Introduction

Dental implants are at the forefront of today's clinical practice, providing patients with answers for the treatment of all forms of edentulism.1 For many patients, it has now become the standard pathway of care.2

With more and more dentists of all levels of experience restoring dental implants there is a need to understand and manage the factors that govern the degree of complexity of dental implant restoration.

The General Dental Council3 states that clinicians must have the skills and knowledge necessary for competence, and provide a good standard of care based on the available up-to-date evidence and reliable guidance. The clinician must take an evidence-informed and patient-centred approach.3

Despite this requirement, there is little assessment of the evidence that supports the most frequently used indices of complexity, or critical analysis of the factors affecting it.

Understanding the potential complexity of any treatment enables the clinician to judge their competency to provide it. Paradoxically, for the inexperienced clinician, it is the plethora of unanticipated factors that compound complexity.

Objective

The aim of this paper is to identify the factors that affect the complexity of implant restoration and to explore the indices that help us to assess it. With this knowledge the growing number of clinicians restoring dental implants will have a better understanding of the available guidance and evidence base, and the differing levels of competence required.

Study design

An electronic search was performed utilising PubMed, mbase, ncbi and Google Scholar. Key search terms were: 'Dental Implant Restoration', 'Complexity of Treatment AND Dental Implants' and 'Complications AND Dental Implant Restoration'.

We restricted our search to English language journals only. The search was restricted to systematic reviews and meta-analysis. Case reports or case series, observational studies, review articles/letters and studies which did not assess dental complexity were excluded from the review.

After title and abstract screening 17 of the 5,970 publications searched fulfilled the above inclusion criteria, and were identified for full text reading. The articles used to support the two published assessment tools were also reviewed to establish the evidence base supporting these guides.

Through consensus discussion drawing on these published articles and clinical experience, the authors generated a list of the key factors that affect complexity.

Assessing complexity

Two key guides are available: International Team for Implantology's Straightforward Advanced Complex tool4 and the Cologne ABC risk score.5 While these guides help identify treatment complexity they provide a limited evidence base from which to base clinical decisions.

The ITI SAC6 was developed following a consensus conference of a multidisciplinary group of 28 clinicians. It was developed to aid clinicians in risk assessing and planning surgical and restorative cases.6 This guide includes a broad spectrum of evidence from 1981 to 2015, including systematic reviews, consensus statements, primary research papers and ITI treatment guides. The appropriate protocols were then developed by a consensus from expert clinicians. The guideline, however, has a potential for bias given its funding by Straumann AG Holdings for use with ITI implants.

The second guide is the Cologne ABC Risk Assessment.5 This is clearly laid out and is based on applying a red, amber, green scale to identified risks for complications, which they relate to complexity. It, like the ITI SAC, was developed by a consensus group, but on comparison, the ITI SAC classification covers restorative processes in greater detail.

The assessment features of the guides are compared in Table 1, and the key articles used to compile the guides is summarised in Table 2.

Table 1 Comparison of assessment features of the ITI SAC4 and Cologne ABC Risk Assessment5 guides
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Table 2 Assessment of the key articles supporting the indices
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Factors affecting complexity

Patient factors

Expectation and communication

High patient expectations lead to increased case complexity and are commonly due to the insufficient provision of information.7 Strietzel reported that 75% patients were given insufficient information regarding all available treatment options, risks, outcomes and costing.

Medico-legally it is essential to discuss the risks and benefits of both having and not having treatment, and this is to be accompanied by a breakdown and explanation of available treatment choices including the range of cost.3 Furthermore, a practitioner must ensure the patient has knowledge regarding the management of complications or failures and this information must be fully documented. Only by providing this can consent be fully obtained.8 This process of informed consent ensures that the patient's expectations are realistic and the complexity is modified.

Communication in any treatment is vital;9 an understanding between the clinician and patient regarding reasonable expectations must be achieved.10 Belser et al. lists patients' expectations as having a long lasting aesthetic and functional result that has as high degree of predictability, involves minimal invasion and risk, with discomfort and is cost effective.11

Oral environment

The oral environment presents a key factor in the complexity of treatment. Dawson and Martin concluded that a healthy mouth, free from active disease should be established before embarking on implant treatment.6

The original reason for tooth-loss should be established; tooth-loss through periodontal disease and bruxism predisposes a patient to an increased risk of complications and will guide the restorative treatment plan. The possibility of modifying the shape of adjacent teeth may also be desirable; virgin teeth adjacent to an edentulous saddle may hamper a clinician's ability to make these adjustments, making the treatment more complex.

