Aesthetic problems associated with implant therapy in partially edentulous patients

Introduction

The aesthetic criteria which should be applied to implantology are in no way different from those applied to general dentistry.

It is respect for these criteria which must drive the construction of an implant-supported prosthesis which, as an integral part of the oral environment, must also contribute to the harmonious relationships of the face as a whole.

In other words, the successful aesthetic result depends upon the harmony which is established between the shape of the teeth, the alignment of the margins, the shape of the lips whilst smiling and the more distant interpupilary line ^{(Chiche and Pinault, 1994)}. Equally, it is necessary to stress that the biomechanical principles required to ensure the function and durability of the implant restorations must not, under any circumstances, be ignored for the sake of aesthetics.

It is the difficulty of taking into account the many parameters, peculiar to each patient, which constitutes the difficulty of this type of treatment.

The aim of this article is to highlight the aesthetic problems involved with each stage of implant therapy, surgical as well as prosthetic.

Aesthetic problems associated with implant surgery

The relationship of the implant to the embrasure

In oral implantology, the requirements for the positioning of the artificial roots vary according to the type of prosthesis.

In the case of an overdenture, either in the maxilla or the mandible, the position of the implants does not affect the final aesthetic result, since the implants are totally covered by the complete denture ^{(Engqvist et al., 1988} ; ^{Bert, 1994)}.

The same is the case of implants constructed on stilts, as described by the Swedish authors, ^{(Adell et al., 1981)}, where implants are judiciously placed between the two mental foramina. The emergence profile of the implants, in all those cases masked by the lower lip, cannot be in balance with the teeth on the prosthesis. The abutments support a pontic on which are set standard teeth using pink autopolymerising resin.

In this type of situation, the position of the implants is guided more by the volume of the alveolar crest than by the prosthesis.

In all other cases of fixed implant-supported prostheses, the emergence of the implant must be located in such a way as to take into account the prosthesis to be fitted ; the reference points are no longer anatomical but prosthetic (^{fig. 1}, ² and ³).

The use of a surgical guide ensures the optimal positioning of implants. This is constructed from a diagnostic wax-up, taking into account the functional and aesthetic requirements of the individual clinical case. The correct mesio-distal position of the implants can easily be determined and the embrasures avoided, whatever the design of the guide used, whether it has a guide tube or not (^{fig. 4}, ⁵ and ⁶). The presence of an abutment at the site of an embrasure severely prejudices the aesthetic result which can only be resolved in two ways :

- if the implant is in the middle of the interdental space, it is possible to mask the metal component with the addition of a pink ceramic facing.

- if the implant encroaches only slightly on the embrasure, it is sometimes preferable to shift the emergence of all the implant crowns in order to correct the error. This correction entails a modification of the design established at the diagnostic wax stage and can lead to overlapping of the prosthetic components.

In both cases, the proposed solutions constitute a compromise to the aesthetic result and must be presented to and accepted by the patient before the final fitting of the prosthesis. If the patient refuses, the implant may be reburied as a « sleeper » and be forgotten, or be removed (^{fig. 5}, ⁶, ⁷ and ⁸).

The harmonious relationships of the margin and crown length

Following implant treatment, an excessively long crown must be avoided, especially in the upper anterior region. This situation can be avoided at the surgical stage by ensuring that the implant is placed in the correct position bucco-lingually. If the implant is placed too far towards the buccal aspect, when it is exposed the point of emergence will be at a more apical level than the adjacent teeth ^{(Parell and Sullivan, 1989)}.

This poor orientation frequently occurs when the extraction socket is used as a surgical guide, notably in cases of immediate implantation or when there has been periodontal disease affecting the buccal aspect of the tooth ^{(Buser et al., 1994)}.

