Partially or completely edentulous patients with a class II jaw relationship often present unique skeletal relationships with challenging prosthetic problems not evident in other types of malocclusions.^1,2  An esthetic and functional occlusion for these patients is often difficult to achieve because of anatomic discrepancies limiting space for occlusal contact, steep guidance factors which can introduce occlusal interferences, and excessive range of motion which can complicate reproducible centric relation and vertical dimension records.  The purpose of this paper is to provide background information and treatment guidelines to facilitate improved prosthetic treatment for partially and completely edentulous skeletal class II patients.

Incidence and Classification

The incidence of class II malocclusions is debatable due to of the lack of agreement on what constitutes a malocclusion.³  Angle's classification system has often been used to classify patients and is based on the horizontal relationship of the first molar, which may or may not be a true indicator of the skeletal relationship.⁴  Cephalometric analysis is a more accurate method to determine which components of the dental and/or skeletal systems are most responsible for creating the malocclusion.  The maxillae and/or mandible may have horizontal and/or vertical growth characteristics that translate into size and positional discrepancies of the teeth, the jaws, or both.  The sagittal skeletal relationship is commonly described by the position of the maxilla and mandible in relation to the cranial base and will tell us if skeletally the jaws are in a class I, II, or III relationship.

The vertical skeletal relationship is often described by the Frankfort Mandibular Plane Angle (FMA) (Fig 1).

Figure 1: Cephalometric landmarks used in FMA determination: Gonion (Go), point on angle of jaw that is most inferiorly, posteriorly, and outwardly directed Menton (Me), lowest point of contour of mandible symphysis Orbitale (Or), lowest point on margin of orbit Porion (P), midpoint on upper edge of external auditory meatus Frankfort horizontal plane, plane intersecting right and left poria and left orbitale.

If the appositional growth of the alveolar processes and facial sutures exceeds the vertical increases from the mandibular condyles, the mandible will rotate posteriorly resulting in a high FMA.  Conversely, if vertical growth at the condyles exceeds the sum of vertical growth components from the facial sutures and alveolar processes, the mandible will rotate anteriorly resulting in a low FMA.  The direction and magnitude of rotation greatly influences prosthodontic treatment, and has been reviewed by DiPietro and Moegelli. ^4,5 (Table I).

Although investigators have described six horizontal and five vertical sub-classifications of skeletal class II patients, we will focus on treatment considerations for the two most common vertical groups, which are being those Class II individuals with a high or low FMA.

Clinical Findings

High Frankfort Mandibular Angle
A high FMA skeletal relationship develops when the anterior components of vertical growth are proportionally greater than condylar growth.  This results in a steeper diagonal growth pattern with anterior face height significantly greater than posterior face height.  The mandible tends to be retrognathic with the maxilla in a near normal position anteroposteriorly.  The maxillae are narrow with well-formed arches, deep vestibules, and a high palatal vault with limited bony palatal bearing area (Figs. 2a-e).

Figure 2: Clinical example of patient with high FMA
a. Profile shows typical short upper lip.	b. Convex profile.
c. Well formed but narrow maxillary ridge.	d. Excessive horizontal overlap.
e. Prominent display of teeth with slight smile.

The upper lip often appears short and the smile line high with considerable display of the incisor teeth and gingival tissue.  Lip seal is difficult to obtain and the lower lip often exhibits a high level of mentalis activity.  These patients are dolichocephalic (long head) and often are described as having the “long-face syndrome” with convex profiles.  The occlusal, palatal, and mandibular planes are divergent with a significant amount of interocclusal distance existing at rest, which tends to accentuate the long-face syndrome.  Furthermore, these patients generally present with more hypotonic muscles of mastication resulting in decreased bite force.¹¹

The retrognathic mandible is generally small and tapering.  If teeth are present, the lateral profile usually exhibits an exaggerated curve of Spee, caused in part by the supra-eruption of the mandibular incisors and the diagonal upward turn of the mandibular ramus.  An occlusal view may demonstrate the "keyhole effect"; the maxillary and mandibular teeth and arches are narrower between the premolars as compared with the distance between the canines, giving the arches the form of a keyhole.

Due to the prognathic-retrognathic skeletal relationship, the mediolateral discrepancy is accentuated in the premolar region; a wider segment of the maxillae must function with the narrower aspects of the more posterior positioned mandible.  Because class II patients with high FMA's tend to have narrower maxillary arches, the mediolateral discrepancy is less severe.  However, it is not uncommon for the maxillary arch to be completely anterior and lateral to the mandible in edentulous patients with low FMA's.

Low Frankfort Mandibular Angle
A low FMA skeletal relationship develops when the anterior components of vertical growth are proportionally less than condylar growth.  In the dentate population, it is most commonly noted in patients with a Class II division 2 malocclusion (Figs. 3a-e).

Figure 3: Clinical example of patient with low FMA
a. Profile shows typical long upper lip.	b. With relatively flat and nearly parallel occlusal plane.
c. With maxillary ridges that are well formed, flat, and tapering with shallow vestibules.	d. Having excessive horizontal overlap.
e. and little display of upper teeth in a smile.

