Biomaterials in Dentistry

Li Wu Zheng, Prince Philip Dental Hospital, The University of Hong Kong, Hong Kong
Jing Yi Wang and Ru Qing Yu, The University of Hong Kong, Hong Kong
© 2018 Elsevier Inc. All rights reserved.

Restorative Dentistry 1
Direct restorative Materials 1
Amalgam 1
Composites 2
Glass ionomer cements 2
Indirect Restorative Materials 2
Metals 2
Ceramics 3
Cements 3
Endodontic materials 3
Denture bases and lining materials 4
Impression materials 4
Waxes 4
Adhesive system 5
Orthodontic 5
Brackets 5
Adhesives 6
Archwires 6
Elastomeric Modules and Chains 6
Periodontics 6
Barrier Membranes 6
Bone Graft Materials 7
3D-Printed Scaffolds 8
Implants Dentistry 8
Implant Materials 8
Metals 8
Ceramics 9
Polymers and carbon compound 9
Oral and Maxillofacial Surgery 9
Autogenous Grafts 10
Bone Substitutes 10
Titanium 10
Tissue Engineering 11
Further Reading 11

The earliest dental materials science, dating back to the early 19th century at Northwestern University, began from the investigation
of dental amalgam. For approximately a hundred years, synthetic restorative materials were the major focus in the field of dental
materials until the end of the last century, when the real potential for biological engineering of tissues and organ systems was
revealed. The comprehension of development and advances in dental biomaterials will benefit both dental practitioners and
patients in selecting appropriate materials for clinical cases and improving treatment outcomes.

Restorative Dentistry

Restorative dentistry can be traced back to ancient times. Materials used for restoration back then include cork, ivory, human teeth,
and metal foils (lead and tin), etc. Nowadays, amalgam, composites, ceramics, metals, and cements are common restorative
materials.

Direct restorative Materials

Amalgam
Amalgam has been using to restore tooth structure since the 19th century. Dental amalgam mainly contains mercury, tin, copper,
and zinc. With proper cavity preparation and manipulation of dental amalgam, this can provide adequate strength to resist

Encyclopedia of Biomedical Engineering https://doi.org/10.1016/B978-0-12-801238-3.11033-5 1

2 Biomaterials in Dentistry

masticatory forces with quite good longevity. Although amalgam has been a great success as a restorative material, its mercury
content has raised concerns regarding the potential adverse effects such as neurotoxicity in recent decades. The exposure to mercury
for dental personnel and environment also adds to the argument of reducing the use of dental amalgam. Moreover, esthetics has
become an increasingly important aspect in the choice of materials, causing patients to be reluctant to choose amalgam’s gray color
material over more esthetic materials, which are becoming more durable. Currently, with the development of other restorative
materials (i.e., composites), the use of dental amalgam as a direct restorative material is phasing down globally, despite its low cost
and good performance.

Composites
Dental composites, or resin-based composites, are synthetic materials that combine polymeric matrix with a dispersion of glass,
mineral, or resin filler particles and/or short fibers by coupling agents. Just like dental amalgam, they are used to restore tooth
structure lost through trauma, caries, or other diseases. Composites can also be used as cements to cement crowns and veneers, etc.
While the amalgam is phasing out in dentistry, composites have become one of the most widely used esthetic restorative materials.
Traditional composites contain relatively large particles of ground amorphous silica and quartz, which gives them good
mechanical properties but makes the surface of the restoration more likely to become rough from daily abrasion. In addition,
many failures of composite restoration are seen at the interface between tooth and composite due to shrinkage or adhesive failure.
To overcome this, microfilled composites, nanofilled composites, and other hybrid composites were developed, using much smaller
particles (at the same time with a large variety in size) to fill in the matrix. With these developments, smoother surfaces are achieved,
wear resistance is increased, and shrinkage is decreased without compromising the mechanical and physical properties.
Composites can be classified as chemically activated (self-cure) resins and photochemically activated (light-cure) resins. The self-
cure resins are supplied as two pastes; polymerization is activated when those two pastes are mixed together. Disadvantages are that
the air may be incorporated into the mix during mixing, thus weakening the material, and the operator cannot control the working
time after mixing. The light-cure resin is supplied as a single paste using the photosensitive initiator system and a light source for
activation. It does not need mixing, which makes it stronger and less staining, and has totally controllable working time. However, it
exhibits higher marginal stress during curing and only cures within limited depth (2–3 mm). Although this incremental curing
demonstrates some advantages, the demand for bulk cure has never stopped. Some new products have claimed that the cure depth
can be up to 4 mm (the cure depth of dual cure resins is unlimited since it is a combination of chemical and light-cure technology),
but their clinical performance has not been fully assessed.

Glass ionomer cements

Glass ionomer cements conventionally are acid-base materials that have been used to esthetically restore tooth structure since the
late 20th century. The powder of a number of glass components mixes with the liquid of polyalkenoic acid to form a paste, then the
acid-base reaction starts and stiffens the paste. The mechanical property does not suit the clinical requirements enough in the
beginning, but slowly improves with time. One of the advantages of glass ionomers is the true bonding between materials and
dentin/enamel; thus they have been widely used for Class V restorations which have high requirements in adhesion, for Class II and
Class III restorations in deciduous teeth, for luting of crowns, and they also can be used as bases or liners. Fluoride release is another
merit of glass ionomers. The disadvantages are that they are moisture-sensitive and have relatively low strength.
The newer glass ionomers are the resin-modified glass ionomers, which were introduced in the 1980s. These are a combination
of conventional glass ionomer cement and light-cure resins to improve some characteristics of conventional glass ionomers such as
increased strength, lower solubility, and less sensitivity to moisture. However, fluoride release of resin-modified glass ionomer is
lower and the biocompatibility is not as good as that of conventional glass ionomers.
Recently, glass ionomers were combined with bioactive glass (BAG) to improve their bioactivity and regenerative capacity. These
materials might be a better choice in tooth restoration compared to conventional glass ionomers or resin-modified ones, due to
their remineralization ability.

