Читать книгу Fundamentals of Fixed Prosthodontics - James C. Kessler - Страница 115

Another consideration in the evaluation of prospective abutment teeth is the root surface area, or the area of periodontal ligament attachment of the root to the bone. Larger teeth have a greater surface area and are better able to bear added stress. The areas of the root surfaces of the various teeth have been reported by Jepsen³ and are shown in Figs 7-8 and 7-9. The actual values are not as significant as the relative values within a given mouth and the ratios between the various teeth in one arch. When supporting bone has been lost because of periodontal disease, the involved teeth have a lessened capacity to serve as abutments. Millimeter per millimeter, the loss of periodontal support from root resorption is only one-third to one-half as critical as the loss of alveolar crestal bone.⁴ The planned treatment should take this into account.

Fig 7-8 Comparative root surface areas of maxillary teeth. The number in parentheses above each tooth is the ratio between the root surface area of the respective tooth and the root surface area of the smallest tooth in the arch, the lateral incisor (based on data by Jepsen³).

Fig 7-9 Comparative root surface areas of mandibular teeth. The number in parentheses above each tooth is the ratio between the root surface area of the respective tooth and the root surface area of the smallest tooth in the arch, the central incisor (based on data by Jepsen³).

The length of the pontic span that can be successfully restored is limited in part by the abutment teeth and their ability to accept the additional load. Traditionally, there has been general agreement on the number of missing teeth that can be restored successfully. Tylman stated that two abutment teeth could support two pontics.⁵ In a statement designated as Ante’s Law by Johnston et al,⁶ the root surface area of the abutment teeth had to equal or surpass that of the teeth being replaced with pontics.⁷

According to this premise, one missing tooth can be successfully replaced if the abutment teeth are healthy (Fig 7-10). If two teeth are missing, a fixed partial denture probably can replace the missing teeth, but the limit is being approached (Fig 7-11). When the root surface area of the teeth to be replaced by pontics surpasses that of the abutment teeth, a generally unacceptable situation exists (Fig 7-12).

Fig 7-10 The combined root surface area of the second premolar and the second molar (A_2p + A_2m) is greater than that of the first molar being replaced (A_1m).

Fig 7-11 The combined root surface area of the first premolar and the second molar abutments (A_1p + A_2m) is approximately equal to that of the teeth being replaced (A_2p + A_1m).

Fig 7-12 The combined root surface area of the canine and second molar (A_c + A_2m) is exceeded by that of the teeth being replaced (A_1p + A_2p + A_1m). A fixed partial denture would be a poor choice in this situation.

It is possible for fixed partial dentures to replace more than two teeth, the most common examples being anterior fixed partial dentures replacing the four incisors. Canine to second molar fixed partial dentures also are possible (if all other conditions are ideal) in the maxillary arch, but not as often in the mandibular arch. However, any fixed prosthesis replacing more than two teeth should be considered a high risk.

As a clinical guideline, there is some validity in the Ante’s Law concept. Fixed partial dentures with short pontic spans have a better prognosis than do those with excessively long spans. It would be an oversimplification to attribute this merely to overstressing of the periodontal ligament, however. Failures from abnormal stress have been attributed to leverage and torque rather than overload.¹ Biomechanical factors and material failure play an important role in the potential for failure of long-span restorations.

There is evidence that teeth with very poor periodontal support can serve successfully as fixed partial denture abutments in carefully selected cases. Teeth with severe bone loss and marked mobility have been used as fixed partial denture and splint abutments.⁸ Elimination of mobility is not the goal in such cases but rather the stabilization of the teeth in a status quo to prevent an increase of mobility.⁹

Abutment teeth in these situations can be maintained free of inflammation in the face of mobility if the patients are well motivated and highly proficient in plaque removal.¹⁰ Crowns that anchor rigid prostheses to mobile teeth do require greater retention than do crowns attached to relatively immobile abutments, however.¹¹ Follow-up studies of these patients with so-called terminal dentitions indicate a surprisingly low failure rate—less than 8% of 332 fixed partial dentures exhibited technical failure in a time span that averaged slightly more than 6 years.¹²

What is the impact of the success of this type of treatment on fixed partial dentures for the average patient? The successful restoration of mouths with severe periodontal disease does have significance in everyday practice. It emphasizes the extreme importance of carefully evaluating the strengths and weaknesses of the remaining dentition on an individual basis.

This should not be a signal for every dentist with a handpiece to start using severely periodontally involved teeth as abutments. One should bear in mind that the successful treatments that have been cited are the work of well-trained and highly skilled clinicians on selected, highly motivated patients.

This type of heroic treatment (herodontics, if you will) is very demanding technically and expensive as well. Performed by a well-trained, skilled clinician on an informed, motivated patient who dreads tooth loss, understands the patient’s role in the success of the treatment, and accepts the risk (and expense) of failure, it can be a good service. “Sold” by a practitioner without special qualifications to an unmotivated and ill-informed patient, this type of treatment easily could result in a lawsuit.

Fig 7-13 There is one unit of deflection (x) for a given span length (p).

Fig 7-14 The deflection will be eight times as great (8x) if the span length is doubled (2p).

Fig 7-15 The deflection will be 27 times as great (27x) if the span length is tripled (3p).

Fig 7-16 There is one unit of deflection (x) for a pontic with a given thickness (t).

Fig 7-17 There will be eight times as much deflection (8x) if the thickness is decreased by one-half (t/2).