate higher rates of implant prosthesis and component complications than implant failures for splinted maxillary prostheses,39 it may be argued that implant diameter, which influences the dimension and design of the abutment interface and screw components, is an important factor influencing therapy success. Regarding implant length, three observations may be made. First, short implant (≤8.5 mm) success is reported.40 Second, longer implants are used in tilted implant protocols to achieve stabilization in denser bone regions of the maxilla.35 Third, the use of extensively longer implants has not proven beneficial to implant outcomes. However, when loading protocols are considered (beyond the scope of this report), longer implants may offer advantages of achieving and maintaining primary and secondary stability.41 An overbearing focus on implant dimension requiring extensive grafting may not be required. This is supported by observations that the placement of short rough-surface implants is not less efficacious in treatment of totally edentulous patients.42 Extremely wide implants may not offer improved success.43
The reported outcomes for implant-supported fixed prostheses reveal that mechanical complications are prevalent.44 Many can be the consequence of excessive mechanical loads that result from bending moments presented by the extended location of prosthetic teeth relative to the implant abutment. Design (or engineering) of a maxillary implant-supported fixed prosthesis is challenged by cantilevers that exist at both the anterior and posterior aspects of the prosthesis. A primary goal of planning and execution of the maxillary implant-supported fixed prosthesis is to maximize the distribution of implants to reduce the extent of anterior and/or posterior cantilevers (Figure 11).
Some approaches to assuring distribution of maxillary implants include grafting, tilted implants, pterygoid implants, and short implants. Further, it may be prudent to consider reducing potentially destructive bending moments by advocating a shorter dental arch and reducing the vertical overjet when designing maxillary implant prostheses.
Current methods for correcting alveolar bony defects prior to dental implant placement include maxillary sinus augmentation45,46 and onlay bone grafting. Sinus augmentation addresses vertical insufficiency in the posterior maxilla and onlay grafting is more often a solution for horizontal insufficiency frequently observed in the anterior maxilla. Sinus grafting enables clinicians to use the posterior maxilla for implant support. While beyond the scope of this report, there are numerous systematic reviews that indicate success of sinus grafting is high and they address variables including biomaterial, height of the residual alveolar bone, rough vs. smooth implants, and the use of membranes or not. Such reviews indicate high success with limited morbidity.47
Regarding onlay grafting to reconstruct the atrophic maxilla, it also may provide for wider and longer implants. This is an important decision when considering the long-term biomechanical attributes of the implant prosthesis and components, as some larger implants house larger and more robust abutments and possess greater strength. However, Chiapasco31 by way of literature review, reported that dental implant survival rates in the reconstructed maxilla were 79.5% (median 82.7%; mean 81.6%) and surgical complications from the iliac crest donor sites include temporal neural disturbances associated with the donor site, as well as temporary pain/gait disturbances. Several of the complicating factors associated with these procedures are the increased duration of therapy, cost, and documented morbidity.48 Regarding vertical bone augmentation techniques, a systematic review encompassing guided bone regeneration, distraction osteogenesis, and onlay bone grafts concluded that data were insufficient to generally advocate this approach.49
Tilting of the dental implants is a method of increasing the distribution of the supporting abutments and reducing prostheses’ cantilever dimension while avoiding vital structures and the morbidity of advanced bone grafting techniques. Implants may be placed in areas with greater bone density (nasal fossa and medial sinus wall) and longer implants may be used, resulting in greater implant stability.50 A formalized therapy was advocated by Malo and colleagues51 and involves use of four implants placed to extend distribution of prosthesis support despite limitations of bone. Irrespective of the number of implants, the use of tilted implants efficiently addressed the important requirement of implant distribution without the need for extensive bone grafting procedures. No additional risk of failure was apportioned to tilted implant when compared to upright implants was found.52 Survival rates are reported between 92.8% and 100%. The outcomes at the level of the associated prosthesis are good; a systematic review indicated high prosthesis survival of fixed dental prostheses supported by tilted and axial implants with mechanical complications that included bridge screw loosening, abutment screw loosening, abutment screw fracture, and material fracture. Replacement of the fixed dental prostheses was not required.53
Bone height is not always available to place a dental implant greater than 8 mm in length, and bone grafting procedures are not always a viable option. Short dental implants can provide many benefits towards both the esthetics and function of the treatment outcome for these patients. Although no consensus exist regarding the definition of a short implant (defined by different sources as less than 10, 8, or 7 mm), short implants have similar survival rates as longer dental implants. When defined as less than 8.5 mm, a systematic review revealed that among relatively short-term studies, the initial survival rates for short implants is high and not related to implant surface, design, or width. As such, short implants may be an alternative to grafting for treatment of the edentulous maxilla.40 In a direct comparison of short implants versus longer implants placed in augmented bone, Pistilli and colleagues demonstrated no difference in implant or prostheses failures.54 Long-term investigations are needed to verify these findings.
Some complications have been related to the poor crown-to-implant ratio and the unfavorable occlusal forces; however, it is important to mention that the sites where these implants are placed are already deficient in bone volume, therefore making the prostheses longer in height and increasing the crown-to-implant ratio. Alternatively, the marked resorption also can provide for increased restorative dimension, affording the construction of a robust prosthesis with large connector dimensions or beam height to counter potentially long spans or selected cantilevers (Figure 12).
