Fixation - Orthopaedic Biomaterials in Research and Practice - page 328

Thus, none of the three major fixation systems achieve the goal of a
true graded seal between implant and bone, but probably all rely, to one
degree or another, on a compliant interfacial zone. A naturally occur-
ring example of this approach is the periodontal ligament, which forms a
compliant interface between a stiff member (alveolar cortical bone) and
a stiffer member (the tooth root).

Temporal development of fixation

Another important factor in the success or failure of fixation is the tem-
poral development of the properties of the interface. This is shown sche-
matically in Figure 13.8. Failure to address these temporal features in
either device insertion or later clinical management may be a contribut-
ing factor to both early and intermediate fixation failures.
Cemented interfaces provide immediate fixation, with stiffness
developing during the setting phase and maturing over the next 24 h.
This fixation will be maintained indefinitely, unless the cement degrades
or the bony support changes. However, since PMMA-type cements pro-
duce a narrow region (0.1-0.5 mm) of devitalized bone, owing to chemi-
cal and possibly thermal trauma, there may be a period, perhaps years
after implantation, when remodeling produces temporary reduction in
mechanical properties and integrity.
Ingrown interfaces develop more slowly, with most studies (in ani-
mals) reporting little or no mechanical integrity before 2 weeks but
nearly complete maturation, in the absence of “significant” motion, in
6-8 weeks. Again, late remodeling, including cancellization, may pro-
duce late degradation of mechanical behavior.
Adhered or ingrown fixation is probably the slowest, with little
adherence seen before 3 weeks and up to 30 weeks without significant
functional loading required, in both animals and patients, to produce a
mature interface. Here there is a real concern about late property changes

100%

?

Cemented

50%

Ingrown Adhered

0%

1 min 10 min 1 h 1 day1 week 1 month 1 year 10 year

Time post-implantation (log)

FIGUre 13.8 temporal development of bond “strength.”