Contrary to what one may think, the extraction socket does not constitute a reliable surgical guide. Centering the implant along the socket often brings the collar of the implant too far buccally. In the case of a tooth which has been removed from the upper anterior segment, the ideal alignment for the implant in a bucco-palatal plane is to position the implant so that its collar is aligned with an imaginary line joining the incisal edges of the adjacent teeth, when viewed from an occlusal direction ^{(Chiche and Leriche, 1998)}.

Loss of interdental papillae

Loss of the papilla can cause failure from an aesthetic viewpoint, if it is in an area exposed by the lips.

The restoration and maintenance of the papilla depends on two factors which must be understood at the surgical stage :

- the presence of bone in the interdental area. Radiographic evidence of interdental bone is the best guarantee concerning the presence of papillae at the end of prosthetic treatment ;

- a distance of at least 1.5 mm in the mesio-distal direction is required between two implants or between a tooth and an implant.

It is generally stated that the minimum space required to place an implant between two teeth is 7 mm ^{(Lekholm and Jemt, 1989)}.

This value represents the space required for the collar of a standard 4.1 mm diameter screw-type Brånemark implant, which must be placed 1.5 mm from the adjacent teeth to allow sufficient space for the formation and maintenance of the interdental papillae.

This 1.5 mm space can, however, be reduced to 1 mm in cases where the implant is situated out of the area where aesthetics is important.

In those cases where tooth replacement is required in the anterior segments, especially in the maxilla, the presence of the papilla is essential for the final aesthetic result. Their presence avoids the appearance of the « black holes » which cannot be satisfactorily eliminated by prosthetic means.

The placement of a standard screw implant is possible in the majority of cases because the average mesio-distal dimension of a central incisor or a canine is between 7.5 and 9 mm ^{(Reynolds, 1968)}.

Sometimes, when the mesio-distal distance between the proximal surfaces of the adjacent teeth is less than 7 mm, or if there is a wide incisive canal, it becomes necessary to choose an implant with a narrower collar ^{(Khayat et al., 1994)}, in order to preserve the minimum 1.5 mm space for each papilla (Screw-vent-Paragon, 3.5 mm/Microminiplant-3I, Narrow Platform - Nobel Biocare, 3.5 mm HL 3.25 , Steri-Oss).

One should emphasise that it is not realistic to expect to treat all cases with the same diameter implant and that it is necessary choose a size appropriate to the mesio-distal space available ^{(Bert and Missika, 1996)}.

Nevertheless, it is still possible to improve and guide the formation of the papillae at the temporary prosthesis stage ^{(Touati, 1995)} :

• On the one hand, one should encourage the formation of the ideal emergence profile of the crown, even if it means experimenting with a degree of compression of the peri-implant gingiva in the region of the interdental space. This compression, shown by a transient blanching of the soft tissues will guide the formation of the papillae in the embrasures.

• On the other hand, the temporary crown must have a contact point which, ideally, is 7 mm from the bone crest. In effect, by analogy with data collected from the natural dentition, this is the dimension which is normally found ^{(Tarnow et al., 1992)}.

Ideal morphology of single and multi-rooted teeth

The morphology of the replacement teeth will be dependent, on one hand, by the diameter of the implant collar and, on the other hand, by the intermediate element (abutment or inlay-core). The nearer the diameter of the implant collar approaches the mesio-distal diameter of the tooth it replaces, the less flare can there be on the intermediate element ^{(Lazzara, 1994)}.

In the anterior segment, the use of standard screw with a diameter of 4.1 mm at the collar enables a successful transition between the diameter of the implant and that of the future single-rooted artificial tooth. This comment is particularly important in connection with the replacement of upper central incisors and both upper and lower canines as we have stated above.

In all cases, the thickness of the peri-implant gingiva represents the transitional space within which the diameter of the implant is adjusted to that of the future prosthesis. An understanding of this enables one to appreciate the influence that the burial of the implant has on the final prosthesis.

If the implant is not buried sufficiently deeply in bone, and if the gingiva is thin, the gingival transitional space is not big enough to mask the flare on the intermediate element. The crown margin cannot be made sub-gingival.

