Fixation - Orthopaedic Biomaterials in Research and Practice - page 333

including its porosity, pore size, shape, and interconnectivity. Larger
pore sizes allow sufficient space for cell migration, adhesion and prolif-
eration, while an interconnected porosity will maintain vascularity by
allowing the free movement of body fluids. Pore sizes of approximately
50 μm or greater have been found to be associated with osseointegra-
tion, where the optimal pore size has been determined to range between
approximately 50 μm and 500 μm. However, these pore sizes have been
based primarily on cobalt-chromium surfaces. Instead, these “optimal”
pore sizes may be smaller for titanium surfaces, which may be more
osteoconductive. The chosen material should further be biocompatible
and possibly bioactive. While porosity may aid in facilitating osseointe-
gration, a consideration with porous coatings is the effect of the material
porosity on fatigue strength, as the voids in the surface can act as stress
concentrations. Further, the increased surface area could potentially lead
to higher corrosion rates, increasing concerns with corrosion related
fracture and metal ion release. Corrosion concerns explain why stainless
steel is not utilized for this purpose. Ceramics, on the other hand, are not
used because of their inherent brittleness, while polymers are limited by
their poor strength and high failure rates.
Irregular surfaces are realized in a number of embodiments, includ-
ing wire mesh, sintered beads, grooves, and voids. The first generation
of porous coatings included CoCr sintered beads, structured titanium
(sintering of pure Ti onto a Ti or CoCr substrate), diffusion-bonded fiber
metal mesh (wire lengths can be woven into a mesh and pressure sintered
onto a solid substrate), and titanium plasma spray, which is fabricated by
casting molten titanium on a substrate surface. Each of these coatings
has performed similarly in a clinical setting in terms of bone ingrowth
behavior, which has been observed to be heavily dependent on intimate
bone contact for long-term biologic fixation. All options in this group are
considered to have a relatively low porosity of 30% to 50% and a high
interface stiffness, making them relatively limited in their capability for
interfacial strength. Further, all traditional porous coatings are theoreti-
cally susceptible to mesh shedding and delamination as they are not able
to be fabricated into a stand-alone structure (i.e., they must be added to
a substrate surface). Specifically, sintered coatings (CoCr and structured
Ti) and diffusion bonded mesh have relatively reduced fatigue strengths
owing to the high-temperature fabrication process. It is estimated that
these coatings will retain less than 50% of their fatigue strength. In com-
parison, plasma-flame sprayed coating is capable of retaining 90% of its
fatigue strength.
In response to these limitations, there is a continued focus of innova-
tions in open-structured metals to replace traditional coated surfaces.
Highly porous foam metals in homogenous and non-homogenous open
cell pore distributions come closer to the ideal porous coating described
above. Current options include titanium- and tantalum-based foams,
which are fabricated through deposition onto a vitreous carbon skeleton.
These metal foams typically exhibit higher porosities, higher coefficients
of friction, and lower stiffnesses in comparison to first-generation coat-
ings. Ranges for these and other properties are provided in Table 13.1.