Biological integration of restorative treatments

Introduction

The quest for long-term success in restoring compromised dental elements has never been as coveted as in this era. New materials and techniques, the everyday growing use of magnification systems for the dental team and the expanded knowledge of the biology of the dento-gingival complex allow dentists and prosthodontists to push forward the long-term predictability of treatments.

To fulfill current patient's expectations we must be able to fabricate biologically integrated prosthetic reconstructions which « disappear » once cemented in between the natural dental elements. Periodontal health is an obvious prerequisite to the delivery of any final dental reconstruction.

In the search to achieve long-term predictable results, it is mandatory to create restorations designed and placed with the utmost respect of the gingival tissues.

Prosthetic precision and location of the margins

The position of the restorative margins in relation to the periodontal tissues has been the object of extensive research (^{Waerhaug, 1953} ; ^{Silness, 1970} ; ^{Newcomb, 1974}). As early as ^{1953, Waerhaug} reported a histological evaluation on periodontal tissue reactions in the presence of prosthetic margins placed within the subgingival environment. The importance of obtaining precise margins became immediately apparent : the zinc phosphate cement traditionally employed for permanent cementation of dental prosthesis, due to its high porosity and solubility, presented an ideal environment for bacterial growth and gingival tissue reactions were promptly observed.

Obviously a smaller gap between the prosthetic margins and the prepared finishing line results in less cement exposure to the periodontal environment. The subgingival placement of a margin, necessary for esthetic purposes, was reported to be directly related with an increase in the gingival index (^{Silness and Ohm, 1974} ; ^{Valderhaug and Birkeland, 1976} ; ^{Valderhaug and Heloe, 1977}).

In order to avoid this complication, it was recommended to keep the margins supragingival whenever possible.

In the last decades, new cements have been developed and advocated for crown cementation and laminate bonding, at the same time the precision of the restoration/tooth margin has improved enormously.

Unfortunately the information available on tissue responses to these new cements is scarce.

With the evolution of new prosthetic materials and with improved capabilities in obtaining precise marginal fits, the margin location has been revisited and the preexisting conclusions have been partially modified. In 1990 a and b, a retrospective study published by Carnevale et al. reanalyzed the influence of the position of the prosthetic margin on the gingival index. Out of the 510 crowns that they examined 64.8 % presented with margins placed 1-2 mm subgingival. In spite of the subgingival location of the finish lines, 76.6 % of the restorations presented with a GI value equal to 0, 22.3 % had a GI value of 1 and only 1.1 % presented with a GI value of 2. The conclusions to be drawn is that the precision of the marginal fit (^{Fuzzi and Carnevale, 1997}) seems more important that its location (^{Lang et al., 1983}) (^{fig. 1}, ² and ³) .

Biological width

Whenever the margins of the restorations extend subgingivally to overcome an affected area or to increase the crowns retention, the risk of violating the biological width (^{Cohen, 1962}) of the tooth is run.

Defined as the sum of the junctional epithelium and the supracrestal connective tissue attachment, this space was measured initially by ^{Gargiulo et al., in 1961}, who reported an average value of 2.04 mm. In ^{1994, Vacek et al.} repeated Gargiulo's study on human autopsy specimens, the value they obtained was slightly inferior to the previous one. Both the authors though agreed on the superior variability of the epithelium compared to the connective tissue. In Vacek's paper, one of the most important findings shows how the biological width slightly differs in value based on the position of the tooth within the arch.

In cases where the restoration margins end up impinging on the biological width, periodontal inflammatory reactions have been reported (^{Parma-Benfenati et al., 1985} ; ^{Tarnow et al., 1986} ; ^{Tal et al., 1989} ; ^{Gunay et al., 2000}) (^{fig. 4}).

The obvious clinical dilemma that often arises is : where can crown margins be placed without violating the biological width ? Some authors (^{Ingber et al., 1977}) have advocated a 3 mm distance of sound tooth structure between the crest of bone and the prosthetic margins to ensure long term success to the restoration (^{fig. 5} and ⁶). This would grant enough space for the existence of junctional epithelium, supracrestal connective tissue fibers and about 1 mm of gingival sulcus. Other authors preferred to have 4 mm or more of space to work with (^{Rosenberg et al., 1980} ; ^{Wagenberg et al., 1989} ; ^{Becker et al., 1998}).

