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Part 1
PRINCIPLES OF MEDICINE,
SURGERY, AND ANESTHESIA
CHAPTER 1
Wound Healing
Vivek Shetty, DDS, Dr.Med.Dent.
Charles N. Bertolami, DDS, D.Med.Sc.
The healing wound is an overt expression
of an intricate and tightly choreographed
sequence of cellular and biochemical
responses directed toward restoring tissue
integrity and functional capacity following
injury. Although healing culminates
uneventfully in most instances, a variety of
intrinsic and extrinsic factors can impede
or facilitate the process. Understanding
wound healing at multiple levels—bio-
chemical, physiologic, cellular, and molec-
ular—provides the surgeon with a framework
for basing clinical decisions aimed at
optimizing the healing response. Equally
important it allows the surgeon to critically
appraise and selectively use the growing
array of biologic approaches that seek to
assist healing by favorably modulating the
wound microenvironment.
The Healing Process
The restoration of tissue integrity, whether
initiated by trauma or surgery, is a phylogenetically
primitive but essential defense
response. Injured organisms survive only
if they can repair themselves quickly and
effectively. The healing response depends
primarily on the type of tissue involved
and the nature of the tissue disruption.
When restitution occurs by means of tissue
that is structurally and functionally
indistinguishable from native tissue,
regeneration has taken place. However, if
tissue integrity is reestablished primarily
through the formation of fibrotic scar tissue,
then repair has occurred. Repair by
scarring is the body’s version of a spot
weld and the replacement tissue is coarse
and has a lower cellular content than
native tissue. With the exception of bone
and liver, tissue disruption invariably
results in repair rather than regeneration.
At the cellular level the rate and quality
of tissue healing depends on whether
the constitutive cells are labile, stable, or
permanent. Labile cells, including the keratinocytes
of the epidermis and epithelial
cells of the oral mucosa, divide throughout
their life span. Stable cells such as fibroblasts
exhibit a low rate of duplication
but can undergo rapid proliferation in
response to injury. For example, bone
injury causes pluripotential mesenchymal
cells to speedily differentiate into
osteoblasts and osteoclasts. On the other
hand permanent cells such as specialized
nerve and cardiac muscle cells do not
divide in postnatal life. The surgeon’s
expectation of “normal healing” should be
correspondingly realistic and based on the
inherent capabilities of the injured tissue.
Whereas a fibrous scar is normal for skin
wounds, it is suboptimal in the context of
bone healing.
At a more macro level the quality of
the healing response is influenced by the
nature of the tissue disruption and the circumstances
surrounding wound closure.
Healing by first intention occurs when a
clean laceration or surgical incision is
closed primarily with sutures or other
means and healing proceeds rapidly with
no dehiscence and minimal scar formation.
If conditions are less favorable,
wound healing is more complicated and
occurs through a protracted filling of the
tissue defect with granulation and connective
tissue. This process is called healing by
second intention and is commonly associated
with avulsive injury, local infection,
or inadequate closure of the wound. For
more complex wounds, the surgeon may
attempt healing by third intention
through a staged procedure that combines
secondary healing with delayed primary
closure. The avulsive or contaminated
wound is débrided and allowed to granulate
and heal by second intention for 5 to
7 days. Once adequate granulation tissue
has formed and the risk of infection
appears minimal, the wound is sutured
close to heal by first intention.
Wound Healing Response
Injury of any kind sets into motion a complex
series of closely orchestrated and temporally
overlapping processes directed
toward restoring the integrity of the
involved tissue. The reparative processes
are most commonly modeled in skin1;
however, similar patterns of biochemical
and cellular events occur in virtually every
other tissue.2 To facilitate description, the
healing continuum of coagulation, inflammation,
reepithelialization, granulation
Part 1: Principles of Medicine, Surgery, and Anesthesia
tissue, and matrix and tissue remodeling is
typically broken down into three distinct
overlapping phases: inflammatory, proliferative,
and remodeling.3,4
Inflammatory Phase
The inflammatory phase presages the
body’s reparative response and usually
lasts for 3 to 5 days. Vasoconstriction of
the injured vasculature is the spontaneous
tissue reaction to staunch bleeding. Tissue
trauma and local bleeding activate factor
XII (Hageman factor), which initiates the
various effectors of the healing cascade
including the complement, plasminogen,
kinin, and clotting systems. Circulating
platelets (thrombocytes) rapidly aggregate
at the injury site and adhere to each other
and the exposed vascular subendothelial
collagen to form a primary platelet plug
organized within a fibrin matrix. The clot
secures hemostasis and provides a provisional
matrix through which cells can
migrate during the repair process. Additionally
the clot serves as a reservoir of the
cytokines and growth factors that are
released as activated platelets degranulate
(Figure 1-1). The bolus of secreted proteins,
including interleukins, transforming
growth factor ß (TGF-ß), platelet-derived
growth factor (PDGF), and vascular
endothelial growth factor (VEGF), maintain
the wound milieu and regulate subsequent
healing.1
Once hemostasis is secured the reactive
vasoconstriction is replaced by a more
persistent period of vasodilation that is
mediated by histamine, prostaglandins,
kinins, and leukotrienes. Increasing vascular
permeability allows blood plasma and
other cellular mediators of healing to pass
through the vessel walls by diapedesis and
populate the extravascular space. Corresponding
clinical manifestations include
swelling, redness, heat, and pain.
Cytokines released into the wound provide
the chemotactic cues that sequentially
recruit the neutrophils and monocytes
to the site of injury. Neutrophils normally
begin arriving at the wound site within
minutes of injury and rapidly establish
themselves as the predominant cells.
Migrating through the scaffolding provided
by the fibrin-enriched clot, the shortlived
leukocytes flood the site with proteases
and cytokines to help cleanse the
wound of contaminating bacteria, devitalized
tissue, and degraded matrix components.
Neutrophil activity is accentuated
by opsonic antibodies leaking into the
wound from the altered vasculature.
Unless a wound is grossly infected, neutrophil
infiltration ceases after a few days.
However, the proinflammatory cytokines
released by perishing neutrophils, including
tumor necrosis factor a (TNF-a) and
interleukins (IL-1a, IL-1b), continue to
stimulate the inflammatory response for
extended periods.5
Deployment of bloodborne monocytes
to the site of injury starts peaking as
the levels of neutrophils decline. Activated
monocytes, now termed macrophages,
continue with the wound microdébridement
initiated by the neutrophils. They
secrete collagenases and elastases to break
down injured tissue and phagocytose bacteria
and cell debris. Beyond their scavenging
role the macrophages also serve as the
primary source of healing mediators.
