Prosthetic failures in dental implant therapy

Abstract Both fixed and removable implant‐supported prostheses are well‐established methods for replacing missing teeth in partially or fully edentulous patients. Numerous systematic reviews have been performed in recent years to evaluate the survival and complication rates of implant‐retained fixed dental prostheses and implant‐retained overdentures, displaying high 5‐year survival rates ranging from 97.1% for fixed dental prostheses to 95%‐100% for implant‐retained overdentures. However, the survival rates only represent the prostheses remaining in use for a defined follow‐up time, and do not account for the potential prosthetic complications that may have arisen and influence the general success of the implant treatment. The most common technical complications of fixed implant‐retained single crowns are crown fracture, fractures of ceramic implant abutments, and esthetic problems. The predominant technical complication at multiple‐unit, implant‐retained fixed dental prostheses is fracture/chipping of the veneering ceramic. Reported technical complications for implant‐retained overdentures are overdenture fracture or chipping of the veneer materials, whereas mechanical complications include implant fracture, attachment failure, and attachment housing or insert complications. To reduce the risk of such failures, a comprehensive pretreatment diagnostic work‐up is essential, including defining the prosthetic goal with the aid of a wax‐up or set‐up and the associated ideal, prosthetically oriented three‐dimensional implant position. Furthermore, selection of the ideal type of prosthesis, including the respective implant components and materials, is important for clinical long‐term treatment success.


| INTRODUC TI ON
Missing teeth can either be replaced by fixed or by removable In the case of single tooth gaps or partially edentulous areas framed by healthy neighboring teeth, fixed implant prostheses are usually indicated, 1 and the decision-making process is straightforward. In edentulous situations, however, the choice of fixed or removable implant prostheses is more complex. A major driver of the decision is facial esthetics (ie, the need for facial tissue support). If both fixed and removable prostheses may be considered, the next factor influencing the selection is the complexity of the surgical interventions required. With pronounced horizontal and/or vertical bone loss, large amounts of hard and soft tissue regeneration may be needed for fixed implant prostheses. Hence, in cases where there is a need for facial tissue support or large bone and/or soft tissue augmentations, removable implant-retained prostheses such as implantretained overdentures are less invasive treatment options. 2,3 Numerous systematic reviews have been performed in recent years to evaluate the survival and complication rates of fixed and removable implant-retained prostheses. [4][5][6][7][8][9] High 5-year survival rates ranging from 97.1% 10 for fixed to 95%-100% for removable prostheses 4 have been reported. In daily practice, however, high survival rates are not the only criterion defining the success of the implant treatment, as they only represent those prostheses remaining in use for a defined follow-up time. 10 They do not show whether or not these prostheses were affected by complications influencing the general success of the implant treatment. 10 It is well known that patients experiencing complications with their fixed or removable implant prostheses are significantly less satisfied with the implant treatment than patients who do not experience problems. 11 In a systematic review conducted by Salvi and Brägger 12 as part of the proceedings of the 4th International Team for Implantology Consensus Conference, prosthetic risk factors were defined as either technical or mechanical risks. Therefore, based on this statement, the prosthetic complications can be considered as technical or mechanical complications. The technical complications represent those relevant to laboratory-fabricated parts such as fracture and chipping of the veneering materials, whereas mechanical complications represent complications relevant to the prefabricated parts, such as implant fracture or abutment failures. 12 As the reviews both fixed and removable implant prostheses exhibit different technical and/or mechanical problems over time. 4,8,10,12,13 | 131 SAILER Et AL.
complications. For simplification, the term "technical complications" will be used for both technical and mechanical problems.

| IMPL ANT-SUPP ORTEDFIXEDDENTAL PROS THE SE S
Today, fixed implant-supported crowns and multiple-unit fixed dental prostheses can be fabricated out of metal ceramics or several dental ceramics. The selection of the material may have an influence on the outcomes.

