MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 80

3.6.1 Issues Limiting Performance of Metallic Biomaterials
3.6.1.1 Wear of Implants. For implant applications, wear of metallic im-
plants is a serious concern for various biomaterials. The examples are different
joints of the human body, where two similar or dissimilar materials come in con-
tact. In a typical HIP prosthesis, a metallic stint is attached to a ceramic ball, and
the ceramic ball moves inside the polymeric acetabular cup. At the joint of the
metal-ceramic interface, fretting fatigue could be responsible for loosening of the
implant. On the other hand, the ceramic ball-polymer cup interface experiences
sliding wear. The presence of body fl uid and different types of proteins may trig-
ger the wear rate in vivo .
Pazzaglia et al. 99 investigated the reason behind the loosening of metal-
plastic total hip prosthesis. Metal-on-plastic total hip prostheses liberate metal
particles due to wear of the femoral stem. Such relative movement between two
surfaces presumably aids the passage of metal particles from the cement-metal
interface to the cement-tissue interface in the absence of direct contact via fi s-
sures in the cement. Metallic wear debris may also be shed from the femoral head
articulating in the cup if the latter is contaminated with an abrasive, such as bone
cement; however, this was not noticed. Irrespective of the mechanism of their
production and release, particles of stainless steel and Co-Cr-Mo alloy at the
bone-cement interface encourage macrophage-related bone resorption 99 . It was
suggested that this aspect represents a contributing and in some cases not incon-
siderable factor in loosening of metal-plastic total hip prostheses.
In their work on tribological behavior of Ti based alloys, Choubey et al. 100
conducted fretting wear experiments on a number of Ti-alloys, in simulated body
fl uid environment. Their results revealed that the COF of Ti-6Al-4V alloy lies
between 0.46-0.50 and COF of Ti-5Al-2.5Fe alloys is 0.3 (see Figure 3.13). The
major wear mechanism was found to be tribomechanical abrasion, transfer layer
formation, and cracking.
3.6.1.2 Corrosion of Metallic Implants. Most of the metallic implants
corrode in contact with body fl uid. Sometimes, these corrosion products are
harmful to the human body and in most of the cases, the mammalian cells cannot
metabolize these corroded waste. Therefore, it is important to study the corrosion
behavior of metallic implants in vitro , that is, prior to in vivo application.
Steel is well known for its excellent strength properties and has been used as
implant materials for a long time. Sivakumar et al. 101 compared the corrosion be-
havior of super-ferritic steel, duplex steel and 316 stainless steel (SS) and found
that super-ferritic and duplex stainless steels can be adopted as implant materials
due to their higher pitting and crevice corrosion resistance. Different alloying ele-
ments in stainless steel may have allergic and carcinogenic effect. To prove this
characteristic, Hierholzer et al. 102 studied the corrosion behavior of SS in infected
plated fractures. They observed that both the absolute concentration of the ions