Implant-supported fixed dental prostheses aim to provide replacements for missing teeth that are able to withstand functional demands and accurately mimic the esthetics of the missing dental units. Ideally, the prosthesis should be indistinguishable from the surrounding dentition. To achieve this goal, the clinician must select the best available materials to meet the challenges posed by the clinical situation. This module will discuss the dental materials that are available for the manufacture of implant-supported fixed dental prostheses and the basic principles involved in selecting the material that is best suited for each case.

After completing this ITI Academy Module, you should be able to describe the ideal properties for dental materials used to fabricate implant-supported fixed dental prostheses or FDPs; list the materials that are available to fabricate implant-supported FDPs; rate available materials against the ideal properties for implant-supported FDP materials; and identify specific situations where special care should be exercised in selecting materials for implant-supported FDPs.

The ideal material for manufacturing implant-supported fixed dental prostheses should meet six basic criteria. First, the material must be biocompatible and not cause undesirable reactions in the surrounding hard and soft tissues. The material must be strong enough to resist functional forces as well as potential parafunctional forces without becoming distorted or damaged. The material should be capable of blending into and mimicking the surrounding dentition. The process of fabricating a prosthesis with the material should be easily achievable using accepted and predictable protocols. Fabrication may follow a conventional path or employ digital design and manufacturing processes. Maintenance of the final prosthesis should be easy to perform, and if the prosthesis is damaged, repair should be possible. Finally, the material should be affordable so that it can potentially be used in the treatment of a broad range of patients.

Materials that come into contact with the hard and soft tissues that surround an implant prosthesis should be biocompatible, that is, they should be safe and not cause adverse reactions within these tissues. This is usually tested by measuring the effect of the material on cell cultures. This graph demonstrates the effect of various metals on a cell culture. In this experiment, titanium and zirconia, the constituents of a novel implant alloy, are the only metals not to significantly inhibit the activity of an osteoblast cell culture.

Dental materials used to fabricate prostheses have physical and mechanical properties that follow natural laws. Strength is a mechanical property of importance to dentistry and is defined as the level of stress that a material can withstand without sustaining a certain amount of initial plastic deformation. When a force (or stress) is applied to a material, it will deform. When this stress is slowly increased, the deformation (or strain) will initially be proportional to the level of stress, following Hooke's Law. This is called 'elastic deformation'. If the stress is removed, the material will return to its basic shape and dimensions. However, every material has a point at which this proportional relationship between stress and strain ceases. This is called the 'proportional limit', and beyond this level of stress the material permanently changes shape in a process called 'plastic deformation'. These relationships remain true for all forms of force: tensile or stretching forces, compressive or crushing forces, and shear or tearing forces - and define a material's tensile strength, compressive strength, and shear strength, respectively. If the stress or force continues to be applied - for example, a wire continues to be stretched with increasing force - eventually the material will break. In this situation the material is said to have reached its 'ultimate tensile stress'.

The hardness of a material is also important for dentistry. Hardness is a physical property that is defined as the ability of a material to resist scratching, indenting, or wear. In the context of materials used for dental prostheses, the hardness of a material may become an important consideration when it exceeds that of the opposing teeth and restorations. In such situations, damage to the opposing dentition through wear may become a problem. This clinical image shows metal-ceramic crowns that exhibit differing levels of wear on the metal surfaces. The right-side crowns have cobalt-chromium alloy frameworks, while those on the left are made with high gold alloy frameworks. The difference in wear on the crowns reflects the differences in the hardness of these materials.

Many dental materials are brittle. Ceramics and some forms of cast metal can be classified as brittle materials. Brittleness is defined as the inability of a material to deform plastically before fracture when significantly stressed. This is usually related to defects within the material's structure or to surface flaws resulting from activities like wear or grinding. Brittle materials are not homogeneous in their structure or in their strength. Fracture usually follows the propagation of cracks through the weakest parts of the material. In the clinical case shown, a zirconia abutment was manufactured and then veneered with compatible porcelain. Unfortunately, a fracture in the ceramic occurred soon after placement of the crown. Uneven seating of the abutment during final torqueing may have led to excessive forces, inducing cracks at the points of stress concentration. These cracks would have propagated when the crown was further stressed while in functional contact, until eventually the fracture occurred. This fracture can be clearly seen in the enlarged image and involves both the zirconia abutment and the veneering porcelain. 'Fracture toughness' is the property of these brittle materials that allows them to resist fracture through crack propagation under an applied stress. This property is influenced by the material itself, but it can also be affected by manufacturing processes as well as handling in both the laboratory and the clinic.

