Reconstructive Techniques – Skin Anatomy And Skin Grafts : Anatomy

Anatomy Of Skin

The skin, which covers the exterior of the body, is the largest end organ. It makes up 15% of the body weight. The skin is made up of several layers. There are primarily two layers to it. The epidermis is the outermost layer, whereas the dermis is the innermost layer. The hypodermis is a third layer that lies beneath the dermis.

Epidermis

The skin's epidermis makes up 5% of the total. Its origin is ectodermal, while the origin of the dermis and hypodermis is mesodermal.  The thickness of the epidermis varies depending on the area of the body.
Melanocytes, Merkel or dendritic cells, and Langerhans cells are some of the additional cells that make up the epidermis in addition to the predominant keratinocytes. Cells that produce melanin pigment are called melanocytes. While Langerhans cells are antigen-presenting cells, Merkel or dendritic cells serve as touch receptors and aid in pressure perception.

The stratum spinosum is the fourth layer. Large, nonproliferative polyhedral keratinocytes are seen in it. These keratinocytes are characterized by spine-like cytoplasmic projections. These are known as stratum spinosum as a result. This layer of the epidermis is also known as the prickle cell layer. Dendritic cells are found within it.

The stratum basale, also known as the stratum germinativum, is the final and innermost layer. The basement membrane divides this layer from the dermis, and desmosomes connect it to the basement membrane. It is the layer that is multiplying the fastest, with cuboidal and columnar cells that convert into keratinocytes and go upward to several layers before reaching the stratum corneum. Nevertheless, Merkel cells and melanocytes are also present in this stratum.

Dermis

95% of the skin is made up of the dermis. Through the basement membrane, it is joined to the epidermis's stratum basale. There are two layers in the dermis.  The top layer, known as the papillary dermis, is made up of collagen fiber and loose connective tissue. This layer permits heat exchange and serves as a major source of nutrients. The reticular layer is the other deeper layer. It has more thickness. It is made up of elastin, collagen fibers, and thick connective tissue.The dermis contains ground substances, sensory fibers, blood arteries, lymphatics, and all of the skin's appendages, including sweat glands and hair follicles.

Hypodermis

• Hypodermis contains subcutaneous fat as well as skin appendages.

Reconstructive Techniques

 There are several reconstructive methods, including flaps, skin grafts, free tissue transfer, composite vascularized tissue transfer, primary closure of the wound, healing of the wound by secondary intention, and skin expanders/substitutes.  Reconstructive treatments were formerly applied using the reconstructive ladder method, which started with the most basic approach and worked up to a more sophisticated one. This approach is no longer employed.

Patient-centered reconstitutive elevators are the favored option these days. The kind of reconstructive procedure to be used will depend on the specific patient, taking into account the patient factor, the likelihood that the surgery would be successful or unsuccessful, the abilities available, and the resources available.

For instance, early skin grafting is recommended for patients with burns because it reduces morbidity and increases patient survival; in contrast, flaps rather than skin grafts are preferred for patients with pressure sores; likewise, flaps are used to reconstruct facial skin tumors because they provide better color matching and aesthetics.

Skin Grafts

Skin grafts include removing a portion of skin from the donor location and transferring it to the recipient area, where it revascularizes. A graft is any tissue that lacks its own blood supply.  Patients who have extensive wounds that cannot be healed by primary closure or who may have an incorrect scar or delayed healing due to secondary intention should consider having skin grafts.

Also Read: Skin Carcinoma – Basal And Squamous Cell  Carcinoma

Types Of Skin Graft As Per Donor

Allograft involves taking graft from several individuals of the same species, whereas autograft involves taking graft from the same individual and applying it to the same individual.  In an isograft, genetically identical twins provide the transplant. 

 Autograft is the greatest option, or gold standard, whereas xenograft uses tissue derived from different species. Like autografts, isografts are a good alternative and produce comparable outcomes.Allografts and xenografts, however, are not frequently utilized due to their high rejection rates. When a child has an extended delivery, allograft might be utilized as a temporary or transient dressing from parent to child.

Types Of Skin Graft Depending Upon Thickness

Depending on thickness, there are primarily two types of skin grafts: split thickness skin graft (STSG), also known as partial thickness skin transplant. This involves extracting an epidermis along with varying amounts of dermis. Another name for it is Thiersch Graft.  A full-thickness skin graft (FTSG) involves removing the complete dermis, including appendages, along with the epidermis. Another name for this is Wolfe's graft.

