Physical Properties of Skin and Its Applications

7/28/2019 Physical Properties of Skin and Its Applications

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Physical Properties of skin and

its applications

Presenters Moderators

Shilpi Bhadani Dr Karoon Agrawal

Triveni Dhaka Dr R K Srivastava

Dr Sunil Sharma


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PHYSICAL PROPERTIES OF SKIN


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Extreme variability of the physical

properties

Differ at the same site in different individuals

Differ at different site on the same individual Differ in different directions at the same site

Change remarkably with age


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Each piece of skin is

uniquely constructed

Uniqueness attributed to

the twin fibrous network

of collagen & Elastin.

Pattern of the interwoven

architecture.


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Collagen is remarkably convoluted structure when

relaxed.

Randomly oriented, straightens out on stretch.

With more load more fibers recruited.

Aligned in one direction

Finally structure of parallel fibers result resistant

to stretch


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Clinical exploits: Allows the surgeon to excise a

certain amount of skin at any site and still close it

primarily.

A point exists beyond which no amount of

additional tension allows the surgeon to

approximate the skin primarily.


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Physical properties

Visco-elastic Properties

Skin Tension Properties

Skin Extensibility

Directional variations


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Visco-Elastic Properties

Creep

Stress relaxation


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CREEP- when a piece ofskin is stretched and forceis

kept constant, skin continues

to expand

Increasing deformation under

constant load.

contrasts with an elastic

material which does not

exhibit increase deformation

no matter how long the load is

applied.


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REASON FOR VISCOELASTIC

PROPERTIES With the alignment of the fibers, tissue fluid and

ground substance are progressively displaced

from the network.

The more fluid in dermis, greater is the amount

of creep obtainable.

Important in tissue expansion.


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CREEPMechanical creep

Stretching of the skin

Disruption of gap junctions

Displacement of interstitial fluid and ground substance

Realignment of collagen

Disruption of elastin

Biological creep

Cell proliferation


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The other major characteristic of

a viscoelastic material is

hysteresis or energy dissipation.

This means that if a visco-elastic

material is loaded and unloaded,

the unloading curve will not

follow the loading curve. The difference between the two

curves represents the amount of

energy that is dissipated or lost

during loading.


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Clinical exploits of Creep

Skin can be stretched by a significant even if small

amount.

Important in scenarios of just too small flap or the

wound that just wont close

Technique of placing hooked retractors on theundersurface of the dermis on each side of the flap or

wound and to pull them in opposite direction with

force (just short of tearing the tissues) for a minute

maximal extension is not obtained at once so load

cycling is done & it is repeated 3 or 4 times.


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STRESSRELAXATION- when a

piece of skin is stretched

for a given distance and

distance is keptconstant, force required

to keep it stretched

decreases

stress will be reduced orwill relax under a constant

deformation.


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TISSUE EXPANSION


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Neumann in 1957 was the first to attempt to exploit

this viscoelastic capacity of skin by implanting a

rubber balloon under the postauricular area for

reconstructing an amputated ear.


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Mechanical Creep : mechanical stretch on the tissues.

Cell is stretched

Collagen fibres stretch out and become parallel.

Elastin undergoes microfragmentation. Interstitial fluid and ground subatance is displaced

Adjacent tissue recruitment.


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Biological Creep : cellular proliferation

Gap junction disruption + tissue surface area

Cell proliferation

Growth of the tissue

Increased synthesis of collagen

Resting tension restores to baseline

Stress relaxation tendency for the resistance of the skin to a stretching forceto decrease when held at a given tension over time


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MOLECULAR BASIS

disruption of cell- matrix adhesion(integrin)

upregulation of Mechanical strain growth factors(EGF,PDGF,TGF-)

transcription of proto-oncogenes.(c-fos/c-myc/c-jun)


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Conformational change in membrane proteins

opening of ca-channels

Protein Kinase C/phospholipase C activation

microfilament contraction

MITOSIS(Via kinases,second messengers and nuclear proteins)

Molecular basis for tissue expansion: clinical implications for the surgeon Takei T. Plast Reconstr Surg 1998;102:247258.


