Journal of International Oral Health

: 2021  |  Volume : 13  |  Issue : 6  |  Page : 549--554

Restorative complications of the single implant-retained crown: A review

James Dudley, Arwa Bamajboor, Mansoor Walipoor, Jane Pellew 
 Adelaide Dental School, The University of Adelaide, Adelaide, South Australia, Australia

Correspondence Address:
Prof. James Dudley
Adelaide Dental School, The University of Adelaide, Adelaide, SA 5000.


Aim: To discuss the literature pertinent to implant and prosthesis success, restorative complications, limitations of reporting on complications, and management strategies for complications of the single implant-retained crown. Materials and Methods: Literature was reviewed from PubMed, Scopus, and Google Scholar databases and included selected systematic reviews. Articles in the English language and published in peer-reviewed journals were included. Results: The accuracy of the literature reporting on implant and prosthesis success was reliant on the methodology and accuracy of measurement within individual studies. The accuracy of the literature reporting on complications suffered from variable definitions and methodologies. Conclusion: There was a comprehensive body of literature that provided a sound evidence base on implant and prosthesis success; however, the inconsistent definition of an implant complication and lack of a universally accepted system for classifying implant complications restricted the clinical application of the literature and hindered inter-study comparisons. Guidelines for managing complications, particularly restorative complications that also allow for individual patient variations, should be developed.

How to cite this article:
Dudley J, Bamajboor A, Walipoor M, Pellew J. Restorative complications of the single implant-retained crown: A review.J Int Oral Health 2021;13:549-554

How to cite this URL:
Dudley J, Bamajboor A, Walipoor M, Pellew J. Restorative complications of the single implant-retained crown: A review. J Int Oral Health [serial online] 2021 [cited 2022 Jan 27 ];13:549-554
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Full Text


The replacement of a single missing tooth with an implant-retained crown often provides functional, aesthetic, and tooth preservation benefits over more traditional treatment alternatives. In western society, the use of dental implants to replace missing teeth is increasing in numbers and rate, with an estimated prevalence in the United States projected to increase to 23% in 2026.[1]

Implant treatment is an elective option, with patients needing to undergo at least one surgical procedure. Ideally, implant treatment should be restoratively driven with the role of the restorative dentist central to success. Clinicians with different levels of training and experience are now placing and restoring dental implants. It has been proposed that operator experience may influence implant success, although this is likely skewed by more difficult patients attending more experienced clinicians.[2],[3] Despite the established long-term single implant and crown survival rates being greater than 90%, complications do occur and require time, effort, and expense to manage.[4],[5]

Clinicians undertaking implant restoration must have detailed procedural knowledge and understanding related to impression taking, abutment and crown materials, methods of crown retention, crown insertion, screws, torque drivers, and restoring screw access channels. Although the literature is filled with individual and generally favorable studies on implant and prosthesis success, the reports on complications create more confusion. This review discusses implant and prosthesis success, restorative complications, limitations of reporting on complications, and management strategies for complications of the single implant-retained crown drawing evidence from the pertinent literature.

 Materials and Methods

A thorough literature search was conducted by using PubMed, Scopus, and Google Scholar databases using PRISMA guidelines [Figure 1]. The following MeSH headings and keywords related to the single implant-retained crown were used: “implant success,” “prosthesis success,” “restorative complications,” and “management of complications.” The literature was limited to full-text English language and peer-reviewed journals Selected systematic reviews directly relevant to the study aims were included. Selected articles were identified and systematically reviewed by all authors. After initial manual screening, 22 articles relevant to the study were reviewed.{Figure 1}


There was a comprehensive body of literature that provided a sound evidence base on implant and prosthesis success. The accuracy of this literature was reliant on the methodology and accuracy of measurement within individual studies.

The accuracy of the literature reporting on complications suffered from variable definitions and methodologies. A consensus on what constitutes acceptable prosthesis life span, maintenance, and repair were identified as limitations. There were no clinical guidelines for managing restorative complications. Overarching clinical guidelines for managing peri-mucositis and peri-implantitis have been developed.



