Key elements in implant restorative dentistry for achieving an optimal esthetic result

Introduction

In the past two decades, the widespread use of dental implants has restored the masticatory function of thousand of patients with missing teeth. Currently the achievement of osseointegration is fulfilled by many implant systems and long term predictable results has been reported by a limited number of implant systems (^{Adell et al., 1981} ; ^{Adell et al., 1990}). However negative long-term outcome has also been reported with the use of dental implants (^{Haas et al., 1996} ; ^{Wheeler, 1996}) and a thorough risk assessment on the use to this therapeutic tool is still unavailable. In the early times of the modern dental implant era, the indications for this treatment modality were reduced to full arch edentulous mandibles. With the development of new armamentarium, the indications have broaden, and partial or even single tooth replacements are widely used nowadays (^{Henry et al., 1996} ; ^{Jemt et al., 1990}).

Once the functional and biological aspects of dental implants have been successfully solved, our patients are demanding more and more not only if implants successfully anchor in the jaws, but also in what will the final esthetic result of the implant supported restoration be. Patients seeking for implant therapy do not really look for more or less millimeters of bone anchorage but the replacement of lost teeth by means of a stable and problem free prosthesis which resembles as much as possible the lost natural dentition. In order to reach these patients expectations dentist must be able to operate within the three areas of knowledge : function, aesthetics and biology.

Biology and function of osseointegrated implants have been widely described in the literature (^{Berglund and Lindhe, 1996}). Therefore the purpose of this clinical review article is to describe the different critical aspects of implant therapy that will determine the final aesthetic results. Emphasis will be placed on the surgical and soft tissue management aspects, which are critical for an optimal aesthetic restoration. Among these critical elements there are three aspects that we consider as fundamental for achieving an optimal aesthetic result : tooth size, location and position of the implants and the state of the surrounding soft tissue.

Tooth size

The tooth size of the final implant supported restoration will depend mostly on the degree of crestal resorption prior to implant placement. This factor will condition the available prosthetic space and thus the size of the prosthetic teeth. If the design of the final restoration is to provide natural looking teeth, they should have average measurements (^{Wheeler, 1984}). In cases where the alveolar ridge is heavily resorbed or the skeletal relationships are inappropriate, the conventional prosthetic design will result in an anaesthetic outcome due to too long teeth (^{fig. 1a}, ^1b, ^1c and ^1d). In these cases, the ideal therapeutic alternatives to provide natural looking teeth in the final restoration will be either a bone regeneration procedure aiming to rebuild the alveolar ridge, or a hybrid prosthetic design where the missing crest is replaced by prosthetic material, either acrilyc or pink porcelain.

The tooth size and the final restorative outlook will also depend to a great extent on the competence of the lip. The height of the lip line during function will determine the degree of resulting aesthetic appearance. Fortunately for patients and dentist, many lost dentitions are accompanied by a competent lip that allows implant supported prosthesis with long teeth to being hidden by a low smile line (^{fig. 2a} and ^2b). On the contrary, high lip lines will demand maximum efforts to achieve excellence in the final prosthetic result. The tissue biotypes (^{Weisgold, 1977} ; ^{Olsson and Lindhe, 1991}) also influence the final result, being a thin phenotype more delicate to manipulate (^{fig. 2a} and ^2b), than « thick » phenotype (^{fig. 3a} and ^3b) where the characteristics of the tissue make it easier to obtain a natural esthetic result.

Implant position

The position and orientation of the implant in the dental arch is critical in order to achieve the desired aesthetic result. Ideally implants should be placed in a position where the head of the implant parallels the incisal edges of the dental arch (^{fig. 4a}) (^{Parel and Sullivan, 1989}). They should be placed with enough inter-implant space to allow for the survival of the inter-dental papillaes (^{Chiche and Leriche, 1998}). Implants should be buried 2-3 mm underneath the adjacent CEJ's in order to allow for an appropriate emergence profile of the restoration.

In many instances there is a compromise between the alveolar bone anatomy and the ideal implant position. If implants are placed only following anatomical priorities the end result will be either ridge lap restorations which impose difficulties for oral hygiene procedures or the need for angulated abutments which result in anaesthetic margins and biomechanical compromise. Several authors (^{Bahat, 1992} ; ^{Garber and Belser, 1995} ; ^{Salama et al., 1996}) have advocated the approach « prosthetic driven implant placement » : which recommends the placement of implants in the ideal position determined by the final prosthesis, which should be designed in the initial diagnostic process.

If implants are misplaced in the provided edentulous space several consequences may occur.

