Biomedical Engineering Reference
In-Depth Information
Investigations into the osteoblast response to nanostructured metals are
limited to HPT-processed titanium. It has been associated with increased
surface wettability and improved pre-osteoblast attachment and growth
rate
in vitro
.
100, 101
Cytoskeletal and extracellular matrix activity is also
increased. Interestingly, the morphological and histochemical pattern of the
fibronectin self-assembly, aggregation and fibrillar network seems similar
to that observed on hydroxyapatite-coated titanium, which is known for its
high osteoinductivity.
Therefore, the introduction of novel technologies to obtain nanostructured
metals provides a promising alternative for enhancing the mechanical properties
of the bulk material, associated with the grain size decrease but they may be
very useful for understanding the physicochemical mechanisms controlling
the cell-substrate interaction in materials modified by severe surface plastic
deformation (ssPD) techniques such as grit blasting and shot peening. For
instance, it has been recently reported that ageing of sandblasted Ti develops
a nanocrystalline structure at the surface layer that significantly improves
the corrosion resistance.
102
6.5.4 Biodegradable alloys
Stainless steels and titanium alloys are widely used in internal fixations
because of their strength and toughness. For young patients, it is generally
recommended that these devices be removed to avoid adverse physiological
responses associated with stress yielding. In the case of children, removal is
definitively required in order to allow developmental bone growth. Degradable
metallic implants are a new category of implants aimed to degrade
in vivo
,
eliminating the need for a second operation to remove the implant. From a
historical point of view, this concept breaks the paradigm requiring biomaterials
to be corrosion resistant.
Magnesium alloys were introduced into orthopaedic and trauma surgery
in the first half of the 20th century.
103
The main advantages in comparison to other metallic biomaterials are
related to their low density (1.7-2.0 g cm
-3
). In addition, fracture toughness
is greater than in ceramic materials, while Young's modulus (41-45 GPa) is
closer to that of natural bone (Table 6.4). although biocompatibility is good,
degradation of magnesium is too fast, losing mechanical integrity before the
tissue has sufficiently healed and producing hydrogen gas in the corrosion
process at a rate that produces an accumulation of hydrogen as subcutaneous
gas bubbles (about one litre of hydrogen per gram of magnesium). Moreover,
such implants could not maintain mechanical integrity over a period that was
long enough for sufficient bony union to develop. Owing to these problems,
their use was abandoned when stainless steels became available.
In order to control the corrosion rate in body fluids, complex alloys
