Fixation - Orthopaedic Biomaterials in Research and Practice - page 322

contact with cortical or cancellous bone, ingrowth is focal, varying in
degree from place to place in an apparently random fashion. However,
portions of the interface within healthy cancellous bone or near to corti-
cal walls tend to be better ingrown. Portions of the surface of the coating
in any particular section may show no bone-coating contact or adjacent
bone.
When in contact with cortical bone (as shown here), “cancellization”
of the bone may be seen near portions of the interface. When in contact
with cancellous bone, there is frequently a bony condensation or pseudo-
subchondral plate parallel to the interface and 1-2 mm away from it,
especially in areas where there has been little ingrowth.
Adhered or ongrowth . Surface coatings are generally very thin, less
than 0.5 mm in thickness, owing to their brittleness, except when pure
titanium or bulk homogeneous materials are used (in experimental ani-
mal models). No fibrous membrane separates coating and bone, although
bioactive ceramics show an amorphous layer 1-100 μm thick containing
coating elements in a gel as well as collagen and polysaccharides. If
placed in cortical bone, as shown here, cancellization may occur, but to
a small degree.
Goals of fixation
The presumed goals of fixation are as follows:
1. Limit of relative motion between loaded implant and supporting
bone (“micromotion”)
2. Production of contact stresses on bone within normal (acceptable)
limits
3. Maintenance of resultant stresses in bone close to normal
physiologic preimplantation levels, to minimize bony adaptive
remodeling
Micromotion
A few words are in order about “micromotion.” Orthopaedic researchers
and clinicians have a fundamentally mechanical approach to problem
solving, as befits those who deal with repair and restoration of the mus-
culoskeletal system with its primary functions of maintenance of struc-
tural integrity and provisions for locomotion.
This naturally leads to seeking a mechanical origin for the loss of
continuity between implants and their bony support, that is, “loosening.”
Most arguments start with an appeal to “micromotion.” This appears to
be an amount of motion between device and tissue that exceeds accept-
able limits, thus leading to loosening.
Charnley, without giving details of the calculation, stated that the
motion between the distal tip of a femoral stem and the surrounding
bone, during loading, was “only about 25 μm (or the length of three red