• When implants are placed too far buccally, the buccal margin of the restoration will be too high compared to the adjacent teeth. At the same time, the thickness of the tissue buccal to the implant shoulder will be thinner. Hence, there will be a tendency for reccession during the formation of the peri-implant biological width (^{Bengazi et al., 1996}) (^{fig. 4b}).

• When implants are placed too far palatally, the emergence of the implant relative to the incisal edge will oblige the construction of a restoration with a ridge lap. This will add difficulties to proper plaque control and will hamper an adequate emergence profile (^{fig. 4c}).

• When implants are placed too close to each other it will prevent the building of interdental papillaes. For the survival of a papilla, a minimum space of 1 mm between teeth and implants, or 2 mm between two implants, must be provided. This situation is of special relevance when the teeth to be replaced are a central and a lateral incisor. In this clinical situation the lack of interdental space (^{fig. 5a}, ^5b, ^5c, ^5d, ^5e and ^5f) may be due to a variety of factors :

- anatomical limitations, such as in the central incisor position. In this position, the most mesial landmark is not the adjacent central incisor but the maxillary central suture line. Therefore it is important to evaluate the presence of a large incisal foramen that will limit the amount of available space for implant placement ;

- wrong diagnostic measurements. It is a common mistake, even in the wax-up, to make the measurements for implant placement location on the buccal aspect of the edentulous space, on the soft tissue border. In this location the radius of the arch is larger and hence it allows for more space. However, the space calculations should be made on the inner part of the arch, parallel to the cingulum of neighbouring teeth, since there is a tendency for buccal resorption of the alveolar crest. In this location the size of the arch is shorter and the available space is thus more limited ;

- bone architecture. The usual osseous resorption pattern results in a flat or even negative architecture, which will make difficult the formation of interdental papillae between the implants. In contrast, the bone will hold the papillae adjacent to the cuspid and the contralateral incisor, worsening the outlook of a negative « inter-implant papillae » ;

- implant convergence. During fixture placement, long shaft burs should be utilised. Otherwise, the crowns of adjacent natural teeth will tilt the handpiece towards the medial aspect of the edentulous space resulting in the insertion of two convergent implants (^{fig. 5c}, ^5d and ^5e). In these situations, at the time of making the restoration the absence of inter-implant space will prevent the formation of an interdental papillae ;

- non parallel adjacent teet. The roots of the cuspid and the incisor adjacent to the edentulous space are usually not parallel. If the two implants are placed simultaneously aiming to a parallel insertion to each of the adjacent teeth, the implants will converge precluding a space for an inter-implant papillae. It is recommended that implants at this location should be inserted independently from the axis of adjacent teeth, following the most appropriate direction with the goal of attaining at least 2 mm of inter-implant space.

• If implants are inserted with the goal of a strict alignment with the longitudinal axis of the restoration, from a biomechanical point of view, the occlusal forces will be transmitted vertically. However, this ideal biomechanical angulation does not always result in the ideal emergence profile of the implant-supported restoration (^{fig. 4d}). The literature does not indicate that more implants failures occur due to misangulation. Moreover, ^{Celleti et al. (1995)} did not report more failures when masticatory forces were applied in a non-parallel direction to the long axis of implants in dogs. Therefore, we would recommend the placement of the head of the implant in the most favourable position for an adequate emergence profile in those locations with aesthetic importance. If there is an inadequate bone or soft tissue volume deficiency, this should be managed by means of hard and/or soft tissue augmentation (^{Bahat et al., 1993} ; ^{Buser et al., 1993} ; ^{Garber and Belser, 1995} ; ^{Israelson and Plemons, 1993} ; ^{Nevins and Mellonig, 1994}). Whether these augmentation procedures should be done prior to or at implant surgery (^{Barone et al., 1999}) will depend on the patient's particular situation and on the skills of the surgeon.

The ideal position and orientation of the implant should be assured by means of a surgical template fabricated from the initial wax-up, which will facilitate the three-dimensional orientation at fixture insertion. The advantages of using a fixed surgical template have been described by our group (^{Sicilia et al., 1998}) when a precise implant placement is important. These advantages include : good orientation and stability during the surgical procedure ; a good contrast for diagnostic imaging procedures ; the provision of enough surgical access and enough tissue reflection for an adequate visibility and handling during bone drilling ; and enough freedom of movement to proceed during the intervention even when the final position is not the initially planned (^{fig. 6a}, ^6b, ^6c and ^6d). We have also evaluated the precision and accuracy of this fixed splint (FST) when compared with a conventional removable splint (MST) (^{Sicilia et al., 2000}), demonstrating a significantly smaller frequency of implants outside the ideal tooth with the FST (7 %) than with the MST (46 %).

