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INTRODUCTION
The great advances in oral rehabilitation techniques and materials in recent years have enabled a significant evolution of the restorative options that can be suggested to each patient. However, the ability to foresee the survival of abutment teeth and the projection of their performance has not been equally developed, thus limiting the scope of such advancements.
Besides a successful diagnosis, case planning should start with a proper prognosis, so that the restorative options correspond to the most predictable solution in each case. Periodontal articles usually approach the prognosis side of the issue,1)(2)(3) while the restorative literature is scarce in this sense.
In general, the dental literature provides some approaches to classifications isolated by discipline, but not a systematic review of all the factors involved in decision-making, nor a classification guide to preserve or extract natural abutment teeth. As for prosthodontics, references are scarce and the evaluation of abutment teeth is more limited. For example, in estimating abutment teeth, Hobo4 only mentions crown-root proportion, root configuration, and area of periodontal ligament.
The field of prosthodontics has evolved over many years of clinical experience; the problem with this form of professional development is that it is based on anecdotal experience, widely regarded as the weakest form of clinical evidence.5
The objective here is to analyze the risk factors that may affect the period of survival of abutment teeth, in order to guide the process of evidence-based clinical decision-making and, to some extent, to try to answer patients′ frequent questions about durability of their treatments. In any case, it is important to avoid mutilating patients arbitrarily, but at the same time not to promise heroic rescues.
Prognostic factor vs risk factor
In the Glossary of Prosthodontic Terms,6) prognosis is defined as the prediction of the likely outcome of a disease or the course of a therapy. Therefore, we must differentiate risk factors, as those that favor the emergence of oral diseases, from prognostic factors, or characteristics that can predict the outcome once a disease is already set.
The lack of classifications covering all aspects to be evaluated when predicting the future of a abutment tooth in the mouth leads to the use of partial classifications and those coming from other areas different of prosthodontic dentistry. Attempts to classify prognosis in categories have generated terms such as favorable prognosis, unfavorable prognosis and questionable prognosis, establishing periodontal limits that tend to be inexact and ambiguous.
It is therefore appropriate to suggest a more comprehensive assessment of prognosis in an attempt to propose a more suitable prognosis classification for prosthetists or oral rehabilitators, who are the ones ultimately deciding on the permanence of one or several abutments.
For the evaluation of prognostic factors in this review, the following categories have been proposed:
1. Findings in dental and periodontal structures of the abutment tooth:
1.1 Dental integrity
1.2 Periodontal factors
1.3 Endodontic factors
1.4 Occlusal factors
2. General conditions and habits:
2.1 Systemic involvement
2.2 Toxic habits
2.3 Oral hygiene habits
3. Treatments needed for conservation and their implications in prognosis.
4. Type of rehabilitation that the abutment will receive.
5. Strategic weight of abutment during rehabilitation.
Findings in dental and periodontal structures
Dental integrity
Extensive crown destruction is the diagnosis that most frequently requires the elaboration of fixed prostheses. Therefore, the extent of such deterioration is a decisive factor in the selection of rehabilitation treatment, as well as in the assessment of the future of the restoration. The etiology of such attrition is diverse, ranging from dental caries and fracture to previous restorations. Regardless of the etiology, it is necessary to determine the extent of the damage and how it modifies the prognosis of the affected tooth.
To this end, it is necessary to evaluate, and whenever possible to quantify, the destruction or degree of root damage in addition to crown damage. Several parameters should be reviewed at this point. While there is not enough literature concerning prognosis in the restorative aspect, it is widely accepted that the main factor to consider is the amount of remaining tooth structure.
It is difficult to quantify the amount of tooth destruction; it is then necessary to resort to some subjective measures; according to the available evidence, the most documented of all is the ferrule effect
The ferrule effect: Since 1990, the ferrule effect has been defined by Sorensen7 as "360-degree metal crown collar surrounding parallel walls of dentine and extending coronal to the shoulder of the preparation".
This item has been widely studied and is still subject to some controversy. The 2002 study by Pierrisnard et al 8) establishes that greater stress on a tooth with endodontic treatment focuses on cervical, while, according to Ichim et al,9) restorations with no ferrule fail mainly because of decementation and subsequent fracture.
