“ABCD” Implant Classification: A Comprehensive Philosophy for Treatment Planning in Completely Edentulous Arches

The World Health Organization categorizes the completely edentulous patient as being physically impaired, handicapped, and disabled.¹  Contrary to popular belief, the number of patients suffering from this debilitating condition is on the increase worldwide, regardless of varying economic standards and general lifestyle.^1–3  Edentulism, though not life-threatening, severely impacts facial appearance, nutrition, and the ability to speak and socialize.²  Significant numbers of complete denture wearers are dissatisfied; with complaints ranging from loose lower dentures, sore spots, and inability to eat various kinds of food.⁴  Ever since the McGill Consensus Conference⁵  concluded that the 2-implant-supported overdenture was the minimal standard of care for patients with edentulous mandibles, dental technology has been in overdrive to develop a variety of solutions to restore completely edentulous patients with implants.⁴  However, patients come to the dental office for teeth, not implants⁶ ; and a prosthetically driven approach is necessary. Multiple surgical procedures such as sinus grafting, and horizontal or vertical augmentations are now available to provide the fixed restorative care patients desire.^7,8  The availability and regeneration of bone for idealized implant placement should not be the lone factor used by the clinician to decide the best prosthetic solution for the patient. The final esthetics, bone volume, space availability, and age are all essential criteria in the treatment planning process. Several classification systems for treatment planning in edentulous situations tend to focus on only a certain parameter such as esthetics or available bone volume, or are specifically designed for the maxilla or mandible.^7–11  The aim of the present article is to provide a scientifically driven approach to best suit a patient's individual needs while carefully understanding the interplay of all the aforementioned factors in the decision-making process.

Completely edentulous situations can be restored with removable overdentures or fixed prostheses, which can be screw/cement-retained with prefabricated or custom abutments.⁴  The complete treatment phase from the presurgical diagnostics to delivery of the final prostheses involves multiple steps and use of various diagnostic aids, surgical techniques, prosthetic components, and material. The clinician faces multiple challenges during the entire rehabilitation process; errors in any step adversely affect the final treatment outcome. This whole process is time-consuming and can be stressful, especially for beginners. The authors have delineated the entire treatment protocol into 4 phases, each of which present a different set of clinical situations that need to be managed accordingly. This delineation will help streamline the flow of treatment.

• Planning and Diagnostic Evaluation. This involves decisions made about the type of prosthesis based on the finalized number and location of the implants.

• Surgical Phase. Decisions regarding need for soft/hard tissue augmentation and free-handed or guided surgery are made depending on the evaluation in the planning phase.

• Provisional Phase. Immediate or delayed loading of the implants based on implant stability and adjunctive surgical procedures performed during the surgical phase.

• Final Restoration Phase. This is the phase focused on achieving ideal esthetics, phonetics, and function for the particular patient. Initiate a maintenance and recall protocol to ensure long-term function of the prosthesis by early detection and resolution of any potential complications.

The decision on the type of final prosthesis at the beginning of the treatment process enables the clinician to develop a precise understanding of the implant type, number and location, type of regenerative procedures required to achieve this ideal implant placement, and use of a particular material and manufacturing technique for the final prosthesis. It also allows for better communication with the laboratory and a realistic accomplishment of the patients' desires while considering the anatomic, structural, and esthetic limitations every case may present.

To simplify this decision-making process and allow predictability in final treatment outcomes, the authors propose the “ABCD” classification system for treatment planning in completely edentulous maxillary and or mandibular arches. This classification of the patient is based on 4 parameters to help choose the final prosthesis design at the planning stage.

We have an aging population due to increased life expectancy while on the other hand we have a large group of younger edentulous patients due to a radical shift in lifestyle choices and resultant periodontal disease or caries.²  Surely, the same treatment plan cannot be applied to the young and geriatric patients.

A holistic approach considering the influence of chronic diseases and degenerative changes in soft and hard tissues is best suited to deal with the dental needs of an aging population. Also, the diminished tolerance to extensive surgical procedures, uncertainty over access to care in the long term, and the ability to maintain satisfactory hygiene during the maintenance phase are areas of concern.¹²  It may be prudent to design a relatively straightforward implant overdenture or a fixed-implant prosthesis in geriatric and medically compromised patients by planning placement of implants in available bone instead of extensive regenerative surgical procedures that may increase morbidity. On the contrary, in the younger age group of edentulous patients, who present compromised bone support; all efforts need to be taken toward regenerative strategies to enable prosthetically driven implant positions. The authors have presented 4 classes based on the age of the patient: A1: young edentulous (<50 years); A2: intermediate edentulous (50–65 years); A3: old edentulous (65–75 years); and A4: geriatric edentulous (>75 years).

The issues to be addressed at the two opposite ends of the spectrum are longevity for the younger age group and simplicity of treatment for the geriatric patient. Without an individualized approach, young patients may end up being treated with lesser number of implants and geriatric ones with more complex regenerative strategies. Understanding the age factor can guide the clinician in choosing the most appropriate option.