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Tissue expander

growth factors

TGF-Cell membrane

Epidermis

Dermis

Subcutaneous- tissue

Stretch


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BIOLOGY OF TISSUE EXPANSION Epidermis -thickness increases during expansion and

usually returns to near normal levels after 4-6 weeks

- reduction in intercellular distance

- reduced undulations of basal lamina


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EPIDERMIS

Histological evaluation of epidermis: (A) before skin extension; and (B) after skin extension: thicker keratinic layer, increased mitotic

activity in the basal layer and slightly thickened epidermis

A B


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Hair follicles/adnexa-no evidence of degeneration

-decreased density

Melanocytes- activity increase during expansion but returns to normal leveafter several months.

Dermis - 30-50% thinning(reticular dermis)occurs and persist at least 9

months after expansion

- collagen content are increased and are arranged in orderly parallel

fashion over the implant surface.


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DERMIS

A

B


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Mitotic activity-increase in epidermis as well as dermis.

- maximum at 96 hrs thereafter progressive

decrease.

Muscle- atrophies during expansion

- increase in no. of sarcomeres and mitochondria perfibres

-focal fibre degeneration with glycogen deposition and

mild interstitial fibrosis.

-returns to normal level after removal.


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Capsule- forms around the implant

- becomes less cellular over time

- thickest at 2 months

- four zones within the capsule(Pasyk):

1.inner zone of macrophages & fibrin layer

2.central zone fibroblast and myofibroblast

3.transitional zone of loose collagen fibres

4.outer zone of blood vessels and collagen


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Although this capsule is often included with the overlying skin

after removal of the expander,capsulectomy has no detrimental

effect on skin flap viability,but may serve to thin the flap if

required and decreases shrinkage and contracture.

Morris SF, Pang CY, Mahoney J, Lofchy N, Kaddoura IL, Patterson R. Effect ofcapsulectomy on the hemodynamics and viability of random-pattern skin flaps raised on

expanded skin in the pig. Plast Reconstr Surg 1989; 84:314322.


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Adipose tissueatrophy with some permanent loss.

upto 50% thinning,some fat cells are

replaced by fibrous tissue.

Blood vessels - decrease in collagen fibres

- increase in elastic fibres.

Angiogenesis secondary to ischaemia of expanded tissues - VEGF

around capsule

- no. and calibre of capillaries

- are similar to delayed flaps.


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INDICATIONS inadequate adjacent tissue for primary closure

inadequate tissue for repair with a local flap

repair of defects that are amenable to repair by a standard flap but with

significant donor or recipient site deformity

to increase the harvest of full-thickness skin grafts/flaps

in previously irradiated or operated areas because of the enhancement of the

vascularity.?


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STRESS RELAXATION-

Does not seem to have much clinical significance,

however does explain that a flap which looks

sutured under tension in the immediate post-op

period is often perfectly viable a few hours later.


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SKIN TENSION


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SKIN TENSION PROPERTIES

Extensibility & tension naturally occuring is often

referred to as elasticity are inter-related butdistinct.

Is a function of the elastic fiber network in a stateof tension.

Tension is important in wound healing. Varies at different anatomic site and at different

directions in the same site.

If a sutured wound is so oriented as to havemaximal tension across itHypertrophic scarresults.


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Plastic surgeons tend to so choose their incisions

that tension across the sutured wound is minimal

and a fine scar results.

Crease lines

Tension lines caused by

Joint movements

Muscle play

Tension across them- nil

Sutured wounds along tension lines- least tension


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There is a critical tension below which scars do notstretch and above which they do.

Decreased skin tension- Scars undetectable

Skin of scrotum, shaft of penis

Elderly (lax skin)

High skin tension- hypertrophic scarring (tensionfrom all directions)

sternum

Shoulder (akin to a coat hanger supporting the skin

of the trunk against gravity)

Eff t f ki t i


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Effects of skin tension on

normal skin

A certain amount of increased tension can exist

without resultant stretching (A contracture

doesnt relieve naturally)

With high skin tension

Stretching

Rupture (striae formation)Blanching of the skin

depending upon the force applied


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STRETCHING

A biologic phenomenon(cf Creep)- growth of skin

Can stretch nine to ten times in most severe cases Skin maintains its thickness

e.g., lymphedema, adiposity

Tissue expansion has the combined effect of creepand biologic stretch.