Once activated, macrophages release a battery
of growth factors and cytokines
(TGF-a,TGF-ß1, PDGF, insulin-like
growth factor [IGF]-I and -II, TNF-a, and
IL-1) at the wound site, further amplifying
and perpetuating the action of the chemical
and cellular mediators released previously
by degranulating platelets and neu-
trophils.6 Macrophages influence all
phases of early wound healing by regulating
local tissue remodeling by proteolytic
enzymes (eg, matrix metalloproteases and
collagenases), inducing formation of new
extracellular matrix, and modulating
angiogenesis and fibroplasia through local
production of cytokines such as throm-
bospondin-1 and IL-1b. The centrality of
Platelet plug
FGF-2
MMP
PDGFPDGF
TGF-3
TGF-2
TGF-1
TGF-1
TGF-1
Macrophage
Fibrin clot
Growth
factors
Blood vessel
Blood vessel
Epidermis
Epidermis
Dermis
Dermis Fibroblast
Fibroblast
Fat
FIGURE 1-1 Immediately following wounding, platelets facilitate the formation of a blood clot that secures
hemostasis and provides a temporary matrix for cell migration. Cytokines released by activated macrophages
and fibroblasts initiate the formation of granulation tissue by degrading extracellular matrix and promoting
development of new blood vessels. Cellular interactions are potentiated by reciprocal signaling between
the epidermis and dermal fibroblasts through growth factors, MMPs, and members of the TGF-ßfamily.
FGF = fibroblast growth factor; MMP = matrix metalloproteinase; PDGF = platelet-derived growth factor;
TGF-ß= transforming growth factor beta. Adapted from Bissell MJ and Radisky D.70
macrophage function to early wound healing
is underscored by the consistent finding
that macrophage-depleted animal
wounds demonstrate diminished fibroplasia
and defective repair. Although the
numbers and activity of the macrophages
taper off by the fifth post injury day, they
continue to modulate the wound healing
process until repair is complete.
Proliferative Phase
The cytokines and growth factors secreted
during the inflammatory phase stimulate
the succeeding proliferative phase (Figure
1-2).7 Starting as early as the third day post
injury and lasting up to 3 weeks, the proliferative
phase is distinguished by the formation
of pink granular tissue (granulation
tissue) containing inflammatory cells,
fibroblasts, and budding vasculature
enclosed in a loose matrix. An essential
first step is the establishment of a local
microcirculation to supply the oxygen and
nutrients necessary for the elevated metabolic
needs of regenerating tissues. The
generation of new capillary blood vessels
(angiogenesis) from the interrupted vasculature
is driven by wound hypoxia as
well as with native growth factors, particularly
VEGF, fibroblast growth factor 2
(FGF-2), and TNF-ß (see Figure 1-2).
Around the same time, matrix-generating
fibroblasts migrate into the wound in
response to the cytokines and growth factors
released by inflammatory cells and
wounded tissue. The fibroblasts start synthesizing
new extracellular matrix (ECM)
and immature collagen (Type III). The
scaffold of collagen fibers serves to support
the newly formed blood vessels supplying
the wound. Stimulated fibroblasts
also secrete a range of growth factors,
thereby producing a feedback loop and
sustaining the repair process. Collagen
deposition rapidly increases the tensile
strength of the wound and decreases the
reliance on closure material to hold the
wound edges together. Once adequate collagen
and ECM have been generated,
Wound Healing
MMPs
Fibrin clot
Blood vessel
Blood vessel
Epidermis
Epidermis
Dermis
Dermis
Fibroblast
Fat
u-PA
t-PA
FIGURE 1-2 The cytokine cascade mediates the succedent proliferative phase. This phase is distinguished
by the establishment of local microcirculation and formation of extracellular matrix and
immature collagen. Epidermal cells migrate laterally below the fibrin clot, and granulation tissue
begins to organize below the epithelium. MMPs = matrix metalloproteinases; t-PA = tissue plasminogen
activator; u-PA = urinary plasminogen activator. Adapted from Bissell MJ and Radisky D.70
matrix synthesis dissipates, evidencing the
highly precise spatial and temporal regulation
of normal healing.
At the surface of the dermal wound
new epithelium forms to seal off the
denuded wound surface. Epidermal cells
originating from the wound margins
undergo a proliferative burst and begin to
resurface the wound above the basement
membrane. The process of reepithelialization
progresses more rapidly in oral
mucosal wounds in contrast to the skin.
In a mucosal wound the epithelial cells
migrate directly onto the moist exposed
surface of the fibrin clot instead of under
the dry exudate (scab) of the dermis.
Once the epithelial edges meet, contact
inhibition halts further lateral proliferation.
Reepithelialization is facilitated by
underlying contractile connective tissue,
which shrinks in size to draw the wound
margins toward one another. Wound contraction
is driven by a proportion of the
fibroblasts that transform into myofibroblasts
and generate strong contractile
forces. The extent of wound contraction
depends on the depth of the wound and
its location. In some instances the forces
of wound contracture are capable of
deforming osseous structures.
Remodeling Phase
The proliferative phase is progressively
replaced by an extended period of progressive
remodeling and strengthening of
the immature scar tissue. The remodel-
ing/maturation phase can last for several
years and involves a finely choreographed
balance between matrix degradation and
formation. As the metabolic demands of
the healing wound decrease, the rich network
of capillaries begins to regress.
Under the general direction of the
cytokines and growth factors, the collagenous
matrix is continually degraded,
resynthesized, reorganized, and stabilized
by molecular crosslinking into a scar. The
fibroblasts start to disappear and the collagen
Type III deposited during the granulation
phase is gradually replaced by
stronger Type I collagen. Correspondingly
the tensile strength of the scar tissue
Part 1: Principles of Medicine, Surgery, and Anesthesia
gradually increases and eventually
approaches about 80% of the original
strength. Homeostasis of scar collagen and
ECM is regulated to a large extent by serine
proteases and matrix metalloproteinases
(MMPs) under the control of the
regulatory cytokines. Tissue inhibitors of
the MMPS afford a natural counterbalance
to the MMPs and provide tight control
of proteolytic activity within the scar.
Any disruption of this orderly balance can
lead to excess or inadequate matrix degradation
and result in either an exuberant
scar or wound dehiscence.