Indirect Restorative Materials

Metals
Crowns, inlays, cast posts and cores, and partial dentures are all examples of indirect metallic restorations. They are produced in the
dental laboratory instead of being carried out in the dental chair. There is a variety of alloys used in dentistry; the main alloys include
noble and precious metal alloys and various base-metal alloys, i.e., CodCr alloys and titanium.

Noble and precious metal alloys

Gold, platinum, iridium, ruthenium, rhodium, and osmium are considered to make up the noble metals, which are very resistant to
corrosion. Silver and palladium are usually referred to as the precious metals, which are expensive. High’gold alloys must have a
gold content >60% and precious metal content no <75%. They can be classified according to their gold content into four types
(I–IV): soft, medium, hard, and extra-hard. In the 1970s, alloys with reduced gold content were rising in the market because of the
rapidly increasing price of noble metals. These alloys can be further classified into medium’gold (40%–60% gold content) and
low’gold alloys (10%–20% gold content). Their mechanical properties are similar to the type III and IV gold alloys, but exhibit
lower ductility. The white color makes them less popular with patients, but they can be used for posts and cores since they will be
 Biomaterials in Dentistry 3

covered with other materials. Silver–palladium alloys are another kind of precious alloy that presents various properties due to their
composition and are less popular than the above-mentioned ones.

Base metal alloys

There are two main kinds of base metal alloys: cobalt–chromium and titanium. Cobalt–chromium alloys have high hardness and
modulus of elasticity, but low ductility. In addition, they are relatively low cost and have very good biocompatibility. Generally,
they are a very popular choice of material in restorative dentistry.
Titanium alloys exhibit high strength, low density, and have great biocompatibility. However, casting is a significant challenge
for these alloys.

Ceramics
Ceramics is a material that is opaque and porous, thus relatively weak. Dental porcelain mainly differs from traditional ceramics in
terms of firing techniques, which make it more suitable for dental restoration. Dental porcelain has very stable chemical properties
and outstanding esthetics which are unlikely to be influenced by time. It has similar thermal conductivity and coefficient of thermal
expansion to enamel and dentine, and exhibits high compressive strength. However, the tensile strength of dental porcelain is very
low (20–60 MPa). In addition, the surface microcracks caused by various reasons during manufacturing are the starting sites of
catastrophic fracture. These drawbacks have limited their use to low-bearing areas, which are anterior regions of both mandible and
maxilla, and made them unsuitable for multi-unit bridges.
To overcome the disadvantages of dental porcelain, three types of dental ceramics have been developed:

• Metal-ceramics (porcelain fused to metal, or PFM), combine the positive mechanical properties of cast dental alloys and
excellent esthetic property of porcelain. The choice of metals is a key element in PFM.
• Reinforced ceramic core systems, which are similar to PFM in that instead of using alloys to support the porcelain, they use
another ceramic material with high strength and toughness yet do not offer esthetic qualities.
• Resin-bonded ceramics involve the ceramic being bonded to enamel and dentine directly, and thus the support comes from its
own tooth structure by resins. Not only the increasing strength and toughness are necessary, but also the adhesive bond, which
eliminates the surface flaws of dental ceramics and thereby reduces the probability of fractures.

Cements
Cements are used in dentistry for various purposes. Some are for cavity lining or bases; others are used as luting agents to lute an
indirect restoration to a prepared tooth.
Basically, a liner is a thin layer of material (0.5 mm) placed on a prepared tooth to protect it, whereas a base acts as the dentin to
withstand the forces applying on it. It is thicker than the liner and also protects the pulp from thermal and chemical stimulates and
galvanic shock. Thus a liner or a base should have good thermally and electrically insulating properties, and not contain irritants. It
should set rapidly, exhibit enough strength to resist fracture, and not move or flow easily while the filling material is being placed.
Ideally, linings should be radiopaque so that any caries around the filling material can be seen. The liner or base must not interfere
with the setting of the filling material or affect the properties of it.
Luting cements share similar properties with linings except for the setting time. There should be enough setting time before the
final seating of the restoration. In addition, it should be strong enough to assist retention and have low solubility.
Commonly used cements are as follows:

• Zinc oxide/eugenol cements are mixtures of zinc oxide (powder) and eugenol (liquid). They are mainly used as a lining or base
under amalgam restorations and as temporary luting cements or filling materials.
• Zinc–phosphate cements have zinc oxide as the major component of the powder and phosphoric acid solution as the liquid.
They are widely used as luting cements and can also be used as linings with adjustment of the powder/liquid ratio to change the
consistency. However, they may have an irritant effect as a liner in deep cavities.
• Calcium hydroxide cements have a low strength and high solubility, and therefore are usually used as linings beneath the base of
zinc phosphate cements or other base materials, and are not suitable for luting. Nevertheless, this material does have other
properties that make it crucial to dentistry such as the fact that it can be used for pulp capping and root canal sealing.