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RUPTURE OF DERMIS (striae)

Increased load is faster than in stretch can

compensate.

Disruption of elastic fibers Loss of elastic recoil and skin tension

Lie at right angle oflangers lines

e.g., pregnancy, cushings disease, over expandingmuscles in people doing body building exercises


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BLANCHING

Force applied causes obliteration of lumina ofdermal blood vessels and obstruct the blood flow

Can lead to necrosis if unrelieved

Seen when a flap is sutured in excessive tension

Skin advanced too far after wide undermining.

Can be relieved by creep phenomenon or bymaking a small incision in dermis right angle to

line of blanching


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SKIN EXTENSIBILITY


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SKIN EXTENSIBILITY

The amount, the skin will stretch before breaking

To allow free movements of body. More in one

direction than others.

Repeatedly extends and relaxes throughout life.

Function of elastic fibers


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SKIN EXTENSIBILITY

Varies with direction, location and age,

maximum in infancy.

Increases from infancy to 40 years of age then

declines rapidly

Converted to skin laxity in elderly

Declines more in skin over forearm than

abdomen in elderly


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SKIN EXTENSIBILITY

When skin is extended, it contracts to equalamount at right angles and if contraction is notpossible, extension is limited

e.g. limb, circumferential skin limitscontraction hence extension is limited

Fixation of skin to underlying structures alsolimits extension

Undermining permits additional extension


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DIRECTIONAL VARIATIONS

Langers lines

Relaxed skin tension lines

Kraissls lines

Lines of maximal extensibility


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LANGERS LINES

HISTORY

Karl Langer (an

anatomist) punctured

holes in fresh cadaverswith an awl

Punctured points had

ellipsoid shape

Connecting the long axesof the ellipses, concept of

lines came into picture


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Before piercing the awl

increases the skin tension

in all directions.

Collagen fibers which get

aligned first are the ones

in the direction of

minimum extensibility

and they are cleft by the

point.


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Langer also noted that completely excised strips

of skin retracted to a side

He had no technique of staining the elastin but

noted that an amorphous hyaline substance wasinvolved

He concluded that tension and retraction, fiber

pattern and cleavage lines are thus related andmutually dependent.


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In the full term foetus or neonate the cleavage

lines encircle the limbs and trunk, apparently

related to the expanding bulk of the fetus.

By 2&1/2 yrs the adult pattern is establishedmainly guided by the movement of the joints

Thus it is the tension that produces the fiber

pattern and not vice versa.


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LANGERS LINES


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LANGERS LINES

Perpendicular to the direction of greatest force

on the skin

Incisions parallel to these- less force acrossthem, better healing


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LANGERS LINES

Short comings

Some are found to run across natural creases,

wrinkles, flexion lines like at lateral to eyelids,glabella, middle of the cheeks

RELAXED SKIN TENSION LINES (RSTL


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RELAXED SKIN TENSION LINES (RSTL

Described by Albert F. Borges

Correspond to the directional pull that exists in the

relaxed skin

Not a visible feature as wrinkle lines

Run transversely in most regions except on the back


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RELAXED SKIN TENSION LINES

Cut obliquely the direction of clavicle, iliac

crest and inguinal ligament

Breast- run transversely, not radiating

Derived by the act of pinching skin and

observing the furrows and ridges formed


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RELAXED SKIN TENSION LINES

Offers best cosmetic result

Most narrow and strongest scar line

Used in placement of scar or in scar revision

Relaxed Skin Tension Lines


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Lines of Maximum Extensibility


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Lines of Maximum Extensibility