Clinical outcomes are often evaluated by success and survival, sometimes erroneously interchangeably. Survival assesses whether an implant or prosthesis is present in the mouth at the time of examination regardless of its condition whereas success assesses an implant or prosthesis against set criteria.[6] One of the first success criteria for implants was proposed by Albrektsson.[7] Since this time, multiple implant and prosthesis success criteria involving various combinations of prosthesis assessment, condition of surrounding soft tissue, occurrence of complications, function, aesthetics, and patient satisfaction measured over appropriate periods of time have been suggested.[8] Objective aesthetic indices that measured aesthetic outcomes such as the Pink Esthetic Score and White Esthetic Score have been proposed.[9] It has been reported that for every additional included success criterion in a study, there was an average of 4.1% reduction in the success rate.[8] This is of key importance when interpreting the literature. Implant success is different from crown success, but both collectively make up treatment success.

Implant and prosthesis success

In a systematic review of single implants, the implant survival rate was 97.2% at five years and 95.2% at 10 years. Of the failed implants, 1.3% were lost before loading whereas 1.5% were lost after loading.[5]

In a systematic review of single implants, the crown survival rate was 96.3% at five years and 89.4% at 10 years.[5] These values were found to be slightly higher than previously reported survival rates[10] and were assumed to be due to the advancement in the implant design and materials and improved prosthetic parts.[5] There have been no reports in the literature of specific tooth sites being associated with greater or lesser prosthesis success rates.

Restorative complications

The approaches to classifying restorative implant complications are inconsistent. Some encompass the complete scope of treatment whereas others focus on the phase of therapy, such as surgical or prosthodontic. Restorative complications can be categorized into technical and aesthetic ones.[5] Although biological complications are generally accepted to constitute a separate category, they may have restorative causes.

Technical complications

The most frequent technical complication in single implant-retained crowns is abutment or screw loosening, with a cumulative incidence of 8.8% after five years.[5] There has been a reported reduction in the incidence of screw loosening, from 25% in 2003 to 3% in 2018 due to advances in screw design, changes in screw materials, the use of torque devices, and the use of recommended torque values.[11],[12]

Several factors are believed to contribute to the likelihood of screw loosening, including the type of implant connection, internal or external hex, anti-rotation features, and the insertion torque value of the prosthetic screw.[13],[14] Screw loosening can be a sign of occlusal overload, which can, in turn, predispose to screw fracture.[15] Both screw loosening and screw fracture can manifest clinically as a loose crown. In addition to careful and appropriate case selection, there have been various suggestions to minimize screw loosening, including:[16]

Providing an “implant protected occlusion” designed to protect by reducing the occlusal load on an implant restoration.[16]

Applying the restorative screw torque as recommended by the manufacturer, with the optimum preload established at 75% of the yield strength of the screw to provide appropriate clamping effect.[17],[18],[19]

Periodic calibration of torque wrenches to maintain accuracy.[20] that allows applying torque to within 10% of the target value.[21]

Following torquing the screw to the desired torque value allowing ten minutes allowing for the “settling effect” before retorquing to the desired value.[18]

The second most common technical complication is the loss of retention of cement-retained crowns, with a cumulative incidence of 4.1% after five years.[5] This complication is associated with inadequate inter-occlusal space, with 4 mm cited as a minimum.[22] There are a variety of factors related to crown retention, including preparation taper, cervico‐occlusal wall height, presence of retentive grooves, surface finish, and type of cement.[14],[23]

Chipping or fracture of the veneering material (usually ceramic or acrylic in bilayered restorations) is another technical complication, with a cumulative incidence of 3.5% after five years.[5] The rate of ceramic fractures and chipping over five years was similar for metal-ceramic (2.9%) and zirconia (2.8%) implant-retained crowns.[24] However, zirconia crowns had significantly more failures due to material (coping) fracture than metal ceramic (2.1% versus 0.2%).[24] Chipping and fractures can be seen more frequently in patients exhibiting parafunctional activity.[25]

Aesthetic complications

Aesthetic outcomes have patient and clinician focused elements. Aesthetic restorative complications have a cumulative incidence of 7.1% after five years.[5] The most common include unsatisfactory tooth shade, shape, or position; poor emergence profile; uncamouflaged labial screw access channel; apical migration of gingival tissues; and loss of interdental papilla.