Endodontic materials
Endodontic materials are used in endodontic treatment, which is the procedure to save the tooth when the pulp and/or
periradicular tissues are injured. These materials can be generally classified into two groups: materials used to maintain the vitality
of the pulp (pulp capping materials), and materials used to disinfect (irrigants and intracanal medicaments) and fill the pulp in root
canal therapy.
Pulp capping materials should be able to induce hard-tissue formation in a superficial way, protect the pulp from further
invasion of bacteria, and not have side effects so that the pulp can be alive. Hard-setting calcium hydroxide cements are the most
commonly used pulp capping materials. This material causes the formation of a 1–1.5 mm thick necrosis layer in the superficial
pulp. The layer will undergo calcification eventually and it is called a dentine-bridge.
4 Biomaterials in Dentistry

Commonly used irrigants and intracanal medicaments include 0.5% sodium hypochlorite solution (as a disinfectant or
antimicrobial), EDTA (as a chelating agent and lubricant), chlorhexidine (antimicrobial), non-setting calcium hydroxide (as an
antibacterial agent), and some antibiotics and anti-inflammation drugs (non-setting Ledermix paste).
The most widely used canal-filling material is gutta-percha. Gutta-percha point is one of the application of gutta-percha, warm
lateral or vertical condensation can be applied to it to soften and compact it. The other use is the hot gutta-percha application
system, which injects the heat-softened gutta-percha into the root canal system.
In addition to gutta-percha, sealer cements are also needed in root canal filling procedures. These cements include zinc oxide-
eugenol cements, calcium hydroxide-containing sealers, glass-ionomer cements, polymers, and mineral trioxide aggregate, etc. Due
to space limitations, details of these materials will not be discussed here.

Denture bases and lining materials

Nowadays, most denture bases are fabricated using acrylic resins. The materials are usually supplied as a powder and liquid. The
major component of the powder is beads of polymethylmethacrylate (PMMA), which can be added to the liquid (mainly
methylmethacrylate monomer) to form a mixture. Shortly after the mixing, the material first has a sandy consistency, then becomes
a sticky mass, and finally comes the dough stage. The dough at this point has lost most of its tackiness and should be packed into the
mold to prevent it from becoming tough and rubbery. Acrylic denture base materials can be classified into different groups
according to their method of curing. Generally, there are heat curing materials, autopolymerizing materials, thermoplastic, light-
activated, and microwave-cured materials. With sufficient thickness, the material showed acceptable mechanical properties;
however, the impact strength and fatigue strength are relatively poor. Dimensional stability is also a problem in acrylic resins in
addition to its thermal insulated nature, which is not an advantage for denture base because it prevents the protective reflexes to
stimuli in oral mucosa. Moreover, the unreacted monomer that remained in the denture base may cause mucosal irritation and
sensitization of tissues.
Denture lining materials can be roughly classified into soft acrylics and silicone rubbers. The hard reline material is to be used
when the denture base has gone through some dimensional change and become less fit so that it needs relining of the fitting surface.
The soft lining materials are to provide a cushion when the denture is under load, and a tissue conditioner is very similar except it is
softer and can only remain soft for 1–2 days, while the softness of the temporary soft lining materials can last for 1–2 months. There
also exist permanent soft lining materials used for patients who cannot bear a hard base. However, no soft lining materials are truly
permanent in practice.

Impression materials
Impression materials are used in dentistry to record the details of intraoral structures to fabricate a reproduction of teeth and soft
tissues for the construction of dental prostheses. These materials should be able to produce an accurate replica of the intraoral
structure, to prevent deformation and be atraumatic when removing from undercuts; they should also have proper setting time and
biocompatibility. They can be categorized as rigid and elastic impression materials. Rigid impression materials include plaster and
compo/zinc oxide-eugenol; however, since they cannot engage the undercuts, their application is limited nowadays. Elastic
impression materials can be further divided into hydrocolloid and elastomeric impression. Hydrocolloid materials include agar,
which is reversible, and alginate, which is irreversible. Elastomeric materials include polysulfide, polyether, condensation-cured
silicone, and addition-cured silicone. The choice of which impression material to use in each case will depend not only on the
specific needs of each case, but also on the impression technique and tray to be used.
Alginate is currently one of the most popular impression materials. It is supplied as dust-free powders. After mixing with proper
amount of water in a rubber bowl with a spatula, it is ready for impression taking. Two to three minutes after the surface tackiness
has been lost, it can be removed from the oral cavity. However, it does not produce very accurate surface detail, and has poor
dimensional stability. A snap-removal technique is required to minimize permanent deformation. It is thus not recommended for
the fabrication of crowns and bridges.
The need for more satisfying impression materials promotes the development of elastomeric impression materials. These
materials have a much lower chance of permanent deformation when removed and are able to reproduce the surface detail very
accurately, but they are hydrophobic and therefore contamination of saliva will result in loss of surface detail to some degree.
Another problem of these materials is that they all undergo setting shrinkage due to polymerization, but in general, the shrinkage is
very low (with polyether and addition-cured silicones being the lowest and condensation-cured silicones being highest).

Waxes
Waxes have a number of applications in dentistry. A major use is as pattern wax—in other words, as modeling wax and/or inlay wax.
In the process of fabrication of dentures or restorations, there is a stage which is to produce the wax pattern of the dental appliances
on the model (indirect technique) or in the mouth (direct technique). This wax pattern determines the size and shape of the
appliance needed. A lost-wax technique is then used to replace the waxes using alloys or polymers. The pattern wax must be able to
shape the appliance accurately, and once it is formed there should be no dimensional change. Also, the wax should be removable
either by boiling or burning, and does not leave a residue.
 Biomaterials in Dentistry 5

Adhesive system
Facilitating adhesive materials in restorative dentistry instead of mainly relying on the retention form of restorative materials has
many advantages. One of the most important advantages is the conservation of tooth structure.

Enamel bonding
The acid-etch technique is most widely used for bonding resins to enamel. In 1955, Buonocore found that by modifying the enamel
surface with the phosphoric acid solution, he could make the surface more receptive to adhesion, which led to the development of
the acid technique. The etching process increases the bonding area by creating etch pits, into which the resin is absorbed by capillary
attraction. The adhesion is mainly based on this micro-mechanical interlocking. The acid-etch technique presents excellent bonding
between the etched enamel and the resin.

