Implant therapy has proven to be a successful treatment modality that is applicable across a wide range of clinical indications for tooth replacement. Even so, complications do occur, and there is clear evidence that prosthodontic complications occur at a higher rate with dental implants than with natural teeth. Commonly referred to as 'hardware complications', it is important to understand why they occur and how best to deal with them. This module will classify implant hardware complications associated with fixed dental prostheses as a basis for elucidating their various causes, best prevention, and management.

After completing this ITI Academy Module, you should be able to define hardware complications; recognize causes of hardware complications; outline approaches for the prevention of hardware complications; and describe management of the most common hardware complications.

A complication can be defined as an unexpected deviation from the normal treatment outcome. In implant dentistry, hardware complications refer to unexpected issues associated with implants or their prosthetic components. For the purpose of diagnosis of cause and management of hardware complications, this overriding definition will be subdivided further.

Hardware complications may be divided into two categories: mechanical complications related to the manufacturing of prefabricated components such as implants and abutments, and technical complications related to dental laboratory procedures and materials involved in construction of implant-supported prostheses.

Mechanical complications are related to prefabricated components that will be described here in generic terms. The implant itself can be divided into the body of the implant and the interface with the prosthodontic components. The prefabricated prosthodontic components are the abutment and the abutment screw. This screw may be incorporated into the abutment or may be a separate component. Variations in implant design are outlined in another ITI Academy Learning Module titled 'Implant Designs and Characteristics'.

There are also other prefabricated prosthodontic components that may be used in connection with the prosthodontic superstructure. These include components associated with provisional prostheses or variations in prosthesis design. Examples of such components are shown here and include a temporary cylinder for a provisional implant-supported prosthesis on the left, a cast-on metal coping to increase the accuracy of the interface connection in the middle, and a transversal screw assembly to assist prosthesis design on the right.

Technical complications are the result of inappropriate construction of implant superstructures or prostheses. This could be related to the design of the prostheses or the process of fabrication by the dental laboratory. For example, this radiograph shows two implant-supported prostheses in which the coping design does not provide for adequate thickness of the veneering porcelain. Technical complications can also be related to the choice or handling of prosthodontic materials. Zirconia, for example, is a very technique-sensitive material and clinicians and laboratory technicians must follow specific guidelines when handling it.

Hardware complications can also be classified according to their magnitude and impact. A minor complication is defined as an issue that can be rectified at chairside and at minimal cost. An example would be loss of the closure filling in the access hole of a screw-retained FDP prosthesis. On the other hand, a major complication is likely to require numerous patient visits and is also associated with higher costs. Loss of veneering porcelain in a longer span FDP, as shown here, would require removal, fabrication of a provisional prosthesis, and repair or even remake of the prosthesis. Hardware complications can also be classified as acute or chronic. Loss of retention of a cemented crown or loosening of a screw-retained FDP would be classified as acute, whereas the chipping and wear observed on this prosthesis would be considered chronic in nature.

Hardware Complication Definitions, Key Learning Points: Hardware complications may be either mechanical or technical in nature. Mechanical complications are the result of defects in design or production of prefabricated components supplied by a manufacturer. Technical complications are the result of inappropriate design or dental laboratory fabrication of custom components or prostheses. Hardware complications can be major or minor as well as acute or chronic.

This Learning Objective will explore the causes of hardware complications. These causes may be directly related to mechanical and technical aspects. They can also be related to contributory factors. These factors include intraoral forces, biophysical differences between teeth and implants, the biomechanics of the oral environment, and the behavior of implants in that environment. Iatrogenic factors can also contribute to hardware complications. These arise out of the clinician's diagnosis, treatment planning, and execution of treatment. Frequently, it is difficult to segregate the direct factors from the contributory factors, and hardware complications are therefore often related to a combination of the factors described above.

Despite the difficulty of determining the precise cause of a complication, it is important to arrive at a diagnosis by identifying all causative factors. Doing so will improve the clinician's chances of avoiding the same complication in the future. Armed with a diagnosis, the clinician can consider the measures required to achieve a predictable and stable long-term outcome. For example, a patient may present with porcelain chipping of their posterior implant prostheses and clear signs of bruxism. In this situation, the clinician may consider metal occlusal surfaces and an occlusal splint for night time protection.

