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1. 1
Calcium phosphate bone cement
Introduction
Definition
Types & comparison
Form
Composition
Setting reaction & mechanism
Manipulation
Properties
Advantages & disadvantages
Applications
Recent advances
Introduction
Types of bone grafts
Autografts: bone from the same individual (the gold standard).
Allografts: another individual of the same species.
Xenografts: bone from different species, usually bovine and coral origin.
Alloplast: synthetic material.
Disadvantagesofautografts
 Morbidity at the donor site.
 Limited availability of bone.
 Require a second surgery at the donor site coupled with additional surgical risks, bleeding
and possible infection.
 Expensive procedure due to the prolonged hospital stay and extended medicare.
Disadvantagesof allografts & xenografts
 Do not provide viable osteogenic cells
 Possible immune response.
 Disease transmission.
Disadvantagesofsynthetic HA (stoichiometric & crystalline)
 Brittleness
 Very slow resorbability. Long-term complications include detachment of the coating from the
implant and peri-implant infection, as HA is known not only as a bioactive mineral but also
as an adsorbent. In other words, HA can adsorb bacteria.

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Definitions
 Osteoconduction means that bone grows on a surface.
 Osteoconductive surface is one that permits bone growth on its surface or down into pores
or channels.
 Osteoinductive material has the ability to induce bone formation by instructing its
surrounding environment to form bone.
 Osteoinduction involves the stimulation of osteoprogenitor cells to differentiate into
osteoblasts (bone-forming cells) that then begin new bone formation.
 Another proposed definition of osteoinduction is the process by which osteogenesis is
induced.
 Osteogenesis is the process of new bone formation.
 Osteointegration describes a process whereby clinically asymptomatic rigid fixation of
alloplastic materials is achieved, and maintained, in bone during functional loading.
(Albrektsson T, Johansson C. Osteoinduction, osteoconduction and osseointegration. Eur Spine J 2001; 10: S96–S101)
Table 1: Existing calcium phosphates (CaPO4) and their major properties.

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Calcium phosphate cement
Calcium phosphate cement (CPC) consists of one or more calcium phosphate powders, which
upon mixing with water or an aqueous solution form a paste that is able to set and harden forming
either a non-stoichiometric calcium-deficient hydroxyapatite (CDHA) or brushite (dicalcium
phosphate dihydrate, DCPD).
Apatite-forming CPCs Brushite-forming CPCs
Hydrolysis of
α-TCP
Acid-base reaction
TTCP + DCPA/DCPD
Acid-base reaction
β-TCP + MCPM/MCPA
End-product: CDHA End-product: DCPD
Better solubility under physiological conditions
Faster reaction & setting
Faster degradation & resorption in vivo
Lower strength
Most commercial products
Form
Powder & liquid
Composition
Powder: one or several calcium phosphate compounds.
Liquid: distilled water or a calcium or phosphate-containing solution.
Setting reaction
 Apatite-forming CPCs: acid-base reaction or hydrolysis.
Examples: Hydrolysis of α-TCP.
Acid-base reaction: TTCP + DCPA/DCPD.
 Brushite-forming CPCs: acid-base reaction.
Examples: Acid-base reaction: β-TCP + MCPM/MCPA.
After hardening, all formulations can form only two main end-products:
 Calcium-deficient hydroxyapatite (CDHA) at pH > 4.2
 or brushite (DCPD) at pH < 4.2.

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Manipulation
Mixing
 Manual mixing: using a mortar and either a pestle or a spatula.
Disadvantage: insufficient and inhomogeneous mixing thus compromises the strength.
 Mechanical mixing: Better.
Placement
 Applied by the fingertips of a surgeon or injected from a syringe.
Properties
 Besides having excellent biological behavior, being injectable and self-setting in vivo at body
temperature are the two main advantages of CPCs as bone substitutes.
1. Setting time
Factors which could reduce the setting time
 Smaller particle size (high specific surface area)
 Low crystallinity
 Accelerators
 Higher setting temperature
 Low liquid-to-powder ratio (L/P ratio).
2. Cohesion and anti-washoutability
 Cohesion is the ability of a paste to keep its geometrical integrity in an aqueous solution.
 A bad cohesion may prevent setting and may lead to negative in vivo reactions due to the
release of microparticles.
 Another definition of cohesion is the ability of a CPC to harden in a static aqueous
environment without disintegrating into small particles.
 The definition of the ‘‘anti-washout ability’’ is similar to that of cohesion, except that the
former is evaluated in a dynamic aqueous environment.
 Numerous biopolymers, such as sodium alginate, hydroxypropyl methylcellulose (HPMC),
hyaluronic acid, chitosan and modified starch, can significantly improve the cohesion and
anti-washout of CPCs.

