Prosthetic Concepts in Dental Implantology

Abstract

This chapter will address evidence-based prosthetic concepts in dental implantology as well as clinical evidence with focus on appropriate logic and technical skills. Those prosthetic factors are as just important as surgical factors, and long-term success can only be achieved if both of those factors are considered, respected, and strictly followed from planning to prosthetic phase of treatment. This chapter will deal with materials selection for prosthetic part, shape, size, and design of supracrestal parts of abutments and their influence on soft tissue and bone stability around dental implants. Furthermore, one of most important decisions is about choosing the proper way of retention: screw- vs. cement-retained restorations, and it will be discussed in detail. Additionally, emergence profile and its function in soft tissues adaptation and adhesion to different prosthetic materials also have important role in long-term success of dental implant restorations.

1. Introduction

In contemporary implant prosthodontics, proper treatment planning prior to dental implant placement is equally important as the prosthetic factors. The good work of oral surgeon could be easily ruined by poor prosthodontic execution, thus changing the dental implant therapy success into therapy failure.

For decades, the scientific literature identified the following criteria for the survival and success of dental implant-based prosthetic rehabilitation:

1. Implant survival: At the time of measurement, the implant is in situ and loaded.

2. Implant success is determined using the Albrektsson criteria [1] with the following modifications: Adell [2], Buser et al. [3], Mombelli and Lang [4], and Misch et al. [5].

   • Implant placed in situ and loaded; no chronic discomfort; no nerve lesion; no peri-implant infection with suppuration; no mobility; no continuous peri-implant radiolucency [1, 3].

   • No probing depth greater than 5 mm in the presence of a bleeding index of 3 [4].

Absence of radiographic peri-implant bone resorption greater than 1.5 mm in the first year of function [2] and greater than 0.2 mm in subsequent years (i.e. 1.7 mm after 2 years); alternative cut-off values for radiographic bone resorption after 2 years of 2 mm (I. Success) and 4 mm (II. Satisfactory Survival) were also evaluated [5].

Currently, implant success is defined by these three criteria [6]:

1. Annual bone lose not more than 0.2 mm,

2. Periodontal probing depth (PPD) no greater than 5–7 mm,

3. No bleeding on probing.

These criteria are based on older studies, previous dental implant designs and restorations that are not biocompatible, and they might need to be re-evaluated. Porcelain fused to metal (PFM) restorations lack the biocompatibility of zirconia, which is widely used today, and current concepts allow bone stability or even growth over time. Therefore, the expected 1.5 mm loss after 1 year and subsequent gradual resorption can be considered relicts of the past [6].

2. Cemented vs. screw-retained restorations

The debate between cemented and screw-retained dental implant restorations is old as the implant prosthodontics itself. There are also different opinions in the scientific literature. Studies have shown that there are no significant differences in survival between the two methods, but screw connection has shown a total of fewer technical and biological complications [7]. But, from the clinical perspective, all cemented dental implant restorations should be checked very meticulously for any cement remnants. Even after many years of function, cement remnants can cause peri-implant mucositis which if undetected and untreated can lead to peri-implantitis with severe crestal bone loss around dental implants (Figure 1). This bone loss is the clinical issue which we are trying to avoid by careful treatment planning and precise execution of clinical and laboratory procedures.

Figure 1.

Dental implant with cement-retained PFM crown; left image—immediately after delivery with no cement remnants visible on the radiograph; right image—patient did not come for regular follow-ups until he noticed bleeding while using dental floss, 9 years later. Unfortunately, dental implant needed to be removed due to severe bone loss.

Even when using screw-retained restoration, we can witness crestal bone loss around dental implants. This can be caused by the inappropriate size and/or shape of titanium base. Too short or too wide titanium base profile for screw-retained crown can compromise transitional zone of connective tissue and junctional epithelium around dental implant restoration leading to crestal bone loss (Figure 2).

Figure 2.

The story of dental implant system, which was newly introduced to the market, with only one available titanium base height at that time (0.5 mm). Subsequent radiographs from top left to bottom right: initial situation; immediately after dental implant placement in augmented socket (two-stage surgery); second stage surgery and healing abutment; at the time of crown delivery (highly polished CAD/CAM zirconia abutment with laboratory cemented screw-retained lithium disilicate crown); 2 years follow-up with crestal bone loss; final radiograph at the time of new crown delivery (1.5 mm high titanium base was available on the market and micro-layered screw-retained zirconia crown with polished subgingival part was made in hope to prevent further crestal bone loss).

The difference in two titanium base height and screw-retained crowns is clearly visible in Figure 3. The impact of different titanium base as well the slight changes in emergence profile shape on the crestal bone level and density in a period of 3 months is shown in Figure 4.

Figure 3.

Left: screw-retained crown with 0.5 mm high titanium base; right: screw-retained crown with 1.5 mm high titanium base for the patient in Figure 2.

Figure 4.

Left: final radiograph at the time of new crown delivery; right: 3 months’ follow-up radiograph with clearly visible bone remineralization and bone density increase around dental implant neck.

