Fixation - Orthopaedic Biomaterials in Research and Practice - page 332

may include both fibrosis, as a classic foreign body immune response, and
bony necrosis secondary to chemical toxicity or to local microvascular
blockade associated with immune hypersensitivity. Here, again, there is a
cyclic effect, with decreased bone quality leading to increased motion and
additional wear debris and corrosion product production. (It is necessary to
mention several local host responses to implants at this point, for complete-
ness. They are discussed more fully in Chapter 14.)
There are processes that may result in loosening, even in the absence
of implant stresses that are, on their face, too high, through an accumula-
tion of wear debris from the articulating interface. Willert and Semlitsch
first pointed out that the so-called “cement disease,” a dissecting fibrosis
at the bone-cement interface, appeared to be related to the production
of wear debris. It was originally thought to be produced by macrophages
and foreign body giant cells, but more recent studies suggest that acti-
vated macrophages secrete an osteoclastactivating factor, thus impli-
cating increased osteoclastic activity in this process. Similarly, with
uncemented implants, “particle disease” refers to an enhanced bone
resorption response or osteolysis owing to wear particles.
Even if stresses in both bone and implant are acceptable, loosening may
result over the long term. Infection with associated increases in corrosion
rate may contribute to bone atrophy or necrosis. Immune responses are
probably not a contributing local factor in loosening of an initially stable
implant. However, the higher surface areas and thus higher intrinsic corro-
sion rates of noncemented metallic devices anchored by ingrowth may cause
this assumption to be questioned in the future. Finally, the presence of an
electrically conducting implant may well “short out” or otherwise interfere
with the normal electrical polarization pattern (represented by the biopoten-
tials; see Chapter 5), leading to adverse bone remodeling effects.
Porous coatings
In an effort to enhance biologic fixation of metallic implant surfaces
and to avoid problems with aseptic loosening of cemented implant sur-
faces, porous coatings have been developed with very promising clinical
results. Porous surfaces are irregular surfaces with substantial surface or
bulk porosity, which are intended to encourage the early and increased
level of bone ingrowth into the implant. The major functionality pro-
vided by a porous surface is twofold: to provide biological anchorage
for ingrowth of bony material and to provide a transition between the
bone and the load-bearing implant that will limit stress shielding and
bone resorption at the implant surface. Mismatch in stiffness between
implanted metallic alloys and bone has been identified as the main rea-
son for implant loosening. Coatings not only have been used primarily
with joint arthroplasty applications but also are utilized in graft substi-
tutes to fill bony defects in spinal fusion devices.
The ideal structure of a porous surface is an open microstructure
resembling cancellous bone in its porosity, stiffness, and frictional
behavior. Ingrowth is directly dependent on the coating architecture,