Implant-supported FDPs must be able to resist repeated high loads. Normal functional biting forces can be as great as 800 newtons. Although there is significant variability and overlap, these forces tend to be higher in males than females, and higher in posterior than anterior sites. This is due to the geometry of the jaw and the position of the muscle attachments in relation to the jaw joints. In effect, the jaw acts as a second-class lever in posterior sites, amplifying the forces on teeth and prostheses in these regions. This is much like a nutcracker, in which the applied force increases the closer the nut is placed to the hinge. Normal functional forces include tooth-to-tooth contact during swallowing and chewing, leading to thousands of loading cycles per day. Parafunctional forces in bruxism can be much higher. Parafunctional contacts tend to be magnified and prolonged when compared with functional loads. This clinical image illustrates the potential power of this repeated heavy loading. Flexure in the teeth has resulted in fracture of the enamel in a number of sites in a process termed 'abfraction'. Wear is also evident from this patient's significant bruxism habit. Implant-supported FDPs placed in this type of environment need to be able to withstand such forces.

An ideal material should also be esthetic so that the implant-supported prosthesis resembles a natural tooth in color and structure to the greatest extent possible. The White Esthetic Score described by Belser and others in 2009 describes the parameters that the clinician should seek to reproduce in an esthetic FDP. Some of these parameters are not related to the material itself; however, the material's ability to mimic natural tooth colors and translucency clearly has an impact on the esthetic outcome. In this clinical case, the patient presented with a failing upper left central incisor, unrestored adjacent teeth, a high smile line, and high esthetic expectations. Her gingival phenotype was medium to thick, so there was a low risk that the abutment material would affect the mucosal color; however, her natural adjacent central incisor is quite translucent, suggesting that the best choice of material might be an all-ceramic prosthesis.

The fabrication of an implant-supported FDP is based on many sensitive steps. Although all FDP materials have some level of evidence supporting their fabrication and use, the level of evidence varies according to the material. Materials used in conventional laboratory protocols have a long history of use, and technicians have been well trained to manipulate these materials and obtain good results. Long-term outcome data is available for conventional materials. New technologies and materials employing digital CAD/CAM techniques have less history, and often only short- to medium-term outcome data has been collected. Additionally, achieving predictable outcomes using these technologies and materials requires additional training and experience. These technologies are constantly being improved, with new materials being brought to market. This issue may complicate the decision-making process, as there may be limited data on the effectiveness of these materials for implant-supported prostheses. Ultimately, the skill of the dental laboratory technician is of prime importance. For example, in cases of high esthetic demand, a veneered rather than monolithic prosthesis will be needed to achieve optimal esthetic results. In such cases, the level of training, experience, and artistic ability of the dental ceramist will likely have a greater impact on the outcome than the choice of material. This is demonstrated in these two clinical images of all-ceramic and traditional porcelain-fused-to-gold implant crowns replacing lateral incisors. Despite the often-stated potential for metal-ceramic crowns to be too opaque for good esthetics, in both cases acceptable results have been achieved. In the hands of a skilled ceramist, the so-called disadvantage of the traditional technique can be overcome.

Implant-supported FDPs must be cleansable to limit the risk of peri-implant disease. Cleansability is related both to the design of the prosthesis and to the materials from which it is made. Some materials have low plaque adherence and are easily maintained. Ceramics tend to have low levels of plaque adherence, even if they are slightly rough. On the other hand, metals and resins must be highly polished to limit plaque adherence, and even so can be hard to keep clean, especially if the tissue-facing surfaces of pontics are concave. It is also desirable to be able to repair FDPs should they be damaged. Fracture of veneering materials is a relatively common complication of implant treatments, and repairing these areas can be problematic. Porcelain repairs may not be possible, and even small to moderate fractures may require removal and replacement of the veneering material. On larger FDPs, this can be difficult and expensive. Resin veneering materials, on the other hand, tend to be easily repaired - often intraorally. Thus, for large reconstructions or in situations where veneering material fracture is to be expected - such as in a patient with a strong bruxing habit - it may be beneficial to use resin materials for the esthetic component of the FDP.