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 The two best methods to increase the cohesion
 Smaller particle size
 Increasing the viscosity
However, they may in some cases compromise the setting time and mechanical properties.
3. Injectability
 Injectability is defined as an ability of a formulation to be extruded through a small hole of a
long needle (e.g., 2 mm diameter and 10 cm length)
 Inj = (WF − WA)/(WF − WE) × 100%
Inj is the percentage injectability
WF is the weight of the full syringe
WA is the weight of the syringe after the injection.
WE is the weight of the empty syringe
 Factors which could increase the injectability
 Increasing the L/P ratio
 Decreasing the viscosity
 Smaller particle size
 Round particles
4. Mechanical properties
4.1. Strength
 Compressive strength of cancellous bone & cortical bone.
 Porosity.
 Type of cement: apatite CPCs > brushite CPCs.
 Sample: dry samples > wet samples.
 Smaller particle size, smaller crystals & denser.
 Additives (reinforcement).
 CPCs can be prepared with compressive strengths comparable to those reported for human
cancellous bone (4-12 MPa) or cortical bone (130-180 MPa).
 The strength decreases globally with increasing porosity.
 With comparable porosities, apatite cements generally have higher strengths than brushite
cements.
 In similar conditions, dry samples have a higher strength than wet samples.
 The smaller the particle size of the starting materials, the faster they will convert into apatite
and the smaller the apatite crystals formed, which, in turn, will lead to more and dense crystal
entanglement and thus to an increase in strength.
 A general conclusion is obtained: crystalline structures that are more compact and
homogeneous, with smaller crystals, seem to give better mechanical properties than less
compact or less homogeneous ones with larger crystals.
 It is difficult to increase strength of the self-setting CPCs without having a negative influence
on the other properties.

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4.2. Fracture toughness
 Fracture toughness (KIc) is a property which is used to describe the ability of a material
containing cracks or notches to resist crack propagation.
 CPCs have low fracture toughness (typically KIc < 0.5 MPa.m1/2).
 KIc values are comparable to the reported values for cancellous bone (0.1-0.8 MPa.m1/2), but
much lower than those for cortical bone (2-12 MPa.m1/2), indicating that more effort is still
required to increase the fracture toughness of CPCs for their application in load-bearing
locations.
4.3. Reinforcement of CPC
 Carbon nanotubes
 Apatite seeds
 CaCO3
 Citric acid & sodium citrate
 Fibers
 Carbon nanotubes (CNT) and multi-walled carbon nanotubes, which find wide applications in
sintered HA–CNT composites, have also been used as reinforcing agents in CPCs.
 By adding certain amounts of apatite seeds, the setting time of CPCs decreased and the
compressive strength concomitantly increased; conversely, however, excess apatite prolonged
the setting time and decreased the compressive strength.
 Carbon nanotubes (CNT) and multi-walled carbon nanotubes, which find wide applications in
sintered HA–CNT composites, have also been used as reinforcing agents in CPCs.
 By adding certain amounts of apatite seeds, the setting time of CPCs decreased and the
compressive strength concomitantly increased; conversely, however, excess apatite prolonged
the setting time and decreased the compressive strength.
 Incorporation of 10 wt% CaCO3 causes a decrease in crystallite size and improved
compressive strength by a maximum of 40% compared to samples free of CaCO3.
 Hydroxyl acids (citric acid or glycolic acid) and their salts (sodium citrate), which allow
easier mixing of the cement and processing with a decreased L/P ratio (relating to a decreased
porosity), thus resulting in improved strength.
 However, these additives generally have optimal concentrations that can be used in the
cement, whereas a higher concentration of such additives can decrease strength.
Fiber-reinforced CPC
 Function: improve strength & fracture toughness.
 Mechanism: bridging, deflection & friction.
 Factors affecting the final properties: length, volume, orientation & fiber/matrix adhesion.
 Types: non-resorbable & resorbable fibers.
 Disadvantage of non-resorbable fibers.
 Problem with resorbable fibers: macropores.
 Solving this problem: fibers having different resorption rates.