Nevertheless, the clinical success of dental reconstructions is determined not only by survival rates, but also by the number of technical or biological complications that develop during clinical function. The optimum materials and techniques for implant-borne reconstructions are frequently debated to increase clinical outcomes. One of the current discussions is about the best fixation method between the implant and the reconstruction. For a patient-centred clinical approach, it is currently uncertain whether cementation or screw retention is the superior option for restoring dental implants. In clinical practice, both cementation and screw retention appear to have advantages and disadvantages. Clinically and technically, cemented implant reconstructions are quite similar to tooth-borne reconstructions. As a result, they may be easier to make and handle in the mouths of patients. However, prefabricated cement-onto or even custom abutments are required. Recently, CAD/CAM (computer-aided design/computer-aided manufacture) technologies enable a wide range of customized abutments to be used, and cemented reconstructions have become the preferred choice in many clinical settings. The difficulty in removing extra cement from cemented crowns and FDPs is one of their drawbacks. More worrying, excess cement has been demonstrated to cause peri-implantitis in the clinical setting [8]. Another notable disadvantage of cemented reconstructions is that, in the event of a problem, they are difficult or impossible to remove without causing damage, such as in the case of technical complications.

The retrievability of screw-retained reconstructions, on the other hand, is a big advantage. Furthermore, biological issues are extremely unlikely to arise if the reconstruction is well-fitting. Because the position of the screw access hole and the surrounding material parameters of the suprastructure must be considered, the horizontal and angular positioning of the implant is more delicate and has less tolerance than when employing screw-retained reconstructions. The fixation screw opening should ideally be located in a non-visible palatal or oral location, which is only possible if a suitable implant site and angulation are available. Furthermore, screw-retained reconstructions require more technical production because the reconstruction core must constantly be customized. Technical issues such as retaining screw loosening or veneering ceramic fracture have been clinically observed.

It is indeed difficult to choose between the two types of reconstructions, and it quite often comes down to personal preference rather than scientific data.

In a systematic review published by Sailer et al. [9], it was discovered that cemented restorations cause much higher bone loss than screw-retained restorations. From a total of 4511 titles, 59 clinical studies were chosen for this review, and the data were retrieved and analysed. For cemented single crowns, the estimated 5-year reconstruction survival was 96.5%, for screw-retained single crowns it was 89.3% (P = 0.091 for difference). The 5-year survival for cemented partial fixed dental prostheses was 96.9%, similar to the one for screw-retained partial fixed dental prostheses with 98% (P = 0.47). For cemented full-arch partial fixed dental prostheses, the 5-year survival was 100%, which was somewhat higher than that for screw-retained FDPs with 95.8% (P = 0.54). The projected cumulative incidence of technical problems over a 5-year period was 11.9% for cemented single crowns and 24.4% for screw-retained crowns. In comparison to the cemented partial fixed dental prostheses, a tendency towards decreased complication was observed with the screw-retained partial fixed dental prostheses (partial fixed dental prostheses cemented 24.5%, screw-retained 22.1%; full-arch partial fixed dental prostheses cemented 62.9%, screw-retained 54.1%). Biological problems such as marginal bone loss greater than 2 mm occurred more frequently with cemented crowns (incidence: 2.8%) than with screw-retained crowns (5-year incidence: 2.8%) (5-year incidence: 0%).

This study found that both types of reconstructions had varied effects on clinical outcomes and that neither fixation approach was clearly superior to the other. Screw-retained reconstructions had more technical issues, while cemented reconstructions had more substantial biological consequences (implant loss, bone loss >2 mm). Screw-retained reconstructions are more easily retrievable than cemented reconstructions, allowing for easier treatment of technical and biological difficulties. These reconstructions appear to be preferred for this reason, as well as their apparent higher biological compatibility.

In contrary, Nissan et al. published a study that compared the long-term outcomes of cement versus screw-retained implant supported partial dentures in a randomized controlled trial and found that cement-retained FPDs had a better outcome [10]. The study group consisted of consecutive patients with bilateral partial posterior edentulism. In a split-mouth design, implants were placed and cemented or screw-retained restorations were randomly assigned to the patients. Examinations for follow-up (up to 15 years) were done every 6 months in the first year and every 12 months in the following years. Ceramic fracture, abutment screw loosening, metal frame fracture, Gingival Index and marginal bone loss were all assessed and reported at each recall appointment. Total of 221 implants were used to support partial prosthesis in 38 individuals. No implants were lost throughout the follow-up period (mean follow-up, 66 47 months [range, 18–180 months] for screw-retained restorations and 61 ± 40 months [range, 18–159 months] for cemented restorations). Ceramic fracture occurred substantially more frequently (P.001) in screw-retained restorations (38% ± 0.3%) than in cemented restorations (4% ± 0.1%). Abutment screw loosening occurred statistically substantially more frequently (P = .001) in screw-retained restorations (32% ± 0.3%) than in cement-retained restorations (92% ± 0.2%). Neither technique of restoration resulted in metal framework fractures. The mean Gingival Index scores for screw-retained (0.48 ± 0.5) restorations were statistically substantially higher (P.001) than for cemented (0.09 ± 0.3) restorations. The mean marginal bone loss was statistically considerably greater (P.001) for screw-retained restorations (1.4 ± 0.6 mm) than for cemented restorations (0.69 ± 0.5 mm).

The long-term clinical and biological outcomes of cemented implant-supported restorations were found to be better to screw-retained restorations in this study. With such contradictory facts, it is difficult to determine which technique is superior. The choice between cement-retained and screw-retained restorations might be philosophical. By opting for cemented restorations, the clinician accepts responsibility for removing all cement residues. Peri-implantitis caused by cement remnants is entirely an iatrogenic condition with no delegation of responsibility to the patient’s oral hygiene habits.