Cost is a consideration in any purchasing situation. Therefore the cost of fabricating an implant-supported FDP should be considered when selecting the materials and technologies to be used. Total costs include the cost of the time required for fabrication, both in the clinic and the laboratory, as well as the cost of materials and the cost of implant components. The major areas where costs can be controlled are in the time used and the materials. If a clinical or laboratory process can be made more efficient so that it takes less time, costs can be lowered. Similarly, selecting materials that have a lower intrinsic cost or are less expensive to produce can also result in saving money. High noble alloys used in the porcelain-fused-to-gold crowns are costly, and their use is becoming an area of cost sensitivity. CAD/CAM technologies have the potential to reduce costs by being more time efficient; however, these digital technologies have significant set-up costs and ongoing upgrade, maintenance, and training expenses. As these technologies become more commonplace and marketplace competition becomes more active, costs will come down.

Ideal Properties, Key Learning Points: Materials must be biocompatible so that they do not cause adverse reactions. A material must have strength and fracture toughness to resist functional and parafunctional forces without distortion or damage. The hardness of the material can affect the opposing dentition and restorations. The material should resemble the surrounding dentition in both color and translucency. Fabrication of a prosthesis with the material should be predictable whether a conventional or CAD/CAM protocol is used; the skill of the dental laboratory technician is a crucial component. Potential for plaque adherence and ability to be repaired are important considerations. The overall cost of a material depends on both its intrinsic cost and ability to be used in a time-efficient workflow.

Dental materials used to make implant-supported FDPs can generally be classified into three groups: metals, ceramics, and resin-based materials. These can be used individually or in combination. Thus, the commonly used technologies include the all-metal prosthesis, the all-ceramic prosthesis, the metal-ceramic prosthesis, and a resin prosthesis on a metal framework the so-called hybrid prosthesis.

Metals used in implant-supported FDPs include titanium, precious and semi-precious alloys based on gold or silver, and non-precious alloys. These materials have many favorable physical properties. They have high compressive and tensile strengths and are resilient and ductile. Many of these metals have similar hardness to dental enamel, so they are not normally associated with excessive wear of the opposing dentition. However, metals are not considered to be esthetically acceptable by most patients. To overcome this shortcoming, veneering ceramics or resins can be used to mimic tooth colors. In general, metals are well tolerated in the mouth.

High gold alloys tend to have the best performance in ceramic bonding, with lower rates of veneering fractures. High gold alloys in implant abutments have been associated with increases in inflammatory infiltrates in surrounding mucosal tissues, but the clinical significance of this is unclear. While it is possible to bond ceramic to titanium frameworks, this process is more technique sensitive and is associated with higher rates of veneering material delamination. However, titanium has the best biocompatibility. More significant soft tissue reactions are associated with non-precious alloys, as observed in patients who have a sensitivity to base metals such as cobalt-chromium or nickel-silver alloys. In such patients, non-precious alloys should be avoided, especially in abutments or parts of the prosthesis that contact soft tissue.

Ceramic materials used in this field include the aluminous and feldspathic porcelains used to veneer other materials to achieve a more lifelike appearance, glass-based materials like the leucite and lithium disilicate ceramics, and oxide ceramics like zirconia. All of these materials are highly biocompatible. In general, they are also esthetically acceptable to patients, but they vary in their level of translucency and therefore in their ability to exactly copy the appearance of surrounding teeth. The glass ceramics tend to be the most translucent. This can be useful in clinical situations where the surrounding teeth also have high translucency. However, these materials have limited ability to mask the color of underlying stained teeth and metal frameworks. The oxide ceramics such as zirconia tend to be the most opaque of this group, but they may need to be veneered with other ceramics to achieve a highly esthetic outcome.

All of these ceramics are somewhat harder than tooth enamel and have been associated with significant wear of opposing teeth. As such, some care should be taken in using these materials in patients with strong bruxing habits. Finally, all ceramics are brittle materials. Some, like the zirconia and lithium disilicate ceramics, have good fracture toughness and can be used for short-span FDPs. However, all of these materials are sensitive to handling errors. The Fifth ITI Consensus Conference noted that studies of ceramic materials used in abutments for implant-supported FDPs "reflect an inherent sensitivity of ceramics to design and processing problems; for example, stress concentration, thin walls, sintering, and residual machining flaws".

