Polymers - Orthopaedic Biomaterials in Research and Practice - page 152

Table 6.3 factors in optimizing bC strength

Factors Outcome

Uncontrollable factors
Aging

Gradual 10% loss of strength resulting from postcuring chemical
changes

Environmental temperature 10% weaker at body temperature than at room temperature

Fatigue Fatigue strength (10 6 cycles) 20%-25% of single-cycle strength

Moisture content Loss of 3%-10% strength owing to water absorption

Strain rate Significant increase in strength with increasing strain rate

Partially controllable factors
Cement thickness

Important; “intermediate” BC thicknesses minimize both fatigue
stresses and shrinkage effects

Constraint Significant; BCs far stronger in compression than tension

Inclusion of blood or tissue Considerable effect; up to 70% loss of strength, depending on

amount

Stress risers (bony bed, prosthesis) Significant; BCs are quite notch sensitive

Fully controllable factors
Antibiotic inclusion 5%-10% loss of strength

Centrifugation/vacuum degassing 10%-25% increase in strength, possible increase in fatigue strength

Insertion Delay may produce up to 40% loss of strength, whereas

pressurization increases strength by up to 20% by reduction of
porosity

Mixing speed Up to 21% loss of strength owing to too slow or too rapid mixing

Radiopaque fillers 5% weaker than unfilled

Note: “strength” and “fatigue strength” are in tension; behavior in compression is different and less sensi-
tive to external conditions.

desirability of maximizing strength. The factors that contribute to BC
strength optimization may be grouped into three classes: those beyond
the control of the surgeon, those partially controlled by the surgeon, and
those fully controlled by the surgeon. These are summarized in Table 6.3.
Despite considerable concern over the behavior of BCs in vivo , par-
ticularly in younger, high-demand patients, the clinical results of series
with attention to cementation technique are excellent. The clinical per-
formance of BCs in comparison with other fixation techniques will be
considered in Chapter 13. However, it is safe to say that BCs will con-
tinue in wide and successful clinical use for the foreseeable future.

Poly(ethylene)

Just as the use of PMMA BCs has markedly reduced loosening and pain
previously associated with total joint replacements, the introduction of
high-performance polymers to replace metal-metal wear pairs with
metal-polymer ones has led to markedly lower wear, and presumably
has reduced the risk of both local and systemic adverse response to metal
implants involving articulating interfaces.