Fixation - Orthopaedic Biomaterials in Research and Practice - page 331

e bone-implant
interface

In
bone

In
implant

Stress

Too high

Acceptable

Too low Too high

“Motion”

Stability

(Infection)
( “Chemical”?)
( ± “Electrical”?)
?

Wear debris
“chemical”

Stress

?

Atrophy
(fracture?)

Necrosis
(remodeling?)

(Immune res.?)
Necrosis

(Pain)
Loosening

FIGUre 13.9 degradation of the bone-implant interface.

and waning as functional requirements change. This is expressed in
Wol f f 's l law*:

The form being given, bone adapts to the loads applied to it .

When a prosthesis is inserted, local stresses are perturbed from nor-
mal (see Figure 13.5). Too high values of stress, applied in times too
short to permit adaptive remodeling, may produce local necrosis. This in
itself may produce mechanical collapse, or possibly collapse may occur
during the resorption phase of remodeling. At this point, a feedback pro-
cess also may occur: as bone mass decreases, local stresses will rise still
further, accelerating the process. However, if stresses are too low, bone
atrophy will occur. This may lead to progressive collapse or to fracture
in a single overload incident.
Both of these phenomena will degrade the interface and lead to
increased relative motion between the implant and bone. This so-called
“micromotion” is a matter of much conjecture, as previously discussed.
There is no agreement as to how much shear strain any of the three
fixation interfaces can sustain before motion is said to take place and
the consequences attributed to it occur. Certainly, any reduction in bone
mass or quality or both will decrease mechanical coupling between the
implant stem and the supporting bone away from the interface.
However, this may occur primarily (Figure 13.9, left side) owing to
less-than-optimal device design or inadequate geometry, resulting in
unacceptable interfacial stresses and motion, before bony changes occur.
This in itself may be enough to constitute loosening. However, it will pro-
duce wear debris in the interface, by fretting processes, and may acceler-
ate corrosion processes. Direct biologic responses to these new products

* This is a necessary paraphrase. There are many forms of “Wolff's Law” given by
various writers, and it is not certain which is the most faithful to his original thesis.