ANB
Class III (33)
2
16
15
Class I (263)
83
145
35
Class II (201)
133
67
1
Class II (218)
Class I (228)
Class III (51)
Wits
ANB
Class III (33)
0
24
9
Class I (263)
41
197
25
Class II (201)
114
87
0
Class II (155)
Class I (308)
Class III (34)
Visual
Wits
Class III (51)
2
32
17
Class I (228)
29
184
15
Class II (218)
124
92
2
Class II (155)
Class I (308)
Class III (34)
Visual
While it is frequently impossible to assign young patients to a specific Angle class, the skeletal relations will aggravate later in life, which is particularly true of Class II patients. Patients with a pronounced distoclusion will develop clinical features that can be easily identified, and, more importantly, they will develop an unequivocal functional pattern. The points raised in this chapter are primarily based on our experience with typical Class II patients who either already presented with characteristic functional patterns before treatment or in whom such a functional pattern tended to become manifest as a result of our treatment.
Angle distinguished between two subcategories of Class II relations. Class II division 1 (II/1) is characterized by a pointed mandibular front; Class II division 2 (II/2) is characterized by cover-bite and deep-bite relations. Both types of malocclusion share the feature of relative mandibular retrusion, and both are considered to be skeletal in nature. Most patients with mandibular retrusion will develop a strictly anterior masticatory pattern. Particularly after the supporting anterior contacts have been lost, the vertical masticatory forces in the anterior segment can no longer be controlled in an adequate manner, so that the posterior segments are increasingly subjected to uncontrolled masticatory loads. This situation is usually associated with inadequate lateral chewing, particularly after the bite has deepened, a considerable anterior overbite has developed, and any laterotrusive movements are definitively prevented by elongated canines. Patients with a narrow maxillary base and pointed front may subsequently develop a functionally open bite or end-to-end occlusion, especially if the maxilla lags behind in terms of growth following any extractions for orthodontic reasons.
Any restorative treatment strategies based on BOI implants will be largely identical even in the presence of different skeletal situations. It is irrelevant, for example, whether the malocclusion is caused by a retruded but otherwise properly developed mandible, or by mandibular micrognathia, or by any combination of both conditions. We have rarely observed anterior masticatory patterns in cases of maxillary macrognathia characterized by large SNA angles, as long as the support zones were still present.
13.2 Development of the Masticatory Pattern
Deep anterior overbites are primary anomalies. The movement relations in these patients can be summarized as “primary articulation problems” (Thielmann 1938). The masticatory pattern plays a major role in the aetiology of these conditions. Since the masticatory pattern is not visible externally, the individual is left without any social feedback. There are no external or internal control mechanisms that would help him or her to maintain or recover a balanced masticatory function. Moreover, masticatory imbalances have a way of deteriorating rather than improving. Unequal utilization will reinforce the propensity for asymmetry and vice versa (see Chapter 10 for details).
Anterior masticatory patterns in the absence of lateral grinding movements have a way of deflecting the mandible in a sagittal direction, with compressive and tensile zones dominating in the caudal and crestal portions of the posterior segment, respectively. Hylander demonstrated that anterior masticatory patterns dominated by chopping movements give rise to a large vertical dimension of the distal mandible, which is actually useless, considering that most of the chewing takes place in the anterior segment. This configuration greatly promotes tooth loss in Class II patients, which will be discussed in the following paragraphs, starting with the mandible:
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Deep-bite and cover-bite situations involve substantially longer movement paths for mouth opening than regular overbite situations. The mouth opening forces are greater, and the contact points are relaxed during caudal flexion of the mandible. These components favour the development of periodontitis.
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Due to the excessively anterior focus of loading during eating, more massive trajectories have to be developed from the jaw angle towards the anterior segment. Biomechanically ideal macrotrajectories cannot be established due to the large crestal width of the teeth. The greater the tendency of the mandible to become deflected by anterior loading, the greater the tendency of the white line to become caudalized. A deep line of load transmission will be associated with narrower segments of the interfering roots, thus greatly facilitating an ideal orientation of the macrotrajectories. In this way, the local bone density has to increase as the only way to compensate for the forces in an economical manner if the bone volume remains constant.
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As a result, the degree of bone mineralization will increase around the basal surfaces and decrease around the crestal surfaces. In other words, the basal integration of teeth is reinforced.
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The crestal bone will become mechanically less resistant despite the large relative surface of the roots. (The term “alveolar bone” is ultimately not applicable because functional stimuli may convert some zones of this structure to statically supporting bone areas).
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The locally inactivated bone will have a smaller remineralization tendency and fewer defences to bacterial attacks, which will further promote the development of periodontitis and potentially subsequent peri-implantitis.
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If suitable conditions for anterior mastication are present, they will be used. The orbicularis oris muscle will be gaining the upper hand relative to the distal masticatory muscles. The mandible, which normally “dances” between the distal and anterior muscles, will develop a strong tendency to advance forward. This movement pattern can also be observed in patients who do not reveal Class II relations but have undergone functional reorientation after tooth loss or wear a denture and experience this functional pattern to be the most convenient one.
Whether or not a partial denture is present, the distal portion the mandibular ridge, whose vertical volume will usually have been reduced by periodontal damage, can flatten further due to resorption or be reduced to a narrow medial knife-edge structure. Both scenarios may emerge once the masticatory activity has been relocated to the anterior segment, regardless of whether the patient wears a denture. The narrow and high medial residual ridge will be reached after the partial denture has been relined or refabricated two or three times and will stabilize the denture laterally, particularly if posterior masticatory patterns are still present and the vertical bone structure is still maintained by lateral pressure related to the denture. The absence of tensile trajectories on the crestal bone of the working side will give rise to flat resorption.
The mandibular ramus will reveal whether the masticatory forces at work are strong or weak. A markedly rectangular jaw angle (Go small) usually indicates the presence of rather powerful masticatory forces. These patients are susceptible to clenching and bruxing, and the attachments of the masseter muscle are clearly visible on the cranial radiograph and OPG.
If the residual ridge is high and narrow on one side of the mandible but vertically resorbed on the other side, the patient’s dental history will usually reveal extended periods of unilateral masticatory loads, which may cause problems in adjusting the masticatory function. Corresponding alterations of the TMJ will be observable in this situation as well.
For a patient to wear a poorly retained denture in the presence of residual anterior teeth and of bone loss takes considerable self-contempt and sufferance. These patients will gradually lose the ability to reliably adjust their TMJs and to correctly judge the force as well as the uniformity and symmetry of their masticatory function. Unlike in Class III patients, the term “hinge axis” is clearly a misnomer in these patients, as the range of motion of both TMJs is considerable and dominated by muscles.
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