Polymers for bone repair - Bone Repair Biomaterials - page 242

is unable to be arranged in a crystalline organized structure. consequently,
this material has lower tensile strength and higher elongation and much
more rapid degradation time than l l-PLA (table 8.2). the semicrystalline
poly( l -lactide) has a modulus about 25% higher than poly( d l -lactide) and a
degradation time of the order of 3 to 5 years. the amorphous poly( d l -lactide)
has a degradation time of 12 to 16 months. 55
the copolymers of l l-lactide and glycolide are amorphous owing to the
disruption of the regularity of the polymer chain by the other monomer. 56 it
is important to note that there is no linear relationship between the copolymer
composition and the mechanical and degradation properties of the materials.
For example, a copolymer of 50% glycolide and 50% d l -lactide degrades
faster than either homopolymer. 57

8.4.4 Degradation of the polymers
simple chemical hydrolysis of the hydrolytically unstable backbone is the
prevailing mechanism for the polymer degradation (Fig. 8.7).
there are two different mechanisms for this polymer degradation to occur:
surface erosion and bulk erosion. in surface erosion, the process is limited to
the surface of the device. the polymeric device will become thinner with time
while maintaining its bulk integrity. 59 Polyanhydrides and poly(ortho esters)
suffer this type of erosion. Alternatively, in bulk erosion, the rate at which
water penetrates the device exceeds that at which the polymer is converted
into water-soluble materials (resulting in erosion throughout the device).
This process occurs in two steps. In the first one, water penetrates the bulk

Hydrolitically
unstable
linkage

￿ ￿ ￿ ￿ ￿

Water
insoluble

Cleavage of backbone linkages
between polymer repeat units

Water
soluble

8.7 Degradation of the polymers by cleavage of hydrolytically
unstable linkages in the polymer backbone, followed by solubilization
of the low molecular weight fragments.