Placement of long, tilted implants into the pterygoid process (or pterygomaxillary region) was introduced a decade before more global consideration of tilted implants55 to avoid bone grafts and to eliminate long posterior cantilevers. The technique, while not widely adopted, is associated with implant survival of less than 90%, even for immediate loaded scenarios.56 These implants can be placed in the pterygomaxillary region or in the pterygoid process. Success rates vary between 71% and 96.3%, comparable to traditional endosseous dental implants. The main limitations of the technique are access for hygiene and use of angled components when pterygoid process–located implants are chosen.57
It may be concluded that when considering the range of options available to clinicians, attaining a goal of wide implant distribution is attainable in most instances. A number of different approaches may be considered and many may offer equivalent opportunities for success. Consistently striving for wide distribution of implants that limit cantilevers and impose lateral loads on sufficiently large components is recommended (Table 4). Clinicians must achieve distribution in a manner that meets the functional demands of a lasting prosthesis and the preferences of the patient as well as the therapeutic team.
Directly addressing the four principles of display, density, dimension, and distribution enables a consistent approach to planning the treatment of maxillary edentulism using dental implants. The result enables construction of an esthetic, functional, and robust prosthesis. Successful treatment of maxillary edentulism using dental implants begins with establishing or refining “ideal” tooth position to support desired esthetics and phonetics. Traditional denture technology remains the basis for this approach. While it is beyond the intent of this report to consider denture construction, there are several approaches to determination of anterior tooth position relevant to this task. Central to this mission is the careful preservation of the midline and occlusal plane. Establishing and preserving the vertical dimension of occlusion is another essential aspect of this task. Any unanticipated or unaccounted for changes in either the horizontal and vertical relationships of the maxillary and mandibular teeth may challenge the eventual restoration of maxillary implants. When denture construction is pursued in preparation for implant rehabilitation of the maxillary arch, patient acceptance of the tooth arrangement is essential and the accepted tooth position must be established at the proper vertical dimension of occlusion and in centric relation.
Established tooth positions inform implant planning. The preferred method to use information regarding tooth position by visualization of the planned prosthesis and bone is the creation of a complex 3D model in implant planning software. Dual scan methods integrating the scanned denture and the CBCT scan of the patient are common, and this permits rapid integration of all necessary data. These complex 3D models permit planning of implant position relative to the planned prosthesis. Both implant dimension as well as the abutment dimension, position, and orientation should be established prior to dental implant surgery (Figure 13 through Figure 15).
Execution of the planned implant surgery can be pursued using a guided surgical approach or by information transferred to an analog guide from the virtual model. The advantages of guided dental implant surgery include implant survival comparable to conventional surgery, compatibility with immediate loading, reduction in postoperative morbidity and complications when mucosa borne guide is used, reduced duration of surgical intervention, and patient satisfaction.58 While the present authors concur that implant placement in the edentulous maxilla may benefit from guided surgical techniques, all surgical procedures should be informed by 3D assessments based on planned prosthesis location.
Prosthesis construction should follow smoothly after well-planned and well-executed implant surgery. As indicated above, the use of abutments defined prior to implant placement should be anticipated. Complex prostheses that fit passively without complication are today expected from CAD/CAM manufacturing methods. Irrespective of the method selected, achieving the plan will assure that the prosthetic rehabilitation will not be encumbered with complex prosthetic solutions to account for implant malposition. A workflow that uses the denture to facilitate the surgical placement of implants using a digital surgical guide, plan the fabrication of the provisional prosthesis, and guide the digital design and CAD/CAM manufacture of a framework or monolithic prosthesis can match the key esthetic, phonetic, and functional parameters established by the initial denture tooth arrangement.
The treatment of maxillary edentulism using endosseous dental implants must account for variability in maxillary anatomy expressed among the edentulous population. Variability in maxillary ridge morphology and bone density are important factors that are accounted for by implant dimension, the use of surface technology, and the inclusion of additional implants. The display of teeth, especially in relation to the residual alveolar ridge, must be carefully managed; ridge lap fixed prostheses impede hygiene and present additional risk factors for peri-implantitis. Broad distribution is a general goal for dental implant planning for the edentulous maxilla. Prosthetic construction should be carefully managed to address the relatively high incidence of complications affecting the edentulous maxillary prosthesis. Occlusal and functional concerns may be ameliorated by careful design, precise management of maxillomandibular relationships, and the use of shallow maxillomandibular relationships of the anterior teeth. The integrated planning should result in a robust prosthesis supported by implants placed in healthy bone that is easily maintained by effective oral hygiene and a carefully communicated maintenance plan.
Dr. Cooper has received research grants from BioHorizons and DENTSPLY, and honoraria from DENTSPLY.
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About the Authors
Lyndon F. Cooper, DDS, PhD
Stallings Distinguished Professor
Department of Prosthodontics
University of North Carolina School of Dentistry
Chapel Hill, North Carolina
Ingeborg J. De Kok,DDS, MS
Department of Prosthodontics
University of North Carolina School of Dentistry
Chapel Hill, North Carolina