Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 206

a fairly uniform surface layer of TiO 2 . Under certain controlled condi-
tions, bonds will form in vivo between bone and high-purity titanium
with appropriate surface preparation (passivation).
Tantalum . This is another metal with a surface layer of Ta 2 O 5 .
Tantalum has been used as a surface coating, applied by a variety of
techniques, and as high surface area porous material resembling cancel-
lous bone. Its high strength, hardness, and cost rule out its use as a bulk
implant material.
Hydroxyapatite . HA is a calcium phosphate ceramic having a close
association with collagen in normal bone. Some workers report bond for-
mation with bone; this depends apparently on HA grain size and purity
and on the experimental model. In some cases, bone growth on or near
implanted HA is more rapid than in association with control implants
of similar composition or of metal. Thus, HA is commonly known as
being “osteoinductive.” This is probably incorrect; a better term might
be “osteoconductive” since HA has not been shown to have any specific
stimulating effect on osteoblasts as does bone morphogenetic protein
or similar biologic molecules. Osteoconductive materials can encourage
bone formation in an osseous environment, while osteoinductive materi-
als can elicit bone formation in an extraosseous environment.
HA is currently used most extensively as a plasma spray coating on
joint prosthesis. Its use here provides a solution for fixation without hav-
ing to deal with complications from PMMA debris. It also provides a
bone-conserving alternative to cement fixation. The consensus among
studies is that bone ingrowth or ongrowth is enhanced with HA, though
it is still uncertain whether HA improves fixation when compared to
bone cement. HA may have some advantages over cement fixation in
younger patients whereas cement fixation may provide improved fixation
in more elderly, severely osteoporotic patients. The main drawbacks to
HA are its brittle behavior and poor tensile strength, which have limited
its applicability as a bone graft substitute.
There is a further caution necessary concerning the literature on HA
and its biologic performance. Strictly speaking, HA is calcium hydroxy-
apatite, with the chemical formula Ca 5 (PO 4 ) 6 (OH) 2 and a Ca/P ratio equal
to 1.67. “HA-type” materials used in research activities vary widely in
composition, with Ca/P ratios as low as 1.5, and are rarely well charac-
terized. These latter materials are usually mixtures of CaHAP and octa-
calcium phosphate, Ca 8 H 2 (PO 4 )6.5H 2 O, a more soluble ceramic. Thus,
it is extremely hard to compare and rationalize the results of studies by
different investigators.
Surface-active glass . A number of glasses and ceramic bodies with
glassy phases have been developed that encourage direct bonding by
bone and, in some cases, soft tissues after implantation. In some cases,
the bond strength may exceed that of the adjacent bone. The necessary
precursor to bonding is a surface reaction that releases Ca 2+ , P 4+ , and
Na + ions and produces a water-rich gel on the implant surface. This gel
is chemically inhomogeneous, usually presenting a silicon-rich surface
layer over a calcium- and phosphorus-enriched underlayer.