An outright cause of a mechanical complication is a flaw in the manufacturing process that predisposes an implant component to failure. For example, fracture of an implant body is a fortunately rare but very serious complication that can result from poor manufacturing. It is important to use implants and associated prefabricated prosthodontic components that meet design and quality requirements for intrinsic strength and durability. Use of materials that do not meet the physical, chemical, or mechanical requirements for implant therapy can lead to mechanical complications. The risk of complications also increases when components are designed in a manner that is incompatible with physical and mechanical laws. This applies in particular to the interfaces between components.

It is important that the clinician is certain that the quality of the manufactured implants and components is consistent. Lack of consistency on the part of the implant manufacturer is a very serious issue. Reputable implant manufacturers publish production quality standards. Examples of measures of production quality are the published acceptable dimensional tolerances of implant components, and the acceptable tolerance of rotation between the implant and abutment. Furthermore, implants and components must be used in a precise manner, and failure on the part of the manufacturer to issue clear and correct instructions regarding their use can increase risk of complications. More information about implant quality and the role of manufacturers can be found in the ITI Academy Learning Module 'Selecting an Implant System'.

Another specific mechanical issue is the so-called cloning of implant and component designs by other manufacturers. Variations in design features, materials, and manufacturing processes may lead to a reduction in the overall quality. Similarly, use of non-original parts in combination with original components can easily lead to mechanical issues. This graph shows the amount of mobility for different implant-abutment combinations. The smallest amount of mobility is seen when the abutment is original, that is, it is from the same manufacturer as the implant. This indicates the tightest tolerance limits of the implant-abutment interface, as shown on the diagram on the previous slide.

An example of the use of non-original or cloned components is shown here. On the left, the original manufacturer abutment and screw are seated into a two-piece implant. On the right, these components have been replaced by a cloned zirconia abutment and screw. Note that the cloned abutment/screw interface (circled in red) is much smaller and sharper than the original design. This pinpoint-type contact between the screw and the zirconia abutment can create very high stress concentration at the interface, potentially leading to fracture of either the abutment or the screw under occlusal function. This type of failure would be mechanical in nature because the source of the problem is the design or manufacture of the components in question. It is important that the clinician is aware of the origin and make of components used.

Technical causes are related to construction of the implant superstructures or prostheses. A systematic review by Pjetursson and colleagues from 2014 compared the survival and complication rates of implant-supported prostheses from papers published up to the year 2000 with those reported in studies published after the year 2000. In spite of overall lower rates of mechanical and technical complications reported in more recent clinical studies, the incidence of reported technical complications is still high. These clinical images show the fracture of a distobuccal cusp due to inadequate support and retention for the teeth and porcelain veneering material. The findings of Pjetursson and colleagues underline the importance of prosthesis design, choice of materials, and fabrication as well as the clinician's certainty that the dental laboratory possesses the required knowledge, skill, and experience to undertake the technical work.

A common technical cause of hardware complications is the design of the coping that supports the veneering materials of an FDP. This clinical example shows a lower left first molar implant-supported crown with inadequate technical design. Similar to the example on the previous slide, the distolingual cusp has fractured off, although no metal is visible. This fracture is likely to be related to inadequate provision of support for the veneering porcelain. This radiograph demonstrates a clear example of unsupported porcelain in a single implant crown. The mesial marginal ridge area of this prosthesis is very susceptible to fracture. A coping should be designed to maintain a 1.5 to 2 mm thickness of veneering porcelain. This illustration shows the so-called cut-back approach to ensuring an even veneering porcelain. The metal substructure design represents an even overall reduction of the full prostheses shape.

The increasing use of ceramic materials in implant dentistry is another specific source of potential complications. As an example, many different types of zirconia with varying microstructures and performance are being introduced. Zirconia should therefore be obtained from a reputable and qualified manufacturer. Furthermore, the dental laboratory as well as the clinician must be aware of the inherent sensitivity of ceramics to design and processing problems. These include stress concentration, thin walls, sintering, and residual machining flaws. It is therefore crucial that the laboratory and clinician adhere to the manufacturer's recommendations. It is also essential that zirconia abutments are not ground, abraded, or adjusted by the clinician or technician following sintering, unless this is recommended by the manufacturer. This clinical example shows a zirconia abutment that fractured during fabrication in the dental laboratory. This could have been caused by misalignment or by over-tightening of the abutment screw into the implant analog.

