Biomedical Engineering Reference
In-Depth Information
residual stress impeding dissolution (Han, Xu, and Lu 2001; Yang and Chang 2001). In
addition, tensile residual stress will promote multiple cracking of the coating, and the com-
pressive residual stress can weaken the bonding and bonding strength at the coating-
substrate interface. Therefore, since both the tensile and compressive residual stresses exert
detrimental influences on HA-coated implants, it is desirable to produce coatings on metal-
lic implants without any unexpected residual stress.
In Vitro
Assay
It is well known that any biomaterial must be thoroughly evaluated to determine its biocom-
patibility/bioactivity and whether it functions appropriately in actual biomedical applica-
tions. Generally, two kinds of evaluation techniques are employed for such purposes:
in
vitro
(in a glass tube) and
in vivo
(in a living organism) tests. Although
in vivo
tests are the
most direct and reliable evaluation methods for biomaterials, their results are normally
difficult to obtain and interpret due to a lack of animal sources and the complexity of dif-
ferent cellular responses. (During
in vivo
tests, the cells that migrate to the implant surface
contain different cell lineage, and the final results are demonstrated by the fact that the
progeny of these cells may form a variety of tissue types adjacent to the implant.) (Boyan et
al. 2001).
In vitro
testing can provide more rapid and relatively inexpensive data compared
with
in vivo
testing. Moreover,
in vitro
testing can provide useful initial screening of mate-
rials and can aid in understanding the performance of a material
in vivo
. These valuable
insights could also help to determine whether an implant/device needs further evaluation
in expensive
in vivo
experimental models and minimize the amount of animals required
in
in vivo
testing (Ratner et al. 1996).
In vitro
tests of biomaterials can be carried out in any
cell-free or cell-containing environment to study their biocompatibility and bioactivity. In
particular, cell-free solutions allow the study of chemical and mineralogical changes of the
material under conditions that simulate the physiological interactions between the mate-
rial surface and the surrounding tissues.
Dissolution Behavior
A prerequisite for any implant used in orthopedic or dental treatment is permanent fixa-
tion to the surrounding tissues with no intervening gaps or fibrous tissues (Vedantam
and Ruddlesdin 1996). According to the
in vivo
and
in vitro
studies as well as more than a
decade's clinical practice with HA-coated prostheses, there is general agreement that the
originally desired benefits of HA coatings, that is, earlier fixation and stability with more
bone ingrowth or outgrowth, can be achieved. However, doubts still exist concerning the
durability of the fixation (Greenspan 1999). One of the most important events occurring at
the bone-implant interface is the resorption of the HA coatings, also called degradation
or coating loss (Bloebaum et al. 1994; Dalton and Cook 1995). Although some resorption
or dissolution is, of course, essential to trigger bone-implant bonding, the fast resorp-
tion could lead to disintegration of the coating, with rapid loss of the bonding strength
between it and the substrates, resulting in delamination, the production of particles, and
loss of mechanical fixation. It is reported that a decrease as high as 31.6% was observed for
plasma-sprayed HA coatings after only 2 weeks' immersion in SBF (Gu, Khor, and Cheang
2003). Other studies have shown resorption of HA coatings up to 2 years after implantation
and a complete loss of a 60-
μ
m-thick HA coating after 4 years (Sun et al. 2001). Aebli et al.
(2003) carried out a histological study of a proximally HA-coated femoral component and
found that the HA coating had completely degraded after 9.5 years' implantation.
