Luck et al.³² could not detect significant differences in cosmesis or infection rate for absorbable vs. nonabsorbable suture materials after 3 months in facial lacerations. Also, no differences regarding the cosmetic outcome could be found in pediatric lacerations after 4 to 5 months.³³ However, nonabsorbable monofilament nylon sutures seemed to diminish the risk of hypertrophic scarring in comparison to absorbable sutures in a randomized clinical trial that looked at 60 patients who underwent midline sternotomy for cardiac surgery.³⁴ Skin was closed in a subcuticular continuous fashion either with a 4-0 braided polyglycolic acid suture or with a 4-0 nonabsorbable monofilamentous polypropylene suture that was removed 8 to 10 days after the surgery. Patients were evaluated 6 months after sternotomy. Interestingly, the benefit of the nonabsorbable monofilament suture was only apparent in the upper half of the sternum. The lower half did not show any difference. The authors concluded that increased tension and mobility of the skin in the lower half of the sternotomy led to the inferior scar appearance independently of the suture material. In terms of cost-effectiveness, it has to be considered that nonabsorbable sutures have a higher cost and require an additional physician visit for removal.³⁵

Compared with sutures, stapling with metal staples is five to seven times faster and produces equivalent cosmetic results when used in an emergency setting.³⁶,³⁷ A 2014 study evaluating 1³⁰ women that underwent an emergency cesarean section found skin closure with staples to have a better cosmesis and shorter duration of surgery, with comparable postoperative pain and wound complications.38 Even in cosmetic surgery on more sensitive areas like the neck, no difference between stapling and suturing could
be detected.³⁰ Two out of five randomized controlled trials evaluating chest and leg wounds after cardiovascular surgery found sutures to be superior cosmetically and were associated with fewer complications.³⁹ However, there are no large trials comparing sutures with staples in elective surgeries in cosmetically critical body regions. Staples puncture the epidermis and therefore carry a potential risk of scarring and wound contamination in comparison to absorbable sutures.⁴⁰ The use of staples has also been associated with increased tension along the incision line, precluding their use for reconstructive flap surgery.⁴¹ Stapling has been found to be safer and easier to use.³⁷ Whether the removal of staples or nonabsorbable sutures is more or equally painful for the patient is not clear.³⁶,⁴² There are also controversial reports about which method is cheaper.³⁷,⁴³

Absorbable staples (tissue half-life of 10 weeks; Insorb) that are placed in the subcuticular tissue without puncturing the epidermis⁴⁰,⁴⁴ are designed to combine the good cosmesis of absorbable sutures with the increased wound closure times of a stapler.⁴⁰ A randomized controlled trial comparing absorbable staples versus absorbable sutures conducted in patients receiving bilateral breast reconstruction found that the wound could be closed faster with staples, while showing a comparable cosmesis.⁴⁵ Absorbable staples also seem to be an adequate wound closure material in sensitive areas like the face and neck,⁴⁰ as well as in immunocompromised patients.⁴⁶ Nevertheless, there are no long-term data regarding absorbable staples and they are a relatively expensive alternative to other wound closure techniques.⁴⁷

Adhesive tapes contain an adhesive material consisting of iso-octo-acrylate and n-vinyl-pyrrolidone.⁴⁸ Microporous strips enable the passage of gas and water from the skin surface, reducing the suitable environment for bacterial growth and wound site infections.⁴⁹,⁵⁰ In clean contaminated wounds (wounds that are clean, but carry a high risk of contamination in locations such the gastrointestinal, respiratory, or genitourinary tract), a lower rate of infection has been reported in wounds closed with adhesive tape in comparison to sutures.⁵¹ The difficulty in approximating the wound edges accurately⁴⁹ and achieving sufficient adhesiveness, which in turn is dependent on correct usage and the presence of a dry wound site, limits the use of adhesive tape.⁴⁹,⁵²