Specialized Healing
Nerve
Injury to the nerves innervating the orofacial
region may range from simple contusion
to complete interruption of the nerve.
The healing response depends on injury
severity and extent of the injury.8–10 Neuropraxia
represents the mildest form of
nerve injury and is a transient interruption
of nerve conduction without loss of
axonal continuity. The continuity of the
epineural sheath and the axons is maintained
and morphologic alterations are
minor. Recovery of the functional deficit is
spontaneous and usually complete within
3 to 4 weeks. If there is a physical disruption
of one or more axons without injury
to stromal tissue, the injury is described as
axonotmesis. Whereas individual axons
are severed, the investing Schwann cells
and connective tissue elements remain
intact. The nature and extent of the ensuing
sensory or motor deficit relates to the
number and type of injured axons. Morphologic
changes are manifest as degeneration
of the axoplasm and associated
structures distal to the site of injury and
partly proximal to the injury. Recovery of
the functional deficit depends on the
degree of the damage.
Complete transection of the nerve
trunk is referred to as neurotmesis and
spontaneous recovery from this type of
injury is rare. Histologically, changes of
degeneration are evident in all axons adjacent
to the site of injury.11 Shortly after
nerve severance, the investing Schwann
cells begin to undergo a series of cellular
changes called wallerian degeneration.
The degeneration is evident in all axons of
the distal nerve segment and in a few
nodes of the proximal segment. Within
78 hours injured axons start breaking
up and are phagocytosed by adjacent
Schwann cells and by macrophages that
migrate into the zone of injury. Once the
axonal debris has been cleared, Schwann
cell outgrowths attempt to connect the
proximal stump with the distal nerve
stump. Surviving Schwann cells proliferate
to form a band (Büngner’s band) that
will accept regenerating axonal sprouts
from the proximal stump. The proliferating
Schwann cells also promote nerve
regeneration by secreting numerous neurotrophic
factors that coordinate cellular
repair as well as cell adhesion molecules
that direct axonal growth. In the absence
of surgical realignment or approximation
of the nerve stumps, proliferating
Schwann cells and outgrowing axonal
sprouts may align within the randomly
organized fibrin clot to form a disorganized
mass termed neuroma.
The rate and extent of nerve regeneration
depend on several factors including
type of injury, age, state of tissue nutrition,
and the nerves involved. Although
the regeneration rate for peripheral nerves
varies considerably, it is generally considered
to approximate 1 mm/d. The regeneration
phase lasts up to 3 months and
ends on contact with the end-organ by a
thin myelinated axon. In the concluding
maturation phase both the diameter and
performance of the regenerating nerve
fiber increase.
Bone
The process of bone healing after a fracture
has many features similar to that of skin
healing except that it also involves calcification
of the connective tissue matrix. Bone is
a biologically privileged tissue in that it
heals by regeneration rather than repair.
Left alone, fractured bone is capable of
restoring itself spontaneously through
sequential tissue formation and differentiation,
a process also referred to as indirect
healing. As in skin the interfragmentary
thrombus that forms shortly after injury
staunches bleeding from ruptured vessels in
the haversian canals, marrow, and periosteum.
Necrotic material at the fracture site
elicits an immediate and intense acute
inflammatory response which attracts the
polymorphonuclear leukocytes and subsequently
macrophages to the fracture site.
The organizing hematoma serves as a fibrin
scaffold over which reparative cells can
migrate and perform their function. Invading
inflammatory cells and the succeeding
pluripotential mesenchymal cells begin to
rapidly produce a soft fracture callus that
fills up interfragmentary gaps. Comprised
of fibrous tissue, cartilage, and young
immature fiber bone, the soft compliant
callus acts as a biologic splint by binding
the severed bone segments and damping
interfragmentary motion. An orderly progression
of tissue differentiation and maturation
eventually leads to fracture consolidation
and restoration of bone continuity.
More commonly the surgeon chooses
to facilitate an abbreviated callus-free
bone healing termed direct healing (Figure
1-3). The displaced bone segments are surgically
manipulated into an acceptable
alignment and rigidly stabilized through
the use of internal fixation devices. The
resulting anatomic reduction is usually a
combination of small interfragmentary
gaps separated by contact areas. Ingrowth
of mesenchymal cells and blood vessels
starts shortly thereafter, and activated
osteoblasts start depositing osteoid on the
surface of the fragment ends. In contact
zones where the fracture ends are closely
apposed, the fracture line is filled concentrically
by lamellar bone. Larger gaps are
filled through a succession of fibrous
Wound Healing7tissue,fibrocartilage,and woven bone.Inthe absence ofany microinstability at thefracture site,direct healing takes placewithout any callus formation.
Subsequent bone remodeling eventual-
ly restores the original shape and internalarchitecture ofthe fractured bone.Func-
tional sculpting and remodeling oftheprimitive bone tissue is carried out by atemporary team ofjuxtaposed osteoclastsand osteoblasts called the basic multicellu-
lar unit (BMU).The osteoblasts developfrom pluripotent mesenchymal stem cellswhereas multicellular osteoclasts arise froma monocyte/macrophage lineage.12Thedevelopment and differentiation oftheBMUs are controlled by locally secretedgrowth factors,cytokines,and mechanicalsignals.As osteoclasts at the leading edge ofthe BMUs excavate bone through prote-
olytic digestion,active osteoblasts move in,
secreting layers ofosteoid and slowly refill-
ing the cavity.The osteoid begins to miner-
alize when it is about 6 µm thick.Osteo-
clasts reaching the end oftheir lifespan of2 weeks die and are removed by phagocytes.
The majority (up to 65%) ofthe remodel-
ing osteoblasts also die within 3 monthsand the remainder are entombed inside themineralized matrix as osteocytes.
While the primitive bone mineralizes,
remodeling BMUs cut their way throughthe reparative tissue and replace it withmature bone.The “grain”ofthe new bonetissue starts paralleling local compressionand tension strains.Consequently theshape and strength ofthe reparative bonetissue changes to accommodate greaterfunctional loading.Tissue-level strainsproduced by functional loading play animportant role in the remodeling oftheregenerate bone.Whereas low levels oftis-
sue strain (~2,000 microstrains) are con-
sidered physiologic and necessary for celldifferentiation and callus remodeling,
high strain levels (> 2,000 microstrains)
begin to adversely affect osteoblastic dif-
ferentiation and bone matrix forma-
tion.13,14Ifthere is excess interfragmentarymotion,bone regenerates primarilythrough endochondral ossification or theformation ofa cartilaginous callus that isgradually replaced by new bone.In con-
trast osseous healing across stabilized frac-
ture segments occurs primarily throughintramembranous ossification.Major fac-
tors determining the mechanical milieu ofa healing fracture include the fracture con-
figuration,the accuracy offracture reduc-
tion,the stability afforded by the selectedfixation device,and the degree and natureofmicrostrains provoked by function.Ifafracture fixation device is incapable ofsta-
bilizing the fracture,the interfragmentarymicroinstability provokes osteoclasticresorption ofthe fracture surfaces andresults in a widening ofthe fracture gap.