 In a split-thickness skin graft, varying amounts of dermis and epidermis are removed. The thickness of a split-thickness skin graft might vary depending on the quantity of dermis present.  A split-thickness skin graft can be further subdivided into thin, intermediate, and thick skin grafts according to the thickness of the dermis. 

The thickness of a thin split-thickness skin transplant is 0.15–0.3 mm. The thickness of an intermediate split-thickness skin transplant is 0.3-0.45 mm, while a thick split-thickness skin graft has a thickness of 0.45-0.6 mm.

Contraction

 A skin transplant has a size reduction during placement. We refer to this as contraction. There are two kinds of contraction. Primary contraction and secondary contraction are what they are. 

Primary contraction is primarily mediated by elastin and is defined as a reduction in the size or contraction of the graft that occurs soon after graft harvesting. Contraction occurs as the elastin recoils. The dermis is where elastin is mostly found. Therefore, the main contraction increases with the amount of dermis in the skin graft. In the same way, the initial contraction decreases with decreasing dermis in the skin graft. 

Primary contraction will be greater in full-thickness skin grafts (FTSG) than in split-thickness skin grafts (STSG). Additionally, initial contraction will be greater in thicker split-thickness skin grafts than in thinner ones.

 Secondary contraction refers to the skin graft's shrinkage or contraction following the graft's healing at the wound bed.Myofibroblasts, which are only present on the recipient bed after the graft is placed, mediate it.
In split-thickness skin grafts (STSG) secondary contraction is more common than in full-thickness skin grafts (FTSG). In this instance, secondary contraction increases with decreasing dermis in the skin graft and decreases with increasing dermis in the skin graft. 

There exists an inverse relationship between secondary contraction and the graft's dermal thickness. Therefore, secondary contraction reduces as split-thickness skin graft (STSG) thickness rises.

Donor site healing

Because STSG donor sites contain dermis and appendages, they reepithelialize. These limbs aid in the regeneration of donor sites.  The STSG donor site can be treated with dressings since secondary intention can repair the wound in 7–14 days.  Large amounts of STSG can be harvested, and the donor site can be used again and again because it closed in 7–14 days due to secondary intention healing, negating the necessity for primary closure. 

The FTSG donor site undergoes total removal of the epidermis and dermis, so it cannot regenerate on its own. Therefore, main site closure is used to heal the donor site of FTSG. In this scenario, only tiny regions can be harvested, and the donor sites cannot be used

Donor sites

In STSG, the arm, back, scalp, gluteal area, and thighs are key donor locations. The most typical donation site for stem cells is the mediastinal region of the thigh.

FTSG is mostly utilized, therefore, for cosmetic reasons. Therefore, post anterior skin, the inside side of the arm, the supraclavicular regions, the hairless groin, and the upper eyelids are donor locations for FTSG.
Compared to other donor locations, a sizable portion of the graft can be extracted from the hairless groin.

Comparison Between STSG And FTSG

We are aware that the dermis contains full-thickness dermis and that there is less dermis in STSG. As a result, initial contraction in FTSG skin grafts is greater than in STSG skin grafts. On the other hand, in the event of a FTSG skin graft, there is less secondary contraction than in STSG.Furthermore, compared to FTSG skin grafts, STSG engraftment or absorption is superior and more consistent since thicker skin grafts demand more nourishment. 

Because half thickness skin transplant experiences greater secondary contraction and deformity than STSG skin graft, FTSG skin graft yields a superior cosmetic result. Because STSG has a higher risk of aberrant or hyperpigmentation due to increased melanocyte proliferation, color matching is also better in FTSG skin grafts.

There is no or very little hair development, no sweating, and no sebum secretion in STSG because they lack sweat glands and hair follicles, whereas FTSG grows with the body. Because of this, these skin grafts can crust, are drier, and are less susceptible to desiccation. Because of this, moisturizing should be done for six months following STSG skin grafts, while FTSG skin grafts result in normal sebum secretion and hair development.While only tiny abnormalities may be addressed in FTSG, vast areas can be harvested and massive defects can be covered in STSG due to donor site healing.

While the donor site of FTSG heals by primary closure, the donor site of STSG can heal in 7–14 days with secondary intention and a straightforward dressing.  FTSG is more resilient. Because the dermis is rich in connective tissue and has viscoelastic qualities, the more dermis, the more durable the skin graft.