Soft tissue

For achieving a final aesthetic therapeutic result, the shape, colour, contour and height of the peri-implant mucosa must be in harmony with the outlook of the implant restoration. Four key elements are important in relation with the soft tissue component on implant supported restorations (^{Rosenquist, 1997}).

Amount of keratinized tissue

The original surgical protocol both for implant insertion and for abutment connection was designed to treat full edentulism with the main objective of restoring lost masticatory function. In this clinical situation, the amount and width of keratinized tissue, in the presence of an adequate plaque control, was not a prerequisite for maintaining peri-implant tissue health (^{Schou et al., 1992} ; ^{Strub et al., 1991} ; ^{Wennström et al., 1994}). In light of this evidence, the second stage surgical protocols used circular or punch scalpels aiming to eliminate the tissue over the implant cover screws, and thus having direct access to the implant heads. However, few studies have evaluated the adequate amount of keratinized peri-implant mucosa needed to provide an aesthetic and plastic outlook of the tissues surrounding dental implants. With the advent of partial or single tooth implant restorative indications in the maxilla, the amount of keratinized peri-implant mucosa has become an important element in order to obtain a good aesthetic result. This quality of the peri-implant mucosa is of important relevance in modelling and colour matching with the adjacent gingiva and in the final dental-alike outcome of the implant supported restorations.

For this reason, both first and second stage surgical protocols in the maxilla have been modified with the aim of maximising the amount of keratinized mucosa. During these surgical interventions, surgical incisions should be designed not only to preserve and maximise the amount of keratinized tissue present (^{fig. 6a}, ^6b, ^6c and ^6d), but to create good periosteal beds. Specially in those situations where connective tissue grafts are needed to augment the amount of soft tissue (^{fig. 6a}, ^6b, ^6c and ^6d) (^{Barone et al., 1999} ; ^{Israelson and Plemons, 1993}). Therefore, palatal partial thickness incisions are recommended for fulfilling both objectives. After the elevation of a partial-thickness flap, a periosteal perforation is carried out to gain access to the alveolar crest in order to allow the placement of implants or abutments.

Volume of the buccal contour

Tooth loss is often followed by osseous resorption, more often on the narrower aspect of the dental alveolus, which is most of the times the buccal plate. In order to prevent this collapse, a simple intervention prior to tooth extraction has been recommended by ^{Langer et al . (1994)}. 3-4 weeks prior to the planned tooth extraction, the tooth crown is sectioned at the level of the bony crest, allowing the marginal tissues for spontaneous gingival growth and full coverage of this root. In the same intervention the root is extracted and replaced by an implant, using an appropriate bone regenerative technique, if needed. The soft tissue obtained during this period prior to the tooth extraction will facilitate the primary closure necessary to achieve the maximum potential for preventing the alveolar collapse and to allow tissue regeneration. Other surgical procedures, such as : the socket seal procedure, also placing an implant during the same intervention of the tooth extraction and adapting a free mucosal graft to the size of the alveolus (^{Landsberg, 1997} ; Sicilia et al., 1999), advanced flaps (Bahat and Handelsman, 1991) and rotated flaps have been described with the goal of preventing the resorption of the alveolar bone after tooth extraction (^{Grunder et al., 1998} ; ^{Rosenquist, 1997}).

When bone resorption is present at the time of diagnosis, the resulting implant-supported restoration will be too large and bulky and therefore unaesthetic. The correction of this resorption will depend on its size. If the deformity is too large, hard tissue reconstruction will be required (^{fig. 7a}, ^7b, ^7c, ^7d, ^7e, ^7f, ^7g and ^7h) . Otherwise soft tissue reconstruction by means of connective tissue grafts, modified roll techniques or apically positioned flaps (^{fig. 8a}, ^8b, ^8c, ^8d, ^8e and ^8f) will provide enough tissue to restore the lost volume. Also depending on the volume to restore, some authors recommend the augmentation procedure : prior to implant placement (^{Buser et al., 1993} ; ^{Nevins and Mellonig, 1994}), at the time of implant placement (^{Rosenquist, 1997}) or at the time of abutment connection (^{Hürzeler and Weng, 1996}).

Height of the mucosal margin

In order to maintain the symmetry with the adjacent teeth a correct level and contour of the soft tissue margin is essential. Several therapeutic approaches, both surgical and restorative, have been recommended in order to allow full control of the height of the gingival margin (^{Bichacho and Landsberg, 1997} ; ^{Davidoff, 1996} ; ^{Rosenquist, 1997}).