The previous slides showed examples of complications that can be directly related to mechanical and technical causes. For a number of them, other factors may have contributed to the complication. For example, occlusal function is likely to have played a role in the fracture of the distolingual cusp. The nature, intensity, and frequency of intraoral forces vary. Whereas occlusal forces arising from opposing tooth contact during mastication and swallowing are usually within physiological limits, forces generated during bruxism, intense sporting activity, and trauma can significantly exceed these limits, leading to sudden peaks of force that put prosthetic materials at risk of fracture. Similarly, repetitive forces that cause continuous jiggling can result in loosening of screws or fatigue fractures. This image shows fracture of the thin walls of the zirconia abutment where the material has been unable to withstand and absorb damaging, repetitive loading by occlusal forces.

The biophysical differences between implants and teeth also play a part. The osseointegration between implant and bone responds differently to occlusal forces compared to the periodontal ligament around teeth. The direct contact to bone affects the capacity of implants and their prostheses for movement and sensory feedback. In turn, this places greater demand on the implant and prosthetic materials for absorption of force. The lack of sensory feedback also affects the patient's ability to avoid occlusal overload.

The biomechanics of the oral cavity in general are also a contributory factor in hardware complications. The exposure to saliva, food, drink, temperature and pH changes, bacterial biofilm, anaerobic environment, and tissue secretion products all take their toll over time and can lead to corrosion, attrition, and material degradation. Similarly, wear caused by function against opposing natural or artificial hard materials and interposed particles is a common feature. Well-intended dental interventions by the patient or dental team will also have an impact. Oral hygiene measures, professional maintenance procedures, and manipulation by the dentist or technicians can alter the integrity of surfaces and materials. A specific example is the use of ultrasonic scalers against the material surfaces.

Closely related to the contribution of dental interventions are iatrogenic causes of hardware complications, that is, complications that can be attributed to clinician failures in diagnosis, treatment planning, and execution of treatment. In the case of the fractured zirconia abutment, the material thickness was inadequate to withstand exposure to occlusal forces. A significant contributory factor, however, was the clinician's decision to use this type of abutment in the posterior part of the mouth where the occlusal forces are greater. Similarly, the excess luting cement shown in this radiograph can be attributed to iatrogenic causes created by the treating clinician at the stages of planning, acceptance, and fitting of the prosthesis. The depth of the prosthesis margin should have been foreseen during the planning stages and corrected via a custom abutment. The cementation margin between custom abutment and crown would thereby have been placed just below the mucosal margin with easy access for removal of excess cement. Equally, use of a much smaller amount of cement would have further reduced the risk of excess. This excess cement has also resulted in a biological complication related to the peri-implant hard and soft tissues. Separate ITI Academy Learning Modules address the 'Principles of Managing Biological Complications' and 'Abutment Selection'.

Implant fractures are, as mentioned, rare - but when they do occur they are invariably serious. In this example, the cause of implant fracture can be traced back to the original planning. The clinical situation shows lower anterior teeth and an extended posterior edentulous space. The decision to replace only the canine and first premolar with an implant-supported prosthesis placed the resulting prosthesis under great occlusal strain. The choice of a reduced-diameter implant further added to the potential for a serious mechanical complication, but the clinical diagnosis and treatment planning was most at fault.

Causes of Hardware Complications, Key Learning Points: Frequently, the cause of a complication is not distinct but rather a combination of mechanical, technical, and contributory factors, including iatrogenic factors. Mechanical causes are related to the manufacturing process and/or the use of non-original parts. Technical causes are related to the design or fabrication of the prostheses, with the inherent sensitivity of ceramic materials playing a significant role. Intraoral forces, biophysical differences between teeth and implants, the biomechanics of the oral environment, and clinician-related factors can contribute to both mechanical and technical complications. Being able to determine the cause of a complication improves the clinician's chances for avoiding the same complication in the future.