Limited mouth opening hinders access to posterior sites. Thirty millimetres of opening is regarded as a minimum requirement for access. Restricted access may not only alter the treatment plan but can also limit the number and position of the implants.6

Aesthetics

Aesthetically replicating a natural tooth is of particular importance when in the aesthetic zone. This was defined at the 'Third ITI Consensus Conference: Esthetics in Implant Dentistry' as 'any dentoalveolar segment that is visible upon full smile, or has esthetic importance to the patient'.12 Most people show over 75% of their incisors and interproximal gingivae,13 therefore anterior implant treatment is considered to be more complex.4

Belser, Buser and Higginbottom define aesthetic restorations as being in harmony with the peri-implant tissues, including tissue health, volume, contours and colour.14 They propose that replacing single teeth without tissue deficiencies can be achieved predictably with implants. Aesthetically challenging cases, replacing soft tissue, and replacing multiple adjacent teeth is more complex.11

A systematic review reported that restoring multiple implants further complicated treatment, and this is associated with increased implant loss following loading.15 This was also reported by Salvi and Brägger who found greater complications with longer supra-structures, though this study was much less robust16.

Intra-arch distance and edentulous space

Consideration must be given not only to the complexity of fabricating the prosthesis, but also to long term maintenance. Mandibular implant retained overdentures are straightforward to restore17 but have a greater risk of long term complications compared with fixed reconstructions,15 although these tend be of a lesser severity.11

Splinting implants for overdentures may again increase the complexity of the restorative treatment, however a systematic review has shown splinted implants have lower maintenance requirements.18

Reduced inter-arch distance may necessitate the creation of the 'ideal' space, complicating treatment.2,6 In a situation with limited vertical height, the selection of a 'bone level' implant may allow for increased restorative options due to the prosthetic components and increased flexibility in the positioning of the prosthetic margin. This makes the treatment less complicated.6

In the case of the edentulous arch, space availability may dictate the restoration of choice. Carpentieri et al. concluded that, with space of less than 10 mm, a fixed prosthesis would be highly indicated; with space of greater than 15 mm, a removable prosthesis would be preferrable.19 The type of restoration has a direct impact on the level of complexity.

Provisional restorations

Excessive inter-arch space may result in an aesthetic compromise. In the aesthetic zone there may be a requirement to prosthetically replace the missing hard and soft tissues, directly affecting the level of complexity.2,6 Pink porcelain may be used to mimic the gingival position, although an aesthetic compromise may still result.

Restricted or increased mesio-distal space complicates treatment. This leads to difficulties in developing contact points, emergence profile, contouring of the prosthetics material, and symmetry. If there is asymmetry of more than 1 mm between contralateral central or lateral incisors, or canines, adjunctive treatments may be required in order to achieve an acceptable aesthetic result.6

Soft tissue

The soft tissue position is dictated by the underlying bone.20 Identifying the tissue biotype helps predict the potential for soft tissue loss; thin biotypes, with highly scalloped gingival margins and prominent papillae, are more prone to tissue loss following extraction.21 Square teeth are associated with thicker biotypes,22 simplifying treatment as less soft tissue infill is required.

A distance of below 5 mm between the contact point and the bone is conducive to soft tissue infill.23 Furthermore, Grunder suggests that the restorative status of the adjacent tooth is the critical factor in dictating papilla height,24 unrestored teeth offering the most predictable prognosis. Adjacent implants, on average, only achieve 3.4 mm (from the bone to the contact point) of tissue infill as reported by Tarnow, thus optimising aesthetics in these cases is more complex.25

The presence of keratinised tissue around an implant is important for peri-implant health. Clinical studies have shown increased bone loss and decreased soft tissue health when keratinised tissue is thin or absent, it therefore follows that the lack of keratinised tissue increases the complexity.26,27 The evidence base for these conclusions is low.