Dentine bonding
Dentine is a heterogeneous tissue composed of about 70% inorganics (hydroxyapatite, HA), 20% organics (collagen), and 10%
water by weight. This makes it much more difficult for bonding compared to enamel. Another problem is that it is a vital tissue and
unlikely to create a dry dentine surface, and will damage the pulp if doing so. Furthermore, the dentine is covered by a smear layer.
However, the strong demand for dentine adhesive systems has promoted the development of dentine adhesive systems.
There are three principal components in dentine-bonding agents: conditioner, primer, and sealer. A conditioner is an etchant,
which is used to remove the smear layer on the dentine. The smear layer is formed because of abrasion or burr cutting. Various acids
were used as conditioners: phosphoric acid, oxalic acid, EDTA, etc. After the conditioner is applied to the dentine, it dissolves the HA
and opens the dentinal tubules, producing a demineralized layer on the surface.
The primer acts as an adhesive to bond the hydrophobic resin to the hydrophilic dentine. The primer should be able to saturate
fully into the demineralized layer, otherwise the remaining demineralized dentine will become a weak region in the restoration. The
coupling agent in the primer is carried in a volatile solvent which is for the water removal in the dentine so that a hydrophobic resin
can be bonded tightly. A sealer is the resin used to fill the cavity, which bonds to the dentine through primer.

Total-etch technique
This technique involves etching the enamel and dentine at the same time, usually with 35% phosphoric acid for 20 s.
Dentine-bonding agents can be categorized according to the number of steps used clinically as three-step, two-step, and single-
step systems. Those consisting of a dentine conditioner, primer, and the sealer are three-step systems. Two-step systems and single-
step systems are developed to make the process more efficient and easier to use. The two-step systems combine either the
conditioner and primer together (self-etching primers) or the primer and the sealer together (one-bottle bond systems). The
single-step systems are supplied in two bottles; the practitioner only needs to mix the two components and applies them to the
enamel and dentine surface for the drying and light-curing. However, the bond strength is lower compared with the aforementioned
two systems.
Apart from the application in restorative dentistry, adhesive systems can also be used for the adhesion of orthodontic brackets
using the acid-etch technique.

Orthodontic

Materials used in orthodontic field mainly include brackets, adhesives used to bond the brackets onto teeth, archwires, and
elastomeric modules and chains.

Brackets
Orthodontic brackets are small orthodontic attachments (metal or ceramic) secured to a tooth for fastening an archwire. Each
attachment is either soldered or welded to a previously placed band enclosing the tooth, or is bonded directly onto the tooth.
Metallic brackets can be further categorized into stainless steel, non’nickel or low’nickel stainless steel, and titanium brackets. The
nickel component in traditional stainless steel has revealed genotoxic effects and may cause some allergic reaction in patients,
therefore non’nickel or low’nickel stainless steels are substitutes for the traditional ones. This type of steel exhibits similar or higher
hardness but may show lower corrosion resistance. Recently, titanium has been used as an alternative to produce brackets because of
its superior biocompatibility and higher corrosion resistance. However, titanium brackets have lower hardness compared to the
stainless steel brackets. Thus, wear presents a problem in titanium brackets.
Ceramic brackets are developed for esthetic purposes. Other esthetic materials have also been used—for example, plastics and
polycarbonate-based materials. Plastic brackets undergo extensive creep deformation and discoloration during use, and a low
hardness limited their use in orthodontic area. As an alternative, ceramic brackets exhibit higher hardness and stiffness; however,
they showed a higher incidence of fracture due to brittleness and aging in the oral environment. Newly developed ceramic brackets
showed better mechanical properties and even more esthetics due to improved transparency, and are more compatible with
intraoral environment.
6 Biomaterials in Dentistry

Adhesives
Adhesives used in the orthodontic field are similar to those in restorative dentistry. The acid-etch technique is commonly used for
bonding. Details can be found in the relevant section.

Archwires
Archwires should move the teeth with light and continuous forces which increase patients’ comfort and optimize the treatment
process. The material used for the archwire should have good elasticity and be elastic for weeks at least when a force is applied on it.
To meet with these criteria and objectives, several alloys have been used as orthodontic archwires. Each has its own advantage in a
particular stage of treatment, but no single alloy suits all the stages involved in a treatment.

• Stainless steel alloys

Stainless steel alloys are very strong and have high corrosion resistance; however, they have lost their popularity due to the new
materials being introduced to the archwire market, but they are still in the market because the advantage of low cost.

• Cobalt–chromium alloys

This material contains mainly cobalt and chromium, but also some iron and nickel. Not only does it show similar stiffness to
stainless steel, but also its formability can be changed by heat treatment. In other words, it can be formed to a specific shape and be
heated to increase the resilience and strength of the material.

• Nickel–titanium alloys

This alloy has a shape memory effect and was first developed by W.F. Buehler in the 1960s. The shape memory effect means that the
nickel–titanium wires remember its original shape before deformation and have the ability to return to that shape when not loaded.
Compared with stainless steel wires, nickel–titanium wires exhibit higher strength and resilience, and a lower modulus of elasticity.
Together with the ability to recover fully from 8% strain deformation (other alloys can recover from only 1% strain deformation),
nickel–titanium wires show some superiorities as orthodontic wires: lighter forces can be applied on teeth, reducing the number of
wire changes required during the treatment. Although they also have some limitations (e.g., restricted formability), they still are a
great improvement in the history of orthodontic wires.

• Beta-titanium alloys
Compared with stainless steels, these alloys applied gentler forces on teeth during deactivation and showed higher springback. They
exhibited better formability over nickel–titanium alloys. However, their stiffness is lower, and they are not resilient enough to
withstand the friction created during the movement of teeth.

Elastomeric Modules and Chains

The use of elastomeric modules and chains is being phased out due to the application of self-ligating brackets, but these modules are
still used to close small gaps in the anterior teeth. They should be able to be stretched easily without great loss of energy, have high
tensile strength and stiffness, and have high resilience to recover their original form fully and rapidly. Further reading is
recommended for the detail of the materials.

