compression molded, extruded, etc. Liquids can be cast into sheets, lenses, and rods
[88, 89]. Even though the mechanical characteristics of PMMA are rather poor, it
allows even distribution of implant loads and forms a strong mechanical bond with
implants. However, there are several limitations to the use of PMMA, such as
brittleness and shrinkage of PMMA, void production in processing [90–92], lack of
adherence to the bone [93, 94], and the exothermic polymerization reaction, which
can damage bone tissue [95, 96].
To minimize this problem, several techniques have been developed. De Santis
et al. [97] have prepared a blend of PMMA and paraffin-based phase change
material (PCM) that is able to store the thermal energy produced during polymerization of methyl methacrylate [98]. Migliaresi and coworkers have used acryl
polymers such as poly(n-butyl methacrylate) (PBMA), for orthopedic applications.
These polymers are characterized by lower exothermic effect, higher fracture
toughness, and superior fatigue life along with lower toxicity to soft tissue and
dental pulp [99]. Use of poly(hydroxyethyl methacrylate) (PHEMA) also showed
improved biocompatibility [100].
Besides having an inert material for fibroblastic cells observed at the
bone–cement interface, PMMA is still the current standard for cement-held
prostheses. Vallo and his group measured the flexural, compressive, and fracture
properties of PMMA bone cements that incorporated various amounts of HAp.
They found that addition of up to 15 wt% HAp increases the flexural modulus and
fracture toughness [89]. It was also observed that, with increasing HAp
incorporation into PMMA, the biological responses of the bone cement increased
and thus increased osteoblast adhesion and response [88]. Kwon and coworkers
reported that a PMMA/HAp composite with 30 wt% of HAp increased the interfacial shear strength at the bone–implant interface 6 weeks after implantation in
rabbits [101]. Several other research studies revealed that, depending on the type of
bone cement, the addition of HAp can improve the mechanical properties of bone
cements [102].
Moursi et al. [103] studied the response of osteoblasts to PMMA/HAp materials.
They found that osteoblasts attached equally well to PMMA/HAp. In contrast, the
increase of osteoblasts on PMMA/HAp was significantly improved, compared to
PMMA, after 8 days in culture.
In addition, to improve adhesion between polymer matrix and HAp, several
copolymer such as PMMA, PBMA, and PHEMA were grafted onto the surface of
HAp using isocyanatoethyl methacrylate (ICEM) or hexamethylene diisocyanate
(HMDI)/hydroxyethyl methacrylate (HEMA) [100]. These three methacrylate
polymers were coupled to HAp particles via covalent bonding with isocyanate
groups.
Skrtic et al. [104] studied methacrylate conversion and volumetric contraction in
photo-polymerized composites with amorphous calcium phosphate (ACP). ACP
was used because it is highly soluble in aqueous media and can be quickly
transformed to HAp. Ideally, on polymerization, the resin systems used in
composites should achieve the contradictory goals of both high vinyl conversion
and minimal volumetric contraction. A high degree of vinyl conversion, with
154
A. Bhowmick et al.
[88, 89]. Even though the mechanical characteristics of PMMA are rather poor, it
allows even distribution of implant loads and forms a strong mechanical bond with
implants. However, there are several limitations to the use of PMMA, such as
brittleness and shrinkage of PMMA, void production in processing [90–92], lack of
adherence to the bone [93, 94], and the exothermic polymerization reaction, which
can damage bone tissue [95, 96].
To minimize this problem, several techniques have been developed. De Santis
et al. [97] have prepared a blend of PMMA and paraffin-based phase change
material (PCM) that is able to store the thermal energy produced during polymerization of methyl methacrylate [98]. Migliaresi and coworkers have used acryl
polymers such as poly(n-butyl methacrylate) (PBMA), for orthopedic applications.
These polymers are characterized by lower exothermic effect, higher fracture
toughness, and superior fatigue life along with lower toxicity to soft tissue and
dental pulp [99]. Use of poly(hydroxyethyl methacrylate) (PHEMA) also showed
improved biocompatibility [100].
Besides having an inert material for fibroblastic cells observed at the
bone–cement interface, PMMA is still the current standard for cement-held
prostheses. Vallo and his group measured the flexural, compressive, and fracture
properties of PMMA bone cements that incorporated various amounts of HAp.
They found that addition of up to 15 wt% HAp increases the flexural modulus and
fracture toughness [89]. It was also observed that, with increasing HAp
incorporation into PMMA, the biological responses of the bone cement increased
and thus increased osteoblast adhesion and response [88]. Kwon and coworkers
reported that a PMMA/HAp composite with 30 wt% of HAp increased the interfacial shear strength at the bone–implant interface 6 weeks after implantation in
rabbits [101]. Several other research studies revealed that, depending on the type of
bone cement, the addition of HAp can improve the mechanical properties of bone
cements [102].
Moursi et al. [103] studied the response of osteoblasts to PMMA/HAp materials.
They found that osteoblasts attached equally well to PMMA/HAp. In contrast, the
increase of osteoblasts on PMMA/HAp was significantly improved, compared to
PMMA, after 8 days in culture.
In addition, to improve adhesion between polymer matrix and HAp, several
copolymer such as PMMA, PBMA, and PHEMA were grafted onto the surface of
HAp using isocyanatoethyl methacrylate (ICEM) or hexamethylene diisocyanate
(HMDI)/hydroxyethyl methacrylate (HEMA) [100]. These three methacrylate
polymers were coupled to HAp particles via covalent bonding with isocyanate
groups.
Skrtic et al. [104] studied methacrylate conversion and volumetric contraction in
photo-polymerized composites with amorphous calcium phosphate (ACP). ACP
was used because it is highly soluble in aqueous media and can be quickly
transformed to HAp. Ideally, on polymerization, the resin systems used in
composites should achieve the contradictory goals of both high vinyl conversion
and minimal volumetric contraction. A high degree of vinyl conversion, with
154
A. Bhowmick et al.