Whether we use standard abutments or custom CAD/CAM abutments, the cementation margin is critical. One of the most common causes of cement residues in soft tissues around dental implant restorations is the widespread clinical practice of setting the implant restoration margin too deep subgingival for aesthetic reasons.

This is typically done to conceal the abutment-crown interface and to allow for eventual peri-implant soft tissue recession. When the margin is deeper than 1.5 mm below the soft tissue level, according to one of the Academy of Osseointegration’s consensus statements, the risk of cement residues is significant [11]. Furthermore, the American Academy of Periodontology recently included residual cement to the list of risk factors for peri-implant mucositis and peri-implantitis [12]. The key challenge is where to put the cementation margin and how deep it should be?

According to the literature, the margin depth should be deep enough to conceal the margin yet shallow enough to allow residual cement to be removed. Because it is difficult to identify the exact ideal cementation margin depth, this statement does not provide sufficient information for safe and successful everyday clinical practice. To identify a safe margin for cementation, several laboratory and clinical trials were conducted.

The study conducted by Linkevicius et al. sought to determine the amount of cement that remained undiscovered following cementation and cleaning of implant-supported restorations [13]. Fifty-three single implant-supported metal-ceramic restorations were used to treat 53 patients. A periodontal probe was used to assess the subgingival location of each implant’s margin mesially, distally, buccaly and lingually, giving 212 measurements. The data were separated into four groups: tissue level (14 samples), 1 mm subgingivally (56 samples), 2 mm (74 samples) and 3 mm (68 samples) below the tissues contour. Metal-ceramic restorations with occlusal holes were made and resin-reinforced glass-ionomer was used to bond them to conventional abutments. After cleaning, a radiograph was taken to determine if all of the cement had been removed. After that, the abutment and crown complex were unscrewed for testing. Adobe Photoshop was used to analyse the photographs of all quadrants of the specimens and peri-implant tissues. Two proportions were determined: (1) the area of cement remnants relative to the overall area of the abutment/restoration; and (2) the area of cement remnants relative to the total area of the implant soft tissue contour.

Excess on the crown groups were group-1 (0.002 ± 0.001); group-2 (0.024 ± 0.005); group-3 (0.036 ± 0.004) and group-4 (0.055 ± 0.007). The amount of undetected excess grew as the margin became deeper subgingivally (P = 0.000), and there was a significant difference between all groups (P 0.05). The soft tissue groups had the following remnants: group-1 (0.014 0.006), group-2 (0.052 0.011), group-3 (0.057 0.009) and group-4 (0.071 0.012). The increase in cement remnants was statistically significant as well as the difference between groups 1 and 2. Radiographic examination revealed cement residues mesially in four cases of 53, or 7.5 %, and distally in six cases of 53, or 11.3 %.

According to the findings of this investigation, the deeper the position of the margin, the more undetected cement was revealed. Dental radiographs should not be considered as a reliable method for cement excess evaluation.

Another study done by Linkevicius et al had the purpose to determine the relationship between patients with a history of periodontitis and development of cement-related peri-implant disease [14]. Between 2006 and 2011, in private practice, 77 patients with 129 implants were selected for this retrospective study from completed implant cases that were scheduled for routine maintenance or had mechanical or biological issues. Researchers analysed implants with extracoronal cement residues and implants without cement residues. The selected cases were then separated into two groups: implants in patients with a history of periodontitis (1) and implants in persons without a history of periodontitis (2). These groups were chosen based on the patient’s treatment history and orthopantomogram. As a control group, a set of 238 screw-retained implant restorations was investigated that were delivered to 66 patients throughout the same period. The incidence of peri-implant disease was assessed in all implant groups.

In 62 of 73 implants with cement residues, peri-implant disease was seen (85%). All implants in group 1 developed peri-implantitis—four cases of early disease and 35 cases of delayed disease. Twenty-one of 30 implants in the periodontally healthy group were diagnosed with peri-implant mucositis, 3 implants developed early peri-implantitis and 11 implants with cement remnants did not develop biological problems. Peri-implant illness was identified in 17 of 56 cases of implants without cement remains (30%). In comparison, just two occurrences of peri-implant disease were discovered in the control group of screw-retained restorations (1.08%).

This study concluded that implants with cement remnants may be more likely to develop peri-implantitis in individuals with a history of periodontitis than in patients without a history of periodontitis.

The literature established that each retention method has a number of advantages and disadvantages. However, there are some clinical situations in which one method of retention is preferable to the other. Shadid and Sadaqa’s review of the literature on screw-retained versus cement-retained fixed implant supported reconstruction identifies several clinical situations in which one method of retention is preferable to the other [15].

Clinical situations that prefer screw-retained restoration:

• Screw-retained large-arch implant reconstructions are preferred, as complications with these long-span prostheses are more common than with short-span prostheses.

• Screw-retained cantilevered prostheses are preferred, as some maintenance of restorative structures or implants is likely to be required during the life of such prostheses.

• Screw-retained restorations are preferred in patients who are at a high risk of developing gingival recession. This enables their uncomplicated removal and subsequent modification of the restorations to reflect the changed circumstances.

• Screw-retained restorations are preferred in patients who are expected to lose additional teeth in the future. This is to facilitate the removal of the restorations and subsequent modification of the restorations.