Periodontics

In recent years, the management of periodontal diseases has evolved from simply the debridement of periodontal pockets to the
regeneration of periodontal tissues. The use of biomaterials has become crucial in the treatment of patients.

Barrier Membranes
A barrier membrane is a device originally used to prevent epithelial migration into a specific area in the guided tissue regeneration
(GTR) procedure. Barrier membranes are divided into two major types: resorbable and nonresorbable (Table 1). In order to achieve
ideal function in a surgical site, the features of a barrier membrane need to meet certain criteria including biocompatibility, tissue
integration, cell-occlusiveness, space-making, and clinical manageability.
Cellulose and ePTFE were the first non-resorbable membranes developed and used in early GTR procedures. The titanium mesh
was developed to reinforce the membrane and shaped for more space. The disadvantages of non-resorbable membranes are that
they have to be removed, which inspired the development of resorbable membranes. Nowadays most practitioners prefer resorb-
able membranes, especially collagen membranes because they are biocompatible and user friendly. They are gradually degraded
without additional surgery once positioned into the defect site. In recent years, acellular dermal matrix, which is human skin
processed in such a way as to remove epidermis and all dermal cells, has been used in root coverage and socket preservation. The
 Biomaterials in Dentistry 7

Table 1 Types of membranes for periodontal regeneration

Resorbable Non-resorbable

Synthetic membrane • Cellulose acetate filter

• Polylactic • PTFE
• Polylactic/polyglycolic • ePTFE
• PL, PG and trimethylcarbonate • Titanium mesh
• PG and TMC • Ethylene cellulose
• Polyethylene glycol • Rubber dam
Natural membrane
• Collagen
• Acellular dermal matrix
• Oxidized cellulose mesh

Table 2 Types of bone graft materials

Allograft Xenograft Alloplast

Fresh frozen bone (FFB) Bovine Hydroxyapatite

Mineralized freeze-dried bone allografts (FDBA) Porcine Tricalcic phosphate
Demineralized freeze-dried bone allografts Equine b-TCP and HA
(DFDBA)
Coralline Bioactive glass
hydroxyapatite
Algae hydroxyapatite Polylactic acid and polyglycolic acid

removal of cells of this graft warrant minimal risk of rejection and inflammation after being grafted. This grafting material acts as a
bioactive scaffold for migration of fibroblasts, epithelial cells, and endothelial cells. It is also a decent material in periodontal plastic
surgeries for increasing the zone of keratinized tissue. Compared to non-resorbable membranes, resorbable membranes are weaker
in space maintaining, thus the appropriate selection of a membrane is crucial for a good clinical outcome.

Bone Graft Materials

Graft materials are nowadays widely used in bone regeneration and reconstruction. Table 2 lists the main categories of graft
materials apart from autograft which is beyond the scope of this chapter. Autologous grafts perform well in osteogenic, osteocon-
ductive, and osteoinductive cases, but require sophisticated operating skills and more surgical time.
While autografts are harvested from the patients themselves, allografts are harvested from an individual other than the patient.
Allograft bone is typically sourced from a bone bank. Allografts are basically osteoconductive and osteoinductive; however, they do
not possess the ability of osteogenesis. The advantage of allograft over autograft is that the former eliminates donor site morbidity.
However, even though the preparation of these freeze-dried bone allografts would reduce the immunogenicity of the grafts, they still
carry a risk of infection and immunoreaction. In addition, the fresh frozen bone is more prone to cause disease transmission and
rejection.
Xenografts are derived from other species such as cows, horses, coral, etc. Deproteinized bovine bone minerals are the most
generally used bone grafts to date, and are frequently used in site augmentation, sinus augmentation, ridge preservation, and
materials for peri-implant defects. They are also often used in conjunction with autografts and resorbable membranes.
Alloplastic grafts are completely synthetic and they usually contain calcium and phosphate. The most used alloplast is HA, which
has a good quality of osteoconduction, hardness, and acceptability by bone. Alloplastic grafts such as calcium phosphate are often
used as an osteoconductive matrix and should be tightly packed in the adjacent host bone to maximize ingrowth. Compared to
bone grafts mentioned above, alloplastic grafts have less of an issue with limited supply and carry minimal risk of disease
transmission.
In addition to being used in oral and maxillofacial surgery, the most common application of bone grafting is in periodontics and
implant dentistry. They are either in block or particulated to adapt better to a defect.
In the development of tissue engineering, barrier membranes and graft materials have been combined with growth factors, for
example, enamel matrix derivative (EMD), to generate better outcomes. EMD is a purified acid extract from the enamel matrix of
porcine fetal tooth. It has been used in treatment of peri-implant and periodontal diseases due to its profound ability to stimulate
the soft and hard tissues surrounding the teeth, and restore cementum, periodontal ligament, and alveolar bone. There are existing
products which mix EMD and Straumann bone ceramic (Straumann Emdogain PLUS, Straumann Holding AG, Basel, Switzerland),
which proved useful in regenerative periodontal surgery such as treatment of vertical bone defects.
8 Biomaterials in Dentistry

3D-Printed Scaffolds
Bone grafting materials are now the treatment of choice to obtain adequate bone volume in periodontal diseases and implant sites.
However, as mentioned above, these grafting materials are not flawless, and all have their pros and cons. With the development of
3D-printing technology, 3D-printed scaffolds have become an attractive alternative to bone grafts in bone augmentation, socket
preservation, and sinus augmentation. The manufactural technique of the “printing” includes inkjet printing, extrusion printing,
and laser-assisted printing with different printing methods using specific biomaterials and generating different resolutions. Using
computer-aided design and computer-assisted manufacturing technologies based on a CT scan of the specific defect to create a
personalized grafting scaffold and well fitted at the defect site, this technique could be a solution worth considering. There is also
great potential for the application of scaffolding matrices in periodontal tissue regeneration as a membrane and grafting material.
Although the efficacy of 3D-printed biomaterials has been demonstrated preclinically, they still have a long way to go before being
widely used in dental clinics.