Although bone union may be ultimatelyachieved through secondary healing bycallus production and endochondral ossi-
fication,the healing is protracted.Fibroushealing and nonunions are clinical mani-
festations ofexcessive microstrains inter-
fering with the cellular healing process.
Extraction Wounds
The healing ofan extraction socket is a spe-
cialized example ofhealing by secondintention.15Immediately after the removalofthe tooth from the socket,blood fills theextraction site.Both intrinsic and extrinsicpathways ofthe clotting cascade are activat-
ed.The resultant fibrin meshwork contain-
ing entrapped red blood cells seals offtheGap healingContact healingOsteoblastBasic multicellular unit
OsteoclastOsteocyteBlood vesselFIGURE1-3Direct bone healing facilitated by a lag screw.The fracture site shows both gap healing and contact healing.The internal archi-
tecture ofbone is restored eventually by the action ofbasic multicellular units.
Part 1: Principles of Medicine, Surgery, and Anesthesia
torn blood vessels and reduces the size of
the extraction wound. Organization of the
clot begins within the first 24 to 48 hours
with engorgement and dilation of blood
vessels within the periodontal ligament
remnants, followed by leukocytic migration
and formation of a fibrin layer. In the first
week the clot forms a temporary scaffold
upon which inflammatory cells migrate.
Epithelium at the wound periphery grows
over the surface of the organizing clot.
Osteoclasts accumulate along the alveolar
bone crest setting the stage for active crestal
resorption. Angiogenesis proceeds in the
remnants of the periodontal ligaments. In
the second week the clot continues to get
organized through fibroplasia and new
blood vessels that begin to penetrate
towards the center of the clot. Trabeculae of
osteoid slowly extend into the clot from the
alveolus, and osteoclastic resorption of the
cortical margin of the alveolar socket is
more distinct. By the third week the extraction
socket is filled with granulation tissue
and poorly calcified bone forms at the
wound perimeter. The surface of the
wound is completely reepithelialized with
minimal or no scar formation. Active bone
remodeling by deposition and resorption
continues for several more weeks. Radiographic
evidence of bone formation does
not become apparent until the sixth to
eighth weeks following tooth extraction.
Due to the ongoing process of bone remodeling
the final healing product of the
extraction site may not be discernible on
radiographs after 4 to 6 months.
Occasionally the blood clot fails to
form or may disintegrate, causing a localized
alveolar osteitis. In such instances
healing is delayed considerably and the
socket fills gradually. In the absence of a
healthy granulation tissue matrix, the
apposition of regenerate bone to remaining
alveolar bone takes place at a much
slower rate. Compared to a normal socket
the infected socket remains open or partially
covered with hyperplastic epithelium
for extended periods.
Skin Grafts
Skin grafts may be either full thickness or
split thickness.16 A full-thickness graft is
composed of epidermis and the entire dermis;
a split-thickness graft is composed of
the epidermis and varying amounts of dermis.
Depending on the amount of underlying
dermis included, split-thickness grafts
are described as thin, intermediate, or
thick.17 Following grafting, nutritional support
for a free skin graft is initially provided
by plasma that exudes from the dilated capillaries
of the host bed. A fibrin clot forms at
the graft-host interface, fixing the graft to
the host bed. Host leukocytes infiltrate into
the graft through the lower layers of the
graft. Graft survival depends on the
ingrowth of blood vessels from the host into
the graft (neovascularization) and direct
anastomoses between the graft and the host
vasculature (inosculation). Endothelial capillary
buds from the host site invade the
graft, reaching the dermoepidermal junction
by 48 hours. Concomitantly vascular
connections are established between host
and graft vessels. However, only a few of the
ingrowing capillaries succeed in developing
a functional anastomosis. Formation of vascular
connections between the recipient bed
and transplant is signaled by the pink
appearance of the graft, which appears
between the third and fifth day postgrafting.
Fibroblasts from the recipient bed
begin to invade the layer of fibrin and
leukocytes by the fourth day after transplantation.
The fibrin clot is slowly
resorbed and organized as fibroblastic
infiltration continues. By the ninth day the
new blood vessels and fibroblasts have
achieved a firm union, anchoring the deep
layers of the graft to the host bed.
Reinnervation of the skin graft occurs
by nerve fibers entering the graft through
its base and sides. The fibers follow the
vacated neurilemmal cell sheaths to reconstruct
the innervation pattern of the
donor skin. Recovery of sensation usually
begins within 2 months after transplanta-
tion.Grafts rarely attain the sensory
qualities of normal skin, because the
extent of re-innervation depends on how
accessible the neurilemmal sheaths are to
the entering nerve fibers. The clinical
performance of the grafts depends on
their relative thickness. As split-thickness
grafts are thinner than full-thickness
grafts, they are susceptible to trauma and
undergo considerable contraction; however,
they have greater survival rates clinically.
Full-thickness skin grafts do not
“take” as well and are slow to revascularize.
Nevertheless full-thickness grafts are
less susceptible to trauma and undergo
minimal shrinkage.
Wound Healing Complications
Healing in the orofacial region is often
considered a natural and uneventful
process and seldom intrudes into the sur-
geon’s consciousness. However, this
changes when complications arise and
encumber the wound healing continuum.
Most wound healing complications manifest
in the early postsurgical period
although some may manifest much later.
The two problems most commonly
encountered by the surgeon are wound
infection and dehiscence; proliferative
healing is less typical.
Wound Infection
Infections complicating surgical outcomes
usually result from gross bacterial contamination
of susceptible wounds. All wounds
are intrinsically contaminated by bacteria;
however, this must be distinguished from
true wound infection where the bacterial
burden of replicating microorganisms
actually impairs healing.18 Experimental
studies have demonstrated that regardless
of the type of infecting microorganism,
wound infection occurs when there are
more than 1 × 105 organisms per gram of
tissue.19,20 Beyond relative numbers, the
pathogenicity of the infecting microorganisms
as well as host response factors determine
whether wound healing is impaired.