Smukler and Chaibi, in 1997, theorized on how a variable genetically predetermined supracrestal gingival tissue will reform after surgical excision (^{fig. 7} and ⁸). The amount of this regrowth seemed to be dependent on the underlying anatomy of the teeth and the bone, and could vary significantly from site to site. This coronal growth of the marginal periodontal tissues after surgical crown lengthening was also demonstrated in a recent clinical study by ^{Pontoriero and Carnevale in 2001}, where different average values of tissue regrowth were observed, when interproximal or buccal/lingual sites were respectively measured.

In the 1953 study, Waerhaug reported on how margins positioned 0,4 mm coronal to the bottom of the sulcus, and not impinging on the connective tissue fibers, showed no signs of inflammation at the microscopic level. From the data of the above mentioned reports it appears how dentists today should not rely only upon a standardized average value of biological width to be used as a guide in determining the location of the prosthetic margins.

Clinical considerations

Several factors need to be kept in mind in restoring or respecting the biological width of the area which needs to be prosthetically rehabilitated. Among these the anatomy of the area (^{Gargiulo et al., 1961}) and its biotype (^{Olsson and Lindhe, 1991} ; ^{Olsson et al., 1993} ; ^{Becker et al., 1997} ; ^{Weisgold et al., 1997}), the location of the furcations whenever present (^{Mardam-Bey et al., 1991} ; ^{Dibart et al.}, submitted for publication) and the amount of the residual tooth structure should be considered (^{fig. 9}, ¹⁰ and ¹¹) .

It is important to understand the esthetic limitations that may result when using a surgical crown lengthening in the anterior sextants of the mouth (^{Tarnow et al., 1992}). To avoid these complications, alternative modalities of treatment such as orthodontic forced eruption may be suggested (^{Ingber, 1976} ; ^{Pontoriero et al., 1987}).

The anatomy of periodontal tissues (soft and hard tissues) may be broadly divided into two categories (^{Ochsenbein and Ross, 1973}) : the thick and flat biotype of periodontium and the thin and scalloped one.

The thin biotype is present in a minority of individuals (^{Weisgold et al., 1997}) and represents a real clinical challenge ; its delicacy mandates for extremely careful handling, since a minor insult to it may result in recession of the gingival margins.

Differently from the thin and scalloped periodontal anatomy the thick periodontal tissues are less prone to recede when injured, and their typical reaction to damage is the development of a periodontal pocket. In approaching the prosthetic restoration of a tooth these different anatomical features must be kept into consideration. Special care needs to be applied in following, with the finishing line of the preparation, the variably scalloped architecture of the bone (^{fig. 12}). The cemento-enamel junction in the healthy periodontium runs parallel to the bone scalloping.

The gingival scallop is influenced by the underlying osseous anatomy and by the anatomy of the teeth (^{Olsson et al., 1993}) and it can be greater or equal to the osseous scallop ; it varies from teeth in anterior locations to posterior ones where it tends to become more horizontal.

It has been reported in the periodontal literature how a violation of the biological width interproximally, such as the creation of a horizontal straight preparation margin in an anterior scalloped interproximal space, may result in reactive pathological changes in buccal locations (^{Kois and Spear, 1992}).

Working in the interproximal areas is demanding and any accidental invasion of the biological width may result in marginal remodelling of the hard and soft tissues (^{Tarnow et al., 1986}).

The results of iatrogenic damage can vary significantly from an increase of the inflammation level (^{Gunay et al., 2000}) to a loss of bone height (^{Tarnow et al., 1986}). This might be related to many variables such as the location of the osseous crest and its thickness, the level of oral hygiene in the area, the patient's individual resistance, the precision in the prosthetic margins and the presence or absence of keratinized tissue (^{Stetler and Bissada, 1987}).

It is important to always evaluate the distance from the proposed prosthetic margin location to the osseous crest and this can be clinically determined through transgingival probing under local anesthesia (^{Kois, 1994}) and, in the posterior areas, with the help of a properly executed vertical bitewing radiograph (^{Reed and Polson, 1984}). Whenever this distance is less than 3 mm extreme caution should be used in placing the margin in a subgingival location and for no reason should it be deeper than 0.5 mm under the marginal gingiva (^{Kois, 1996}) (^{fig. 13} and ¹⁴).