| Singleimplant-retainedcrowns
Metal-ceramic, single implant-retained crowns were the gold standard for decades, yet today all-ceramic implant crowns fabricated out of lithium disilicate or zirconia ceramics are successfully used as alternatives. 8 In addition, leucite-reinforced glass ceramics, alumina ceramics, or resin-matrix ceramics can be considered for the fabrication of single implant-retained crowns. 8 The clinical outcomes for implant-retained single crowns, as well as for their supporting implants, are very good. The overall 10-year survival rate of implants supporting single crowns was demonstrated to be excellent at 95.2%, independent of which crown material was used. 5 However, the overall 10-year survival rate of the crowns was slightly lower at 89.4%. 5 At the crown level, the survival rate was influenced by the materials used for their fabrication, as shown in a more recent review. 8  Interestingly, crowns cemented to the supporting implant abutments suffered less from screw loosening than screw-retained crowns 15 Further evaluation of the literature showed that both the crown design (screw-retainable or cementable) and the implantabutment connections (external or internal) have a significant influence on the risk of screw loosening. 10 The stability of the screw joint can be influenced by the prosthetic implant axis. It has been shown that more screw loosening occurred with angulation-correcting implants than with straight implants. 16 Hence, the appropriate three-dimensional position of the implant is a crucial parameter with screw-retained implant prostheses to decrease the risk of complications. Furthermore, the number of retaining screws should be limited to one, as double screw systems exhibited a higher risk of screw loosening. 17 In addition, manufacturer-recommended torque values should be adhered to. 18 Finally, implants with internal implant-abutment connections are preferred to external connection systems, to reduce the risk of screw loosening. 19 Today, implant-retained crowns are more frequently screwretained than cemented, 15,20 following current recommendations to reduce the risk of peri-mucositis and peri-implantitis via excess cement. 20 However, despite all these improvements, a long-term stable solution eliminating screw loosening has not yet been found, and this risk has to be taken into consideration when treatment planning.

| Loss of crown retention
Loss of retention as a result of de-cementation is the second most frequent complication with implant single crowns, occurring in 4.1% of cemented crowns after 5 years of function. 5 The incidence reported in publications has decreased from 7.3% prior to 2000 to 3.1% after 2000. 10 One possible reason for this improvement may be the more recent increase in the use of resin cements, indicated for the cementation of all-ceramic crowns to the underlying titanium or zirconia/alumina abutments.
The restorative material plays an important role with incident technical problems. Metal-ceramic crowns are not dependent on adhesive cementation to the substrate (the abutment) in order to receive sufficient strength for clinical function as they already have excellent material stability. For this reason, metal-ceramic crowns are mainly cemented with conventional cements like zinc phosphate or glassionomer cement. Ceramic crowns exhibit a reduced fracture strength compared with metal-based crowns, and need to be chemically bound to the underlying substrate for improved clinical strength. 21 Resin cements provide a chemical bond between the ceramic crowns and the underlying materials, thereby reinforcing the ceramic crowns. It has been shown that the 5-year rate for loss of retention of ceramic crowns was only 1.1%, 8 whereas for metal-ceramic crowns the rate for loss of retention was five times higher at 5.5%, as reported in earlier reviews. 22 The main disadvantage of resin cements is that they are very viscous, mostly translucent, and not radio-opaque. In addition, they exhibit chemical bonding to the abutment substrate after curing.
Hence, the removal of excess cement is significantly more difficult than with nonadhesive, opaque conventional cements. 23 Research has shown that the position of the crown margin is an important influencing factor for remnants of excess cement: the deeper the crown margin then the larger the amount of excess cement. 24 The removal of excess cement is difficult even at shallow crown margins. 15,24 During treatment planning for cemented implant crowns, the appropriate three-dimensional position of the implant has to be carefully considered. To reduce the risk of complications associated with excess cement, screw retention of fixed implant prostheses is recommended. 15,24 2.1.3 | Chipping or fracture of the veneering material Chipping of the veneering ceramic is the third most frequent complication with fixed implant prostheses. The rates reported range from 3.2% to 25.5%, 10 with an overall 5-year complication rate of 3.5%. 5 Veneering ceramics are silica-based ceramics with excellent esthetic properties; however, they have very low fracture strength values. 25 They are applied to different metallic or ceramic framework materials, establishing a bond between the veneering ceramic and the framework material important for clinical performance. 25 Several factors influence the risk of chipping of the veneering ceramic.
The framework material plays an important role in preventing high chipping rates. It has been shown that veneered alumina or lithium disilicate crowns experienced chipping in 1.8% and 3.5% of cases after 5 years of function, respectively, whereas veneered zirconia crowns exhibited very high chipping rates of 11.8% over the same time frame. 8 By comparison, the incidence of chipping in metal-ceramic crowns was 3.5% 5 ( Figure 1). Furthermore, the oral cavity is a very challenging environment for the performance of dental materials, most specifically for ceramics.
Humidity, chemical attacks like acidic food or drinks, and changing temperatures lead to accelerated aging of ceramics. 25 With aging, the risk of fracture or chipping increases. In addition, inherent defects and pores within the ceramic resulting from the manual veneering procedures 26 further increase the risk of fracture or chipping. The long-term integrity of the veneering ceramic is also influenced by the occlusion/function, 27 as the forces applied to implant restorations are significantly higher than those applied to tooth-borne restorations. It has been shown that the tactile sensitivity of dental implants is 8.7 times lower than that for natural teeth, 28 hence the occlusal load on implant-retained crowns is almost nine times higher than when supported by natural teeth.
Given the above, chipping of the veneering ceramic may not be avoided as a complication at veneered restorations. Therefore, current concepts may involve avoiding veneering materials by fabricating restorations out of monolithic ceramics. However, clinical studies on the monolithic lithium-disilicate and zirconia ceramics are still scarce, and conclusions cannot yet be drawn. One review reported on a 5year cumulative survival rate of monolithic lithium-disilicate implant crowns of 91%. 8 No medium-to long-term data on monolithic zirconia implant crowns are currently available. 29 Therefore, it remains unknown whether or not monolithic implant crowns will have fewer problems with chipping. More research and development are needed before clinical recommendations on monolithic implant crowns can be made.