The continual presence of a bacterial
infection stimulates the host immune
defenses leading to the production of
inflammatory mediators, such as
prostaglandins and thromboxane. Neutrophils
migrating into the wound release
cytotoxic enzymes and free oxygen radicals.
Thrombosis and vasoconstrictive
metabolites cause wound hypoxia, leading
to enhanced bacterial proliferation and
continued tissue damage. Bacteria
destroyed by host defense mechanisms
provoke varying degrees of inflammation
by releasing neutrophil proteases and
endotoxins. Newly formed cells and their
collagen matrix are vulnerable to these
breakdown products of wound infection,
and the resulting cell and collagen lysis
contribute to impaired healing. Clinical
manifestations of wound infection include
the classic signs and symptoms of local
infection: erythema, warmth, swelling,
pain, and accompanying odor and pus.
Inadequate tissue perfusion and oxygenation
of the wound further compromise
healing by allowing bacteria to proliferate
and establish infection. Failure to
follow aseptic technique is a frequent reason
for the introduction of virulent
microorganisms into the wound. Transformation
of contaminated wounds into
infected wounds is also facilitated by
excessive tissue trauma, remnant necrotic
tissue, foreign bodies, or compromised
host defenses. The most important factor
in minimizing the risk of infection is
meticulous surgical technique, including
thorough débridement, adequate hemostasis,
and elimination of dead space.
Careful technique must be augmented by
proper postoperative care, with an emphasis
on keeping the wound site clean and
protecting it from trauma.
Wound Dehiscence
Partial or total separation of the wound
margins may manifest within the first
week after surgery. Most instances of
wound dehiscence result from tissue failure
rather than improper suturing techniques.
The dehisced wound may be
closed again or left to heal by secondary
intention, depending upon the extent of
the disruption and the surgeon’s assessment
of the clinical situation.
Proliferative Scarring
Some patients may go on to develop aberrant
scar tissue at the site of their skin
injury. The two common forms of hyperproliferative
healing, hypertrophic scars
and keloids, are characterized by hypervascularity
and hypercellularity. Distinctive
features include excessive scarring,
persistent inflammation, and an overproduction
of extracellular matrix components,
including glycosaminoglycans and
collagen Type I.21 Despite their overt
resemblance, hypertrophic scars and
keloids do have some clinical dissimilarities.
In general, hypertrophic scars arise
shortly after the injury, tend to be circumscribed
within the boundaries of the
wound, and eventually recede. Keloids, on
the other hand, manifest months after the
injury, grow beyond the wound boundaries,
and rarely subside. There is a clear
familial and racial predilection for keloid
formation, and susceptible individuals
usually develop keloids on their face, ear
lobes, and anterior chest.
Although processes leading to hypertrophic
scar and keloid formation are not
yet clarified, altered apoptotic behavior is
believed to be a significant factor. Ordinarily,
apoptosis or programmed cell death is
responsible for the removal of inflammatory
cells as healing proceeds and for the evolution
of granulation tissue into scar. Dysregulation
in apoptosis results in excessive
scarring, inflammation, and an overproduction
of extracellular matrix components.
Both keloids and hypertrophic scars
demonstrate sustained elevation of growth
factors including TGF-ß , platelet-derived
growth factor, IL-1, and IGF-I.22 The
growth factors, in turn, increase the numbers
of local fibroblasts and prompt exces-
Wound Healing
sive production of collagen and extracellular
matrix. Additionally, proliferative scar
tissue exhibits increased numbers of
neoangiogenesis-promoting vasoactive
mediators as well as histamine-secreting
mast cells capable of stimulating fibrous
tissue growth. Although there is no effective
therapy for keloids, the more common
methods for preventing or treating these
lesions focus on inhibiting protein synthesis.
These agents, primarily corticosteroids,
are injected into the scar to decrease
fibroblast proliferation, decrease angiogenesis,
and inhibit collagen synthesis and
extracellular matrix protein synthesis.
Optimizing Wound Healing
At its very essence the wound represents
an extreme disruption of the cellular
microenvironment. Restoration of constant
internal conditions or homeostasis at
the cellular level is a constant undertow of
the healing response. A variety of local and
systemic factors can impede healing, and
the informed surgeon can anticipate and,
where possible, proactively address these
barriers to healing so that wound repair
can progress normally.23
Tissue Trauma
Minimizing surgical trauma to the tissues
helps promote faster healing and should
be a central consideration at every stage of
the surgical procedure, from placement of
the incision to suturing of the wound.
Properly planned, the surgical incision is
just long enough to allow optimum exposure
and adequate operating space. The
incision should be made with one clean
consistent stroke of evenly applied pressure.
Sharp tissue dissection and carefully
placed retractors further minimize tissue
injury. Sutures are useful for holding the
severed tissues in apposition until the
wound has healed enough. However,
sutures should be used judiciously as they
have the ability to add to the risk of infection
and are capable of strangulating the
tissues if applied too tightly.
Part 1: Principles of Medicine, Surgery, and Anesthesia
Hemostasis and Wound
Débridement
Bleeding from a transected vessel or diffuse
oozing from the denuded surfaces
interfere with the surgeon’s view of underlying
structures. Achieving complete
hemostasis before wound closure helps
prevent the formation of a hematoma
postoperatively. The collection of blood or
serum at the wound site provides an ideal
medium for the growth of microorganisms
that cause infection. Additionally,
hematomas can result in necrosis of overlying
flaps. However, hemostatic techniques
must not be used too aggressively
during surgery as the resulting tissue damage
can prolong healing time. Postoperatively
the surgeon may insert a drain or
apply a pressure dressing to help eliminate
dead space in the wound.
Devitalized tissue and foreign bodies
in a healing wound act as a haven for bacteria
and shield them from the body’s
defenses.23 The dead cells and cellular
debris of necrotic tissue have been shown
to reduce host immune defenses and
encourage active infection. A necrotic burden
allowed to persist in the wound can
prolong the inflammatory response,
mechanically obstruct the process of
wound healing, and impede reepithelialization.
Dirt and tar located in traumatic
wounds not only jeopardize healing but
may result in a “tattoo” deformity. By
removing dead and devitalized tissue, and
any foreign material from a wound,
débridement helps to reduce the number
of microbes, toxins, and other substances
that inhibit healing. The surgeon should
also keep in mind that prosthetic grafts
and implants, despite refinements in biocompatibility,
can incite varying degrees of
foreign body reaction and adversely
impact the healing process.
