Biomedical Engineering Reference
In-Depth Information
A crystalline copolymer of LA and ε-caprolactone was reduced to fragments by 2
years (den Dunnen et al. 1993b).
Since the hydrolytic scission of the backbone bonds in these polymers leads to
production of acidic monomers, it is expected that a local inflammatory response
should take place during tube degradation. The intensity of acid release from the
implant and some of its consequences have been studied (Gibson et al. 1991).
Little information has been typically provided in the literature about in situ deg-
radation rates or half-lives of these tubes following implantation in the PNS. The
lack of information makes it difficult to reach definitive conclusions on the effect of
tube degradability in these studies. Another complication stems from the occasional
swelling or cracking of degradable tubes during the study. A third complicating fac-
tor in this group of investigations is introduced by the common lack of negative tube
controls, such as silicone tubes, which makes it difficult to separate effects arising
from tube degradation rate from those due to its chemical composition. Controls
used in this group of studies have been mostly positive (i.e., the autograft and the
normal nerve). It is recalled that the autograft can be harvested from a number of
anatomic locations and is often considered a standardized treatment.
The degradation rate of the tube wall was carefully controlled at three levels,
yielding three different polymers, and was thoroughly characterized in a study of
the 8-mm gap in the rat sciatic nerve (Borkenhagen et al. 1998). The total weight
loss in 24 weeks was 33, 74, and 88 % for the three polymers. No significant dif-
ferences in fidelity of regeneration were observed after 24 weeks between the three
tubes. Although in this study swelling was controlled, degradation led to the devel-
opment of large cracks along the length of all tubes during the first 4 weeks. Tube
cracking eventually causes loss of tubulation, an experimental configuration that
is known to have a profoundly negative effect on regeneration across a gap (see
Table
6.1
). Accordingly, it is pertinent to question whether the regeneration data
in this study should be interpreted entirely in terms of the well-documented differ-
ence in degradation rate among tubes, which appeared to have no direct effect on
regenerative activity; an alternative explanation is based on the observed loss of
tubulation in all three tubes resulting from a tube failure process, which may have
dominated the healing process and may have hypothetically concealed possibly sig-
nificant effects of the degradation rate on regenerative activity.
Tube degradability was considered as a variable in a study of the 6-mm gap in
the mouse sciatic nerve in which tubes made of two biodurable polymers, silicone
and poly(tetrafluorethylene), and two degradable polymers, a copolymer of LA and
ε-caprolactone, and collagen (detailed structural features not specified), were com-
pared (Navarro et al. 1996). The results showed a superiority of the two degradable
tubes in terms of frequency of reinnervation; however, tube degradability per se
was not unambiguously shown to be a contributing factor in the extent of morpho-
logical or functional recovery. It is possible that the extensive data reflect an effect
of chemical composition rather than of degradability of tube on the structural and
functional properties of regenerated nerves.
Equivalence to an autograft control was observed in a number of studies. A tube
fabricated from PGL mesh was observed after 36 weeks to lead to regenerates of
