PARTIALLY EDENTULOUS ARCHES

To organize treatment plans in a consistent approach, a classification of patient conditions is necessary. Because more than 65,000 possible combinations of teeth and edentulous spaces exist in a single arch, no universal agreement exists regarding the use of any one classification system. Numerous classifications have been proposed for partially edentulous arches. Their use allows the profession to visualize and communicate the relationship of hard and soft structures. This chapter reviews a classification for diagnosis and treatment planning for patients who are partially or completely edentulous and require implant prostheses. By using this classification, which the author first presented in 1985, the doctor is able to convey the dimensions of the bone available in the edentulous area and also indicate the strategic position of the segment to be restored.¹⁵

HISTORY

Cummer,¹ Kennedy,² and Bailyn³ originally proposed the classifications of partially edentulous arches that are most familiar to the profession. These classifications were developed to organize removable partial denture (RPD) designs and concepts. Other classifications have also been proposed^4–14 (including one by the American College of Prosthodontists), none of which has been universally accepted. The Kennedy classification, however, has been taught in most American dental schools.

The Kennedy classification divides partially edentulous arches into four classes.² Class I has bilateral posterior edentulous spaces, Class II has a unilateral posterior edentulous space, Class III has an intradental edentulous area, and Class IV has an anterior edentulous area that crosses the midline.

The Kennedy classification is difficult to use in many situations without certain qualifying rules. The eight Applegate rules are used to help clarify the system.¹¹ They may be summarized in three general principles. The first principle is that the classification should include only natural teeth involved in the final prosthesis and follow rather than precede any extractions of teeth that might alter the original classification. This concept, for example, considers whether second or third molars are to be replaced in the final restoration. The second rule is that the most posterior edentulous area always determines the classification. The third principle is that edentulous areas, other than those determining the classification, are referred to as modifications and are designated only by their number. The extent of the modification is not considered.

CLASSIFICATION OF PARTIALLY EDENTULOUS ARCHES

The implant dentistry bone volume classification developed by Misch and Judy in 1985 may be used to build on the four classes of partial edentulism described in the Kennedy-Applegate system.^15,16 This facilitates communication of teeth positions and the primary edentulous sites among the large segment of practitioners already familiar with this classification, and it enables the use of common treatment methods and principles established for each class. The implant dentistry classification for partially edentulous patients by Misch and Judy also includes the same four available bone volume divisions previously presented for edentulous areas. Other intradental edentulous regions that are not responsible for the Kennedy-Applegate class determination are not specified within the available bone section, should implants not be considered in the modification region. However, if the modification segment is also included in the treatment plan, then it is listed, followed by the available bone division it characterizes (Box 19-1).

Box 19-1 Implant Dentistry Classification of Partially Edentulous Arches

Class I: Partially Edentulous Arch with Bilateral Edentulous Areas Posterior to Remaining Natural Teeth

Division A

1. Edentulous areas have abundant bone width (>6 mm), height (>12 mm), and length (>7 mm) for endosteal implant(s).

2. Direction of load is within 30 degrees of implant body axis.

3. Crown height <15 mm.

4. Root form implants and independent prostheses often are indicated.

Division B

1. Edentulous areas have moderate available bone width (2.5 to 6 mm) and at least adequate bone height (>12 mm) and length (> 6 mm).

2. Direction of load is within 30 degrees of implant body axis.

3. Crown height <15 mm.

4. Surgical options include osteoplasty, small-diameter implants, and/or augmentation.

Division C

1. Edentulous areas have inadequate available bone for endosteal implant (or implants) with a predictable result, because of too little bone width (C–w), length, height (C–h), or angulation of load.

2. Crown height >15 mm.

3. Surgical options for C–w include osteoplasty or augmentation; for C–h, options include subperiosteal or disc implants or augmentation.

4. Root forms may be considered with augmentation and/or nerve repositioning.

Division D

1. Edentulous areas have severely resorbed ridges, involving a portion of the basal or cortical supporting bone.

2. Crown height >20 mm.

3. Surgical options usually require augmentation before implants can be inserted.

Note: If the bilateral edentulous areas are not within the same division, then the right side is described first (e.g., Class I, Division A, B).

Class II: Partially Edentulous Arch with Unilateral Edentulous Area Posterior to Remaining Teeth

Divisions A to D (Same as for Class I)

Class III: Partially Edentulous Arch with Unilateral Edentulous Area with Natural Teeth Remaining Anterior and Posterior

Divisions A to D (Same as for Class I)

Class IV: Partially Edentulous Arch with Edentulous Area Anterior to Remaining Natural Teeth and Crossing the Midline

Divisions A to D (Same as for Class I)

Treatment Planning: Class I

In Class I patients, distal edentulous segments are bilateral and natural anterior teeth are present. The majority of these arches are only missing molars, and almost all have retained at least the anterior incisors and canines.¹⁷ Therefore, once restored to proper occlusal vertical dimension, the natural anterior teeth contribute to the distribution of forces throughout the mouth in centric relation occlusion. More importantly, when opposing natural teeth or in fixed implant prosthesis, they also permit excursions during mandibular movement to disclude the posterior implant-supported prostheses and protect them from lateral forces. However, many of these mandibular Class I patients oppose a maxillary denture, in which case bilateral balance is more appropriate.

The Class I patient is more likely to wear a RPD than Class II or III patients because mastication and/or support of an opposing removable prosthesis is more difficult when not wearing a mandibular prosthesis. The posterior soft tissue–supported Class I partial dentures are designed to either primary load the edentulous regions or the natural anterior teeth. The clasp design, which places less force on the tooth (e.g., bar clasp including t, y rpi), will place more force on the bone. The RPDs, which place more force on the abutment teeth (e.g., precision partial dentures), will place less force on the bone. In either case, the removable prosthesis often accelerates the posterior bone loss. In addition, a partial denture that is not well designed or maintained distributes additional loads to abutment teeth and may even contribute to poor periodontal health. The combinations of these conditions lead to bone loss in the edentulous regions and poorer adjacent natural abutments.^18–20 As a result, it is this author’s observation that long-term Class I patients who have been wearing an RPD often exhibit Division C ridges and mobile abutment teeth.

