Fixation - Orthopaedic Biomaterials in Research and Practice - page 323

blood cells).” Others considering the dynamics of tissue ingrowth into
porous surfaces have suggested that motion may be unacceptable if it
exceeded 0.5 D p′ , or 35-50 μm, and maybe even allowable up to 150 μm.
Thus, it is clear that when considering micromotion, we are talking
about relative motion that is of the same order of magnitude as cellular
dimensions. It is unclear whether it is even appropriate to consider these
effects directly as motion (relative strain) or in terms of local increases
in stress. This is especially true as detailed mechanical analysis of the
cement-bone interface, for instance, is almost impossible, owing to its
microstructural complexity. Thus, knowledge of macroscopic movement
does not lead easily to certain knowledge of the magnitude or direction
of micromotion.
Despite these misgivings, it is clear that the term micromotion will
continue to be used as shorthand for the mechanical causative aspects of
chronic device loosening.

a consideration of fixation

Are these stated goals of fixation credible ones? Or are they chimeric,
whose pursuit is foredoomed to failure?
An experiment by Weightman (Figure 13.5) casts a good deal of light
on the generic problem (Weightman 1975). In this experiment, a cadaver
femur was instrumented with strain-measuring devices (strain gauges)
along its lateral and medial faces. It was then secured distally and loaded
in a more-or-less anatomic direction. Surface (outer fiber) stresses in the
cortical bone were calculated from the strain gauge outputs, assuming

1715 N

Lateral surface Medial surface

1

2

Femur
and
prosthesis

Femur
and
prosthesis

Intact
femur

C C ´

3´

3

Intact
femur

4

4´

-5 5 -10 -5 5 10

σ (MPa) σ (MPa)

FIGUre 13.5 femoral cortical stress. (Weightman, b., stress analy-
sis. in swanson, s.a.v., freeman, m.a.r. [eds.]. The Scientific Basis of
Joint Replacement . p. 18. 1975. Copyright Wiley-vCh verlag Gmbh &
Co. KGaa. reproduced with permission.)