Biomedical Engineering Reference
In-Depth Information
immediately underneath the dermis is a fatty tissue (panniculus adiposus). Located
even deeper underneath the dermis there is, in both rabbit and guinea pig, but in
the sheep as well (Horne et al. 1992), a layer of striped muscle (panniculus carno-
sus) that is responsible for skin-twitching movements. This striped muscle layer
is absent in the human except in the muscles of the jaw and of facial expression
(Billingham and Medawar 1951). Where the role of contraction is dominant, the
contribution of epithelialized scar formation to defect closure is minimal, as indi-
cated in Table
4.3
. Porcine skin is more firmly attached to subdermal tissues than is
true in rodents; in that respect, the porcine skin resembles human skin more closely
(Kangesu et al. 1993a).
In adult human skin defects, contraction appears to contribute approximately
40 % to closure (Ramirez et al. 1969). This experimental result is in qualitative
agreement with the generally accepted clinical reality that contraction plays a much
smaller role in the human than in the rodent. In the human, skin is attached rela-
tively firmly to nondeformable fascia and the latter is tethered to major musculature
or bone; the
panniculus carnosus
is largely missing in the human (Billingham and
Medawar 1951, 1955; Peacock and Van Winkle 1976). Low skin mobility in the
human has important clinical consequences since wounds close to a much larger
extent by scar formation than is true with rodents. Added to the cosmetic deformity,
itself reducing the quality of human life significantly, the limiting effect of a rela-
tively large, stiff scar on joint motion can be occasionally crippling. Examples are
cases where contracting skin edges cause a healed chin to be pulled down to the
chest or where a joint is dislocated from the traction acting on it by the combined
processes of contraction and scar formation (Rudolph et al. 1992). These are the
long-term consequences for the adult human of spontaneous repair of a large-skin
defect by contraction and significant scar formation.
There is probably an extensive variation in the contribution of contraction in
wounds located in different anatomical sites. Few measurements are reported in the
literature. In one case, it has been reported that contraction of excisional wounds in
the human sacrococcygeal pilonidal sinus contributed 88 % to wound closure, while
deposition of scar contributed the remainder (Berry et al. 1998).
Even though closure of a defect in the peripheral nerve has not been studied as
extensively as in skin, the histological observations appear sufficient to describe
approximately each term in the defect closure rule for this organ. For example, fol-
lowing transection of the peripheral nerve, the initial area of the defect is the cross
section of one of the stumps. In a spontaneously healed nerve, contraction induces a
reduction in this area down to approximately 50 % of the initial cross-sectional area
while formation of a neural scar closes up the remaining area (Holmes and Young
1942; Weiss 1944; Weiss and Taylor 1944a; Sunderland 1990; Chamberlain et al.
2000a). The absence of direct quantitative observations in many cases is stressed.
The conjunctiva and the underlying stroma form the smooth tissue layer that
covers the internal surface of the eyelid (Burkitt et al. 1993). Excision of the con-
junctiva and the entire stroma down to the bare sclera is an anatomically well-
defined defect that spontaneously leads to contraction and synthesis of scar