In agreement with what was previously stated by Kois in 1996, and later on proved in a prospective 2-year human study by ^{Gunay et al., in 2000}, it is the opinion of the authors that efforts should be made in keeping the margins at least 2 to 2.5 mm from the osseous crest. Additionally, it is important to gain access to the subgingival area of the tooth preparation as atraumatically as possible, and to ensure this positioning a retraction cord in the sulcus becomes very useful (^{Dragoo and Williams, 1981}) (^{fig. 15}, ¹⁶ and ¹⁷).

To surgically lengthen the area that will host the prosthesis, a careful evaluation of the preexisting anatomy is once again extremely beneficial in the attempt to envision the final desired result. The biotype of the periodontium must be considered in planning for the procedure and in cases of thin and scalloped gingiva in the aesthetic zone alternative procedures, like forced eruption with fiberotomy, should be explored in order to maximize the esthetics of the final outcomes (^{Kozlovsky et al., 1988}).

Several authors (^{Steiner et al., 1981} ; ^{Dowling et al., 1994}) have shown variations in the post surgical healing of the gingiva based on the individual thickness of the periodontal tissues.

It is therefore expected a tendency of thin and scalloped gingiva to recede in a more apical position when compared to thick tissues that will tend to grow back to a more coronal position. In evaluating the case preoperatively, it is important to locate the furcation openings and to consider the root trunk anatomy (^{Becker et al., 1998}).

If ostectomy needs to be performed, furcations sometimes represent a limitation to the extent of bone which can be removed ; in those circumstances a violation of the biological width may arise whenever the prosthetic margin will end up being too close to the furcal peak of bone (^{Dibart et al.,} submitted for publication).

It is often advantageous to re-prepare the dental elements during surgical crown lengthening procedures to optimize the emergence profile at the dento-gingival junction, to barrel in the furcation area whenever needed, to clearly establish the apical extent of restorative materials which may have been used in the build up of the prosthetic abutment, and to open up the embrasure spaces in cases of root proximities (^{Carnevale et al., 1990a} ; ^{Carnevale et al., 1990b}).

The intra-operatory preparation should be performed, if necessary, only on endodontically treated teeth and it will result in about 1 mm of bone resorption during the postoperative healing (^{Carnevale et al., 1990a} ; ^{Carnevale et al., 1990b}). In the interradicular areas it is advisable to leave the connective tissue fibers attached to the cementum, in order to minimize the postoperative bone resorption, and the risk of developing a maintenance problem in the furcation (^{Levine and Stahl, 1972}) (^{fig. 18}, ¹⁹, ²⁰, ²¹, ²², ²³, ²⁴ and ²⁵) .

It is possible to try to anticipate the postoperative regrowth of the gingival tissues (^{Smukler and Chaibi, 1997}) in the area undergoing crown lengthening procedure, by transgingival probing down to the level of the bone. In some instances the transgingival probing under local anesthesia may be done in the controlateral healthy tooth/teeth relying on the physiological symmetries present in the mouth. The dimension of this « columns of tissue » will tend to reform around the interested tooth/teeth after completion of the post-surgical healing process.

With this assumption in mind, and measuring the residual amount of tooth structure above the crest of bone, the amount of ostectomy that is needed in order to re-accommodate the pre-measured columns of tissue can more precisely be determined.

In this way it may be possible to approximately predict the final amount of sound tooth structure available above the gingival tissues for the restorative manœuvres.

This approach is much less predictable when dealing with a thin and scalloped periodontium and extreme care should be applied in trying to determine the final outcomes of surgical procedures in such circumstances. Fortunately only 15 % of the patients will present with this type of anatomy (^{Weisgold et al., 1997}). When restorative materials are used in the prosthetic build-up of the cervical portion of the abutment, it is important to realize that the amount of tooth structure required above the level of the crest of bone may increase to a value of 4-5 mm.

Creating these conditions becomes important in order to locate the margins of the restoration on sound tooth structure and to create a ferrule effect able to reduce the potential risk of fracture (^{Eissman et al., 1976}).

Only through the preoperative evaluation of the anatomy of the area to be treated and carefully analyzing the extent of damages to the tooth, or teeth to be rehabilitated, will it be possible to produce biologically integrated prosthetic restorations.