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 The incorporation of fibers into brittle cement improves fracture toughness as well as tensile
and flexural strength.
 In the fiber-added composite cements for civil engineering, three mechanisms of fiber
reinforcement (fiber bridging, crack deflection and frictional sliding) appear to be operative
(Fig. 1).
Fig. 1. Schematic illustration showing three mechanisms of fiber reinforcement (fiber bridging, crack
deflection and frictional sliding) in fiber-added composite cements.
 Specifically, first, when the matrix starts to crack, the fibers bridge the crack to resist its
further opening and propagation.
 Secondly, crack deflection by the fibers prolongs the distance over which the crack
propagates, consuming more energy in newly formed surfaces.
 These two mechanisms have also been reported to be the major contributors to the fracture
toughness of human bone, which is a composite consisting of hard mineral nanoparticles
(carbonated apatite) and a fibrous polymer (collagen).
 Finally, the frictional sliding of fibers against the matrix during pullout further consumes the
applied energy and increases the fracture resistance of the composite.
 Due to the chemical similarity between CPCs and cements for civil engineering, it is strongly
expected that, by adding fibers, the above toughening mechanisms can also be achieved in
CPCs.
 A fiber with a high tensile strength is essential.
 Fiber length, volume fraction, orientation and fiber/matrix adhesion, are also critical for the
final properties of the composite.
 The mechanical properties gradually increased with increasing fiber volume fraction.
 However, a plateau or a decrease following the increase was often observed in the
mechanical properties at high fiber volume.
 This is mainly because the high volume of fibers may compromise their workability, making
it difficult for them to be mixed and wetted by the CPC paste, leaving space between fibers
and matrix.

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 A similar evolution in mechanical properties was also found in CPCs with increasing fiber
length. The authors ascribed the decrease in mechanical properties to the heterogeneous
distribution of the long fibers.
 The addition of chitosan lactate into the CPCs produced a stronger matrix to support the
fibers to better resist crack propagation.
Types of fibers
1) Non-resorbable fibers: collagen fibers, polyamides, carbon fibers and glass fibers.
2) Resorbable fibers: natural or synthetic polyesters such as polylactide (PLA), poly(lactic-
co-glycolic acid) (PLGA) and polycaprolactone (PCL) or chitosan.
 The incorporation of non-resorbable fibers into resorbable CPCs could cause fiber release
into the surrounding tissues, with the subsequent biocompatibility risks.
 As for resorbable fibers, macropores produced from fiber degradation can promote bone
ingrowth. However, these macropores are detrimental to the mechanical stability of CPCs
before new bone grows into the pores.
 To solve this problem, several types of fibers having different resorption rates could be used
simultaneously. The fast degradable fibers create pores for bone ingrowth, while fibers with a
low degradation rate provide strength to the implant.
5. Bioresorption
 Brushite-forming CPCs are soluble in physiological conditions, and therefore they degrade
and resorb in vivo faster than apatite-forming CPCs.
 It might take from 3 to 36 months for different formulations to be completely resorbed and
replaced by bone.
 At physiological pH, the in vitro solubility of DCPD is approximately 100 times higher than
that of β-TCP; roughly, the same order of magnitude applies for the in vivo resorption
kinetics of these calcium orthophosphates.
 Porosity of self-setting calcium orthophosphate formulations is a very important factor for
their biodegradability.
 Osteoclastic cells degrade the hardened calcium phosphates layer-by-layer only (from outside
to inside) due to lack of macroporosity and interconnected pores.
Bioresorption mechanism
 Active resorption: by the cellular activity of macrophages, osteoclasts and other types of
living cells (phagocytosis).
 Passive resorption: due to either dissolution or chemical hydrolysis (brushite-forming
formulations only).
 Dissolution might be both chemical and physical.
 The former occurs with calcium phosphates of a low solubility (those with Ca/P ratio > ~1.3).
 The latter occurs with calcium phosphates of a high solubility (those with Ca/P ratio < ~1.3).
 For example, for MCPM, MCPA, DCPD and DCPA the solubility product are several times
higher; therefore, they might be physically dissolved in vivo, which is not the case for α-TCP,
β-TCP, CDHA, HA and TTCP.