Biological complications with restorative causes

Biological complications include soft tissue presentations such as fistula, excessive swelling, hyperplasia, peri‐implant mucositis, and peri-implantitis. Peri‐implant mucositis is characterized by bleeding on gentle probing, erythema, swelling, or suppuration[26] without loss of supporting bone,[27] whereas peri‐implantitis is defined as “a plaque‐associated pathologic condition occurring in the tissue around dental implants, characterized by inflammation in the peri‐implant mucosa and subsequent progressive loss of supporting bone.”[26]

The cumulative incidence of soft tissue complications is 7.1% after five years.[5] There is a reported 5.2% cumulative incidence of greater than 2 mm bone loss around single implant-retained crowns,[5] although it has been reported to be as high as 17.8%.[27] The cumulative incidence of greater than 2 mm bone loss is slightly higher for cement-retained crowns than screw-retained crowns.[5]

The major risk indicators for the development of peri-implantitis include a history of severe periodontitis, poor oral hygiene, and lack of regular supportive care after implant restoration.[26],[28] An inaccurate implant to abutment (microgap) or abutment to crown (marginal gap) connection and over-contoured crown are common causes of plaque (or “bacterial biofilm”) accumulation and can cause peri‐implant mucositis and peri-implantitis.[26],[29],[30] Excess cement and cement remnants are also associated with peri‐implant mucositis and peri-implantitis,[30] with 85% of implants with implant disease having cement remnants.[31]

Limitations of reporting on complications

There is no universally accepted system for classifying implant complications, which appears congruent with the inconsistent definitions of an implant complication proposed in the literature. Some examples are “unexpected deviations from the normal treatment outcome”[32]; “may represent an increased risk of failure but are either of temporary significance or are amendable to correction”[33]; “chair time is required after incorporation of the prosthesis”[6]; “a reversible or irreversible unfavourable condition.”[34] Importantly, a complication does not necessarily imply an error was made during the planning, treatment, or review stages, or that there was a negative impact on the patient.

Complications (unexpected events) are distinct from failures (requiring intervention), maintenance (expected events), adjustments (interventions to continue service) and re-treatment.[35] It remains unclear as to what constitutes acceptable maintenance and repair.[35]

An example of a poorly placed implant can illustrate the difficulties in reporting complications in implant dentistry. A poorly placed implant may lead to an unsatisfactory aesthetic outcome, potential associated restorative complications in the attempts to camouflage the implant positioning, removal of the poorly placed implant, and replacement of the implant in a more optimal position. The interventions could be summarized by the original error as one single complication (the poorly placed implant) or by the interventions required to address the error as three or more complications that may, in addition, involve aspects of maintenance, adjustments, and re-treatment. Alternatively, the patient could accept the unsatisfactory aesthetic outcome and elect not to pursue further management, resulting in the recording of no complication.

A further challenge is that the same adverse occurrence (e.g. a loose abutment screw) may have several outcomes, ranging from simple to severe in screw-retained and cement-retained crowns. The reporting of complications in isolation without consideration of the steps that are needed to remedy the complication does not reflect the complete picture. There can be overlap of a single complication into multiple categories.

At times, it is not always easy to determine whether a complication has actually occurred, for example in the case of an aged crown where the color has departed from that of adjacent teeth. Further, confusion exists when the aged and worn prosthesis requires modification (or replacement) and whether this constitutes a complication.

Inter-study comparisons of different complications are often difficult and reporting of units of frequency can be problematic, as occurrence is reported by the number of units treated and per unit of time.[36],[37] The reporting of complications does not consider the complexity of the initial treatment. Average numerical value reports of complications related to certain procedures may not be applicable to all clinical situations or operators.

The literature reports on implant treatment samples that are non-standardized. Patients are selected for implant treatment with a goal of providing successful implant treatment, and then they agree to undergo implant treatment procedures. Patients who are not suitable or do not wish to undergo the required procedures or are unable to afford the treatment are excluded. The only inclusion criterion is that implant treatment has been completed; the degree of difficulty and risk are not represented. The samples are not randomized, rather they are selected for success.