Resin-based materials include acrylics, composite resin materials similar to those used clinically in dentistry, and polyether ether ketone or PEEK. These are rarely used alone as definitive prostheses because they tend to be weak and soft, physical properties that fall short of the requirements for long-term loading. When reinforced with metal frameworks, these materials can be useful as definitive prostheses because they are easily made using conventional techniques and can be easily repaired. It should be noted that these acrylic resin-veneered frameworks are prone to wear with time, so patients should be advised that renewal of the resin part of their prosthesis will be needed on a semi-regular basis.

The biocompatibility of resin-based materials varies according to the material. Acrylics can still leach out monomer after curing, causing some local irritation to surrounding tissues. Thus, it is advisable to keep acrylic materials away from soft tissues as much as possible. Composite resins tend to be well tolerated, as they do not release toxic chemicals after they have been cured. PEEK appears to be stable over time, but there is little data on its biocompatibility. PEEK components are often supplied as bases for provisional prostheses to be veneered with composites or acrylics. Unfortunately, bonding acrylic or composite materials to PEEK is unreliable, so these prostheses are strictly for short-term use.

Available Materials, Key Learning Points: Metals tend to be biocompatible, with minimal wear on opposing teeth, but require veneering in order to be esthetically acceptable. Ceramic veneering of titanium is technique-sensitive and prone to delamination. Ceramics are esthetic but brittle materials that are highly sensitive to design and handling errors. Ceramics vary in translucency; some ceramics may require veneering with more translucent materials to achieve a highly esthetic outcome. Resins are too weak to be used alone as a definitive prosthesis. Resin materials can be reinforced with metal for a definitive prosthesis; PEEK can be veneered with other resins for a provisional prosthesis.

Metal-ceramic crowns based on high noble alloys have a good evidence base in implant dentistry and have been the mainstay of implant-supported FDPs for many years. These prostheses have adequate biocompatibility. While some studies have shown an increase in inflammatory cell infiltrates adjacent to gold abutments, this does not appear to be associated with higher failure or complication rates. Metal-ceramic crowns are certainly strong and hard - possibly too hard, as the veneering ceramic can be associated with excessive wear of the opposing teeth. Esthetics can be good; however, this type of construction tends to be somewhat more opaque than natural teeth. Even so, a skilled dental ceramist can achieve a very good result in the right circumstances, as in the clinical case shown here. The processes involved in manufacture of this type of prosthesis are well established and understood. While conventional casting techniques are most common, digital manufacture of the metal framework is possible. The veneering process is still performed conventionally. These materials are easily cleaned because plaque adherence to glazed porcelain is low. However, repair of fractured ceramic veneering is problematic and often requires removal of all veneering material from the metal framework followed by re-application of the porcelain, a process that can be difficult and expensive. Finally, the cost of precious alloys is becoming an issue, driving patients and practitioners to seek less expensive options.

Prostheses made from porcelain fused to non-precious metal frameworks have many of the properties of porcelain-fused-to-gold prostheses, and skilled technicians can still achieve acceptable esthetic outcomes. The main areas of difference lie in the intrinsic cost of the metal and in its biocompatibility. Some individuals have sensitivity to base metals and will experience an inflammatory reaction when in contact with these alloys. These patients will often report sensitivity to non-precious metals used in jewelry. The use of non-precious alloys in such individuals, especially as abutments or frameworks in close contact with mucosal tissues, should be avoided.

Monolithic zirconia prostheses have been promoted as viable alternatives to metal-ceramic crowns for general use. These ceramic prostheses are well tolerated and strong enough for use in most sites in the mouth. They are suitable for single-tooth replacements and short-span FDPs. From an esthetic point of view, these materials tend to be somewhat opaque, but they can be acceptable in less esthetically challenging situations, such as this clinical case. Manufacture of these prostheses relies on CAD/CAM techniques, which are now commonly available and have established reliability. Plaque adherence is low. If these prostheses fracture, repair is usually not possible and they must be remade. Finally, the raw materials for these prostheses are fairly low in cost. The major cost drivers are the purchase of CAD/CAM design and manufacturing equipment and the training costs associated with adopting this new technology. After establishing this capability, there are some continuing expenses for upgrades and maintenance; however, as this technology becomes more widespread and competition within the marketplace becomes more intense, the overall cost of these prostheses is expected to decline.