Lines of Maximum

Extensibility (LMEs) are

perpendicular to RSTLs

RELAXED SKIN TENSION


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LINES

Z- Plasty for scar revision

Breaks the line of scar and changes its direction

Objective is to place it as near as possible to theRSTL



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LINES

LINES OF MAXIMAL EXTENSIBILITY


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Perpendicular to RSTL

Parallel to muscle fibers

Skin is most extensible along these lines

Limberg flap



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Limberg flap

Rhombus marked so that two sides are as close to

parallel to these lines

Donor site which has short diagonal also parallel,

is chosen to facilitate primary closure of donor site


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KRAISSLS LINES


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KRAISSLS LINES

Took photographs of elderly men with very

loose facial skin and were compared

A sketch was made

Individual variations seen due to differences incontour or muscle development

KRAISSLS LINES


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KRAISSLS LINES

SKIN FOLDS AND WRINKLES


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SKIN FOLDS AND WRINKLES

Orthostatic lines

Dynamic lines

Gravitational lines

Combination types

Orthostatic Lines


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Orthostatic Lines

Natural furrows present at birth

Due to skin excess for the purpose of flexion

and extension

Posterior and anterolateral aspect of neck

Dynamic Lines


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Dynamic Lines

Result of repetitious right angle pull on the

skin by the muscles of expression

First linesForehead by action of frontalis muscle

Adolescent years


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Dynamic Lines


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Dynamic Lines

Next lines

Laugh lines or crows feet

Orbicularis oculi muscle

In twenties

Also vertical glabellar lines

Paired corrugator supercilli muscles


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Dynamic Lines


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Dynamic Lines

Circum-oral lines

Fifth decade

First in upper lip then in lower lip

Orbicularis oris muscleVertical lines

Dimple like depression in chin

Mentalis muscle

Prominent in some


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Gravitational lines


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Gravitational lines

Usually after 40 yrs of age

Originate from underlying bony prominences

like orbital rims, zygoma Sunken eyes with drooping eyelid skin

Hollow cheeks

Double chin

THE AGING SKIN


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THE AGING SKIN

Collagen

Swell, later fragment and atrophy

Synthesis and degradation reducedMore insoluble (mature)

More resistant to degradation due to increased

cross-linking

More hydroxyproline

THE AGING SKIN


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THE AGING SKIN

Elastin

Increase in number and thickness

Less extensible

Loss of elasticity

homogenization of the ground substance

Increase in fibrous connective tissue

THE AGING SKIN


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THE AGING SKIN

Lack of tone

Changes in collagen and ground substance

Atrophy of errector pilorum muscles

Wrinkling (more in exposed areas)

Environmental factors more important

Related to sun exposure

THE AGING SKIN


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G G S

On exposure to UV radiation

Increased cross-linking of collagen

strength and also makes it brittle

Resist tensile forces but weakened to resistshearing forces

THE AGING SKIN


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Skin changes on sun exposureFlattening of epidermis

Disappearance of rete ridges

Dermal papillae flattened

Basement membranes shortened

Decrease in size of dermis


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TISSUE EXPANSION


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EpidermisThickens

Normalizes in 4-6 wks

Hair follicles- compressed, no degenerationMelanocytic activity- increased

Reduced intercellular distance

Decreased basal laminal undulations

TISSUE EXPANSION


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DermisThins out

Persists till 36 wks after expansion is completed

Increased collagen content

Increased mitotic activity in fibroblasts

Capsule- progressive collagenization in 3 months

Large compact orderly placed collagen bundles

Myofibroblasts develop adjacent to the capsule

TISSUE EXPANSION


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Strain induced cell proliferation- matrix dependent

1 Stress deforms ECM

2 Changes in integrins and alter integrin

expression

3

Integrins - pathways to signal cellproliferation

PHYSIOLOGY

Constant mechanical stress applied to skin over time


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Constant mechanical stress applied to skin over time

causes plastic deformation-tissue creep(due to disruption of

elastic fibres).

Gibson reported about The mobile microarchitecture of dermal collagen

according to which application of a load to skin causes it to stretch,and when

the load is removed it relaxes back.However,when an excesive load isapplied,it stretches without subsequent relaxation.

(Gibson T. Br J Surg 1965;52:764770.)


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Documents

Physical Properties of Skin and Its Applications