• In situations where there is little interocclusal space, adequate retention for cement-retained restorations may be impossible, as these restorations require a vertical component of at least 5mm to provide retention and resistance form. However, screw-retained restorations can be used with as little as 4 mm of interocclusal space. Additionally, screw-retained restorations can be directly attached to implants without the use of an intermediate abutment, reducing the amount of interocclusal space required for these restorations.

• In situations where it is difficult or impossible to remove excess cement (e.g. if the final restorative margin will be greater than 3 mm subgingivally, the use of screw-retained restoration is indicated). In this situation, an alternative to screw-retained restoration is to fabricate a custom abutment for cement retention with a restorative margin that follows the gingival contours.

• Screw-retained restorations are preferred in cases where technical or biologic complications are anticipated, as they allow for easy removal of the restorations, thereby resolving the issues.

Clinical situations that prefer cemented restoration:

• Cement-retained restorations are preferred for single-unit and short-span implant restorations, assuming that implant table size, implant number and abutment screw torque can be optimized. In such cases, screw retention would be used only if the implant’s long axis was excessively palatal in the anterior region.

• Cement-retained crowns are preferred in cases involving small diameter crowns where screw access may jeopardize the crown’s integrity.

• Cement-retained restorations are preferred in situations where the occlusal surface will be compromised in terms of aesthetics or occlusal stability as a result of the presence of a restorative material sealing the screw access.

• If the divergence between the implant axis and the retaining screw of the angled abutment receiving the restoration is less than 17 degrees, conventional screw retention of the restoration using premachined abutments is not possible.

Very valuable information for clinicians was identified in a more recent review by Hamed et al, which comprised 12 clinical research (randomized controlled trials, clinical trials, prospective studies and retrospective cohort studies) with at least 2 years’ follow-up time and published between 2010 and 2020 [16]. One of the most important advantages of cement-retained restorations is it’s the passivity and simplicity in manufacturing process in comparison with screw-retained restorations. This feature comes to light especially when zirconia is used as material for framework. The review indicates that the cement-retained implant approach is appropriate when enhanced predictability, a patient’s desire for superior aesthetic outcomes and a cost-effective method are present. Due to the significant complications associated with screw-retained restorations in terms of technical and prosthetic outcomes, cement-retained implant restoration results in more successful outcomes. Whereas a biological complication associated with the cemented implant promotes the use of screw-based implant reconstruction. Additionally, the screw-retained repair is more suitable for multiple unit implantation for patients with restricted inter-arch space. For instance, screw retention reconstruction is advised when inter-arch space is restricted (less than 4 mm) and retrievability is necessary. Similarly, cement retention can be used to compensate for inappropriately angled implants and when occlusion is easier to control without the hole.

It must be emphasized that prosthodontics plays a crucial role in maintaining mucosal homeostasis. Plaque accumulation and the soft tissue reaction are directly related to design, structural connections (screw-retained or cement-retained) and characteristics of materials. Proper prosthetic design with an appropriate emergence profile that promotes excellent oral hygiene and prevents plaque accumulation is unquestionably critical in preventing peri-implant mucositis [17]. According to de Tapia et al, when peri-implant tissue inflammation arises, the prosthetic design should be evaluated and, if necessary, adjusted to correct design issues that may obstruct good hygiene and to reduce biomechanical stress factors that may be involved [18].

3. Emergence profile

The ninth edition of the Glossary of Prosthodontic Terms (GPT9) defines ‘emergence profile’ and ‘emergence angle’ identically for natural teeth and implant prosthesis [19]. Emergence profile is defined as the contour of a tooth or restoration, such as the crown on a natural tooth, dental implant or dental implant abutment, as it relates to the emergence from circumscribed soft tissues. Emergence angle is the angle between the average tangent of the transitional contour relative to the long axis of a tooth, dental implant or dental implant abutment.

However, extrapolating these words to implant prostheses remains ambiguous at the moment, as there are no established outcome metrics or protocols to support quantitative measurements. Emergence profile and emergence angle are currently defined in terms of ‘circumscribed soft tissues’. While these can be clearly characterized and quantified in the relatively narrow periodontal sulcus, they present considerable complications when it comes to implant measurements [20].

The term ‘implant supracrestal complex’ has been recently proposed in order to describe the anatomic complex of human tissue, mechanical components and bacteria extending through the transmucosal part of an implant prosthesis. The paradigm of the ‘implant supracrestal complex’ aims to describe the human tissue in parallel with the design features of the implant-abutment-prosthesis complex and assists in identifying the role of design elements in health and disease of the peri-implant tissue [21]. The review article by Mattheos et al. investigated seven focus questions regarding emergence profile, emergence angle and/or ‘implant supracrestal complex’ [21]:

1. Is any particular design of the implant supracrestal complex’s emergence profile or emergence angle associated with an increased risk of peri-implant mucositis or peri-implantitis?

2. Is there evidence that peri-implant mucositis and peri-implantitis are more prevalent in bone-level implants than in tissue-level implants?

3. Is there evidence that certain components of the implant supracrestal complex increase the incidence of peri-implant mucositis or peri-implantitis by obstructing oral hygiene access?

4. Is there evidence that an increased risk of peri-implant mucositis or peri-implantitis is associated with the ‘implant supracrestal complex’ tissue height (total vertical tissue height and/or peri-implant sulcus depth)?