| Fractures of ceramic abutments
Fracture of ceramic abutments is a rare complication, 19,30 and the rates were 2.0% (95% confidence interval 0.5%-7.4%) and 1.9% (95% confidence interval 0.7%-4.8%) for internally connected ceramic abutments in the literature. 19 The reviews demonstrated no differences in the survival rates of metallic and ceramic implant abutments for implants with external connections. Furthermore, no differences were found when comparing anterior and posterior regions, 30 or internally and externally connected ceramic abutments. 19 However, ceramic abutments exhibited more fractures than metallic abutments, a technical complication that inevitably leads to the failure of the implant restoration. 19 Fracture of an internally connected ceramic abutment predominantly occurs in the internal part of the implant-abutment connection, 31 and in situations where the remnants cannot be removed from the internal connection, it may be necessary to remove the implant. For this reason, in internal connection implant systems, the application of ceramic abutments should only be recommended for the esthetic anterior regions.
Nowadays, the combination of internally connected titanium-base abutments with zirconia abutments may serve as an alternative solution. A laboratory study showed significantly higher fracture strength values for titanium base-supported zirconia abutments (hybrid solution) compared with externally or internally connected one-piece zirconia abutments, 31   It has been shown that metallic abutments and metal-ceramic implant crowns caused a grayish discoloration of the mucosa in both laboratory and clinical studies. 33,34 The amount of discoloration and its effect on esthetic outcomes may be associated with the thickness of the mucosa. 34 A critical soft tissue thickness of 2 mm was defined, with a grayish shine-through of the metallic implant components in cases with thin soft tissues of < 2 mm. 34 The color of tissues with thicknesses of > 2 mm was not influenced by the abutment or restorative materials. Hence, in esthetically important clinical situations, the clinical recommendation was to either use whiteish ceramic zirconia abutments and ceramic implant restorations in these cases, or to increase the thickness of the peri-implant mucosa to values > 2 mm with soft tissue grafts. 34 Interestingly, recent studies showed that the bright white color of zirconia also induces a soft tissue discoloration, leading to brightening and a pale appearance of the tissues (Figure 2A-L).
As has been shown, discolorations at the level of peri-implant soft tissues, as well as at the level of the implant restoration, can be perceived by both experts and laypersons, 35,36 therefore the esthetic outcome of implant restorations is key to their success. For this reason, several studies have focused on the ideal color of implant abutments and restorations. It has been shown that light pink or warm orange colors are more favorable than white. [37][38][39] The influence of recent ceramic materials (ie, the colored and translucent lithium disilicate and zirconia ceramics) used for monolithic implant single-unit and multiple-unit fixed dental prostheses has yet to be investigated.