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 Therefore, biodegradation of the latter materials is only possible by osteoclastic bone
remodeling and is limited to surface degradation since cells cannot penetrate the microporous
structure.
 Osteoclastic cells resorb calcium phosphates with Ca/P ratio > ~1.3 by providing a local
acidic environment which results in chemical dissolution.
Advantages
1) Self-setting ability in vivo.
2) Can be injected directly into the bone defects, where they intimately adapt to the bone cavity
regardless its shape (injectable, moldable & perfect adaptation).
 Note: Being injectable and self-setting in vivo at body temperature are the two main
advantages of CPCs as bone substitutes.
3) Minimal invasive surgery, quicker recovery & less pain. Shorter hospital stays cheapen the
expenses (low cost).
4) Biocompatible, bioactive & osteoconductive.
5) Bioresorbable & can be replaced by newly formed bone.
6) Can be loaded with drugs (drug delivery system).
7) Easy manipulation.
Disadvantages
1) Brittleness and low fracture toughness limit their application to non-stress bearing areas.
2) Lack of macroporosity and interconnected pores, which prevents fast bone ingrowth, and the
cements degrade layer-by-layer from the outside to the inside.
3) Can washout from surgical defect if excess of blood. Compression during setting is
recommended. In addition, formulations containing sodium alginate have been studied to solve
this problem.
 Unfortunately, the perfect grafting material does not exist. The self-setting CPCs are not an
exception to this statement.
Applications
 Bone substitute &repair of bone defects in non-stress bearing areas, e.g., craniofacial
applications.
 Bone filler for gaps around oral implants.
 Bone augmentation.
 Drug delivery system: for the treatment of bone diseases, e.g., tumours, osteoporosis and
osteomyelitis, which normally require long and painful therapies.
Other dental applications
 Direct pulp capping: Compared to calcium hydroxide, both materials were equally capable of
producing secondary dentin at ~24 weeks.
 Pulpotomy.

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Recentadvances
1) Two liquids or pastes
2) Automix
3) Premixed CPC
4) Fibers having different resorption rates
1. Two liquids or pastes
 An aqueous one and a non-aqueous one based on glycerine.
 Advantage: easier to homogenize than powder with liquid.
2. Automix
 Twin-chambered syringe that allows injection immediately after mixing.
 Advantages
 More homogenous mix without air bubbles.
 More reproducible.
 Immediate injection into the bone defects.
 Fast.
 Less contamination risks.
3. Premixed CPC (water-settable cement)
 Already prepared paste, which is stored until use.
 A non-aqueous but water-miscible liquid is used to prepare the paste.
 Hardens after being injected into the bone defect.
 Advantages: fast & no mixing.
 Disadvantages: sensitive to moisture during storage & poor mechanical properties.
4. Fibers having different resorption rates
 The fast degradable fibers create pores for bone ingrowth, while fibers with a low
degradation rate provide strength.
References
1. Zhang J, Liu W, Schnitzler V, Tancret F, Bouler JM. Calcium phosphate cements for bone substitution:
Chemistry, handling and mechanical properties. Acta Biomaterialia 2014; 10: 1035–1049.
2. Dorozhkin SV. Self-setting calcium orthophosphate formulations. J Funct Biomater 2013; 4: 209–311.
3. Chow LC. Next generation calcium phosphate-based biomaterials. Dent Mater J 2009; 28: 1–10.
4. Ambard AJ, Mueninghoff L. Calcium phosphate cement: review of mechanical and biological properties. J
Prosthodont 2006; 15: 321–328.
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