Tissue Perfusion
Poor tissue perfusion is one of the main
barriers to healing inasmuch as tissue
oxygen tension drives the healing
response.24,25 Oxygen is necessary for
hydroxylation of proline and lysine, the
polymerization and cross-linking of procollagen
strands, collagen transport,
fibroblast and endothelial cell replication,
effective leukocyte killing, angiogenesis,
and many other processes. Relative hypoxia
in the region of injury stimulates a
fibroblastic response and helps mobilize
other cellular elements of repair.26 However,
very low oxygen levels act together with
the lactic acid produced by infecting bacteria
to lower tissue pH and contribute to
tissue breakdown. Cell lysis follows, with
releases of proteases and glycosidases and
subsequent digestion of extracellular
matrix.27 Impaired local circulation also
hinders delivery of nutrients, oxygen, and
antibodies to the wound. Neutrophils are
affected because they require a minimal
level of oxygen tension to exert their bactericidal
effect. Delayed movement of neutrophils,
opsonins, and the other mediators
of inflammation to the wound site
further diminishes the effectiveness of the
phagocytic defense system and allows colonizing
bacteria to proliferate. Collagen
synthesis is dependent on oxygen delivery
to the site, which in turn affects wound
tensile strength. Most healing problems
associated with diabetes mellitus, irradiation,
small vessel atherosclerosis, chronic
infection, and altered cardiopulmonary
status can be attributed to local tissue
ischemia.
Wound microcirculation after surgery
determines the wound’s ability to resist the
inevitable bacterial contamination.28 Tissue
rendered ischemic by rough handling, or
desiccated by cautery or prolonged air drying,
tends to be poorly perfused and susceptible
to infection. Similarly, tissue
ischemia produced by tight or improperly
placed sutures, poorly designed flaps, hypovolemia,
anemia, and peripheral vascular
disease, all adversely affect wound healing.
Smoking is a common contributor to
decreased tissue oxygenation.29 After every
cigarette the peripheral vasoconstriction
can last up to an hour; thus, a pack-a-day
smoker remains tissue hypoxic for most
part of each day. Smoking also increases
carboxyhemoglobin, increases platelet
aggregation, increases blood viscosity,
decreases collagen deposition, and decreases
prostacyclin formation, all of which negatively
affect wound healing. Patient optimization,
in the case of smokers, may
require that the patient abstain from smoking
for a minimum of 1 week before and
after surgical procedures. Another way of
improving tissue oxygenation is the use of
systemic hyperbaric oxygen (HBO) therapy
to induce the growth of new blood vessels
and facilitate increased flow of oxygenated
blood to the wound.
Diabetes
Numerous studies have demonstrated that
the higher incidence of wound infection
associated with diabetes has less to do with
the patient having diabetes and more to do
with hyperglycemia. Simply put, a patient
with well-controlled diabetes may not be
at a greater risk for wound healing problems
than a nondiabetic patient. Tissue
hyperglycemia impacts every aspect of
wound healing by adversely affecting the
immune system including neutrophil and
lymphocyte function, chemotaxis, and
phagocytosis.30 Uncontrolled blood glucose
hinders red blood cell permeability
and impairs blood flow through the critical
small vessels at the wound surface. The
hemoglobin release of oxygen is impaired,
resulting in oxygen and nutrient deficiency
in the healing wound. The wound
ischemia and impaired recruitment of
cells resulting from the small vessel occlusive
disease renders the wound vulnerable
to bacterial and fungal infections.
Immunocompromise
The immune response directs the healing
response and protects the wound from
infection. In the absence of an adequate
immune response, surgical outcomes are
often compromised. An important assessment
parameter is total lymphocyte count.
A mild deficit is a lymphocytic level
between 1,200 and 1,800, and levels below
800 are considered severe total lymphocyte
deficits. Patients with debilitated immune
response include human immunodeficiency
virus (HIV)-infected patients in
advanced stages of the disease, patients on
immunosuppressive therapy, and those
taking high-dose steroids for extended
periods.31 Studies indicate that HIVinfected
patients with CD4 counts of less
than 50 cells/mm3 are at significant risk of
poor wound outcome.32 Although newer
immunosuppressive drugs, such as
cyclosporine, have no apparent effect on
wound healing, other medications can
retard the healing process both in rate and
quality by altering both the inflammatory
reaction and the cell metabolism.
The use of steroids, such as prednisone,
is a typical example of how suppression of
the innate inflammatory process also
increases wound healing complications.
Exogenous corticosteroids diminish prolyl
hydroxylase and lysyl oxidase activity,
depressing fibroplasias, collagen formation,
and neovascularity.33 Fibroblasts reach the
site in a delayed fashion and wound strength
is decreased by as much as 30%. Epithelialization
and wound contraction are also
impaired. The inhibitory effects of glucocorticosteriods
can be attenuated to some
extent by vitamin A given concurrently.
Most antineoplastic agents exert their
cytotoxic effect by interfering with DNA
or RNA production. The reduction in protein
synthesis or cell division reveals itself
as impaired proliferation of fibroblasts
and collagen formation. Attendant neutropenia
also predisposes to wound infection
by prolonging the inflammatory
phase of wound healing. Because of their
deleterious effect on wound healing,
administration of antineoplastic drugs
should be restricted, when possible, until
such time that the potential for healing
complications has passed.
Radiation Injury
Therapeutic radiation for head and neck
tumors inevitably produces collateral
damage in adjacent tissue and reduces its
capacity for regeneration and repair. The
pathologic processes of radiation injury
start right away; however, the clinical and
histologic features may not become apparent
for weeks, months, or even years after
treatment.34 The cellular and molecular
responses to tissue irradiation are immediate,
dose dependent, and can cause both
early and late consequences.35 DNA damage
from ionizing radiation leads to mitotic
cell death in the first cell division after
irradiation or within the first few divisions.
Early acute changes are observed
within a few weeks of treatment and primarily
involve cells with a high turnover
rate. The common symptoms of oral
mucositis and dermatitis result from loss
of functional cells and temporary lack of
replacement from the pools of rapidly
proliferating cells. The inflammatory
response is largely mediated by cytokines
activated by the radiation injury. Overall
the response has the features of wound
healing; waves of cytokines are produced
in an attempt to heal the radiation injury.