| Multiple-unitimplant-fixeddentalprostheses
In contrast to single implant crowns, the selection of materials for multiple-unit implant-fixed dental prostheses is limited to metal ceramics and zirconia ceramics. For multiple-unit fixed dental prostheses, zirconia displayed an inferior performance compared with metal ceramics, which are considered to be the gold standard. 40,41 In a recent review, metal-ceramic, multiple-unit, implant-fixed dental prostheses exhibited cumulative survival rates of 98.7%. 42 By comparison, zirconia-ceramic, multiple-unit, fixed dental prostheses had a significantly lower 5-year survival rate of 93%. 42 Another review reported better 5-year cumulative survival rates for partial and full-arch zirconia, multiple-unit, implant-fixed dental prostheses of 98.3% and 97.7%, respectively. 7 In both reviews, the predominant technical complication was chipping of the veneering ceramic. Metal-ceramic, fixed dental prostheses had a 5-year chipping rate of 11.6%, while for zirconiaceramic, fixed dental prostheses the rate was 13.9%, with the difference being statistically significant. 42 The predominant technical/mechanical complication at multipleunit, implant-fixed dental prostheses is fracture/chipping of the veneering ceramic. Fracture of the ceramic framework and screw loosening are less frequent, but nevertheless are clinically relevant complications.

| Chipping of veneering ceramic
Chipping of the zirconia veneering ceramic was found in 34.8% of multiple-unit, zirconia-fixed dental prostheses in one review 7 and in 50% of the fixed dental prostheses in another. 42 Chipping of the veneering ceramic was reported for 8.8% of metal-ceramic, implantfixed dental prostheses 22 (Figure 3A-C).
As with single-unit, zirconia restorations, this problem remains unsolved, although the monolithic zirconia, implant-fixed dental prostheses appear to offer a promising alternative. 43,44 Randomized controlled clinical trials with longer follow-up periods are needed before clinical recommendations can be made.

| Fracture of zirconia frameworks
Fracture of zirconia frameworks was observed in 4.7% of restorations after 5 years of function, a complication that very seldom occurred with metal-ceramic, multiple-unit, fixed dental prostheses (0.2%). 42 The extension of multiple-unit, fixed dental prostheses is a crucial factor influencing the outcomes of zirconia as a framework material.
Indeed, fractures only occurred with full-arch, zirconia fixed dental prostheses; no fractures were observed with partially fixed dental prostheses. 7 It has been shown previously that the size and the shape of the connectors are the most relevant parameters for the stability of multiple-unit, zirconia fixed dental prostheses. The new types of monolithic translucent zirconia ceramics exhibit better esthetic properties than the previous yttria-stabilized tetragonal zirconia polycrystal framework materials, yet lower strength values. 45 For predictable outcomes, manufacturers' recommendations need to be followed when designing these restorations. 45 Unfortunately, no long-term research is yet available for monolithic, multiple-unit, zirconia fixed dental prostheses.

| Screw loosening
Screw loosening is a rare complication with both the metalceramic and the zirconia-ceramic, implant-supported, multipleunit fixed dental prostheses. 7 overdentures were reported to compromise patient satisfaction and masticatory efficiency less than fixed prostheses. [48][49][50] High overall survival rates for implant-retained overdentures have been reported for 5 years, ranging from 95% to 100%. 51,52 However, the survival rate should also be considered alongside complication rates to judge overall clinical success.