Implants Dentistry

The application of dental implants can be traced back to AD 600, when humans used bamboo stakes to replace missing teeth. Since
then, dental practitioners struggled with materials until 1952, when Professor Branemark developed a threaded implant design
made of pure titanium and found that titanium apparently bonded irreversibly to living bone tissue. Subsequently, the materials of
dental implants have developed further to meet varied clinical requirements.
To produce a high-performance dental implant to replace a missing tooth, the properties of the biomaterials, including the bulk
properties, surface properties, and biocompatibility, have to be taken into consideration (Table 3).

Implant Materials
Dental implants are usually classified according to their placement situation with the tissue. There are essentially three categories:
endosseous implants, subperiosteal implants, and transosseous implants. Four classes of materials—metals, ceramics, polymers,
and carbon compound—are used in modern dental implants (Table 4).

Metals
Metals are currently the most popular material in dental implants and they are almost exclusively titanium based. The characteristic
properties of titanium meet many requirements in a dental implant, such as primarily high strength and high corrosion resistance,
which ensure good biocompatibility. Thus titanium is viewed as the material of choice in dentistry. Both commercial pure titanium
(CPTi) and Ti–6AL–4U alloy possess excellent corrosion resistance; Ti–6AL–4U alloy has a slightly higher modulus of elasticity than
pure titanium, but it is more expensive. The disadvantage of titanium material is that there might be esthetic issue due to its gray
color; in addition to that, there is an unesthetic display of metal when soft tissue recession occurs. For high load-bearing zones such

Table 3 Properties of an implant biomaterial

Properties Function

Bulk properties
• Modulus of elasticity of Uniform distribution of stress
18 GPa Minimizes the relative movement at implant bone interface
• High creep deformability Materials with high creep values could better tolerate high masticatory forces
• High tensile, compressive, Prevent fractures
and shear strength Improve functional stability
Improved stress transfer
• High yield strength, high Prevent brittle fracture under cyclic loading
fatigue strength
• A minimum ductility of 8% Crucial for contouring and shaping of an implant
• Hardness and toughness Decrease the incidence of wear
Prevents fracture of the implants
Surface properties
• Surface tension and surface Determines the wettability of implant
energy
• Surface roughness Alteration in surface roughness improve cell attachment to the implant
Biocompatibility
• Good corrosion resistance Implant bio-material should be corrosion resistant as corrosion could result in
roughening of the surface, weakening of the restoration, release of elements
from the metal or alloy, and toxic reactions
 Biomaterials in Dentistry 9

Table 4 Classification of implant materials

Types Materials

Metallic Pure titanium (CPTi)

Titanium alloys (Ti–6AL–4U)
Co–Cr alloys
Stainless steel
Precious metals
Ceramic and ceramic coated Bio-inert ceramics
Bioactive and biodegradable ceramics
Polymers PMMA, PTFE, PE, PSF
Carbon compound

as in posterior areas with lower esthetic requirements, implant materials with high strength such as CPTi or titanium alloys should
be considered.
Stainless steel has also been used for many years; however, it is contraindicated in patients who are allergic to nickel, as it
contains 7%–8% nickel to stabilize the austenitic structure. Meanwhile, the toxicity of nickel is still a concern. Apart from that, the
low cost and good mechanical properties are its significant advantages.

Ceramics
Because of their better biologic inertness compared with metals, and their good strength, ceramics have been used as dental implant
materials. Ceramic materials are classified into two types: inert ceramics, which are non-reactive materials such as aluminum oxide
(AL2O3), carbon, and zirconia (ZrO2); and bioactive and biodegradable ceramics such as glass ceramics, BAG, and calcium
phosphate ceramics (CPCs).
Inert ceramics from aluminum, zirconium oxides, and titanium have been used for endosteal plate form, root form, and pin type
of dental implants. This type of material is high in compressive, tensile, and bending strength, with better physical characteristics
such as similar color as natural tooth, and minimal thermal electrical conductivity. However, its inherent brittleness has limited its
application. Modern dentistry has placed more emphasis on bioactive and biodegradable ceramics as these materials possess similar
constituents to those of normal tissue, with excellent biocompatibility, and a similar modulus of elasticity to bone. They could be
gradually resorbed and even replaced by tissue over time. As a matter of fact, they are primarily used as scaffolds or to coat metallic
implants because of their good tissue bonding property.
Bioactive and biodegradable ceramics, such as calcium phosphate ceramics (CPCs), have attracted attention in the development
of implant materials. Calcium phosphate materials have been applied as bone augmentation and replacement materials as well as
in implant materials. They could be formed to serve as structural support under relatively high-magnitude loading conditions for
implant applications, or as alloplastic grafts and mixed with drugs, collagen, and growth factors such as bone morphogenic proteins.
In addition, CPC coating on metallic surfaces has been used in endosteal and subperiosteal dental implant designs to improve the
longevity and surface biocompatibility of an implant. Mixtures of HA, tricalcium phosphate (TCP), and teracalcium phosphate can
be either plasma sprayed or coated to produce a bioactive surface on an implant. In fresh extraction site or a newly grafted site,
HA-coated implants seem a better choice for its greater implant bone interphase and higher shear bond strength.

Polymers and carbon compound

Polymeric implants were used in the past few decades. Due to their low mechanical strength and susceptibility to fracture, they are
nowadays only used as adjuncts to enable stress distribution along with implants. Carbon compounds have an excellent quality of
biocompatibility as well as a modulus of elasticity similar to that of bone. However, their low compressive strength results in their
application only as coatings on metallic and ceramic materials.