Class I patients often have mobile anterior teeth, because long-term lack of bilateral posterior support caused by the wearing of a poorly fitting RPD, or none at all, has resulted in an overload to the remaining dentition. Therefore these patients often require a posterior implant prosthesis to be independent from the mobile natural teeth. In addition, the occlusal scheme must accommodate the specific conditions of mobile anterior teeth. This requires increased implant support in the posterior segments when compared with most Class II or III patients, as well as greater attention and frequency for occlusal adjustments.

The treatment plan must consider the factors of force previously identified and relate them to the existing bilateral edentulous condition. Osteoplasty cannot be as aggressive in the Class I patient to increase bone width, compared with the Class IV or fully edentulous patient with implants primarily in the anterior regions, because of the opposing anatomical landmarks (maxillary sinus or mandibular canal). Augmentation procedures are often required to improve posterior bone volume, increase the implant surface area, and permit the fabrication of an independent implant restoration.

Financial concerns may require the staging of treatment over years. The posterior region with the greatest volume of bone usually is restored first, if no bone grafting is required. In this manner, implants of greater size and surface area can resist the unilateral posterior forces while the patient awaits future treatment. If many years pass before implants are to be inserted in the lesser available bone, then continued resorption may require augmentation before reconstruction.

If both posterior segments require bone grafting, the patient is encouraged to have both posterior segments augmented at the same time. In this way, the autologous portion of the graft may be harvested and distributed to both posterior regions, decreasing the number of surgical episodes for the patient.

Division A Treatment Plans

When patients are placed in a Class I, Division A category, an independent implant-supported fixed prosthesis is usually indicated. Two or more endosteal root form implants are required to replace molars with independent prostheses. The greater the number of teeth missing, the larger the size and/or number of implants required. Posterior available bone is limited in height by the mandibular canal in the mandible or the maxillary sinus in the maxilla. The first premolar-positioned implants must avoid encroachment on the apex of the canine root and yet avoid the anterior loop of the mandibular canal or maxillary sinus.

Division B Treatment Plans

Class I, Division B patients have narrow bone in posterior edentulous spaces and anterior natural teeth. A fixed prosthesis is also indicated in these categories. Available bone height is restricted by the mandibular canal or maxillary sinus. Therefore osteoplasty to increase bone width has limited applications. Endosteal small-diameter root form implants may be placed in the mandibular posterior Division B edentulous ridge. If narrow-diameter root forms are used, then a greater number than for the Division A ridge is indicated, and the use of one implant for every missing tooth root with no cantilever is recommended.

The patient missing molars and both premolars requires additional implant support. Four Division B root forms may be the foundation of an independent fixed partial denture (FPD) in the mandible, depending on the other stress factors. If stress factors are too great (as a result of parafunction) or bone density is poor (as in the maxilla), then the Division B bone should be augmented to Division A before larger-diameter implant insertion. The anterior teeth in Class I patients should provide disclusion of the posterior implants during all excursions when opposing natural teeth or a fixed prosthesis. Molar endosteal implants should not be rigidly cross-splinted to each other in the Class I patient. Flexure of the mandible during opening may cause a rigid splint to exert lateral forces on the posterior implants. Therefore independent restorations are indicated.

Division C Treatment Plans

When inadequate bone exists in height, width, length, or angulation, or if crown/implant ratios are equal to or greater than 1, the practitioner must consider several options. The first treatment option is to not use implant support, but rather to orient the patient toward a conventional removable partial prosthesis. However, although this condition is easiest to treat with a traditional soft tissue–borne restoration, bone loss will continue and can eventually compromise any restorative modality.

The second option is to use bone augmentation procedures. If the intent of the bone graft is to change a Division C to a Division A or B for endosteal implants, then at least some autogenous bone is indicated. Augmentation is used most often in the Class I maxilla, where sinus grafts with a combination of allografts and autogenous bone are a predictable modality. Implants may be placed after the graft has created a Division A ridge, and the treatment plan follows the options previously addressed.

In the mandible, the third option for the Class I, Division C patient is to place unilateral subperiosteal implants or disc implants above the canal (Figure 19-1). The disc implants support independent posterior fixed prostheses bilaterally.

Figure 19-1 In Class I, Division C, options include small-dimension implants such as disc implants, which can be placed in minimal heights of bone above the mandibular canal. Two independent fixed prostheses are supported by implants.

The fourth treatment option in the mandible is nerve repositioning and endosteal implants in Class I patients who are poor candidates for bone augmentation or subperiosteal implants. Risks of long-term paresthesia exist that may include hyperesthesia and pain. Reports in the literature concern dysesthesia and fracture of the severely atrophic mandible.^21,22 In addition, the gain of height in the C–h mandible may only permit the placement of implants 10 mm high, still insufficient to compensate for the increased crown height and resultant unfavorable crown/implant ratio.

Division D Treatment Plans

Class I, Division D usually occurs most often in the long-term edentulous maxilla. A sinus graft is usually performed before implant placement. Class I, Division D ridges are rarely found in the mandibular partially edentulous patient. When observed, the most common causes are from trauma or surgical excision of neoplasms. These patients often need autogenous bone onlay grafts to improve implant success and prevent pathologic fracture before prosthodontic reconstruction. After the graft is mature and the available bone improved, the patient is evaluated and treated in a manner similar to other patients with favorable bone volume.