Biomedical Engineering Reference
In-Depth Information
Figure 4.16
Phosphate glass fibres in the system P
2
O
5
-CaO-Na
2
O-SiO
2
; fibre
diameter is around 30
m. (Image provided courtesy of C. R ussel, Jena, Germany.
Copyright (2012) Otto-Schott-Institut.)
μ
be heated to temperatures above
T
g
and then drawn into fibres. This
method is particularly interesting for producing fibres of highly disrupted
polyphosphate or invert phosphate glasses, which show a high tendency
to crystallise. Crystallisation of glass fibres can have deleterious effects
on fibre production and mechanical properties, and it also influences
glass solubility, degradation rates and cell response.
One of the major challenges during development of both conventional
and degradable implant materials for fracture fixation is the mechanical
compatibility between implants and bone. Metals and alloys have been
used successfully for internal fixation; however, these implants do not
degrade over time, and the rigid fixation from bone plating can cause
stress protection atrophy resulting in loss of bone mass and osteoporosis.
While the elastic modulus of cortical bone ranges from 17 to 26 GPa,
common alloys have moduli ranging from 100 to 200 GPa. This large
difference in stiffness can result in high stress concentrations as well as
relative motion between the implant and bone upon loading. Degradable
polymers, such as poly(lactic acid) (PLA), are currently used as sutures
or degradable fracture fixation materials such as screws. Their main
advantages over metallic implants are avoidance of a second operation
to remove the fixation device and avoidance of stress shielding by a
gradual load transfer from the degrading polymer to the regenerating
bone. However, their lower stiffness (for PLA screws, around 3 GPa)