Management of complications

It is not the intention to present an all-encompassing review of all complication management strategies, rather to outline some guiding principles for the previously discussed most common complications. It is important for the restorative dentist to be aware of the essential concepts even if management is via referral to an appropriate specialist.

Loose but undamaged and intact screws can generally be retightened with good success.[38] Damaged or fractured screws require judicious removal and replacement. A screw fractured at the base of the head, leaving the shank and threads otherwise undamaged, is usually free to unwind out of the internal implant configuration by using retrieval tools, as all preload has been lost. The challenge can be accessing the residual screw without causing damage to surrounding components. The routine replacement of screws is not recommended, but the routine assessment of screw tightness is recommended to minimize additional and more severe complications.[38]

Strategies to minimize chipping and fractures include providing adequate thickness for the ceramic, avoiding heavy occlusal contacts, reducing the occlusal table and cuspal heights, considering metal occlusal surfaces or monolithic zirconia, patient risk education and providing a protective nightguard.[25] The extent and location of chipping or fracture influences whether the crown is polished, repaired, or replaced. Repair procedures may be conducted intra-orally or extra-orally.

Although classifications and techniques have been proposed for the intra-oral repair of natural tooth crowns,[39] there is no supporting evidence for the clinical success of this procedure for implant-retained crowns. An intra-oral repair has cost and time benefits but may not have longevity. When planning extra-oral repairs of chipping or fractures, the expected ease of crown retrievability is a key consideration in this decision, with screw retention offering significant benefits. When an implant-retained crown is successfully removed from the implant, it may be necessary to make an implant-level impression or supply the recent implant model to the laboratory to facilitate the repair, particularly if contact points or occluding surfaces are involved.

The management of aesthetic failures in implant dentistry is a complex area due to the often specific and individual nature of aesthetics and the lack of standardization in the classification of aesthetic failures in the literature, resulting in difficulties in providing generalized guidelines.[5],[40] The cause of an aesthetic restorative issue should always be first identified. The management options are largely dependent on the specific issue and associated risk factors, the management of further risk, and consideration of the patient’s desires and expectations. Some examples of techniques to manage aesthetic restorative complications include remaking the crown, restorative camouflage using pink porcelain or a ceramic (more aesthetic) abutment to mask tissue discoloration, soft tissue grafting, or removing and replacing malpositioned or diseased implants.

Peri-mucositis and peri-implantitis is commonly managed by reference to specific protocols, such as that proposed by Heitz-Mayfield et al. (2018)[27] involving:

Thorough assessment and diagnosis

Control of modifiable local and systemic risk factors for peri‐implantitis

Nonsurgical debridement

Early reassessment of peri‐implant health, generally within one to two months

Surgical access if resolution has not been achieved, including open flap debridement, thorough surface decontamination of the implant and associated prosthetic components, option of regenerative/reconstructive or resective approaches, and appropriate postoperative anti‐infective therapy

Supportive care tailored to the patient risk profile, most likely three to six monthly

The effectiveness of this approach has been demonstrated with an estimated 76% to 100% implant survival rate at five years after therapy and 70% to 99% at seven years.[27]

Limitations and future recommendations

This narrative review was limited to selected literature and was not all-encompassing or blinded. The accuracy of the literature reporting on implant and prosthesis success was reliant on the methodology and accuracy of measurement within individual studies. The accuracy of the literature reporting on complications suffered from variable definitions and methodologies, and there is a need to develop an accepted and standardized system of classifying implant complications. Guidelines for managing complications that also allow for individual patient variations should be developed.


Clinicians undertaking implant restoration must have a detailed procedural knowledge of materials and techniques as well as an understanding of the success, complications, and associated complication management strategies. There is a comprehensive body of literature that provides clinicians with a sound evidence base on implant and prosthesis success. The lack of a universally accepted system for classifying implant complications restricts the clinical application of the literature and hinders inter-study comparisons. Guidelines for managing complications that also allow for individual patient variations should be developed.



Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

Author contributions

Not applicable.

Ethical policy and institutional review board statement

Not applicable.

Patient declaration of consent

Not applicable.

Data availability statement

Data are available upon a valid request to the corresponding author.


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