RECIPIENT BEDS / C / I

Grafts lack a source of blood of their own. It draws blood from the bed of the receiver. The blood supply needs to be adequate for the graft to absorb well. Therefore, recipient beds with inadequate blood flow, such as regions that have already been subjected to radiation or irradiation, naked bones without periosteum, cartilages without perichondrium, and tendons that are not covered with peritenon, are inappropriate and should not be used for skin grafting. 

Recipient beds should not be placed in infected areas, particularly if the hemolytic streptococci or bacterial colony count is more than 105.
Bones without periosteum, cartilages with perichondrium, tendons covered in peritenon, and healthy soft tissues, fats, and muscles are appropriate for use as recipient beds. 

Graft Uptake/survival

 Graft uptake occurs after the transplant is inserted. The initial step in graft uptake is called adherence, during which a fibrin link forms between the recipient bed and the graft. The graft is first nourished by diffusion, or plasma imbibition, for the first 48 hours. The skin transplant is nourished by the plasma exudates from the host capillary, which diffuse over the fibrin clot. 

Revascularization is the second stage of graft absorption. It involves many procedures. The first step in revascularization is inosculation, which is the alignment of the recipient bed's and the graft's capillaries, also known as kissing capillaries, which aids in graft nourishment.

Neo-angiogenesis, also known as neovascularization, is the process by which new capillaries and veins grow out of the recipient bed following inoculation. Ultimately, lymphatic drainage is formed, and edema is reduced.

Remodeling or restructuring is the next stage of graft absorption. Within seven days of grafting, fibroblasts replace the first established fibrin linkages, causing collagen links to form between the recipient bed and the graft. After 10–14 days of grafting, there will be a secondary contraction and the graft will fully adhere to the wound bed.

After 4-5 weeks, the graft undergoes re-innovation, which is finished 12–24 months after grafting. The sensation that returns initially is pain, which is followed by temperature and light touch. Patients who have had grafts are more vulnerable to thermal harm because their temperature sense recovers later. Because FTSG has more dermis and appendages, it will re-innovate better and sooner. 

Also Read: Vascular Anomalies - Hemangiomas and Malformations

Graft Harvesting

Humby's knife

 The Reese dermatome, electric power, air power, or Humby's knife can all be used to harvest STSG. Dermatomes are more adept at controlling thickness.Multiple tiny punctate bleeding spots near to each other are observed when thin STSG is taken, whereas larger, sparsely distributed bleeding spots are observed when thick STSG is performed. A scalpel is used to remove out FTSG.

Graft Expansion

There are situations when there is insufficient graft to cover sizable surface regions. Therefore, the purpose of graft expansion is to give the graft a larger surface area. One way to achieve graft expansion is by meshing, which involves cutting slits with number 11 or number 15 blades or utilizing a mesher machine to create hole fenestration.
Meshing offers a number of benefits. Mesh grafts are useful when there is porosity of donor skin or the recipient bed is sub-optimal or irregular. They increase the surface area of the graft by 1.5 times or by 50%, help contour the graft on uneven or irregular recipient beds, and help escape fluid so that it doesn't get collected and form hematoma or seroma.

One drawback of meshing is that it causes more secondary contraction, which can result in less attractive outcomes like crocodile skin or a checkerboard pattern. 

Graft Failure

The graft is inserted, but it is rejected or does not take up. We refer to this as graft failure. Hemostoma, seroma, bacterial count greater than 105/gm of tissue, or hemolytic streptococcus infection are the causes of graft failure because these bacteria produce a lot of proteases, which break apart fibrin clots. Hemostasis is the most frequent cause. Shearing across the graft or graft and graft bed movement are possible additional reasons. Over the graft bed, the graft needs to be immobilized. Tie-over dressing or bolstering can be used to stop shearing. Gauze is layered atop paraffin gauze and stitched shut to create bolster dressing.
• To avoid graft failure, vacuum-assisted closure, splinting, and immobilization of the joint is known as vacuum dressing. It is beneficial in adherence of graft to the wound bed.

Poorly vascularized or irradiated recipient beds, arterial insufficiency, venous congestion, and lymphatic stasis are additional reasons why grafts fail.

Hope you found this blog helpful for your NEET SS Surgery Skin and Subcutaneous preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.