MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 73

The fabrication of synthetic analogues of bone is a great challenge to mate-
rial scientists. As mentioned in section 3.3, bone itself is a composite material in
nature with a polymeric phase collagen.
In THR applications, one of the major problems arises with the mismatching
of stiffness of the prosthesis and the femur bone. The metal stem has fi ve to six
times higher stiffness than host bones, which causes stress shielding problem 67,68,69 .
As a solution, some researchers introduced CF/PS 70 and CF/C 71 polymer compos-
ites to replace the metal stem. Improved creep property, more strength, and mod-
erate stiffness are the major characteristics of this type of polymer composites. To
mimic the mechanical properties of the femur bone, the use of another polymer
composite, such as CF/PEEK stem prosthesis, are successfully achieved 72,73,74 . A
few examples of other polymers for THR application include CF/Epoxy, CF/C,
and CF/PE, and so on. However, lower stiffness, E-modulus, and poor mechanical
strength associated with polymer composites implants limits their long-term use
as suitable bone replacement materials. To overcome this problem, ceramic-
polymer composites have been developed as hard tissue replacement materials 75 .
3.5.2 Polymer-Ceramic Composites
Major advantages of the ceramic materials are their good biocompatibility, high
hardness and improved corrosion and wear-resistance properties. However, a
large mismatch in E modulus (stiffness) between the hard tissue and ceramic im-
plant materials effectively caused stress shielding or stress protection problems,
as the bone is insuffi ciently loaded compared to the implanted materials 76 . Also,
the stress shielding affects the healing and remodeling process, leading to a de-
crease in bone density. This is because of the fact that bone growing cells (osteo-
blast) progressively become less productive and form a lower strength porous
bone, which is called trabecular or cancellous bone.
Another major shortcoming of ceramic biomaterials is their lower fracture
toughness value, which limits their use as load-bearing implant materials. In this
case, low modulus materials, such as polymers could be combined with ceramic to
obtain combination of desired material properties. Building on several years of
research, HAp/HDPE, SiO 2 /silicone rubber, HAp/EVA, BCP/PMMA, HAp/
PLA, HAp/PEEK, bioactive glass/PMMA, nano-HAp/Poly (hexamethylene adi-
pamide) composite materials are being developed 77 .
Among various ceramic containing polymer biocomposites, hydroxyapatite
reinforced HDPE composites are extensively studied for their potential use as
hard tissue replacement materials and has been successfully used in orbital sur-
gery and developed as an analogue material for bone replacement. The closer
property matching of HAp/HDPE composite (E Modulus: 2-5GPa, Tensile
strength: 18-25 MPa) to bone, reduces the problem of bone resorption, which is
associated with the use of metal/ceramic implants.
Bonfi eld et al. 78 was the fi rst to develop hydroxyapatite reinforced high den-
sity polyethylene (HDPE) biomaterial for skeletal applications and coined a
trade name HAPEX TM for HAp/HDPE composite. In a refi ned processing route,