But in the presence of ferrule, post length has a minimal effect on force distribution, and the most important thing is the amount of remaining coronal dentin.10)(11) Some studies, on the other hand, do not conclude that ferrule influences the survival rate of any type of pole 12) nor it is a significant preoperative parameter for the prognosis in molars.13) It should be noted, however, that in the first study the reported ferrule magnitude was inaccurate, and in the second study all the samples had a ferrule of at least 3 mm, questioning the results.
In a recent literature review conducted by Juloski et al,14) the authors concluded that the presence of 1.5 to 2 mm ferrule has a positive effect on the fracture strength of endodontically treated teeth and, in case of fracture, this would be rather favorable; also, the ferrule effect should be sought even in a partial manner when it cannot be achieved in the entire periphery, thus reducing the impact of the post-core system, the cementing agent, and the final restoration on the abutment. The in vitro study by Tan,15) on the other hand, found out that although it is better to have partial ferrule, when it is not uniform (from 0.2 to 2 mm), the mechanical behavior is less adequate than when achieving a homogeneous cervical ferrule of 2 mm, as confirmed by the Zhi-Yue′s study,16) which was also conducted on maxillary central incisors.
Remnant dentin
On the other hand, the preservation of at least one coronal wall significantly reduces the risk of failure. When all coronal walls are lost, the beneficial effect of the 2 mm ferrule is also lost or becomes less relevant.13)(14)(15)(16)(17
It is also important to minimize internal root wear, preventing it to extend further than 1/3 of the mesiodistal or bucco-lingual root diameter, since root structural reliability will be at risks.
Periodontal factors
Bone remnants/reduced periodontium: many periodontal predictors have been evaluated, finding out that the amount of remaining bone is one of the most influential. A 10-year retrospective study on molars with root resection therapy found out that having more than 50% of bone support is a good predictor of remaining roots.18) Despite the favorable results, in most studies on teeth with reduced periodontium19)(20)(21)(22) the choice of abutment teeth is still one of the most critical determinants for the outcomes of restorations.
Dentists commonly increase the number of ferruled abutment teeth but there is no biological or scientific support for such clinical behavior; on the contrary, this decision increases biological and technical risks. Given the impossibility of establishing a predictable mathematical rule (Ante′s Law/ quantification of periodontal ligament cover), both clinical judgement and careful selection of the case become more important, evaluating other factors besides the amount of remaining periodontium.23
In the study by Ghiai and Bissada,24) tooth mobility was the most important predictor regardless of tooth under treatment, while the works by McGuire and Nunn 25)(26) evaluated 100 periodontal patients under treatment and in a maintenance program (2,509 teeth), in order to assess the evolution of prognosis according to several proposed parameters. They found out that smoking, initial probing depth, initial root shape, initial furcation involvement, endodontic involvement, tooth malposition, and lack of use of dental protectors in patients with parafunction, had significant influence in the worsening of the prognosis to 5 years. In addition, bone loss percentage, parafunctional habits, and diabetes were marginally associated with the probability of a tooth worsening in 5 years.25)(26
It is also important to consider tooth type before establishing prognosis, since uniradicular teeth respond better to therapy, and therefore their prognosis will be better than that of multiradicular teeth.27) These differences are influenced by the difficulty in accessing and the degree of involvement of furcation areas. The severity of the involvement of bifurcations, as well as their location in the arch and in the affected molar directly influence the success of therapy and therefore the abutment tooth prognosis.
Endodontic factors
In a recent study,28) the cumulative 4-year survival rate was similar among teeth receiving treatment for the first time and those with retreatment (95.4% vs 95.3, respectively). The presence of preoperative pain was an important factor, especially in those that were lost in the short term; apical permeability reduced tooth loss and the extrusion of gutta-percha had no great significance in the first 22 months, although it did later.
The same authors also evaluated periapical response and the associated prognostic factors, by quantifying roots with complete apical healing after new and repeated endodontic treatment,29) finding out that complete apical healing occurred in a similar proportion in both first-time and retreated root canals. This suggests that the endodontic retreatment itself does not worsens prognosis, with the exception of other technical considerations.