Patients with a low FMA are brachycephalic (short head) and present with a skeletal deep bite and a less convex facial profile.  Overclosure of the mandible often results in an averted lower lip and deep labiomental sulcus.  The mandibular plane, occlusal plane, and palatal plane are relatively flat and nearly parallel compared with high-FMA profiles.  The ridges are well formed, flat, and tapering with shallow vestibules.  The palatal shelves have a gentle taper leading to a broad, flat palatal bearing area. The mandible is well formed and less likely to exhibit the keyhole effect.  The upper lip appears long, limiting the display of teeth and gingivae.  The curve of Spee is often accentuated due to lack of incisal contact leading to supra eruption and a deep vertical overlap of the anterior teeth.¹²

More residual ridge resorption may be evident in patients with a low FMA.⁷  Tallgren reported considerably more alveolar bone loss for edentulous patients with a low FMA over a 25 year period.  Because patients with low FMA's often have well-formed mandibles with limited vertical bony height and shallow vestibules, residual ridge resorption (RRR) can be clinically significant with time.  A contributing factor to RRR could be the potentially greater and more vertically directed forces of mandibular closure seen in skeletal class II patients with a low FMA.

Treatment Guidelines

Four basic occlusal concepts are important for most removable prosthodontic patients but are essential for edentulous skeletal Class II patients.

1. Centric relation must be recorded and used as a reference position to relate the mandible to the maxilla

2. the mandibular posterior denture teeth must be placed over the lower residual ridge

3. freedom of movement must be provided during eccentric movements

4. multiple occlusal contacts must be established in centric relation and in eccentric positions.

Recording centric relation can be difficult for skeletal class II patients, especially patients with a high FMA.^8,9  This difficulty may be the result of the more acute direction of muscular contraction during closure of the mandible or because skeletal class II patients often function considerably forward from centric relation.  Directing the patient to position their tongue posteriorly while completing a centric relation record will facilitate the recording of a more retruded position of the mandible.

The mandibular posterior denture teeth should be positioned over the edentulous ridge, and adjustments made with the maxillary occlusal table.^1,10  Some type of tooth or resin veneer may be necessary to extend the occlusal table either facially (for esthetics) or lingually (for function).  Some authors advocate the use of palatal-lingual ramps or a buccal facade of tooth-colored acrylic resin added to the maxillary posterior denture teeth to increase the occlusal table palatally or to improve esthetics by establishing proper lip support .⁹  (Figs. 4 a,b and 5 a,b )

Figure 4:

a.

Clinical example of a skeletal class II patient with the typical pattern of excessive vertical overlap of the anterior dentition.  The goal of multiple eccentric contacts is best accomplished by using anatomic teeth, a steep occlual plane, a more pronounced curve of spee, and the possible use of a balancing ramp.

b.

Same patient in a protrusive movement.  Despite the use of anatomic teeth and a steep occlusal plane, a balancing ramp on the lower denture was necessary for protrusive balance.

Figure 5:

a.

Clinical example of skeletal class II patient with a microg- nathic and retruded mandibular arch where modification to the maxillary teeth was completed to provide improved esthetics and function.

b.

Maxillary veneers were added to the labial of functionality positioned lingual teeth to improve esthetics.

The authors suggest the use of a lingualized occlusal scheme with anatomic teeth in the maxilla, and employing a compensating curve in harmony with the recorded condylar inclination.  A steep condylar guidance, as most high FMA patients will have, will require an increased upturn of the compensating curve in the molar region.  A compensating curve and cuspal inclinations facilitate the objective of multiple occlusal contacts, especially in protrusive balance.  Although the use of non-anatomic teeth arranged in a flat plane allows excellent freedom of movement, multiple occlusal contacts in eccentric movement are compromised, especially in protrusive balance.

Multiple occlusal contacts is an important goal with class II patients.²  At rest, most skeletal class II patients will posture the mandible in a more anterior position, primarily because of esthetics.  Rest position is an intra border position and voluntary closure to occlusal contact is often into centric occlusion.  Positions other than centric relation are usually not repeatable if the patient is edentulous in either arch and tooth guidance has been lost.  Therefore, multiple occlusal contacts are essential both in centric relation and eccentric positions for the stability and comfort of the prostheses.  This is especially important for skeletal class II patients with a low FMA.  The mandibular molars of these patients are usually in line with the force of contraction of the muscles of mastication.  Comfort with the prostheses can be a problem because of limited aligned space available for support and the potential for more forceful closure.

Summary

Patients with a class II relationship due to a skeletal etiology comprise approximately 15% of the population with the two largest sub-categories being high and low FMA groups.  Occlusal relationships in skeletal class II patients are often challenging because of arch size discrepancies which limits the potential occlusal contact area, steep guidance factors which can introduce occlusal interferences, and excessive range of motion which can complicate centric relation records.  Our purpose has been to provide background and treatment suggestions to improve the management of complete or partially edentulous patients with a skeletal class II skeletal jaw relationship.