5. Is there evidence linking the material used in the abutment and/or prosthesis to an increased risk of peri-implant mucositis or peri-implantitis?

6. Is there evidence that the design and placement of implant-abutment-prosthesis junctions are associated with an increased incidence of peri-implant mucositis or peri-implantitis?

7. Is there evidence that the kind of prosthesis retention (cement or screw) increases the incidence of peri-implant mucositis or peri-implantitis?

The conclusions from this review article can be summarized as follows:

1. The highest rate of peri-implantitis (37.8%) occurred when a convex profile was combined with a restoration emergence angle of >30 degrees for the bone-level implants, but the same was not confirmed for tissue-level implants. The highest prevalence of peri-implantitis occurred in the combination of bone-level implants, emergence angle ≥30, convex profile and splinted-middle implant prosthesis.

2. Most of analysed studies did not find any significant difference in the prevalence of peri-implant mucositis/peri-implantitis or the respective clinical outcomes measures between tissue-and bone-level implants. Few of the analysed studies reported different prevalence of peri-implantitis between bone-level and tissue-level implants, yet no statistical comparison was attempted or if then being statistically insignificant.

3. The complete resolution of inflammation in cases affected by peri-implant mucositis was achieved in 66.6% of the patients who were treated with additional prosthesis contour modification versus only in 9.6% of the patients who received standard peri-implant mucositis treatment. Modifying the contour of the prostheses to improve access for oral hygiene significantly improved the clinical outcomes after standard mechanical treatment of peri-implant mucositis.

4. Sites with a shallow mucosal tunnel showed greater and faster resolution of inflammation after treatment compared with the deep ones.

5. Analysis of abutment material (titanium, zirconia or gold) and peri-implant tissue health outcomes measures reported no, or insignificant, differences.

6. Results from different studies concluded that the use or not of intermediary abutments on an external connection implant was not found to have any influence on the prevalence of peri-implantitis after 5 years. Marginal bone loss was significantly lower at superstructures connected to abutments compared with those at implant level. No significant difference was found between abutments with different surface topography.

7. Of the five analysed papers which suggested a difference, two papers found cement-retained restorations to be related to higher risk of peri-implantitis, while two found cement-retained restorations to be related to higher risk of peri-implant mucositis and one found screw-retained restorations to be related to higher risk of peri-implant mucositis.

Additionally, the authors state that there are insufficient data with bone-level implants to conclude that a large emergence angle in combination with a convex abutment or prosthesis may result in peri-implantitis. Additional study is necessary to characterize the emerging profile in respect to the real degree of peri-implant soft tissue and to interpret these results more accurately. A single randomized clinical study found no difference in the risk of peri-implant mucositis between tissue- and bone-level implants. Prosthesis modification may be an effective and necessary adjunct to anti-infective therapy for peri-implant mucositis in implant-supported prostheses with limited access to oral hygiene. At the moment, there are no data to suggest that increasing the vertical height of the peri-implant soft tissues alone increases the risk of peri-implantitis. However, it has been shown that treating established peri-implant mucositis is more difficult in the presence of a deep peri-implant sulcus. It has not been shown that the presence or absence of a prosthetic abutment, or the material of the abutment (Titanium or Zirconia), alters the risk of peri-implantitis. The evidence relating the kind of prosthesis retained and the risk of developing peri-implantitis is equivocal.

From a clinical standpoint, properly shaped dental implant restorations are critical for the treatment’s aesthetics and biological success. The primary challenge is the shift from a round dental implant to the cervical shape of the missing tooth. This transition is accomplished through the use of implant abutments. Su et al. [22] characterized this contour as having two adjacent but distinct zones within the dental implant abutment and crown, an apically located subcritical contour zone and a coronally located critical zone. The critical zone refers to the portion of the dental implant abutment and crown that lies between the free gingival margin and the deeper subcritical zone. This zone is circumferential in form, approximately 1 mm wide in the apicocoronally direction and is often convex or flat in shape. The critical zone may or may not contain a variety of restorative materials, depending on the kind of restoration (cemented or screw-retained). The subcritical zone is positioned apically to the critical zone and may be concave, convex or flat in shape. Changes in the shape of the critical and subcritical contour zones should be planned carefully in accordance with the dental implant site, soft tissue thickness and materials used. If the crown form cannot be adjusted, reshaping the subcritical zone can improve both the aesthetic and biological success of the treatment.

There are numerous strategies for peri-implant soft tissue conditioning, including immediate temporary restorations, custom-made healing abutments and gradual remodelling of soft tissues through modification of critical and subcritical zones of the temporary implant crown. Figure 5 shows the emergence profile shaping with custom-made temporary PMMA crown on PEEK abutment after the implant was integrated.

Figure 5.

Emergence profile shaping with temporary PMMA crown on PEEK abutment. Upper-left: initial clinical appearance with stock healing abutment; upper-right: size and shape of soft tissue emergence profile after removal of stock healing abutment; middle-left: lateral view of temporary PMMA crown on PEEK abutment; middle-right: frontal view of temporary PMMA crown on PEEK abutment; lower-left: clinical appearance 2 months after temporary crown delivery; lower-right: newly formed and shaped emergence profile with soft tissue maturation.