Zirconia prostheses can be veneered with porcelain to achieve a more esthetically pleasing and lifelike result, as shown in this example of implant-supported crowns on lateral incisors. Biocompatibility is good, and strength is adequate. Some reports note increased incidence of veneering material fracture when the substrate is zirconia; however, ceramic systems have been refined over time, and veneer fracture is less of an issue with current systems. While the frameworks for these prostheses are made using CAD/CAM technologies, the veneering process is still done conventionally, and this requires skilled technicians for the best results. These prostheses are easily cleaned by patients, as ceramics have low plaque adherence. Repairs of damaged prostheses can be difficult and costly, especially if the veneering material needs to be replaced. Finally, these materials can be somewhat less expensive than those made with conventional techniques because the CAD/CAM part of the fabrication process is more time and labor efficient.

Lithium disilicates are a class of glass-based ceramics that are characterized by excellent translucency and good fracture toughness. The esthetic outcome of stained monolithic prostheses made from lithium disilicate is usually very good; porcelain veneering to further improve esthetics and characterization is also possible. In the clinical case shown, lithium disilicate implant-supported crowns have been used to replace the missing lateral incisors, achieving harmonious esthetic integration with the surrounding natural teeth. In all other parameters, these prostheses perform in a similar manner to the zirconia materials discussed previously.

The resin-metal hybrid prosthesis has the distinction of being the original implant-supported treatment, dating back to the original work of Brånemark and coworkers. It is based on proven prosthodontic techniques and materials. In this clinical case, a cast gold alloy frame supports traditional denture teeth and pink acrylic resin serving as the soft tissue replacement. This combination can achieve the same degree of esthetics as a conventional complete denture prosthesis and is often sufficient to meet the patient's needs. More recently, cast gold frames have been replaced by milled titanium or cobalt-chromium structures; however, the veneering resin materials have mostly remained unchanged. Biocompatibility of acrylic resins can be an issue for some individuals due to the leaching of monomer from the resin. These prostheses survive reasonably well in the functional environment in which they are placed, but problems with wear and fracture of the teeth and pink acrylic can occur, especially in patients with strong parafunctional habits. However, if damaged, repair is usually straightforward, and total replacement of the acrylic portion is possible. Adequate space for hygiene access is essential for success, as the metal and acrylic resin materials have some potential for plaque and calculus accumulation. Finally, manufacturing costs are reasonable, especially since the technique has moved away from cast gold frameworks, and maintenance and repair costs are reasonably low.

Finally, resin materials can be used to make relatively low-cost provisional prostheses. These are often used in the development of the transition zone and emergence profile of implant prostheses in esthetic sites, because additional material can be easily added to the provisional crown to improve tissue displacement. Although these materials lack sufficient strength to be used as definitive prostheses, acrylic or composite resin provisional crowns can be maintained for many months. Esthetics is often limited, although the clinical case shown represents the pinnacle of the technician's art when employing composite resin materials for provisional prostheses. In all other areas, their properties are in accordance with previous discussions.

Rating Material Options, Key Learning Points: Metal-ceramic prostheses have a long record of use and reasonably esthetic outcomes, but some patients experience sensitivity to base metal alloys. Monolithic zirconia and lithium disilicate can be used to fabricate strong prostheses using CAD/CAM technology. Lithium disilicate results in a more translucent prosthesis than monolithic zirconia; both materials can be veneered with porcelain to improve the esthetic result. Fractured veneering porcelain on zirconia or lithium disilicate requires remake of the prosthesis. Resin-metal hybrid prostheses have a long record of use and reasonable costs, but fracture of the acrylic resin is not uncommon. Resin materials are ideal for provisional prostheses because additional material can be added to develop the emergence profile for the definitive prosthesis.