To avoid this possibility, the sitting of the implant in a vertical plane must be determined and established according to dental criteria : the implant collar should be located 2 to 3 mm apical to the cement-enamel junction of the adjacent central incisor ^{(Parrell and Sullivan, 1989)}. Compliance with this rule leads, in many cases, to implant threads being left exposed which must then be covered using graft techniques ^{(Collins and Nunn, 1994} ; ^{Friberg, 1995)}, or by guided bone regeneration ^{(Lazzara, 1989)}.

This situation is frequently encountered in cases of trauma, agenesis, or loss of the alveolar crest following a long period of edentulousness. It is essential to plan the ideal position for the single-tooth implant prior to the surgery, taking into account the residual volume of the alveolar crest, in order that the best reconstructive technique can be utilised ^{(Garber and Belser, 1995)} (^{fig. 7}, ⁸, ⁹, ¹⁰, ¹¹ and ¹²).

In the posterior segments, the use of a standard screw implant carries the strong possibility of affecting the morphology of multi-rooted teeth, especially of molars where the relationship of the diameter of the collar and the diameter of the future tooth is important (^{fig. 13} and ¹⁴).

The degree of divergence of the walls of the intermediate component will be variable and depends on the vertical height between the arches :

- if the height is small, the bevel must be even greater, because the tooth to be replaced is wide. This situation creates a major obstacle for access for adequate cleaning, leading to plaque accumulation ;

- if the height is normal, the bevel will be more gradual and the amount of divergence of the walls is such as to avoid creating a stagnation area, thus permitting better plaque control.

In the majority of cases, a standard collar in the molar areas leads to the creation of a premolar morphology, not in response to the non-proven clinical concept of reduction of occlusal areas, but to avoid creating too greater bevel on the inlay-core.

The use of wide diameter implants, between 4.5 and 6 mm, permits more gradual and aesthetic transgingival emergence profiles, even when the inter-arch height is small. This ensures that, in the molar regions, the re-establishment of an ideal occlusal morphology, in accordance with the principles of a functional occlusion (^{fig. 15} and ¹⁶).

Risk factors associated with the implant-supported prosthesis

The ideal emergence profile

The emergence profile is equally a determining factor which, by directly influencing the coronal morphology, contributes to the final appearance of the prosthesis.

An ideal emergence profile, identical to that of a natural tooth, depends initially on the insertion of the prosthetic component into the implant and its transgingival portion. There are many such components on the market and they can be classified into four categories :

- machined non-adjustable titanium abutments such as Cera-One (Nobel Biocare), the Single Tooth Abutment (3I) ;

- machined adjustable titanium abutments which can be modified by removal of material such as the Dia System (Steri-Oss), titanium Inlay-core (3I) ;

- ceramic abutments that can be adjusted by removal of material such as Ceradapt (Nobel Biocare) ;

- machined abutments made from precious metal alloys which can be cast, referred to as « anatomical inlay-cores », GLA (Paragon), Gold UCLA abutments recommended by nearly all manufacturers.

It is this last type of component which we use most frequently in all situations where the aesthetic result is important.

Of great importance is the accurate fit of this component and the emergence profile of the post in relation to the diameter of the tooth which it replaces ^{(Lewis, 1992)}.

In effect, as has already been emphasised, there is sometimes a large discrepancy between the mean diameter of an upper central incisor which is 9 mm and that of a standard screw type implant of the Brånemark system^® with a collar of 4.1 mm diameter.

The ability to adjust castable abutments gives great flexibility to this system. One can add more or less wax on the model in order to establish the ideal mesio-distal contour of the replacement tooth.

Lack of visible cervical margins

Even if the margins do not always need to be placed subgingivally in the posterior regions of the mouth, it is usually the case for anterior implants.