The main causes of endodontic failure and therefore decreased prognosis of abutment include non-treated root canal anatomies and the increasing presence of second mesiobuccal canals compared to initial treatment.30)(31) Therefore, prognosis will be proportional to the ability to remove any object or obstruction, and the possibility of completely cleaning up root systems. Teeth retreated for large apical lesions usually have worse prognoses.32)(33
When an endodontically treated teeth needs to be retreated, one of the decisive prognosis factors is the fact that the first treatment respected root anatomy. This relationship is inversely proportional to up to two times the results of the new treatment.34
The prognosis of an endodontic retreatment improves if the cause of repetition is related to obturation errors or to the sealing method; consequently, the endodontic prognosis will be linked to the real possibility of restorative or prosthetic sealing.35
The 1991 study by Vire 36) assessed the causes of failure in 116 endodontically treated teeth that were extracted during a one-year period in a private clinic. Only 8.6% of failures were of endodontic origin, the most frequent of all being failure by crown fracture in uncrowned teeth, and periodontal failure (59.4 and 32%, respectively). However, failures of endodontic origin happened sooner than the others in all cases.
The 2011 prospective study by Ng 29) concludes that, after evaluating 11 endodontic factors, some elements are more determinants than others for tooth prognosis. Therefore, absence of apical lesions, smaller apical lesions, absence of fistula, no material surpassing, good obturation length and irrigation with EDTA and then with NaOcl, presence of satisfactory final restoration, and not using temporary material, all can significantly contribute to improving the prognosis of a abutment tooth.
Occlusal factors
One of the factors to be considered in prognosis is occlusion. It is important to note that, beyond the individual conditions of each tooth structure, they should work as part of the dynamics of the entire masticatory system.
Measuring masticatory force can provide useful data for the evaluation of the function of mandibular muscles and their activity. It is also useful in assessing the performance of artificial dentures.37) In terms of facial morphology, variations in the magnitude of the masticatory forces and the impact of occlusion in the performance of restorations should be considered. Patients with more vertical rami and more acute goniac angles have higher masticatory force. It is almost a consensus that patients with elongated faces have lower masticatory force. In addition, patients with short faces have thicker masseters associated with stronger bite.38)(39)(40) From these studies we can conclude that patients with short faces develop higher masticatory force, which could influence the performance of restorations installed in these patients.
The studies by Zivko-Babic et al 41) confirm the findings of other authors that forces in the molar area are considerably higher than in the anterior area and that men usually have more masticatory force.
Another important occlusal aspect is related to the loss of posterior support and the influence of it on remaining teeth. It can be stated that the fewer functional anterior pairs, the greater the gaps between teeth, the contacts in anterior teeth, and the overbite.42
On the other hand, the prognosis of an endodontically treated tooth is worse in a partially edentulous arch when compared to the same tooth in a fully dentate arch.43
As for the occlusal schemes, the occlusal scheme design has evolved thanks to clinical experience and there is no evidence indicating that a given design is notably superior to others. The available evidence suggests that complex neurophysiological mechanisms allow the muscular system to accommodate to oral and dental changes44 as well as to other emerging conditions.
General conditions and habits
Systemic involvement
In terms of systemic aspects, some factors are more related than others with the prognosis of teeth in the mouth. Diabetes mellitus, for example, is related to periodontal disease in a bi-directional way, so that more teeth are lost in diabetic patients as a result of a periodontal status affected under such metabolic condition. In this same sense, however, a constant relationship between type-2 diabetes and tooth decay has not been demonstrated.45
Toxic habits
In order to refer to the influence of toxic habits on the prognosis of teeth, we take the habit of cigarette smoking as a reference since it is the most frequent and influential factor, though not the only one.
A study that assessed 100 periodontally treated patients (2,484 teeth) with maintenance for 5 years 25) found out that smoking decreased the likelihood of improvement in 60% and doubled the likelihood of worsening the prognosis in 5 years.
Heavy smokers experienced more tooth loss compared with those who had not smoked. Tobacco smoking, the amounts of packs smoked per day, the years the patient has been smoking, and the years that have passed since quitting the habit were significantly associated with tooth loss due to its relationship to periodontal disease.46
Oral hygiene habits
Patients with high risk of caries (Caries Management by Risk Assessment, CAMBRA) should be carefully rehabilitated. Although the risk of recurrence of caries and tooth loss associated with these patients cannot be accurately predicted, it should be considered a sign of alarm for the prognosis of rehabilitation.
