Chemical properties

Bonding  [ionic, covalent, metallic, molecular, hydrogen]

  • Refers to properties of materials that are dictated by the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together. The type of bonding and structure helps determine what type of properties a material will have.
  • Ceramics usually have a combination of stronger bonds called ionic (occurs between a metal and nonmetal and involves the attraction of opposite charges when electrons are transferred from the metal to the nonmetal); and covalent (occurs between two nonmetals and involves sharing of atoms). The strength of an ionic bond depends on the size of the charge on each ion and on the radius of each ion. The greater the number of electrons being shared, is the greater the force of attraction, or the stronger the covalent bond. These types of bonds result in high elastic modulus and hardness, high melting points, low thermal expansion, and good chemical resistance. On the other hand, ceramics are also hard and often brittle (unless the material is toughened by reinforcements or other means), which leads to fracture.
  • Metals have generally weaker bonds than ceramics, which allows the electrons to move freely between atoms. Think of a box containing marbles surrounded by water. The marbles can be pushed anywhere within the box and the water will follow them, always surrounding the marbles. This type of bond results in the property called ductility, where the metal can be easily bent without breaking, allowing it to be drawn into wire. The free movement of electrons also explains why metals tend to be conductors of electricity and heat.
  • Plastics or polymers of the organic type consist of long chains of molecules that are either tangled or ordered at room temperature. Because the forces (known as van der Waals) between the molecules are very weak, polymers are very elastic (like a rubber band), can be easily melted, and have low strength. Like ceramics, polymers have good chemical resistance, electrical and thermal insulation properties. They are also brittle at low temperatures. The following table provides a general comparison of the properties between the three types of materials.


  • Refers to the original base components of the material:
  • Hydroxyapatite (HA) [Ca10(PO4)6(OH)2]
  • Tri-calcium phosphate (TCP) [Ca3(PO4)2
  • Biphasic: percentage combination of HA & TCP in same material
  • Hybrid: One of the above with added material such as Si, Mg or Bioactive glass

Composition has an effect on:    

  • Mechanical properties (impactability, strength, stiffness, Young’s modulus)
  • Biological properties (osteoconduction)
  • Degradability speed
  • Rules of thumb:

Strength:               TCP less brittle in dry formulation compared to HA

Strength:               TCP quicker loss of mechanical strength compared to HA in vivo

Resorption:          TCP chemically less stable compared to HA

Resorption:          TCP possesses high resolution characteristics compared to HA

Degradation:       TCP easily resorbed by osteoclasts compared to HA

Degradation:       TCP faster degradation (12-18 months) compared to HA (2-10 years)


  • Refers to the degree of structural order in a material, less order provides a more amorphous material

Crystallinity has an effect on:     

  • Mechanical properties (hardness, density)
  • Biological properties (osteoconduction)
  • Degradation properties (speed and type of degradation)
  • Rules of thumb:

Strength:              High crystallinity provides better stiffer material

Resorption:         Amorphous porous materials enhance bone ingrowth but also biological degradation

Degradability:   High crystallinity leads to slower degradability due to resistance in dissolution

Calcium-phosphate (Ca/P) ratio

  • Refers to be a measurement of Ca-P ceramics composition
  • Ca/P ratio Ca-P granules between 1.67 (HA) and 1.5 (TCP)
  • Ca/P ratio Ca-P cements between 2.0 (TTCP) and 1,0 (DCPH)
  • Rules of thumb:

Strength:               High Ca/P ratio provides higher strength when compared to low Ca/P ratio

Degradability:    High Ca/P ratio 1,67 (HA) leads to slower degradability as compared to Ca/P ratio of 1,5 (TCP)