The use of castable abutments, as opposed to other components, such as non-adjustable machined titanium abutments or titanium abutments which can be adjusted by reduction, allow precise definition of the finishing line and better adaptation to the implant and its environment. When the finished margin must be subgingival, it is possible, at the wax-up stage, to define the buccal depth of the shoulder at 1.5 mm by following the gingival scalloping and to extend this at an even depth to the region of the interdental papillae. This margin may possibly be subgingival on the palatal aspect. The casting also allows a realistic anatomical stump to be prepared which is perfectly shaped on all its surfaces (^{fig. 17}, ¹⁸, ¹⁹ and ²⁰).

The flexibility of this method which allows adaptation to the local conditions is important when the gingival contour is important.

In this situation, the depth of the sulcus in the proximal area is proportionately greater than the slope between the crests of the papillae, and the most apical part of the buccal marginal gingiva is significant.

When using castable abutments to support a cemented ceramic crown, it is easier to control the removal of excess cement during cementation. The margins are fixed at a pre-determined depth which do not exceed 2 mm around the whole periphery, whatever the thickness of the peri-implant gingiva.

Finally, when everything has been assembled, this system offers great variations in the angulation of the cast posts, thus allowing correction of the axis of the future crown in relationship to the implant.

Concealing the screws at the occlusal surface

From the point of view of the patient, the presence of the access holes for the screws in the occlusal surfaces of the canines can prejudice the aesthetics. Some patients have difficulty in appreciating that after many months of treatment, alternating between surgical sessions and sophisticated prosthetic treatment, in response to their functional, psychological and aesthetic requirements by the placement of prosthetic appliances, the holes are sealed with, at best, composite.

From the clinician's viewpoint, apart from their unaesthetic appearance, the presence of the holes begs two questions :

- How can one adjust the occlusion when, in the most favourable cases, they are in the middle of the occlusal surfaces, ie in the fossae which must articulate with the opposing teeth ?

- If these holes in the bonded crown are obturated with composite, on which surfaces will they occlude and for how long ?

These problems, as much functional as aesthetic, can be solved by construction of fixed implant-supported prostheses cemented on abutments.

To reiterate, the use of a castable abutments has direct aesthetic advantages. When it is screwed to the implant, this component behaves as a conventional post and can receive a bonded crown, fixed with a cement such as Temp-Bond^®.

Like all abutments using screws, if any problems arise, it can be simply unscrewed so that you are left with the basic substructure.

Its retention on the implant is assured by a screw, preferably of gold, tightened to 32 Ncm with a torque wrench. This tool is sufficiently reliable to avoid the use of any cement or composite at the subgingival junction of the implant/post, whatever the thickness of the gingiva.

Finally, the emergence of the screw can be lingual or buccal and not affect either the occlusion or the final aesthetic result since the crown has not been screwed on.

The concept of cemented crown on abutments has the advantage of being fully removable : for ease of removal, all that is necessary is to place a notch in the palatal surface of the crown into which can be inserted the end of a crown remover. The cementation can be done with a « temporary cement », the retention of which can be adjusted by the addition of Vaseline, or when filling the abutment access hole for the post screw ^{(Koka et al., 1995)} (^{fig. 21}, ²² and ²³).

Conclusion

Every treatment carries with it some risk which must be evaluated throughout the whole of the treatment period. In implantology this risk is even greater, in that there is a single surgical procedure where the implants are placed, the exact positioning in space determining irreversibly the final result.

The major difficulty in connection with the aesthetics of implant-supported prostheses resides in the multidisciplinary nature of the work. There must be perfect co-ordination between all members of the operative team : the surgeon, the prosthetist and the technician.

Their respective competencies, good communication as well as constant reference to surgical and prosthetic guidelines offers the best guarantee of an implant-supported prosthesis which is both functional and aesthetic.

Demande de tiréts à part : Frédéric A. CHICHE, 49, boulevard de Courcelles, 75008 PARIS - FRANCE.

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