FUNCTIONALLY GRADED ALL CERAMIC HIP JOINT - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 320

Materials for load bearing implants such as hip or knee implants should
be able to resist peak loads as high as ten times body weight and an average load
of three times the body weight [Black, 1992]. Therefore, only bioceramics like
Al 2 O 3 , ZrO 2 and their composites are considered as suitable ceramics for ball-
heads and acetabular cup inserts (liners) in total hip joint replacement and will
be discussed in this paper. Compared to metals and polymers, advantages of
these ceramic implants are the excellent bioinertia, the extremely low wear rate
and the reduced osteolysis. Other ceramics as SiC and Si 3 N 4 have also an excellent
hardness, mechanical strength and corrosion resistance, but have been much
less investigated as biomaterials. Part of the reasons for this is their more compli-
cated processing routes, in particular with respect to densifi cation, where high
temperature and—in the case of silicon nitrides—also high gas pressure is
necessary. Noteworthy are the biomorphic silicon carbide ceramics which have
inherited their microstructure and in particular their pore structure from the
wood from which they have been prepared. Biomorphic SiC is fabricated by
molten - silicon infi ltration of carbon templates obtained by controlled pyrolysis
of wood. The bioactivity of such a material coated also with a bioactive glass
layer has been reported by Gonzalez et al. (2003). Silicon nitride has been another
silicon based ceramic which has hardly been investigated with respect to its
potential applications as biomaterial. The role of the surface fi nish of the materi-
als on the proliferation of osteoblast cells on reaction bonded silicon nitride has
been highlighted by Kue et al. (1999). In a more recent study by Neumann et al.
(2006), the potential application of silicon nitride for a miniplate osteofi xation
system for the midface has been investigated. One of the conclusions was
that Si 3 N 4 ceramics showed a good biocompatibility outcome both in vitro and
in vivo .
In the early 1970s, ceramic components for total hip joint replacements
(THR) were introduced. The hip joint consists of a ceramic ball-head attached to
a metal stem and an acetabular ceramic or polyethylene insert, which is attached
to a metallic cup, as illustrated in Figure 10.1. Today, more than 350,000 ceramic
components for total hip joint replacement have been implanted.
In general, the material combinations for the femoral head-acetabular
cup pair comprise metal - metal, metal - polyethylene, ceramic - polyethylene and
ceramic-ceramic combinations. The currently used material combinations are
summarized in Figure 10.1 . The metal - polyethylene and ceramic - polyethylene
couplings are still considered to be the most suitable in most cases. The wear rates
and the consequent risks of loosening of the implant are proportional to the time
and motorial activity. In fact, for elderly or barely active patients, the use of poly-
ethylene must not be considered a limitation. The alternative ceramic-ceramic
and metal-metal solutions are chosen for patients with a high motorial activity
and it is stated that ceramic-ceramic couplings are appropriate in almost 20% of
cases, while metal-metal couplings only in fi ve percent of all cases. Indeed metal-
metal bearings carry the risk that the released metal ions and debris may cause
(among other potential negative effects) allergic reactions leading at least to dis-
comfort to the patient. Because of the long-term biocompatibility of the debris