Certain clinical situations merit additional consideration when selecting the most appropriate material. As previously described, bruxism results in higher forces being exerted on teeth and implant-supported prostheses. This loading is more frequent and more prolonged than functional loading. This situation has been identified by Brägger and coworkers as contributing to greater risks of technical complications such as screw loosening and prosthesis fractures. To minimize risks, the clinician can rely on several strategies. A clinician can choose to use stronger materials or materials that are more readily repaired. In situations where opposing natural teeth may be damaged by wear, one should avoid using materials that are much harder than enamel. For example, as in the clinical case shown, a metal occlusal surface may be less damaging to the opposing teeth than a porcelain one.

Long-span FDPs can undergo flexure, and the potential effects of this flexure on the prosthesis can be problematic. The flexure in a beam or rod for a given load is defined by the length of the beam and the thickness of the material. In a three-point loading situation, such as an implant-supported FDP being loaded in the pontic area, the flexural stress is proportional to the length between the support points and is inversely proportional to the thickness of the beam. This effective stress will result in a distortion of the beam, where the distortion (or strain) is described by the stress-strain curve discussed in the first Learning Objective. The elastic modulus of the material, known as Young's modulus, defines the relationship between stress and strain during elastic deformation. This modulus can be expressed as the slope of the proportional part of the stress-strain curve and essentially describes the stiffness of a material. The steeper the slope, the stiffer the material, that is, for a given stress a stiff material will deform less than a more elastic material.

In the context of implant-supported FDPs, flexure can lead to problems. Implants lack periodontal ligaments and are therefore less resilient than teeth. As a result, any load must be managed within the prosthesis and the implants. Excessive flexure of the FDP is associated with problems such as fracture of ceramic veneering materials and screw loosening. Strategies to minimize these risks include using stiffer materials like non-precious alloys in metal-ceramic FDPs and increasing the vertical thickness of the connectors.

Full-arch cases, especially those in which fixed implant-supported prostheses are employed in both arches, can have loading-related problems that impact on the choice of materials. Lacking periodontal ligaments, implants also lack any proprioception relating to contact between the prostheses and teeth. In a mixed tooth and implant situation, the surrounding and opposing teeth can give the patient some sense of tooth contact and loading. In full-mouth rehabilitations with implants, there are no natural teeth to provide this feedback, and this can lead to problems with prosthesis wear and fracture. Limiting these potential problems requires prudent selection of materials. It is recommended to choose materials that have some inherent resilience, like resin-based materials. Additionally, due to the additional risk of damage to the prosthesis, it is advisable to plan ahead for likely future repairs. Segmented reconstructions or the use of resin-metal hybrid prostheses that can be more easily repaired are sensible strategies. In this clinical case, a young patient with complete agenesis of the secondary dentition has been treated with implant-supported FDPs. Although the patient's bruxing status was unknown, some difficulties with wear or fracture were anticipated. To reduce risk and to make for easier repairs, the upper arch was restored using segmented metal-ceramic FDPs, while the lower arch was restored with a resin-metal hybrid prosthesis. In this situation the hybrid prosthesis serves as the sacrificial element, that is, it undergoes wear in preference to the upper FDPs. When needed, the hybrid prosthesis can be readily refurbished at an acceptable cost in time, effort, and money.

Special Considerations, Key Learning Points: Material strategies for bruxing patients include choosing stronger or more easily repaired materials while avoiding materials that are harder than enamel. Because long-span FDPs can undergo flexure, stiffer materials like non-precious alloys and thicker connectors are recommended. Materials for full-arch prostheses should preferably have resilience and the ability to be easily repaired.

Dental Materials Selection for Fixed Dental Prostheses, Module Summary: Important properties of a material for implant-supported FDPs are biocompatibility, strength, esthetic potential, cost, and ease of fabrication, maintenance, and repair. Metal-ceramic crowns and FDPs are strong, well tolerated by most patients, and acceptably esthetic, although somewhat opaque. Glass ceramics such as lithium disilicate and oxide ceramics such as zirconia are strong, esthetic, and biocompatible materials for single crowns and short-span FDPs. Disadvantages of ceramic materials are the potential for wear of the opposing dentition and the need for prostheses to be remade upon fracture or chipping. Resin materials are weak and prone to wear, but resins can be reinforced with metal to serve as full-arch hybrid prostheses or used for fabricating provisional prostheses.