Biomedical Engineering – From Theory to Applications

1. Introduction
18
Orthopaedic Modular Implants
Based on Shape Memory Alloys
Daniela Tarnita1, Danut Tarnita2 and Dumitru Bolcu1 1University of Craiova,
2University of Medicine and Pharmacy, Craiova,
Romania
Intelligent materials are those materials whose physical characteristics can be modified not
only through the charging factors of a certain test, but also through diverse mechanisms
involving a series of additional parameters like luminous radiation, temperature, magnetic or
electric fields etc. The use of intelligent materials in medical sciences offers to the economic
medium the safest way to launch effective, highly-feasible and especially biocompatible
products on the internal and international markets. The most important alloy used in
biomedical applications is Ni–Ti, Nitinol (Nickel Titanium Naval Ordinance Laboratory), an
alloy of an almost equal mixture of nickel and titanium, which is able to fulfil functional
requirements related not only to their mechanical reliability but also to its chemical reliability
and its biological reliability. Superelastic Nitinol alloys are becoming integral to the design of a
variety of new medical products. The very big elasticity of these alloys is the most important
advantage afforded by this material, but by no means the only or most important one. To
highlight the value of superelastic Nitinol to the medical industry, we can present other
properties: biocompatibility, kink resistance, constancy of stress, physiological compatibility,
shape-memory deployment, dynamic interference, fatigue resistance hysteresis, and MRI
compatibility (Duerig et al., 1999; Friend & Morgan, 1999; Mantovani, 2000; Pelton et al., 2000;
Ryhanen et al., 1999). These properties were used for manufacturing medical products
including stents, filters, retrieval baskets, and surgical tools.
There are many metals exhibit superelastic effects, but only Nitinol based alloys is
biologically and chemically compatible with the human body (Kapanen et al., 2002;
Raghubir et al., 2007; Shabalovskaya, 1995; Yeung et al., 2007). In vivo testing and
experience indicates that Nitinol is highly biocompatible, more so than stainless steel. The
extraordinary compliance of Nitinol clearly makes it the metal that is most similar
mechanically to biological materials. This improved physiological similarity promotes bony
ingrowths and proper healing by sharing loads with the surrounding tissue, and has led to
applications such as hip implants, bone spacers, bone staples, and skull plates. NiTi
applications in orthopaedics currently include internal fixation by the use of fixatives,
compression bone stables used in osteotomy and fracture fixation, rods for the correction of
scoliosis (Yang et al., 1987), shape memory expansion staples used in cervical surgery
(Sanders et al., 1993), staples in small bone surgery (Mei et al., 1997), and fixation systems
for suturing tissue in minimal invasive surgery (Musialek et al., 1998). Several types of