In clinical cases like the one shown in Figure 5, additional challenge may emerge during copying and transferring emergence profile shape to either digital or conventional stone cast model. In both ways, the clinician needs to act fast due to fast tissue begin to collapse immediately after removing temporary crown (or custom-made healing abutment). In conventional prosthodontics impression, fast and predictable way is open tray pick-up transfer customization intraorally or extra orally with flowable composite resin material. This technique with intra oral customization with flowable composite resin material is shown in Figure 6.

Figure 6.

Left: intraoral customization of open tray pick-up transfer and final impression with preserved emergence profile size and shape for final crown fabrication.

Additionally, the significance of the emergence profile and the interest of clinicians and researchers have increased significantly in recent years. Gomez-Meda et al. [23] defined a more detailed classification of emergence profile surfaces and areas. This article discusses the esthetic biological contour concept (EBC), which consists of distinguishing important zones of emergence profiles and recommending detailed design principles for those zones. The clinical significance of EBC is that it promotes aesthetic outcomes and a favourable biological response to implant-supported restorations when designed properly. The EBC concept denotes three zones that correspond to the subgingival contour of an implant restoration’s emergence profile (Figure 7). Each of these zones will come into contact with a distinct type of tissue and therefore must be designed differently.

Figure 7.

Schematic presentation of three zones of EBC concept: E—esthetic zone (blue), B—bounded zone (green) and C—crestal zone (red).

The EBC concept is divided into three zones:

• E Zone (esthetic zone) is a subgingival area that is 1 mm wide and located apical to the free gingival margin. It should be shaped similarly to the crown of the extracted or contralateral tooth to resemble a natural crown. Its contour should be convex and support the free gingival margin in the proper position, establishing the implant crown’s cervical morphology. This zone is adjacent to sulcular epithelium, a type of stratified squamous epithelium.

• B Zone (bounded zone) is the emergence profile area apical to the E zone that is approximately 1–2 mm wide in cases where the dental implant is ideally placed 3–4 mm apical from the free gingival margin zenith point. Although the B zone is normally concave, in patients with deficient soft tissues, connective tissue graft may be required to improve gingival phenotype, crestal stability and aesthetics. This biologic boundary zone is in contact with junctional epithelium, which is a non-keratinized epithelium.

• C Zone (crestal zone) is a 1–1.5 mm wide area immediately coronal to the implant neck. However, its dimensions vary depending on the depth of the integrated dental implant. In this area, the abutment contour should be flat or slightly concave to avoid putting pressure on the bone tissue surrounding the restoration. Figure 2 illustrates the detrimental effect of this pressure on crestal bone stability. Additionally, certain dental implant designs (i.e. tissue level implants) incorporate this zone into the implant body. This zone is critical for the stability of the crestal bone because it is in contact with connective tissue.

Each of the zones described in the EBC concept serves a distinct purpose in the emergence profile’s design. Understanding the significance and unique design features of the EBC zones enables the provision of aesthetic and biologically sound interim and definitive implant restorations.

4. Materials selection

Several new dental materials have entered the market over the last decade. They offer an aesthetic, functional and economical alternative to metal-ceramics, the most frequently used material for prosthodontic restorations. This is especially true for zirconium oxide and lithium disilicate ceramics. The incorporation of CAD/CAM manufacturing technology into daily work has resulted in a significant reduction in dental technicians’ labour costs. Furthermore, these increased aesthetic standards have resulted in an increase in the use of metal-free restorations at the expense of metal-ceramic restorations. These events also influenced the materials used and the manufacturing process for custom-made dental implant abutments, effectively eradicating stock dental implant abutments. Additionally, the titanium bases for implant abutments have been redesigned to incorporate anti-rotation properties and a cylindrical shape, allowing for more efficient extraoral cementation of prosthodontic restorations. Such prosthodontic restorations, particularly following the introduction of angulated screw access to the abutment screw, resulted in an increase in the proportion of screw-retained restorations versus cemented restorations. All these advancements are now being used more frequently in clinical practice, but they have also prompted scientists to explore new materials and techniques. Given the time, material, human and technical resources required to conduct a high-quality long-term prospective or retrospective study, there is still insufficient solid evidence of these new materials and technologies’ clinical benefits and effectiveness. However, prior research and the subjective clinical experience of numerous clinicians indicate that the new materials will eventually justify their partially uncritical use in clinical practice.

From the clinician’s perspective, 5- or 10-year success or survival rates are not the only criterion to consider when planning and implementing implant-prosthodontic treatment. Additionally, the clinician should consider the frequency with which technical and biological issues may emerge when specific materials are used.

With so much conflicting information and data, clinicians may depend on review articles that structurally describe and analyse more scientific studies on a given subject. Pjetursson et al. recently published a statement paper about material selection for implant-supported restorations [24].

4.1 Metal-ceramic implant-supported restorations

For a long period of time, metal frameworks veneered with feldspathic ceramic have been used in dentistry. They are well-researched and documented restorations that can be used for single crowns and fixed partial dentures. The metal framework provides a high-strength core, protecting the whole restoration against tensile and flexural stress during chewing function. Besides the conventional casting technique, metal framework nowadays can be produced by milling or an additive laser printing process. There are two important published meta-review papers that examine the clinical outcomes, success and survival rates of metal-ceramic implant-supported restorations, as well as the complications rates.