Oral and Maxillofacial Surgery

In terms of maxillofacial diseases, the most crucial concern, apart from treatment of the actual disease, is the esthetic requirements
and functional demands of the oral and maxillofacial region. The need to reconstruct the facial region for the reason of tumor
resection, trauma congenital disorders, or even esthetic need is one of the main challenges of maxillofacial surgeons. Autologous
bone or soft tissue grafts are routinely used in reconstruction design nowadays. However, the donor site morbidity, the medical
condition of the patient to tolerate a major surgery, the special training and sophisticated technical requirements of a surgeon, and
the relatively high rate of infection and graft resorption are always concerns. Apart from the widely used autologous grafts, many
other biomaterials have been used alone or in combination with bone grafts. As mentioned above, allografts, xenografts, and
alloplastic materials have been used as bone substitutes in periodontics and implant surgeries to restore small defects. However, will
these biomaterials be an appropriate choice of treatment for major head and neck reconstruction?
10 Biomaterials in Dentistry

Autogenous Grafts
To date, autogenous bone is the gold standard for maxillofacial reconstruction. An autogenous graft is prepared from a healthy part
of the patient’s own bone and grafted onto the defect area. A graft of cortical bone ensures a good structural support and a reduced
resorption, while a graft of cancellous bone ensures early revascularization. The types of autogenous grafts include free bone graft,
particulate cancellous bone marrow graft, pedicle composite flaps, microvascular free-flap transfer, bone marrow aspirates, and
plasma-rich protein. Among these, microvascular free-flap transfer performs noticeably well in major defect reconstruction since it is
vascularized with adequate blood supply and excellent in functional and esthetic reconstruction. However, the morbidity associated
with the harvesting procedures of these autogenous grafts, the limited availability when the volume of the defect is huge, and the
prolonged operation time have all inspired the continuous seeking of alternatives.

Bone Substitutes
Bone substitutes include allografts, xenografts, and alloplastic grafts. Allografts are costly and proved to have a high rate of
resorption with poor mechanical properties due to chemical and radiation pre-treatment. Therefore, allografts are limited to
small and medium defects, and are not suitable for reconstruction of critical-size defects in the maxillofacial region.
Xenografts are less costly than allografts. They gained popularity in reconstruction of small bony defects because of their slow
resorption property. A common example of a xenograft is a kind of natural porous bone mineral called Bio-Oss™ (Geistlich,
Wolhusen Switzerland), which is derived from bovine bone and is commonly used in periodontics. However, as with allografts,
xenografts are not suitable for the reconstruction of critical-size defects due to their poor mechanical properties and lack of
vascularization.
Alloplastic grafts contain a wide range of materials (Table 2), some of which have been used for bone regeneration. Ceramics
such as HA and calcium sulfate have been used in the restoration of periodontal defects and small correction of maxillofacial regions
such as sinus augmentation, but they still possess some limitations when it comes to repairing major defects in the maxillofacial
region.
While ceramics slowly find their way to repair small defects in maxillofacial surgery, BAG has also proved its usefulness and
bright future in the reconstruction of small cranial-facial bone defects. BAG is a silicate glass-based material which undergoes a
specific surface reaction. It forms a bond with hard and soft tissues when implanted. BAG demonstrates controlled resorption in
optimal time, efficient bioactivity, and the ability to modulate cell migration, with a cortical bone-like modulus of elasticity.
However, similar to ceramics, it is not suited for reconstruction of continuity defects of jaw bones due to the lack of the required
mechanical properties. It has been applied for contour refinements as inlay for frontal bone, nasal dorsum, mandibular angle, and
alveolar ridge augmentation.
Polymeric materials used in maxillofacial surgery are mainly synthetic polymers. Although natural polymers such as collagen
(used in periodontal regeneration in combination with other grafting materials), alginate, hyaluronic acid, peptide hydrogen, and
chitosan are biocompatible, they could only be used in combination with other materials because of their disadvantage in being
water soluble. Synthetic polymers could be prefabricated as well as contoured with a burr in a surgery. Prefabricated polymers that
have played a vital role in maxillofacial surgery include hard-tissue replacement polymers, PMMA, and porous polyethylene
(MEDPOR; Stryker, Kalamazoo, Mich., USA). PMMA, as the first substitute used for adult cranial reconstruction, presented a
9.5% infection rate in cranial reconstruction. However, in recent years it has also been used as a framework in combination with
bioglass fabricated into a customized porous implant to repair calvarial and midface bony defects. This promising material proved
good functional and esthetic outcomes with no observation of long-term complications. Porous polyethylene implants are also
commercially available and have been used for nasal and malar augmentation, orbital floor reconstruction, and calvarial defect
reconstruction. They are dense implants with a pore size of 100–250 mm. However, these materials still have limitations such as the
risk of infection, exposure, and extrusion.
Resorbable poly(L-lactide) (PLLA) and polyglycolide polymers (PGA) have also been widely used in maxillofacial surgeries.
Their combination product copolymers, which take in the most positive characteristics of them both, are poly(lactide-co-glycolide)
(PLGA) and PLLA-PGA; these are widely used as bioresorbable plates and screws for the fixation of bones. These resorbable plates
have gained popularity in treatment of pediatric mandibular complex fractures, since they function well in realignment and stable
positioning of rapidly healing fracture segments with no secondary removal operations needed. They have also been used in internal
fixation of adult maxillofacial fracture in certain anatomy sites when strength is not much of a concern. The limitations of these
resorbable materials are their limited ability to withstand masticatory forces and the chances of inflammation when the materials
begin to degrade. In addition, PGA and PLGA have been combined with HA or b-TCP to form a composite scaffold in an attempt to
increase the degradation time and improve the material’s mechanical properties.