The study by Frisk et al 47) in a population sample from Sweden strongly suggests the hypothesis of significant association between endodontically treated teeth (which often receive prostheses) and recurrent tooth decay.
Finally, the patient′s ability and interest in oral hygiene is an important overall factor in the prognosis. The negative influence of not using dental floss and the habit of cigarette are decisive in tooth survival.48)(49
Treatments to preserve abutment teeth and their implications for prognosis
After assessing the existing conditions, there should be some consideration on the procedures that are needed in order to conserve abutment in the mouth. Firstly for economic reasons, and in the second place because the favorable prognosis of abutment teeth may stop being that favorable after some procedures, ranging from gingivectomies to surgeries of bone resection, amputations, and root resections.
If the necessary procedures include amputation, hemisection, or bone resection, the degree of root fusion and the amount of bone remnants must be considered.50)(51
The amount of tooth structure exposed above the crest following surgical crown lengthening must be of 4 mm, which is enough to provide a stable dentogingival bond and appropriate biological width to allow tooth preparation and the establishment of an end line.52) This may be too much for some abutment, so the root length of some teeth in the mouth as well as root trunks must considered.
Orthodontic extrusion for crown lengthening is the ideal option for esthetic reasons, but it has the same biomechanical effect on the optimal or ideal minimum crown-root proportion.4) In this sense, 5.0 mm of bone loss significantly increases the concentration of stress and deformation of root dentin;53 therefore, the crown-root ratio is an important prognostic factor for both the pillar tooth attached to the bone resection and for adjacent teeth that will be affected with this procedure.
Now, root trunks must be taken into account in the case of multirradicular teeth. While one tooth with multiple roots well anchored in bone and differing among themselves can better resist bone loss associated with surgical crown lengthening, there must be special consideration in order to avoid involvement of any furca that did not exist at the beginning or to avoid worsening an already existing furca. It is worth remembering the types of root trunks identified as A, B, and C according trunk′s radio and root lengths. The study by Hou and Tsai 54) shows some predictions of these root trunks depending on tooth type. Their findings after evaluating 166 maxillary molars and 200 mandibular molars are summarized like this:
1. Short root trunks are commonly found in buccal and the long ones in mesial, both in first and second maxillary molars.
2. In mandibular molars, the shortest trunks are in buccal, while the longest ones are in lingual.
3. In general, root trunks were generally bigger in second molars compared with the first ones.
In addition to considering the prognosis risk involved in the osteotomy needed to amputate or resection roots (see root trunk) plus the osteotomy needed to restore ferrule and biological width (see above), we must consider the prognosis for the implant in case of failing of the therapy to preserve natural tooth, now in an affected bone environment.
Type of rehabilitation
In addition to the types of assessment already described, clinicians must take into account the type of rehabilitation the abutment is receiving. The prognosis of a abutment tooth and its rehabilitation are closely connected. The most favorable force distribution for the periodontium of abutment teeth is a fixed prosthesis, recommended as the best option of rehabilitation in teeth with reduced preiodontium.55
The 2013 longitudinal study by Tada et al56 about the prognostic factors affecting the survival period of abutment teeth of removable partial denture evaluated 147 patients provided with 236 new RPD (846 analyzed abutment), finding out that the survival rate of direct, indirect and no abutment teeth to 5 years was 86.6, 93.1 and 95.8% respectively. Following a multivariate analysis, the study concluded that the highest tooth loss risk is for remnant teeth used for RPD, as confirmed by other studies.57
Endodontically treated teeth are at high risk when used as the primary abutment of distal extension removable partial prosthesis, with 4 times more probabilities of failing than the same teeth not serving as abutment for dentures with a free-end saddle.58
On the other hand, if the abutment tooth is going to be added a single crown or a bridge, this is going to affect oral hygiene. It has been shown that abutment for single crowns have better performance concerning plaque accumulation.59) If in addition the compromised teeth are in the posterior area, it can be even more difficult to achieve efficient hygiene.