The best method of managing complications is to reduce the risk of occurrence in the first place. A preventive approach should therefore be adopted from the outset. The relevant prosthodontic principles include careful consideration of the number of teeth to be replaced to meet the functional needs of mastication, phonetics, and occlusal stability. Assessment of the prosthodontic space is essential to identify limitations or challenges that must be overcome to ensure adequate dimensions of the implant-related prosthodontic hardware. The implant type, number, positions, angulations, and dimensions should be determined by the prosthodontic plan as should the need for adjunct surgical measures to facilitate optimum prosthodontically determined implant placement. For example, a bone augmentation procedure may be required in the presence of advanced bone resorption to avoid a lingual or buccal implant position that would complicate the prosthesis.

Where prefabricated prosthodontic components could lead to risk of complications, alternatives including custom options should be considered. At the same time, the prosthodontic design should avoid unnecessary complexity. It is also sensible to draw up contingency plans in the event of complication and failure. Prosthodontic planning principles for various clinical situations are set out in detail in separate ITI Learning Modules.

Specific planning points for prevention and management of hardware complications were outlined by the 5th ITI Consensus Conference in a set of Treatment Guidelines. In addition to the prosthodontic planning principles, the Treatment Guidelines offer the following recommendations: To reduce risk of implant fracture the clinician should use appropriately designed and manufactured implants that have been thoroughly tested and documented. Prosthetic screws are manufactured to specified tolerances and, to reduce risk of fracture or loosening, it is important to avoid mishandling and misfit and to protect against occlusal forces. Whilst metal abutments rarely fracture, greater caution should be observed when choosing ceramic abutments as they are not appropriate for all indications. Framework fractures are also rare but fracture of the veneering materials is not. Careful attention to the final prostheses design and contours as well as provision of adequate support will reduce the risk of fracture.

Because occlusal forces are causative factors for hardware complications, occlusal considerations are also part of planning for prevention. An anterior deep overbite with resulting proclined implants is one example of an occlusal situation that can lead to overload and hardware complications through increased lateral forces.

When planning for prevention, an important occlusal consideration is the presence of bruxism. Planning considerations include retrievable implant prostheses designs, as seen in this clinical example of a screw-retained lower left first molar implant crown. Metal occlusal surfaces may be preferred over ceramic occlusal surfaces for patients exhibiting bruxism. Caution should be exercised with cantilevers due to the lateral loads that bruxism can generate. For example, occlusal forces on this cantilevered full-size molar unit has led to the extreme complication of fracture of both supporting reduced-diameter implants. Finally, it is sensible to provide a hard resin protective occlusal appliance for use during sleep. The occlusal appliance seen in situ in this clinical image has been adjusted to the guidelines for a mutually protected occlusion to reduce the risk of unfavorable stress on the teeth and implants in the opposing arch as well.

The 5th ITI Consensus Conference also recommended that clinicians, technicians, and manufacturers employ a tracking system for implants and prosthodontic components. The patient should also keep a record of all components used. This information is useful in the treatment of existing complications, especially if the patient's implant clinician is unavailable. This information can also prevent complications caused by the use of non-original components. Clinicians should be aware that not all implant systems have the same level of documentation; therefore clinicians should know or be informed of the origin of all components used. Variation in quality also applies to custom-fabricated CAD/CAM prosthetic solutions, and it is important to note that use of state-of-the-art CAD/CAM technology, per se, does not eliminate hardware complications.

A further important point in reducing risk of hardware complications is to adopt a systematic protocol for delivery of implant-supported prostheses. An example of such a step-by-step protocol is the following: First, evaluate the prosthesis prior to the delivery appointment to check for misfit. A rehearsal of the prosthesis assembly should follow to ensure familiarity and to avoid mishandling upon insertion in the mouth. Then, insert the prosthesis intraorally and verify that it is clinically acceptable. If not, make careful modifications to render the prosthesis, and particularly the occlusal scheme, clinically acceptable. Finally, the prosthesis should be fitted and baseline records taken to facilitate continuing care. These records include the cement used or the torque values applied to abutment and prosthetic screws. They also include a radiograph to document prostheses fit and marginal levels of peri-implant bone.