The cytokines lead to an adaptive response
in the surrounding tissue, cause cellular
infiltration, and promote collagen deposition.
Damage to local vasculature is exacerbated
by leukocyte adhesion to endothelial
cells and the formation of thrombi that
block the vascular lumen, further depriving
the cells that depend on the vessels.
The acute symptoms eventually start
to subside as the constitutive cells gradually
recover their proliferative abilities.
However, these early symptoms may not
be apparent in some tissues such as bone,
where cumulative progressive effects of
radiation can precipitate acute breakdown
of tissue many years after therapy. The late
effects of radiation are permanent and
directly related to higher doses.36 Collagen
hyalinizes and the tissues become increas-
Wound Healing
ingly fibrotic and hypoxic due to obliterative
vasculitis, and the tissue susceptibility
to infection increases correspondingly.
Once these changes occur they are irreversible
and do not change with time.
Hence, the surgeon must always anticipate
the possibility of a complicated healing
following surgery or traumatic injury in
irradiated tissue. Wound dehiscence is
common and the wound heals slowly or
incompletely. Even minor trauma may
result in ulceration and colonization by
opportunistic bacteria. If the patient cannot
mount an effective inflammatory
response, progressive necrosis of the tissues
may follow. Healing can be achieved
only by excising all nonvital tissue and
covering the bed with a well-vascularized
graft. Due to the relative hypoxia at the
irradiated site, tissue with intact blood
supply needs to be brought in to provide
both oxygen and the cells necessary for
inflammation and healing. The progressive
obliteration of blood vessels makes
bone particularly vulnerable. Following
trauma or disintegration of the soft tissue
cover due to inflammatory reaction, healing
does not occur because irradiated
marrow cannot form granulation tissue.
In such instances the avascular bone needs
to be removed down to the healthy portion
to allow healing to proceed.
Hyperbaric Oxygen Therapy
HBO therapy is based on the concept that
low tissue oxygen tension, typically a partial
pressure of oxygen (PO2) of 5 to
20 mm Hg, leads to anaerobic cellular
metabolism, increase in tissue lactate, and
a decrease in pH, all of which inhibit
wound healing.64 HBO therapy entails the
patient lying in a hyperbaric chamber
and breathing 100% oxygen at 2.0 to
2.4 atmospheres for 1 to 2 hours. The
HBO therapy is repeated daily for 3 to
10 weeks. HBO increases the quantity of
dissolved oxygen and the driving pressure
for oxygen diffusion into the tissue. Correspondingly
the oxygen diffusion distance
Part 1: Principles of Medicine, Surgery, and Anesthesia
is increased threefold to fourfold, and
wound PO2 ultimately reaches 800 to
1,100 mm Hg. The therapy stimulates the
growth of fibroblasts and vascular
endothelial cells, increases tissue vascularization,
enhances the killing ability of
leukocytes, and is lethal for anaerobic bacteria.
Clinical studies suggest that HBO
therapy can be an effective adjunct in the
management of diabetic wounds.65 Animal
studies indicate that HBO therapy could be
beneficial in the treatment of osteomyelitis
and soft tissue infections.66,67 Adverse
effects of HBO therapy are barotraumas of
the ear, seizure, and pulmonary oxygen
toxicity. However, in the absence of controlled
scientific studies with well-defined
end points, HBO therapy remains a controversial
aspect of surgical practice.68,69
Age
In general wound healing is faster in the
young and protracted in the elderly. The
decline in healing response results from
the gradual reduction of tissue metabolism
as one ages, which may itself be a
manifestation of decreased circulatory
efficiency. The major components of the
healing response in aging skin or mucosa
are deficient or damaged with progressive
injuries.37 As a result, free oxidative radicals
continue to accumulate and are harmful
to the dermal enzymes responsible for
the integrity of the dermal or mucosal
composition. In addition the regional vascular
support may be subjected to extrinsic
deterioration and systemic disease
decompensation, resulting in poor perfusion
capability.38 However, in the absence
of compromising systemic conditions, differences
in healing as a function of age
seem to be small.
Nutrition
Adequate nutrition is important for normal
repair.39 In malnourished patients
fibroplasia is delayed, angiogenesis
decreased, and wound healing and remodeling
prolonged. Dietary protein has
received special emphasis with respect to
healing. Amino acids are critical for wound
healing with methionine, histidine, and
arginine playing important roles. Nutritional
deficiencies severe enough to lower
serum albumin to < 2 g/dL are associated
with a prolonged inflammatory phase,
decreased fibroplasia, and impaired neovascularization,
collagen synthesis, and
wound remodeling. As long as a state of
protein catabolism exists, the wound will
be very slow to heal. Methionine appears to
be the key amino acid in wound healing. It
is metabolized to cysteine, which plays a
vital role in the inflammatory, proliferative,
and remodeling phases of wound healing.
tein.
Serum prealbumin is commonly
used as an assessment parameter for pro-
40,41 Contrary to serum albumin,
which has a very long half-life of about
20 days, prealbumin has a shorter halflife
of only 2 days. As such it provides a
more rapid assessment ability. Normal
serum prealbumin is about 22.5 mg/dL, a
level below 17 mg/dL is considered a
mild deficit, and a severe deficit would be
below 11 mg/dL. As part of the perioperative
optimization process, malnourished
patients may be provided with
solutions that have been supplemented
with amino acids such as glutamine to
promote improved mucosal structure
and function and to enhance whole-body
nitrogen kinetics. An absence of essential
building blocks obviously thwarts normal
repair, but the reverse is not necessarily
true. Whereas a minimum protein
intake is important for healing, a high
protein diet does not shorten the time
required for healing.
Several vitamins and trace minerals
play a significant role in wound healing.42
Vitamin A stimulates fibroplasia, collagen
cross-linking, and epithelialization, and will
restimulate these processes in the steroidretarded
wound. Vitamin C deficiency
impairs collagen synthesis by fibroblasts,
because it is an important cofactor, along
with a-ketoglutarate and ferrous iron, in
the hydroxylation process of proline and
lysine. Healing wounds appear to be more
sensitive to ascorbate deficiency than uninjured
tissue. Increased rates of collagen
turnover persist for a long time, and healed
wounds may rupture when the individual
becomes scorbutic. Local antibacterial
defenses are also impaired because ascorbic
acid is also necessary for neutrophil superoxide
production. The B-complex vitamins
and cobalt are essential cofactors in antibody
formation, white blood cell function,
and bacterial resistance. Depleted serum
levels of micronutrients, including magnesium,
copper, calcium, iron, and zinc, affect
collagen synthesis.43 Copper is essential for
covalent cross-linking of collagen whereas
calcium is required for the normal function
of granulocyte collagenase and other collagenases
at the wound milieu. Zinc deficiency
retards both fibroplasia and reepithelialization;
cells migrate normally but do not
undergo mitosis.44 Numerous enzymes are
zinc dependent, particularly DNA polymerase
and reverse transcriptase. On the
other hand, exceeding the zinc levels can
exert a distinctly harmful effect on healing
by inhibiting macrophage migration and
interfering with collagen cross-linking.
