Biomedical Engineering Reference
In-Depth Information
Observers often describe this failure as a “spontaneous” loss of the graft or as lack
of graft “take” or as a detachment (avulsion).
It has been occasionally suggested that KC sheet grafts have been avulsed after
being displaced by the contracting dermal edges of the defect, especially in rodent
models where contraction is a dominant mode of defect closure (Billingham and
Reynolds 1952; Banks-Schlegel and Green 1980; Ogawa et al. 1990). Data from
two studies can be used to test this suggestion. Both were carried out in swine, a
model in which contraction is a much less dominant mode of defect closure than in
the rodent. In one study, the dermis-free defects were grafted with KC sheets as de-
scribed above (Carver et al. 1993b); in the other, the grafts were placed on a defect
that was prevented from contracting by use of a specially built rigid frame (splint)
(Kangesu et al. 1993b). Extensive mechanical failure of KC sheet grafts was ob-
served in both studies. The data showed that avulsion occurred even in the absence
of contraction. The hypothesis that failure of KC sheet grafts was due to contraction
of defect edges was clearly not supported by the data.
In skin, the BM is located between the epidermis and the dermis, and it is often
modeled as an efficient adhesive layer that keeps the epidermis and dermis (the
adhints, in the analogy of an adhesive joint) securely bonded together. A direct dem-
onstration of the contribution of the BM to the mechanical stability of skin was
made by preparing specimens of the dermis with and without a BM, followed by
incubating KC sheets in contact with these two surfaces. The presence of a BM in
the first group of specimens was confirmed by staining for laminin and type IV
collagen. In this vitro study, when the dermis lacked a BM, the KC sheet could be
pulled away from its surface with negligible force; instead, the KC sheet was torn,
suggesting a strong bond, when it was pulled from a dermis which had a BM (Guo
and Grinnell 1989).
Another hypothesis for failure can be based on the documented inability of KC
sheets to synthesize the undulating BM pattern (rete ridges) that characterizes the
normal epidermis (Carver et al. 1993b). The presence of an intact, extensive rete
ridge pattern has been associated with resistance to shear and peel forces (Brig-
gaman and Wheeler 1975; Lavker 1979). As before, the dermoepidermal junction
in skin is modeled simply as an adhesive joint, in which the BM plays the role of
the adhesive while dermis/epidermis are the two adhints. Other factors remaining
constant, the strength of the adhesive bond increases with the BM surface area (in-
terfacial area for adhesion). This model predicts qualitatively that, in the absence of
rete ridges, the extensive interfacial area of the BM is lowered significantly and the
strength of the adhesive joint is accordingly reduced to the point where mechani-
cal failure occurs much more readily. The readiness with which suction blisters
can be raised on the skin of elderly subjects can be accounted for, according to
this model, by the known absence in these subjects of a rete ridge pattern (Kiistala
1972; Lavker 1979).
In a clinical investigation the structural basis for the fragility of KC sheet grafts
was studied by observing just where the failure occurred following formation of
a standard blister both in the epidermis synthesized by grafting KC sheets and in
normal skin. Blisters formed much more readily in an area grafted with a KC sheet