The meta-review analysing metal-ceramic single crowns [25] included 30 studies with a total of 4542 crowns, with 83% of cement-retained crowns and 17% of screw-retained crowns, respectively. The meta-analysis estimated an annual failure rate of 0.35% (95% CI: 0.19%–0.66%), which corresponds to a 5-year survival rate of 98.3%. The respective complication rates were 13.3%, which means that one out of eight metal-ceramic single crowns showed some technical, biologic or aesthetic complication or failure. Only 86.7% of the metal-ceramic implant-supported single crowns showed no complications over the 5-year follow-up period. The 5-year incidence rate of peri-implantitis and soft-tissue complications was 5.1%, and significant bone loss of more than 2 mm at marginal bone level was 3.3%. Technical complications, including fracture of abutments or abutment screws, were rare complications, with an incidence rate of 0.2%. Abutment screws loosening was more frequent, with a 5-year complication rate of 3.6%. The incidence of ceramic fractures and chipping was 2.9%, and framework fractures were only reported to be 0.2%.

Another meta-review by Sailer et al was analysing multiple-unit metal-ceramic fixed partial dentures [26] and included 16 studies with a total of 993 fixed partial dentures supported by 2289 dental implant abutments, with 73% of cement-retained fixed partial dentures and 27% of screw-retained fixed partial dentures, respectively. The annual failure rate for metal-ceramic fixed partial dentures was 0.26% (95% CI: 0.10%–0.64%), corresponding to a 5-year survival rate of 98.7%. The respective complication rates were 15.1%, meaning that one out of six fixed partial dentures had some kind of complication. Hence, 84.9% of fixed partial dentures were free of any complications over the 5-year follow-up observation period. The 5-year rate of peri-implantitis and soft tissue complications was estimated to be 8.5%. The significant bone loss incidence rate was reported to be 2.6%. Among technical complications, the incidence rate was reported as follows: abutment screws loosening was 5.3%, ceramic fractures or chipping was 11.6% and framework fractures were 0.5%.

Both metal-ceramic single crowns and multiple-unit fixed partial dentures are well researched with very good long-term success rates. They can be used as a treatment option in a wide spectrum of clinical indications, especially in clinical cases with high clinical implant crowns, cantilever types of implant restorations and implant-supported fixed partial dentures with distal units extending more than 8 mm, fixed partial dentures with more than two pontics and in cases with small connector height due to limited interocclusal space.

4.2 Zirconia-ceramic implant-supported restorations

Increasing aesthetic demands have led to the development of different subtypes of zirconia ceramics. With their appearance, they adequately imitate not only the appearance but also the structure of hard dental tissues. In addition, new generations of zirconia ceramics have excellent biocompatibility and improved mechanical properties. Previous generations of zirconia ceramics had an opaque whitish appearance and had to be veneered to make the restoration look aesthetically pleasing. Newer generations of zirconia ceramics come in multilayer blanks or blocks with different levels of translucency and can be used as monolithic restorations.

The previously mentioned meta-reviews also analysed zirconia-ceramic implant-supported single crowns and fixed partial dentures.

The review by Pjetursson et al [25] analysed eight studies with a total of 912 zirconia-ceramic implant supported single crowns for an average 5-year follow-up period. Of all the included single crowns, 51% were cement retained and 49% were screw retained. The annual failure rate for implant-supported zirconia-ceramic single crowns was 0.49% (95% CI: 0.21%–1.18%), which corresponds to a respective 5-year survival rate of 97.6%. The estimated 5-year complication rate was 16.2%, meaning that only 83.8% of implant-supported zirconia-ceramic single crowns were free of any complications over the complete 5-year observation period. The most frequent complication rates were: 5.3% for peri-implantitis and soft tissue complications, 4.4% for marginal bone loss of more than 2 mm, 2.8% for ceramic fractures or chipping and 2.1% for framework fracture.

Another meta-review by Sailer et al [26] included three studies with a total of 175 zirconia-ceramic fixed partial dentures and an average follow-up period of 5.1 years. Only 15% of all restorations were cement-retained and 75% were screw-retained. The annual failure rate for implant-supported fixed partial dentures was 1.455 (95% CI: 1.06%–1.98%), which corresponds to a respective 5-year survival rate of 93.0%. The most frequent complications were soft tissue complications with a 10.1% incidence rate and framework fracture with a 4.7% rate.

According to the previously mentioned research and numerous other published articles, today we cannot consider veneered zirconia-ceramic as the material of choice for implant-supported fixed partial dentures. They show a high degree of risk of chipping or catastrophic fracture of the restoration framework. The study by Larsson et al. [27] states that the frequency of chipping and framework fractures of fixed partial dentures is up to 50%, which is a clinically unacceptable value. These problems are largely eliminated by the use of monolithic zirconia-ceramic, which with its aesthetic properties satisfies everyday clinical applications. In addition to the lack of chipping, implant-supported restorations of monolithic zirconia-ceramic show greater fracture resistance because the framework of such structures has larger dimensions compared with the framework of coated veneered zirconia-ceramic restorations. Evidence of this is a recent systematic review paper by Pjetursonn et al. [28] that analysed the 3-year survival and failure rates of veneered and monolithic zirconia-ceramic implant-supported restorations. The estimated 3-year survival rates were 96.3% (95% CI: 93.9%–97.7%) for veneered zirconia-ceramic single crowns and 96.1% (95% CI: 93.4%–97.8%) for monolithic zirconia single crowns. Veneered single crowns showed significantly (p = 0.017) higher annual ceramic chipping rates (1.65%) compared with monolithic single crowns (0.39%). Interestingly, the location of the single crowns, anterior vs. posterior, did not influence survival and chipping rates.