F I G U R E 4 Summary of different implant-retained overdenture attachment types
The most frequently used bar designs can be either cast or milled. 53 In general, the attachment systems consist of a metal or plastic retainer (the female part or matrix) and an attachment part (the male part or patrix). While the matrix is embedded to the prosthesis, the patrix is attached to the implant. 54

| Definitions
The 2. prosthesis screw-tightening or replacing not more than once a year after the first year.
3. Activation, repair, and replacement of either matrix or patrix within the limit of two replacements in the first year and a maximum of five replacements in 5 years.
4. Denture relining once in 5 years, considered as maintenance rather than a complication.

Need of activation/loss/fracture of patrix or matrix retention component
The need for activation, replacement, or repositioning of a retention component, either the matrix or patrix, is the most frequently encountered event occurring with implant-supported overdentures in both jaws. [57][58][59][60] "Time in function" is a more relevant factor than the attachment type. The incidence of a dislodged, worn, or loose matrix (or its housing) was more common after the first year with ball

Complication type Definitions
Mechanical complications Finally, an important factor in clinical situations that may compromise the retention of solitary anchors is the implant angulation.
To reduce the incidence of patrix/matrix repairs, the use of ball, locator, and magnet attachment types may be indicated for an implant divergence of 10-40 degrees. 60,61 There is a difference regarding the aftercare requirements be- Bar fractures are rare technical complications; however, in the case of a bar failure, renewal of the prosthesis may be required.
According to a literature review there are six reported essential causes for metal framework fractures, including implant overdenture bars. 64 These are inadequate metal thickness, poor solder joints, excessive cantilever length, alloys with inadequate strength, patients' parafunctional habits, and improper framework design. 64 Some of these are directly related to the bar itself, such as bar material, fabrication methods, or sensitivity ( Figure 5).
Finally, the occlusal load and fabrication procedure have an impact on complications rates. 57,58,61 The passive fit of prosthetic components and evenly distributed occlusal forces, without exceeding materials' resistance and eliminating parafunctional load, reduce the incidence of problems during aftercare.

| Riskfactorsfortechnicalcomplications
To reduce the risk of prosthetic complications with implant-retained overdentures, the selection of the attachment, the optimal number and location of implant fixtures, as well as consideration of the clinical factors, such as available prosthetic space and the opposing dentition, all need to be evaluated.

| Attachment type
Factors affecting the clinician's preference with regard to attachment types can be variable. A recent global survey of 116 prosthodontists from 33 countries showed that the most commonly preferred attachment type was the locator attachment, 65 and clinicians often made their selection based on subjective criteria such as their expertise, personal comfort, and their dental technician's preference, or as influenced by marketing strategies. 66 Nevertheless, each attachment system comes with its own clinical prerequisites and has different indications. Existing prosthetic space, inter-implant distance, implant position and angulation, and number of implants are the main factors that should dictate the implant attachment of preference. Moreover, the maintenance requirements and complication rates arising can be related to such factors. 64,67 The consequence of ill-positioned implants is, that the insertion path of the prosthesis and its fit will not be optimal, and this will result in a higher incidence of need for matrix change, or wearing of the patrix ( Figure 6). In these cases, bar attachments are preferred to correct the axis deviations and achieve a better way of insertion. 68 An incorrect selection of attachment will inevitably result in both higher maintenance requirements and complication rates.
The effect of attachment type on prosthetic maintenance and complication rates, as well as retention, stability, and patient satisfaction, has been studied in various clinical studies and reviews. 54,59,67,69,70,71 In a recent systematic review and meta-analysis, Leao et al 70