After fitting of the prostheses, it is recommended that regular maintenance appointments are scheduled as part of continuing care. These appointments should include a careful occlusal review. Positional changes to teeth over time, together with occlusal attrition, can result in heavier contacts on implant prostheses. If required, it is also recommended that clinicians undertake any required adjustments to the prostheses to restore and optimize the respective static and dynamic occlusal schemes. Occlusal guidelines recommend that contacts on implant prostheses are limited to centered and graded contacts in static occlusion. In dynamic occlusion anterior guidance should, where possible, be delegated to adjacent natural teeth whilst dynamic posterior contacts should be avoided. Any adjustments should include meticulous polishing of worn ceramic surfaces to reduce the risk of fracturing the veneering material as well as their abrasive effect against opposing teeth and restorations.

Prevention of Hardware Complications, Key Learning Points: Prosthodontic planning can prevent many hardware complications through careful assessment of the prosthodontic space, diagnosis of the need for adjunct surgical procedures prior to implant placement, and consideration of occlusal forces. ITI Consensus Conference Treatment Guidelines recommend using implants of known quality, using ceramic abutments only when indicated, and ensuring that all veneering materials are adequately supported. It is important that both the patient and clinician maintain a record of all implants and components to aid in the treatment of complications should they occur. A step-by-step protocol for delivery of implant-supported prostheses and regular maintenance visits are further steps to reduce the risk of hardware complications.

A loosened occlusal or abutment screw is the most common complication that one can expect to deal with on a fairly routine basis. Screw-retained prostheses are popular with many practitioners, particularly when retrievability is desired. However, occlusal screw loosening is a risk of this method of anchorage. Screws that have worked loose under function are also at increased risk of fracture. Therefore, when screw-retained restorations are utilized, the patient should be instructed to return to the clinic immediately if he or she senses any mobility in the crown or FDP so that the screw or screws can be retightened or replaced. Screws that have come loose multiple times should be replaced with new screws, as frequent loosening will cause the surfaces of the screw threads to become burnished and thereby less capable of maintaining tightness. Repeated screw loosening may also be a sign that the prosthesis does not fit accurately. The resulting excessive strain on the screws causes them to work loose. In this example, neither of the two implant-supported crowns are seated fully against the abutments. The strain arising from the inaccurate fit has translated into loosening of the abutment screw in the more distal implant.

Tightening of occlusal and abutment screws should always be done with a torque control device to insure that the screws are being tightened to the correct torque. The screw head should be protected by an inert and easily recognizable material such as PTFE tape before the access cavity is closed with a light-cured composite filling material. The static and dynamic occlusion should be checked to insure that it conforms with the overall occlusal scheme and with specific recommendations for implant-supported FDPs. The ITI Academy Learning Module 'Occlusion on Fixed Implant Prostheses' covers this topic in detail.

A specific clinical occurrence is the loosening of the abutment screw from the implant body under a cement-retained crown that remains affixed to the abutment. It can prove very difficult to remove the crown from the loosened abutment, and care should be taken not to inadvertently fracture the abutment screw or cause other damage to the implant-abutment interface. If possible, the most efficient option is to open an access hole through the cemented crown to gain access to the abutment screw. This approach does, however, presuppose knowledge of the exact crown-abutment assembly and point of access to the screw. It is a sensible idea to keep the working cast with notation of these details, and it is also possible to construct indices that will guide precise reentry to the screw.

Incomplete seating of prostheses during cementation can also lead to strain on all hardware components. In this clinical example the contact tension between two adjacent implant crowns has prevented full seating during cementation of the more distal prosthesis. This should have been detected clinically when the prostheses were fitted and on the radiograph taken at baseline. Clinically, the excessive contact tension should have been detected when it was impossible to pass floss through the contact area. Radiographically, the incomplete seating should have been obvious as a clear gap at the mesial margin.

For cemented implant-supported prostheses, loss of retention is a common occurrence. Prior to recementation it is important to analyze why the cementation has failed. It should be noted that most dental cements have been formulated for bonding to natural teeth and may not perform in a similar or appropriate manner when used for implant-supported FDPs. Prior to recementation the interior fitting surfaces of the prostheses should be cleaned thoroughly. Then the prostheses should be checked for full and correct seating and marginal fit.