Morton et al.'s consensus statement suggests soft tissue augmentation can be effective in replacing lost tissue,28 however the need for soft tissue grafting increases complexity. Systematic reviews by Rotundo et al.29 and Poskevicius and Galindo-Moreno30 both illustrate soft tissue gains are possible at the time of implant placement, but highlight that tissue loss does occur with remodelling.

Occlusion

The incisal relationship effects complexity: a complete overbite, significant overjet or retroclination increases complexity. This leads to the potential for adverse horizontal forces on abutment components.6 Severe malocclusion can lead to the need for adjunctive orthodontic or restorative therapy before embarking on implant treatment; if left untreated, this can lead to mechanical complications due to non-axial loading or the ability to apply excessive force to the restoration.6,31 In contrast Koyano and Esaki, found limited evidence to advocate a particular occlusal scheme.32

Parafunctional forces increase the risk of complications due to the application of excessive force.2 In addition to this, the reduced proprioceptive feedback from a restored dental implant can lead to up to four times greater loading of an implant-supported restoration compared with a natural tooth.2,32,33 The presence of parafunctional activity may necessitate a reduced occlusal table width, avoidance of non-axial forces, and the use of an occlusal splint.

There is an increased complexity when providing full-arch prostheses and bilateral free end saddles when compared with single tooth restorations.34 Cantilevered restorations, which can place considerable off-axis force on an implant, present a significant technical risk factor.35 Brägger's findings are supported by Pjetursson's meta-analysis.36

Human factors

A key factor influencing the complexity of treatment is the clinician.6 The role of the clinician is to assess, investigate, diagnose, and advise the most appropriate treatment options. They then are required to carry out treatment to an appropriate standard of care, while continually ensuring that the patient is fully informed and able to consent to the treatment provided.6

Complex cases often involve a multidisciplinary approach. The use of protocols and tools, such as those discussed, aimed at identifying high-risk or complex cases can aid clinicians in accurately assessing these cases at an early stage.

Team communication

There is a causal relationship between communication and complications.37 Effective communication can prevent active errors and reduce complexity. Team briefings play a vital role,37 as does the implementation of customised safety checklists and creating opportunities 'for all team members to speak up and exchange information'.38 A non-prejudicial environment for all professionals to voice concerns further improves safety and reduces complexity.39

A method for reporting errors is paramount and provides the best tool for exposing their cause, thus enabling action to be taken and lessons learnt.37 An implant log, as recommended by the GDC,3 is a valuable tool that may be used by the operator for reflective analysis and learning.

Eradicating human error is not possible but can be managed. Belser et al. advocate systematic therapeutic protocols to develop the highest degree of predictability.11 A thorough treatment plan therefore should provide a comprehensive step-by-step guide from assessment through to treatment completion and maintenance, thus assessing and managing its complexity.40 Furthermore, it should minimise the incidence of unanticipated outcomes, thereby simplifying treatment in the subsequent stages of restoration and maintenance.

Afsharzand highlighted the frequency of poor communication between the dentist and laboratory, which inevitably leads to increased treatment complexity.41

Systemisation provides defences, barriers and safeguards against error, thereby reducing complexity.42

Work environment

There are many elements conducive to providing a properly functioning work environment. Training and development has traditionally been viewed as fundamental to reducing error and complexity: 'Education has traditionally been based on the assessment of technical proficiency rather than human interaction'.43 The GDC's new curriculum framework document recognises leadership and team work as core education.44

Formal and appropriate training is a vital and a necessary requirement.45 However, training is only one aspect that enables clinicians to assess complexity. Ongoing evaluation and appraisal builds a strong working environment, providing support to all team members.

Technical factors

Impression techniques

Impression taking and casting must be accurate; any errors will complicate the fitting of the prosthesis.46 Correct selection and seating of impression copings is critical with evidence suggesting open tray impressions are most accurate.47 The magnitude of inaccuracies multiplies with each additional implant, thus increasing the complexity, an effect further compounded by distortion when constructing the suprastructures.48

Ill-fitting suprastructures may be non-passively seated on implants, possibly causing long term complications such as screw loosening49 or potentially bone loss.50 Passivity therefore must always be checked; the Sheffield Test51 can help, but is relatively subjective. Splinting impression components together while taking impressions reduces errors47 but distortion of the splinting material must also be considered.52