| Number of supporting implants
The possibility of using two implants to support mandibular overdentures was first introduced by van Steenberghe et al (1987) 72 . Ever since then, mandibular overdentures have been extensively studied regarding the optimal number of supporting or retaining implants.
Two consensus conferences concluded that a mandibular overdenture supported by two implants is the "gold standard" treatment for edentulous patients. 46 Even although there is consensus regarding the number of supporting implants for mandibular overdentures, the number of implants needed to support a maxillary overdenture remains controversial (

| Maxilla vs mandible
Both maxillary and mandibular overdentures were introduced to dental practice more than 30 years ago. [82][83][84][85] However, the number of patients with maxillary complete edentulism who seek implant therapy is lower than for mandibular edentulous patients, because of their greater satisfaction with complete dentures. 86 Accordingly, patients edentulous in the maxilla who are willing to undergo implant therapy are more often the ones with compromised denture stability as a result of advanced bone resorption. 62  Overall, a higher incidence of technical problems has been detected in maxillary overdentures. 61

| Available prosthetic space and opposing dentition
In general, more vertical and horizontal prosthetic space is required for the components supporting implant-retained overdentures than for implant-supported fixed dental prostheses. Where implantretained overdentures are considered as a treatment option, the jaws should accommodate enough space for the attachment, the housings/bar clips, and prosthesis thickness. 94 Lack of sufficient prosthetic space will lead to inadequate dimensions of both attachments and prosthesis. However, because of limited existing data, direct correlation of the inter-arch space with overdenture survival and success rates is not possible at present. 53 Limited clinical evidence demonstrates that if the inter-implant distance is < 8-10 mm then the proper placement of bar clips is jeopardized and, accordingly, clip loosening occurs more frequently. 95 Another potential risk factor for complications with implantretained overdentures is the opposing dentition. Complete edentulism has been shown to occur earlier and more frequently in the maxilla than in the mandible (40% vs 27%, respectively), 62 and maxillary overdentures are opposed to a natural dentition more often than mandibular overdentures. It is difficult to find agreement in the literature regarding the effect of the opposing dentition on the complication rates of overdentures. Nevertheless, in a number of clinical studies evaluating maxillary implant-retained overdentures, the opposing dentition seems to account for increased rates of complications or failure. 88,96,97,98,99 The natural dentition can create higher forces and may lead to an increased need for maintenance and higher complication rates in opposing implant-retained prostheses.

| Prosthetic material
A frequent technical complication in implant overdenture treatment is base fracture, hence, the design and materials play a crucial role in outcomes. 100,101 Denture-base reinforcement is recommended to prevent technical complications of implant-retained overdentures, because it improves the implant overdenture stiffness and decreases denturebase deformation. Materials used for denture-base reinforcement are metal, high performance polymers, and carbon and glass fibers ( Figure 7). [102][103][104][105] It has been shown that reinforced implant-retained overdentures exhibit a reduced risk for fracture compared with nonreinforced implant-retained overdentures. The cobalt-chromium framework is still the gold standard for framework fabrication today. [106][107][108] However, nonmetallic framework materials like high performance polymers, such as polyetheretherketone, polyetherketoneketone, and glass fibers, are under investigation, as they may be beneficial because of their lower weight, better esthetics, and superior bonding ability to acrylic denture-base materials. [109][110][111] However, more data are needed before any recommendations can be made on these newer materials.

| CON CLUS IONS
This review of the literature on fixed and removable implantretained prostheses demonstrates that technical complications F I G U R E 7 Types of prosthetic reinforcements for IODs. IODs, implant-retained overdentures; PEKK, polyetherketoneketone; PMMA, polymethylmetharcylate cannot be avoided in either type of implant-retained prosthesis.
Technical complications can lead to the failure of implant treatment.
To reduce the risk of this failure, a comprehensive pretreatment diagnostic work-up, including defining the prosthetic goal with the aid of a wax-up or set-up and the associated ideal, prosthetic-oriented three-dimensional implant position, is crucial. Furthermore, selection of the ideal type of prosthesis, including the respective implant components and materials, is important for the clinical long-term success of the reconstruction.

ACK N OWLED G M ENT
Open Access Funding provided by Universite de Geneve.