The authors have integrated the Bedrossian⁸  classification of bone zones and modified it. Although this parameter is focused on the more complex maxilla, the concept can be extended to the mandibular arch as well. The absence of bone in the posterior mandible will indicate either regenerative procedures if conventional axial implants are to be placed or whether tilted implants should be used in the interforaminal region. In situations with considerable loss of bone in the anterior and posterior regions of the mandible, a subperiosteal or transosteal approach can be undertaken if the age and systemic condition of the patient preclude extensive regenerative surgeries.

The classification of bone zones based on bone availability in each zone is as follows: B1: all zones; B2: only incisors and bicuspids; B3: incisors and zygoma; and B4: only zygoma (quad).

So, when bone is available in all zones (B1), 6 to 8 axial implants may be placed to minimize cantilevers. In category B2, 2 or 4 axial implants in the incisor region and 2 tilted implants in the bicuspid zone may be considered. For B3, the incisor zone management may remain the same with 2 tilted implants in the zygoma. In category B4, it could be quad zygoma.

The static and dynamic positions of the upper lip and its tonicity are important determinants in the decision-making process regarding the type of prosthesis and resulting esthetics. The authors have adopted the Tjan et al¹³  smile line analysis to categorize the cosmetic display parameter in the ABCD protocol. They stress the fabrication of a provisional complete denture to preempt the position of the anterior teeth, their visibility, and inclination with respect to the residual ridge, esthetics, and phonetics that are to be achieved in the final prosthesis. Following are the classifications: C1: low lip line (< 75% of anterior teeth); C2: medium lip line (75%–100% of anterior teeth and interproximal gingiva); and C3: high lip line (complete anterior teeth and continuous band of gingiva).

The maximum lip mobility that the patient can elicit will determine if the transition line between the future prosthesis and the residual ridge will be visible. If yes, a crestotomy must be performed during surgery to hide the transition line behind the envelope of the upper lip. When the prosthesis is in full display during smiling, the junction between the pink and white of the prosthesis must be managed appropriately to provide a good esthetic result. In low lip line cases, these factors will not matter much and hence the overall bone reduction can be kept to a minimum, sufficient to create a good soft tissue bed to house the intaglio of the prosthesis.

If the ridge is visible on smiling and the interarch space is less, a metal-ceramic profile prosthesis can be used and the visibility of the gingiva will give it a more natural appearance. However, if the interarch space is greater due to advanced atrophy, the lip may require support from the prosthesis. In such cases an overdenture may be the design of choice over a fixed prosthesis. This decision will depend on other factors mentioned in the classification previously such as the age of the patient and availability of bone, which will determine the number and position of implants.

Regardless of the degree of ridge resorption, similar to traditional complete denture fabrication, the position of incisal edge of upper incisors is determined by esthetics, phonetics, and lip dynamics.^4,13,14  The most predictable way to finalize the length of upper incisors is to follow the concept of locating the cuspid zero.¹⁵  This method allows a more accurate judgment of the incisor length compared to the traditional methods that show a wide gender variation.

The technical success of prosthetic materials depends on certain minimum space requirements that must be fulfilled. If materials are used in thinner sections there will be more failures due to breakage. The available restorative space is measured from the implant prosthetic platform to the proposed incisal edge in the anterior region and occlusal plane in the posterior region and this space will govern the selection of the prosthesis.^4,16 

The authors have categorized the interarch space into 4 classes (Figure 1): D1: minimal (10–12 mm); D2: moderate (12–15 mm); D3: moderate (15–18 mm); and D4: excessive (>18 mm).

The choice of final restorative material will depend on the available space (Table). In minimal resorption cases (D1), only the anatomical crowns of the missing natural teeth will be replaced. For D2 and D3 classes, it would also appear to replace a part of the resorbed alveolus. In these cases, as the degree of resorption has increased, the use of pink ceramic becomes important if the patient has a high lip line. Fixed restorative options would be a porcelain-fused-to-metal (PFM) prosthesis/monolithic zirconia/layered zirconia, which could be screw/cement retained based on location of screw access openings.⁴  In D3 cases either a hybrid prosthesis with resin teeth or a combination bridge (screw-retained framework with cement-retained crowns) can be used. The overall choice depends on the economics as well as the position of the implant access holes.

As the interarch space is restricted, overdentures for D1 class should be avoided. In D2 cases, the Locator attachment is preferable. In the maxilla the implants must be splinted with a bar attachment, which is unfeasible with the limited interarch space.⁴ 

As the degree of resorption increases further (D3), use of traditional designs of PFM make the prosthesis too heavy and sometimes difficult to produce. Removable prostheses could be designed with a ball and socket or a locator attachment. A milled low-profile bar attachment is a possibility; however, the choice should be made with caution.⁴ 

In D4 cases the degree of resorption has advanced to such great levels that using any form of fixed prosthesis will make the design biomechanically unfavorable. The preferred restorative choice is an overdenture with different attachments.⁴ 

The four factors (ABCD) need to be carefully analyzed at the presurgical diagnostic step. Different permutations of these will help the clinician arrive at a definitive treatment plan for a patient and also anticipate and resolve any complications that may arise during the successive stages of the rehabilitation process.