Due to the risk of retained excess cement and the ensuing biological complications, cement should be applied in a thin layer to the inner surfaces of the prostheses. It is also preferable to use a type of cement that is brittle and non-adhesive to ensure that any excess can be easily removed. For deeper margins it is advisable to take a follow-up radiograph to confirm correct seating and absence of excess cement. If excess cement is detected, it must be removed immediately and completely to correct the problem. If left in situ or undetected, a chronic fistula or acute abscess may be the first, and also serious, sign of the complication. If so, this usually requires mucoperiosteal flap elevation to expose the area and to allow thorough cleansing of residual cement.

When an occlusal or abutment screw fractures, care should be exercised in trying to retrieve the broken stump of the screw. Use of rotary instruments should be avoided if possible, as they can easily damage the internal threads of the implant itself. To retrieve a broken screw the clinician must have good access and visibility, which may require elevation of a mucoperiosteal flap. A very sharp hand instrument such as a probe or a scaler should be used to engage the broken stump of the screw, and a counterclockwise motion should be applied gently to allow the broken piece to be rotated out of the implant. If the sharp instrument does not dislodge the screw stump, an ultrasonic scaler can be used with the same counterclockwise rotation. If neither of these techniques succeeds in removal of the broken screw, a screw retrieval kit may be obtained from the implant manufacturer.

Porcelain fracture is unfortunately too common an occurrence. When it does occur one should determine, if possible, the cause of failure. Poor technical skill or a material mismatch may have resulted in inadequate bonding between the ceramic and the underlying coping, or an inadequate coping design may have given rise to unsupported porcelain. Regardless of the cause, the prosthesis must be removed and replaced. Intraoral repair of veneering ceramic is not a good option as it will likely result in failure soon after being placed. The best course of action is to start over with a newly designed coping with well-applied and well-fired porcelain.

In the case of hybrid dental prostheses, fracture of denture teeth, composite resin, and/or methylmethacrylate base material is not uncommon. Such complications are usually easy to correct by removing the prosthesis and replacing the missing teeth and gingival acrylic. Non-cross-linked resin surfaces must be available to facilitate chairside repair. Fully cross-linked composites may require laboratory repair.

Implant fracture is a rare complication and, as such, does not fit into the category of common complications. It is, however, a very distressing event for the patient and clinician alike, and its management is rarely straightforward. It therefore warrants cautionary comments within the context of this Learning Module. Implant fracture may, as previously discussed, be the result of unacceptable quality of the implant as purchased from the manufacturer or be the result of inappropriate implant type, number, or position. When implant fracture does occur, there is usually little alternative but to remove the broken implant. This should be carefully planned and carried out by a surgeon with suitable experience so as to limit the damage to the implant site. Reconstruction via hard tissue grafting may prove necessary. However, the cause of the complication should be diagnosed first, and any reconstruction procedures should be subject to a redesigned prosthodontic and surgical plan that appropriately addresses and corrects the cause.

Management of Common Hardware Complications, Key Learning Points: Screw loosening is a common complication that should be addressed immediately to prevent subsequent further damage to the screw or other components. Loosened abutment screws under cemented crowns are more difficult to treat. When recementing a prosthesis, meticulous removal of excess cement should be followed by a post-cementation radiograph. Removal of a fractured screw requires patience and a scaler or screw retrieval kit. When ceramic veneering has fractured, best practice is to remove and replace the prosthesis. Fractured implants should only be removed by an experienced clinician; further diagnosis and treatment planning must occur before the implant can be replaced.

Principles of Managing Hardware Complications Associated with Fixed Dental Prostheses, Module Summary: Prosthodontic complications may be categorized as mechanical and technical in nature, and additional factors such as occlusion or iatrogenic issues can contribute to their occurrence. It is frequently difficult to determine the nature of a complication in any specific clinical situation, but diagnosing the cause is key to prevention of future complications. The clinician should be able to recognize situations in which the risk of complications is increased. Careful prosthodontic planning and use of well-documented implants and original components can reduce the risk of complications. The clinician should be prepared to treat prosthodontic implant complications as they occur.