Modern digital impressions are quicker and simpler than conventional impressions53 and eliminate model casting, although there is currently no strong in vitro evidence in favour of their accuracy.47

Provisional restorations

Provisional restorations play a key role in maintaining and developing soft tissues.20 It is clear that the need to use provisional restorations increases the level of restorative complexity. Removable appliances are easier to fabricate than fixed provisionals4 although the latter may allow for better tissue management and ultimately a more aesthetic definitive restoration.54

The use of provisional restorations varies between cases, and in some cases, no provisional restoration is required at all. Belser, Martin et al. concluded that inappropriate or non-use of provisional restorations may lead to aesthetic complications.12

If there is a need to use provisional restorations to develop symmetry of the hard and soft tissues and a 'natural appearance' of the papilla between prosthetic teeth, then this increases the complexity.6,55 In the case of replacement of multiple missing teeth, a lack of adjacent teeth results in the need for alternative methods to create the appearance of natural papillae. This may include tissue-shaping with provisional restorations, ovate pontics and the use of pink porcelain.6,55 The junction between the prosthesis and mucosa may present further aesthetic challenges, and may be acceptable or unacceptable dependent on whether this area is seen during normal function or smiling, this in turn has a bearing on the level of complexity.

Type of retention

The prostheses can be definitively restored with either screw or cement retention. Shadid and Sadaqa provide a useful overview of retention although this is low level evidence due to their non-systematic selection of literature.50 This study suggests that the depth of the restoration margin below the gingivae must be considered for a cemented prosthesis to enable removal of excess cement.56 If this is not achieved, it can lead to peri-implant inflammation.56

Screw retention is safer for deeper margins and gives the optimal marginal fit.57,58 This is not always suitable due to the position of the screw access channel, which compromises the occlusal integrity of the crown. Screw retention is arguably more complex and raises concerns about passivity for larger reconstructions; however retrievability is often cited as its overriding benefit.14

Wittneben, Millen and Brägger researched articles in multiple languages from a broad spectrum of clinical settings with a strict inclusion and exclusion criteria. Their review also contained meta-analysis with a long follow-up, and they found fewer technical complexities and biological complications with screw retained prostheses than cemented ones. Implant survival was not significantly different.59

Each case must be assessed individually; however both cement and screw retention increase the complexity of implant restoration or maintenance if incorrectly utilised.

Loading protocol

Dental implants can be restored immediately (within one week), early (within two months) or conventionally (after two months).60 Emerging evidence suggests immediate loading is appropriate when adequate primary stability is achieved.60,61

Moraschini et al. conducted a meta-analysis of RCTs looking at immediate or conventional loading of single implants in the posterior mandible. They concluded that there was no significant statistical difference between immediate and conventional loading of dental implants. However, the development of a large number of RCTs was required in order to monitor immediate loading techniques.62

There is significant heterogeneity regarding the timing of immediate loading and the quantity or measurement of primary stability. Without a clear consensus and evidence, immediate loading is still unpredictable and as such, is considered to greatly increase treatment complexity.4 Immediate loading should only be attempted by experienced clinicians;63 conventional protocols should be followed in practice while the clinician is inexperienced.

Early loading in the edentulous maxilla is considered to be more complex than in the mandible, primarily due to the relatively increased volume of cortical bone in the mandible.

Evidence is emerging for the immediate restoration of the edentulous arch using a splinted, passive-fitting restoration, however the complexity associated with planning, fabrication and delivery of such a prosthesis is considered to be high.6,63

Conclusions

A thorough understanding of complexity is of particular importance for the inexperienced clinician, who must be able to scrutinise their own competence in providing treatment, know when to refer a patient, and when to seek additional mentoring.

There are many interconnecting factors which affect the complexity of dental implant restoration, and these have been explored in this paper. Furthermore the two widely used indices for the assessment of complexity have been investigated, and although these offer a good guideline as to the level of complexity, there is a lack evidence to support their use. Based on the findings of this paper a practical guide to aid practitioners in reducing complexity has been proposed (Table 3).

Table 3 Guidance to reduce complexity in dental implant restoration
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The development of evidence-based treatment and protocols is necessary to further develop the current indices, and these need to be expanded to include other critical areas, such as human factors.