The following clinical cases will demonstrate the application of this classification. The first case is of a young edentulous patient (Figures 2 through 8), who had lost all her teeth due to periodontal disease.

• A: A1 (40 years),

• B: B2,

• C: C3, and

• D: D2, leaves us with 14 mm of interarch space.

Considering the patient's age, long-term function was desired from the prosthesis. The case was engineered with 7 implants in each arch to support a fixed screw retained PFM prosthesis. Also, the lip line was high (C3), and the ridge crest was visible in maximum smiling position. A crestotomy was planned during surgery to hide the transition line, after ensuring there was sufficient bone available (B2) for axial implant placement in the anterior region. The prosthetic pink-and-white junction was managed and made age appropriate for the young patient. The posterior maxilla was grafted to achieve a better bone dimension.

Application of the classification allowed the authors to plan all parameters of the surgery. The need for additional bone was addressed with appropriate sinus management techniques to allow for optimal engineering of the case. In a geriatric patient, the same case could have been managed with tilted implant placements without any sinus intervention. The classification allowed for a time tested, robust approach and not a preconceived plan of tilted implants. Although tilted implants show great success rates at 10 years,¹⁷  the authors felt that expecting them to survive for several decades without any complications would be unreasonable. The classification also allowed the final prosthesis material choice to be made before starting treatment.

The second case is of a female patient (Figures 9 through 16), who is 70 years old.

• A: A3 (70 years),

• B: B2,

• C: C3, and

• D: D3.

All these parameters were weighed against the age of the patient. Instead of regenerating bone with sinus grafting in the posterior maxilla, a simpler solution was provided with 2 tilted and 2 axially placed implants. Since the lip line was high (C3), it was decided to make the final prosthesis as a screw retained framework with individual cement retained crowns with a specifically individualized pink zone to make the pink-white junction esthetically perfect.

There is considerable ambiguity in literature regarding data on preference for a fixed or removable implant prosthesis with each design having distinct advantages. Removable designs are economical and suited for cases with severe resorption, high lip lines, need for hygiene maintenance and a certain degree of freedom in positioning of implants.^18–20  Fixed prostheses allow increased retention, enhanced masticatory ability, and reduced maintenance appointments.¹⁸  Given the risks and benefits of these rehabilitation options, comprehensive guidelines for selecting a particular prosthesis for a patient become mandatory. Too often clinicians face scenarios where implants have been placed only to discover that the type of prosthesis promised to the patient and the available restorative space do not match.⁴  Past classification systems tend to focus on a certain parameter relating to esthetics, bone volume, virtual implant planning procedures, or providing guidelines for either fixed/removable restorations.

Interestingly, all these classifications focus on the maxillary arch and use a particular parameter: esthetics in the case of the lip-tooth-ridge classification by Pollini et al,⁹  fixed maxillary restorations such as in Bedrossian et al,⁸  Bidra and Agar,²¹  virtual planning by Avrampou et al,¹¹  or only removable prostheses such as by Ahuja and Cagna.¹⁰  Most classifications tend to undervalue the age factor in the final prosthesis choice. The mandibular arch is largely neglected and removable prostheses designs unexplored to a considerable extent. One of the earliest classifications in implant dentistry given by Misch⁷  is based on the amount of hard and soft tissue structures replaced and the support obtained from the implants and or soft tissues.

The authors felt a need to develop a holistic classification that would encompass all the different criteria without diluting the importance of any parameter and at the same time enable the clinician to foresee potential complications at the diagnostic stage. The ABCD classification uses the 4 vital parameters of age, bone volume, cosmetic display, and degree of resorption to create an algorithm that satisfies the treatment needs of every patient. Various permutations of the 4 parameters can be used to arrive at a solution that streamlines the further phases of the rehabilitative process. An accurate diagnostic assessment of aesthetics is important to avoid disappointment in the later stages of treatment, especially after implants have been placed. The amount of lip support required, the degree of exposure of the ridge, and trial denture teeth on smiling along with the interarch space available help the clinician decide the type of prosthesis. These 2 factors dictate the number and position of implants and, if adequate bone is not available, the need for regenerative surgery as well. The age factor, when weighed against these parameters, helps determine the risk-benefit ratio of undertaking complex surgical procedures for achieving ideal, fixed restorations or the need to opt for relatively straightforward removable prosthesis designs.

The ABCD classification system uses the interplay between the 4 vital parameters of diagnosis to arrive at a precise, predictable, esthetically, and functionally favorable treatment plan for the edentulous patient. It is a scientifically driven, individualized approach to rehabilitate every patient in a manner best suited to their needs and expectations.

Abbreviations

ABCD classification:

“age, bone, cosmetic display, degree of resorption” classification

LTR:

lip-tooth-ridge

PFM:

porcelain fused to metal

The authors declare no conflicts of interest.