Biomedical Engineering Reference
In-Depth Information
range of the critical axon elongation can accordingly be reported only as upper or
lower bounds.
Once adopted as an estimation method, the procedure described here normalizes
data in the literature and has certain powerful consequences. It enables obtaining
estimates of
L
c
from a large number of publications in the literature where only a
single experimental point (measured
%N
at a selected
L
) has been reported. Use of
this approximation makes available a large database of independent observations
and leads to conclusions about the regenerative activity of devices and the mecha-
nism of regeneration that are worth reporting until the data available in the literature
become more complete. A tabulation of values of critical axon elongation estimated
from diverse data from independent studies appears in Table
6.1
.
We now examine data that were obtained with different species. An analysis
similar to the one described above for the rat leads to the conclusion that mouse
sciatic nerve data from several independent investigators who studied the silicone
tube configuration can also be used to construct a standard curve for the mouse
(Fig.
6.1
a; top, left curve). The mouse data are consistent with a significantly lower
value for critical axon elongation than for the rat, namely,
L
c
= 5.4 ± 1.0 mm. Rat and
mouse data were superposed simply by plotting
%N
data against the reduced length,
L
/
L
c
, the ratio of the gap length,
L
, at which an observation of
%N
was made in a
rodent species, divided by the critical axon elongation,
L
c
, for that species. We ob-
serve that the data superpose (Fig.
6.1
; bottom). The ability to superpose data from
two species suggests that the relation between frequency of reinnervation and gap
length reflects a general phenomenon that may not be species-specific.
Data obtained with different species show large differences (Fig.
6.1
; bot-
tom). The data show that the critical axon elongation for the mouse sciatic
nerve,
L
c
= 5.4 ± 1.0 mm, is significantly lower than that for the rat sciatic nerve,
L
c
= 9.7 ± 1.8. This finding immediately questions in a quantitative fashion the prac-
tice of comparing the effectiveness of nerve chambers that were studied with a
variety of animal species by simply citing the absolute gap length along which
regeneration had been observed. Clearly, a report of an absolute gap length, fre-
quently encountered in the literature, is not meaningful by itself. Data from different
animals can only be compared directly if the disparate anatomical lengths have been
normalized.
There is a further consequence. Division of the gap length,
L
, at which an obser-
vation of
%N
was made for a given species, by the critical axon elongation,
L
c
, for
the same species, to yield the reduced gap length,
L/L
c
, we obtain a single S-shaped
relation between
%N
and gap length for both species (Fig.
6.1
; bottom). The super-
position of data achieved in this manner describes the behavior of the device from
both mouse and rat in a single reduced curve. Although detailed data with other
species were not available when this review was prepared, it is suggested that this
simple scaling law may extend to other species as well. For example, it might be
possible in future studies to compare quantitatively, regeneration data obtained with
a given device in the rat at a 20-mm gap length and that conducted in a monkey or
a human across a 50-mm gap. Such a potential correlation would be very valuable.