Advances in Wound Care
An increased understanding of the wound
healing processes has generated heightened
interest in manipulating the wound
microenvironment to facilitate healing.
Traditional passive ways of treating surgical
wounds are rapidly giving way to
approaches that actively modulate wound
healing. Therapeutic interventions range
from treatments that selectively jumpstart
the wound into the healing cascade,
to methods that mechanically protect the
wound or increase oxygenation and perfusion
of the local tissues.45,46
Growth Factors
Through their central ability to orchestrate
the various cellular activities that
underscore inflammation and healing,
cytokines have profound effects on cell
proliferation, migration, and extracellular
matrix synthesis.47 Accordingly newer
interventions seek to control or modulate
the wound healing process by selectively
inhibiting or enhancing the tissue levels of
the appropriate cytokines.
The more common clinical approach
has been to apply exogenous growth factors,
such as PGDF, angiogenesis factor, epidermal
growth factor (EGF), TGF, bFGF,
and IL-1, directly to the wound. However,
the potential of these extrinsic agents has
not yet been realized clinically and may
relate to figuring out which growth factors
to put into the wound, and when and at
what dose. To date only a single growth factor,
recombinant human platelet-derived
growth factor-BB (PDGF-BB), has been
approved by the United States Food and
Drug Administration for the treatment of
cutaneous ulcers, specifically diabetic foot
ulcers. Results from several controlled clinical
trials show that PDGF-BB gel was effective
in healing diabetic ulcers in lower
extremities and significantly decreased
healing time when compared to the placebo
group.48,49 More recently, recombinant
human keratinocyte growth factor 2 (KGF2)
has been shown to accelerate wound
healing in experimental animal models. It
enhanced both the formation of granulation
tissue in rabbits and wound closure of
the human meshed skin graft explanted on
athymic nude rats.50,51Experimental studies
suggest potential for the use of growth factors
in facilitating peripheral nerve healing.
Several growth factors belonging to the
neurotrophin family have been implicated
in the maintenance and repair of nerves.
Nerve growth factor (NGF), synthesized by
Schwann cells distal to the site of injury,
aids in the survival and development of
sensory nerves. This finding has led some
investigators to suggest that exogenous
NGF application may assist in peripheral
nerve regeneration following injury.52
Newer neurotrophins such as brain-derived
neurotrophic factor and neurotrophin-3 as
well as ciliary neurotrophic factor appear to
support the growth of sensory, sympathetic,
and motor neurons in vitro.53–55 Insulinlike
growth factors have demonstrated similar
neurotrophic properties.56 Although
most of the investigations hitherto have
been experimental, increasing sophistication
in the dosing, combinations, and delivery
of neurotropic growth factors will lead
to greater clinical application.
Osteoinductive growth factors hold
special appeal to surgeons for their ability
to promote the formation of new bone. Of
the multiple osteoinductive cytokines, the
bone morphogenetic proteins (BMPs)
belonging to the TGF-ß superfamily have
received the greatest attention. Advances in
recombinant DNA techniques now allow
the production of these biomolecules in
quantities large enough for routine clinical
applications. In particular, recombinant
human bone morphogenetic protein-2
(rhBMP-2) and rhBMP-7 have been studied
extensively for their ability to induce
undifferentiated mesenchymal cells to differentiate
into osteoblasts (osteoinduction).
Yasko and colleagues used a rat segmental
femoral defect model to show that
rhBMP-2 can produce 100% union rates
when combined with bone marrow.57 The
union rate achieved with the combination
approach was three times higher than that
achieved with autologous cancellous bone
graft alone. Similarly, Toriumi and colleagues
showed that rhBMP-2 could heal
mandibular defects with bone formed by
the intramembranous pathway.58 The
widespread application of osteoinductive
cytokines depends in large part on a better
understanding of the complex interaction
of growth factors and the concentrations
necessary to achieve specific effects.
Gene Therapy
The application of gene therapy to wound
healing has been driven by the desire to
selectively express a growth factor for controlled
periods of time at the site of tissue
injury.59 Unlike the diffuse effects of a
Wound Healing
bolus of exogenously applied growth factor,
gene transfer permits targeted, consistent,
local delivery of peptides in high concentrations
to the wound environment.
Genes encoding for select growth factors
are delivered to the site of injury using a
variety of viral, chemical, electrical, or
mechanical methods.60 Cellular expression
of the proteins encoded by the nucleic
acids help modulate healing by regulating
local events such as cell proliferation, cell
migration, and the formation of extracellular
matrix. The more popular methods
for transfecting wounds involve the in vivo
use of adenoviral vectors. Existing gene
therapy technology is capable of expressing
a number of modulatory proteins at
the physiologic or supraphysiologic range
for up to 2 weeks.
Numerous experimental studies have
demonstrated the use of gene therapy in
stimulating bone formation and regeneration.
Mesenchymal cells transfected with
adenovirus-hBMP-2 cDNA have been
shown to be capable of forming bone when
injected intramuscularly in the thighs of
rodents.61,62 Similarly bone marrow cells
transfected ex vivo with hBMP-2 cDNA
have been shown to heal femoral defects.63
Using osteoprogenitor cells for the expression
of bone-promoting osteogenic factors
enables the cells to not only express bone
growth promoting factors, but also to
respond, differentiate, and participate in
the bone formation process. These early
studies suggest that advances in gene therapy
technology can be used to facilitate
healing of bone and other tissues and may
lead to better and less invasive reconstructive
procedures in the near future.
Dermal and Mucosal Substitutes
Immediate wound coverage is critical for
accelerated wound healing. The coverage
protects the wound from water loss, drying,
and mechanical injury. Although autologous
grafts remain the standard for replacing
dermal mucosal surfaces, a number of
bioengineered substitutes are finding their