Value or strategic weight of abutment
When prognosis is favorable, it is an ideal situation and does not require so much thinking. But when the prognosis is not that good, the strategic weight of the pillar should be considered in the full scheme.
When a abutment tooth is really strategic for rehabilitation of the entire mouth, reasonably we should try to keep it.
For example, it may be essential to conserve a abutment to avoid free end-saddles. Patients with removable free end-saddles tend to experience more abutment loss, and the survival period is even shorter when the free-end saddle is bilateral.56) Other situations may be considered, such as avoiding intermediate abutment affecting the passivity of restorations, as well as long fixed restorations with pontics having more than 2 units.
If the conservation of a questionable abutment puts the restoration at risk, its permanence (though always desirable) is not essential in the design, so extracting it does not unfavorably alter the treatment strategy and its extraction should be immediately considered.
Some of the considerations and questions that we must ask ourselves before making the best decision in removing or retaining teeth includes: will the teeth be beneficial or detrimental to the planned prosthesis? Will they decrease or increase the long-term costs? Will they decrease or increase the complexity of the restoration? Will they reduce visits and reviews or impose additional dental treatments to patient?60
On the other hand, the WHO established as a goal for the year 2000 that people aged 80 years should have at least 20 teeth, which is considered enough for the adult patient to eat and enjoy meals with a wide range of foods.61) The literature even mentions the conservation of bicuspids (referred to as the ultimate occlusal preservation goal) in adult patients, as well as short arches as an option.62)(63) All teeth should rationally be conserved as much as possible but the short arch should also be considered up to the second bicuspids as a treatment option when the most posterior teeth negatively affect the aforementioned considerations and the risk of conservation is greater than the benefit.
Some authors have suggested an organization chart to group all these prognostic factors classifying them by levels, including severity of periodontal disease, restorative factors, and furcas involvement, to name just a few.64) However, each factor seems to be equal to all the others and the decision to remove or conserve is again subjective. In oral rehabilitation, we must review all of them and, if possible, evaluate them in importance and hierarchical levels.
Considerations and analyses may vary for each particular case, but the most important thing is not to ignore unchangeable prognostic factors that threaten the longevity of restorations and the pillar teeth themselves.
SUGGESTIONS
Further studies are needed in order to establish a categorization of prognostic factors based on levels of association with tooth survival, from standardized and well-controlled clinical studies.
For full mouth rehabilitations, it is recommended to use implants in a strategic way, to plan shorter bridges each time, either supported by natural teeth or by implants, and to only include abutment with good prognosis for long fixed prostheses.65) The study by Mish 66) on 2,98 natural teeth adjacent to 1,377 implants showed minimal complications in these natural teeth and a 10-year survival.
CONCLUSIONS
It is important to accurately guide decision-making in order to minimize failure risks and to solidly base success on a prior review of influencing factors. That way, the purely instrumental part-often challenging-is the technical completion of a series of responsible previous analysis that are consistent with the outcome expected by the patient.
More information is needed on the proportional value of each factor, or whether they all have the same percentage in decision-making.
The answer to the question of whether or not preserving a abutment tooth must take into account measuring the extension of tooth damage in all its aspects, questioning the necessary procedures to keep it, along with impacts for patient in terms of costs, time and morbidity, and the high or low strategic value of the tooth in question. This process can be done in this order of steps, or even start from the latter, i.e. to ask first for the degree of strategic value before starting procedures and creating costs, especially if the equation includes the new role that the pillar tooth will play in the proposed rehabilitation.
Re-prognostication: this name can be given to the process needed after the initial corrective period, in order to evaluate the prognosis once again after initial therapy on the abutment tooth and the response to the endodontic, surgical, and periodontal treatments. A second evaluation period following the initial corrections would be a useful suggestion and must be constantly made since it is a dynamic process.
Abutment teeth with no cervical ferrule or in which it cannot be properly achieved to allow a bracket effect on remnant tooth should be reconsidered as a support for definitive restorations.
The quantity and quality of remnant tooth, periodontum status and the quality of the endodontic treatment should be carefully estimated before predicting the performance and survival of a abutment tooth from the biological point of view.
From the mechanical point of view, occlusion can lead to failed restorations on abutment teeth with acceptable dental remnants.