To respect or restore the delicate biological equilibrium of the involved tissues it is necessary to fully evaluate the clinical scenario on a case by case analysis.

Tissue reactions to restorative procedures

In preparing a tooth to accept a prosthetic restoration many are the steps during which the periodontal tissues may be involuntarily damaged. The care that is required to avoid invading the biological width has been already discussed, and we will not dwell further on this issue. It is anyways important to remember that other potential iatrogenic insults may harm the periodontium and hinder the creation of a biologically integrated prosthesis.

^{Loë (1968)} reported on the periodontal damages that a rotary instrument may cause to the epithelial and connective tissue attachment of dogs during tooth preparation. Later some human histology came from the work of ^{Dragoo and Williams in 1981} and subsequently in ¹⁹⁸². The main conclusions of the two human studies were that any restorative procedure performed subgingivally, on elements affected by periodontal pathology may worsen the condition, while any lesion caused to healthy gingiva by rotary instruments, retraction cords or electrosurgery can be reversible within a month.

The only comment to the research reported is that, by admission of the same authors, in all cases there was clinical inflammation at 4 weeks on the marginal gingiva surrounding the temporary crowns. Moreover in the histological sections they presented the tissue was never free from an inflammatory infiltrate.

^{Ingraham et al., in 1981}, comparing different techniques to obtain accurate impressions of subgingival margins highlighted the difficulties in controlling electrosurgery versus gingival curettage with rotary instruments.

An accidental trauma caused during restorative procedures on the connective tissue fibers may be irreversible and cause an apical shift of this attachment apparatus (^{Tarnow et al., 1986}).

Intrasulcular restorative manœuvres can be accomplished successfully without resulting in damages to the tissues, and independently from the type of technique or marginal preparation (^{Carnevale et al., 1990a} ; ^{Carnevale et al., 1990b} ;^{Fuzzi and Carnevale, 1997}). To achieve this goal it is of utmost importance to accurately execute every clinical step atraumatically until reaching the creation of a precise marginal fit.

Postoperative stability of the gingival tissues

When a surgical procedure is needed to recreate a biologically acceptable relation of the restoration with the periodontal tissues, a proper healing time must be allowed.

The healing process after periodontal surgery is variable in different individuals and several are the factors that seem to play a role in it. Smoking and patient's age have been respectively reported as capable of impairing or slowing down the healing sequence of events (^{Preber and Bergström, 1990} ; ^{Holm-Pedersen and Loë, 1971}). While the literature reports how tissues after surgery will re-grow coronally to a certain height of about 4 mm, the timing of this phenomenon is still controversial.

It is usually suggested to wait at least 6 to 8 weeks to take the final impression during restoration of posterior dental elements, this to ensure an acceptable level of maturation of the connective tissue (^{Ramfjord et al., 1966}). ^{Rosen and Gitnick in 1964} advocated the placement of the margins above the gingival tissues at 6 to 8 weeks ; then in presence of a thick tissue anatomy the progressive maturation of it will eventually bring them in a subgingival location.

In cases where esthetics is important a healing time of 5 to 6 months has been recommended to achieve stability of the gingival margins (^{Wise, 1985} ; ^{Dowling et al., 1994}).

^{Bragger et al. (1992)}, in their 6-month research of healing after crown lengthening, reported that « once defined by surgery the mean changes of the level of the free gingival margin were minimal ». When analyzing more closely his results though it appears that only 38 % of the operated areas maintained stable gingival levels between 6 weeks healing time and 6 months post-surgery. In 33 % of the cases a coronal shift of the gingival margin could be noted while in 29 % of the cases the margins were displaced apically. From this study it seems clear that in the esthetic zone a 6 months healing time might not be enough to grant stable gingival levels.

This seems to be confirmed in the previously quoted clinical study from ^{Pontoriero and Carnevale in 2001}. Moreover van der Velden already in 1982 concluded how « the displacement of the gingival margin in a coronal direction has not yet been completed by 6 months ».

In essence, the indication that arises from the body of information which is available today is that stable gingival margin levels might take up to 1 year to form. This should be kept into consideration whenever a precise subgingival placement of the finishing line is desired and it becomes extremely important in the attempt to maximize the esthetics of our reconstructions.

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