When a clinician needs to choose between veneered or monolithic zirconia-ceramic implant-supported restorations, the following factors must be considered: aesthetic demands, location in the dental arch, physical properties of the material, possibility for surface modification and abrasion (wear) properties of the material [24].

4.3 Lithium-disilicate reinforced glass-ceramic implant-supported restorations

To improve the physical properties of glass-ceramic and make it more suitable for prosthetic restorations, lithium disilicate or, in rare cases, leucite fillers were added. Nowadays, there are several techniques for the production of lithium-disilicate reinforced glass-ceramic, such as heat pressing and CAD/CAM milling from prefabricated blanks. Due to its mechanical properties, lithium-disilicate reinforced glass-ceramic can be used for both implant-supported single crowns and short-span fixed partial dentures in the anterior region of the dental arch. A systematic review article by Pjetursson et al. [28] evaluated five studies reporting on veneered leucite or lithium-disilicate reinforced glass-ceramic implant-supported single crowns (a total of 110 crowns) and 14 studies on monolithic leucite or lithium-disilicate reinforced glass-ceramic implant-supported single crowns (a total of 484 crowns). The mean follow-up period for veneered single crowns was 8.1 years and 2.6 years for monolithic single crowns, respectively. Results show a low annual failure rate of 0.80% (95% CI: 0.14%–4.64%) for veneered crowns and 1.02% (95% CI: 0.51%–2.05%) for monolithic reinforced glass-ceramic single crowns. This means that 3-year survival rates were 97.6% for veneered single crowns and 97.0% for monolithic single crowns. The study also reported that monolithic reinforced glass-ceramic crowns had the lowest annual complication rate of 1.7%, and veneered reinforced glass-ceramic crowns had an annual complication rate of 2.6%. In comparison, annual complication rates for monolithic zirconia-ceramic single crowns were 3.6% and for veneered zirconia-ceramic single crowns were 4.5%.

Considering these meta-review results, it is reasonable to conclude that lithium-disilicate reinforced glass-ceramic implant-supported crowns are the treatment of choice for high aesthetic-demanding clinical cases in the maxillary anterior region Figure 8 shows such a clinical case with tooth #21 replaced by a dental implant where the implant-supported crown was made on a titanium base abutment customized with a zirconia CAD/CAM abutment and a lithium-disilicate reinforced glass-ceramic crown.

Figure 8.

Clinical case with tooth #21 replaced by a dental implant where the implant-supported crown was made on a titanium base abutment customized with a zirconia CAD/CAM abutment and a lithium-disilicate reinforced glass-ceramic crown.

5. Conclusions

Proper treatment planning prior to dental implant implantation is just as critical in current implant prosthodontics as the prosthetic components. The wonderful work of the oral surgeon may quickly be destroyed by inadequate prosthodontic execution, resulting in the failure of dental implant treatment.

The controversy between cemented and screw-retained dental implant restorations is as ancient as implant prosthodontics itself. Additionally, there are divergent views in the scientific literature. Although no substantial difference in survival has been shown between the two procedures, screw connection has demonstrated a total of less technical and biological problems. It is presently unknown whether cementation or screw retention is the preferable choice for restoring dental implants from a patient-centred clinical perspective. Both cementation and screw retention seem to have benefits and downsides in practical practice. Choosing between cement-retained and screw-retained restorations may be a matter of philosophy. By choosing cemented restorations, the physician is responsible for completely eliminating all cement residue. Peri-implantitis induced by cement remains is a completely iatrogenic disease with no blame assigned to the patient’s oral hygiene practices.

The emergence profile of a tooth or restoration, such as a crown on a natural tooth, a dental implant or a dental implant abutment, is described as the shape of the tooth or restoration in relation to its emergence from restricted soft tissues. Clinically, appropriately designed dental implant restorations are crucial for both the aesthetics and biological effectiveness of the procedure. The biggest difficulty is adapting the circular dental implant to the cervical form of the lost tooth. This shift is made possible by implant abutments. Changes in the critical and subcritical contour zones should be carefully considered in relation to the dental implant location, soft tissue thickness and materials employed. If the crown shape cannot be altered, altering the subcritical zone may significantly enhance the treatment’s cosmetic and biological success.

Over the recent decade, many innovative dental materials have reached the market. They provide an attractive, practical and cost-effective alternative to metal-ceramic restorations, the most often used material in prosthodontics. This is particularly true for ceramics made of zirconium oxide and lithium disilicate. Both metal-ceramic single crowns and multi-unit fixed partial dentures have a lengthy track record of success. They can be used to treat a wide variety of clinical indications but are particularly useful in cases requiring high clinical implant crowns, cantilever-type implant restorations, implant-supported fixed partial dentures with distal units extending beyond 8 mm, fixed partial dentures with more than two pontics and cases requiring a small connector height due to limited interocclusal space. When a clinician must choose between veneered and monolithic zirconia-ceramic implant-supported restorations, the following factors must be considered: aesthetic requirements, location in the dental arch, material physical properties, surface modification capability and material abrasion (wear) properties. Considering the findings of this meta-analysis, it is acceptable to infer that lithium-disilicate reinforced glass-ceramic implant-supported crowns are the treatment of choice for clinical situations requiring a high level of aesthetics in the maxillary anterior area.

Conflict of interest

The author declares no conflict of interest.