Titanium
Titanium not only has outstanding performance in implant dentistry, it is also a common choice for maxillofacial surgeon for its
desirable mechanical properties and good biocompatibility. It is used as a bone tray for mandibular reconstruction, for cranioplas-
ties, orbital floor implants, condylar reconstruction, and titanium plates and screws for internal fixation of fractures. Its high
mechanical properties and lightness render it a good choice when used as fixation plates/screws for major force-bearing fracture sites
 Biomaterials in Dentistry 11

Table 5 A brief summary of biomaterial application in maxillofacial repair and reconstruction

Maxillofacial region Repair/reconstruction requirement Materials commonly used Alternatives

Extra-oral
Maxillofacial fracture Strong mechanical strength, stable Titanium plates/screws –
positioning
Pediatric complex Stable positioning, biocompatibility Resorbable plates/screws Titanium plates/screws
fracture
Maxillofacial bone defect Reconstruction, protection Autogenous grafts Titanium mesh, polymers, injectable
cement, tissue engineering
Facial augmentation Restoration of bone volume and Autogenous grafts, titanium, bioactive glass, Resorbable polymer, polymer face filler,
contour, esthetic polymers 3D scaffolds
Intra-oral
Ridge augmentation Restoration of function and esthetic, Titanium, deproteinized bovine bone minerals Bioactive glass, injectable cement
implant placement ceramic, tissue engineering
Maxillary sinus lift Implant placement Mineral composite Tissue engineering
Temporomandibular joint Function, esthetic Cast cobalt–chromium–molybdenum alloy, –
replacement titanium with polyethylene

Modified from Deb, S. (2015). Biomaterials for oral and craniomaxillofacial applications. S. Karger AG.

such as mandibular angle fractures. This metallic material has also been used in combination with polymeric materials such as
polyethylene; the combined material possesses not only the mechanical strength offered by titanium but also the porous
biocompatible surface offered by polyethylene. Titanium has the advantage of visibility on post-op with minimal distortion in
MRI images, yet the other side of the coin is that it produces artifacts and interfere with the interpretation of MRI images just like
other metallic objects. Other limitations of titanium are that the thermal conductivity of the metal would bring discomfort to
patients in cold weather, and the gray color would be an esthetic issue when soft tissue is thin and pervious to light.
The functional and esthetic reconstruction in the maxillofacial region has always been a vital task for surgeon and researchers.
The application of biomaterials for maxillofacial repair and reconstruction are summarized in Table 5.

Tissue Engineering

Biomaterials have been widely used in almost all fields of dentistry; however, none of them is able to completely restore or replace
the structure and function of missing tissues. The burning needs in restoration and reconstruction in clinic inspire and promote the
development of tissue engineering, a brand new technique using a combination of scaffolds, cells, and biologically active molecules
to assemble functional constructs that restore, maintain, or improve damaged tissues for medical or dental purposes. Tissue
engineering evolved from the field of biomaterials, but has been growing in scope and importance and is now an independent
field. The great potential of tissue engineering in dentistry has advanced the study and clinical trials in periodontal regeneration,
dental pulp regeneration, and maxillofacial reconstruction; however, the imperfect technique, high cost, risk of biological contam-
ination, and ethical issues are concerns that need be solved before the engineered tissues can be widely used in clinics.

Further Reading

Agarwal S, Gupta A, Grevious M, and Reid RR (2009) Use of resorbable implants for mandibular fixation: A systematic review. Journal of Craniofacial Surgery 20(2): 331–339.
Anusavice KJ, Shen C, and Rawls HR (2013) Phillips’ science of dental materials. London, UK: Elsevier Health Sciences.
Asa’ad F, Pagni G, Pilipchuk SP, et al. (2016) 3D-printed scaffolds and biomaterials: Review of alveolar bone augmentation and periodontal regeneration applications. International
Journal of Dentistry 2016: 1239842.
Carpena Lopes G, Narciso Baratieri L, de Andrada C, Mauro A, and Vieira LCC (2002) Dental adhesion: Present state of the art and future perspectives. Quintessence International
33(3).
Craig RG and Powers J (2002) Restorative dental materials, 11th edn. St. Louis: Mosby.
Darby I (2011) Periodontal materials. Australian Dental Journal 56(Suppl. 1): 107–118.
Deb S (2015) Biomaterials for oral and craniomaxillofacial applications. S. Karger AG, Basel.
Dhuru VB (2004) Contemporary dental materials. Oxford: Oxford University Press.
Eliades T (2007) Orthodontic materials research and applications: Part 2. Current status and projected future developments in materials and biocompatibility. American Journal of
Orthodontics and Dentofacial Orthopedics 131(2): 253–262.
Jones JR (2015) Reprint of: Review of bioactive glass: From Hench to hybrids. Acta Biomaterialia 23(Suppl): S53–82.
Lyngstadaas SP, Wohlfahrt JC, Brookes SJ, et al. (2009) Enamel matrix proteins; old molecules for new applications. Orthodontics & Craniofacial Research 12(3): 243–253.
McCabe JF and Walls AW (2013) Applied dental materials. Chichester: John Wiley & Sons.
Misch CE (2008) Contemporary implant dentistry. St. Louis: Mosby Incorporated.
12 Biomaterials in Dentistry

Payne KF, Balasundaram I, Deb S, et al. (2014) Tissue engineering technology and its possible applications in oral and maxillofacial surgery. British Journal of Oral & Maxillofacial
Surgery 52(1): 7–15.
Profeta AC and Huppa C (2016) Bioactive-glass in oral and maxillofacial surgery. Craniomaxillofacial Trauma and Reconstruction 9(1): 1–14.
Sam G and Pillai BR (2014) Evolution of barrier membranes in periodontal regeneration—"Are the third generation membranes really here?” Journal of Clinical and Diagnostic
Research 8(12): Ze14–17.
Van Noort R and Barbour ME (2013) Introduction to dental materials4: Introduction to dental materials. Elsevier Health Sciences.
von Recum AF (1998) Handbook of biomaterials evaluation: Scientific, technical and clinical testing of implant materials, 2nd edn. London, UK: Taylor & Francis.