Surgically, the level of the gingival margin can be modified with the use of displacement flaps either coronally, apically or self-folding, in a similar manner as it was described on edentulous ridges for fixed prostheses (^{Abrams, 1980}).

Restoratively, varying the buccal profile of the restoration can modify the height of the gingival margin. If restorative material is added to the buccal profile, the gingival margin will be moved apically. If the material is removed, the gingival margin will be displaced occlusally (^{fig. 9a}, ^9b, ^9c and ^9d). It is recommended to modify the height of the soft tissue margin during the phase of temporary restorations. Besides, a natural contour of the restoration will be achieved as long as the implant is placed in an adequate position (^{fig. 4a}). The head of the implant should be 2-4 mm underneath soft tissue margin in order to allow for the transition between the round section of the head of the implant and the natural section of the replaced natural tooth (^{Jansen and Weisgold, 1995}) (^{fig. 10}). This transition can be carried out either with custom abutments (^{Rieder, 1996}) or specifically designed abutments (^{Daftary and Bahat, 1994}). An easy way to obtain the desired emergence profile is to use highly polished provisional resin restorations and then, add, reduce and reshape the restorative material until you reach the ideal section of the desired tooth.

Level and size of the inter-dental papillaes

If we want an artificial tooth to look natural, we must provide papillae of correct shape and size. This is critical in the anterior part of the maxillae (^{Rosenquist, 1997}). When treating a case of partial edentulism, the interproximal bone height is the key factor in the final aesthetic result (^{Salama et al., 1998}), since the underlying bone will hold the level of the soft tissue around the restoration. Some authors (^{Salama et al., 1998} ; ^{Tarnow et al., 1992}) have proposed that in order to predictably obtain a papilla of adequate height, the distance from the crestal bone to the interproximal contact point between the adjacent restorations should be 5 mm. In situations where more space is present, the aim of creating a papillae by means of soft tissue reconstructive procedures have not been predictable (^{Jemt, 1997}). On the contrary, advanced surgical procedures and/or orthodontic treatment (^{Salama et al., 1998}) that will achieve reconstruction of the lost alveolar crest, will reduce this distance to the ideal position of the contact point allowing for the presence of inter-dental papillae and thus resembling the natural dentition (^{fig. 7a}, ^7b, ^7c, ^7d, ^7e, ^7f, ^7g and ^7h)). In those situations where there is adequate underlying bone to hold the level of the soft tissues, different surgical techniques can result in the creation of inter-implant papillae : buccal repositioned flaps with approximal pedicles (^{Palacci, 1992}), multiple C incisions and apically repositioned flaps. In the same manner that the buccal contour of the gingival margin can be modified by adding or reducing the amount of restorative material, the interdental papillae can be enlarged by the lateral compression created with the restorative material. A pre-requisite is the presence of an adequate amount of keratinized mucosa sorrounding the implant and a precise position of the implant relative to the adjacent teeth or implants (^{fig. 11a}, ^11b, ^11c, ^11d, ^11e, ^11f, ^11g, ^11h, ¹¹ⁱ, ^11j, ^12a, ^12b, ^12c, ^12d, ¹³, ^13b, ^13c, ^13d, ^13e, ^13f, ^13g et ^13h).

When handling surgically the soft tissues with the aim of improving the final aesthetic result, it is important not only to reach the desired height or contour, but also to maintain it over time. The maintenance of the soft tissue profiles will depend not only on the patient's oral hygiene practices, but also on the specific soft tissue biotype. In « thin » biotypes (^{Olsson and Lindhe, 1991} ; ^{Weisgold, 1977}) there is a higher tendency to recession after treatment (^{fig. 2a} and ^2b), whereas in « thick » phenotypes (^{fig. 3a} and ^3b) the tissue is more prone to maintain its initially placed position.

Conclusion

Implant restorative therapy is aimed not only to restore function but also to provide patients with a naturally looking dentition. Strategic placement of implants should be guided by the final restorative position, which must be established in the initial diagnostic process and not mediated by the position of the existing bone. Reconstructive hard and soft tissue procedures will provide the adequate conditions for obtaining the desired position and angulation of the implants that will allow patients to enjoy a final aesthetic implant supported restoration (^{table I}).

Acknowledgement

We are fully indebted to the work of the following fine restorative dentists which have provided the final restorations shown in this clinical review : M. Palacios, J. Gil, P. Sainz, C. Albea, B. Izaguirre, T. Martin, J. Ochagavia, V. Ojeda, M. Arnaiz.

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