A liquid adhesive (n-alkyl-alpha-cyanoacrylate tissue adhesives, e.g., Dermabond) polymerizes once applied on the wound, bridging the edges of the wound together.⁵³ The film is water resistant and sloughs off about 9 days after application.⁵³,⁵⁴ It is supposed to be used in small lacerations replacing sutures that are 5-0 or smaller.⁵⁴ For the closure of surgical or traumatic wounds, Dermabond was considered to be faster,⁵⁵,⁵⁶ less painful,⁵⁵ and with a similar,⁵⁷,⁵⁸ equal,⁵⁶ or improved⁵⁹,⁶⁰ cosmesis compared with standard wound closure methods. No higher rates of dehiscence have been detected.⁵⁶,⁵⁹ Whereas the experience of the surgeon is a key factor for a good cosmesis using suturing,⁶¹ it seems to be negligible for the outcome with tissue glue.⁶² Another positive side effect is that liquid tissue adhesive seems to inhibit bacterial growth and wound site infection to a certain degree.⁶⁰,⁶³,⁶⁴ Tissue adhesives are now being commonly and successfully used for facial wounds, groin wounds, hand surgery, blepharoplasty, laparoscopic wounds, hair transplantation, and lacrimal punctum closure.⁶³,⁶⁵ In addition, it can be a good method for wound closure in patients who are at risk for keloid or hypertrophic scar formation.⁶⁶ Tissue glue is not suitable for patients at risk for delayed wound healing (diabetics or patients with collagen vascular diseases),⁵⁹ in wounds that are in difficult areas (moist, hairy), or wounds that are complicated by edema, infection, high tension, or bleeding.⁶⁷,⁶⁸ Depending on the situation (size of wound, experience of the doctor with different wound closure techniques), liquid tissue adhesives can be more costly than conventional wound closure techniques.⁵²

Laser techniques to optimize wound healing and scar formation are a promising and highly investigated alternative to currently established procedures.⁶⁹ In laser tissue welding (LTW), the laser energy is directed to opposed wound margins, causing their partial liquefaction (conversion of photonic energy into heat energy), which is followed by their fusion.⁵² In this process, collagen fibers from both sides of the wound are intertwined,⁷⁰ leading to an immediate wound seal, thereby promoting a faster reepithelialization without the need for a large formation of granulation tissue.⁵²,⁶⁹

In laser tissue soldering (LTS), a solder (a protective proteinaceous barrier such as a semisolid/solid serum albumin) is added to enhance the fusion of opposed wound margins.⁷¹ Data suggest that the use of wavelength-specific dye absorbers such as indocyanine green and adhesive proteins such as albumin to LTW may lead to an increased strength and speed of wound closure than that achieved with traditional suture techniques.⁷¹,⁷²,⁷³

Laser-assisted tissue bonding is expected to be a fast and efficient technique for wound closure, potentially decreasing scar formation and complications associated with standard surgical alternatives.⁵² LTS has shown very promising results in various tissues (e.g., skin, vessels) in animal models regarding strength and tissue quality.⁷³,⁷⁴,⁷⁵ Temperature-controlled LTS in rats showed comparable histologic and cosmetic results to those of standard sutures.⁷⁰ In a porcine model, the scars after LTS were almost undetectable 7 days following surgical skin incision.⁷⁵ A 2001 study compared the results of LTS with conventional suturing for hypospadias repair in 183 boys. The authors reported fewer complications (fistulas and stenosis) in the LTS group and concluded that LTS is an acceptable and faster method of tissue closure in hypospadias repair.⁷⁶ Furthermore, the reduced need for suture materials appeared to decrease local inflammation.⁷⁶ However, only a few studies regarding LTW and LTS have been conducted in humans.⁵² The concerns of low tensile strength, the
potential for excessive thermal damage to the skin, and the high costs of the equipment will likely exclude this technology from use in routine clinical practice.⁵²

In addition to their applications in connecting wounded tissues, lasers are a promising treatment option for existing keloids and hypertrophic scars (see Chapter 13), though the mechanism of action of laser treatment is still not fully understood. Suggested mechanisms include the thermo-induced breakdown of collagen fibers as well as the mediation of growth factor expression.⁷⁷,⁷⁸,⁷⁹,⁸⁰,⁸¹ In comparison with other treatment options such as intralesional 5-fluorouracil (5-FU) and corticosteroids for keloidal and hypertrophic scars, pulsed dye laser therapy has shown comparable results without side effects (see Chapter 10).⁷⁷,⁷⁸ Argon lasers, the first lasers used in the treatment of keloids, were dismissed in favor of carbon dioxide, neodymium-doped yttrium aluminum garnet (Nd:YAG), and pulsed dye lasers because of unsatisfying results.⁸² As a monotherapy for keloids and hypertrophic scars, nonfractionated carbon dioxide laser ablation was associated with recurrence rates of 90% and higher,⁸³,⁸⁴ whereas the nonablative Nd:YAG laser showed recurrence rates of 17%.⁸⁵ Laser-assisted scar treatment also seems to be effective in treating associated symptoms like itching, redness, and pain.⁷⁷,⁸⁶ A review article on the treatment of keloids and hypertrophic scars published in the Journal of the American Medical Association stated that pulsed dye lasers are an emerging therapeutic option owing to their high efficacy, yielding fewer adverse effects.⁸²

Recently, using a picosecond pulse width infrared (IR) laser that targets tissue water, Amini-Nik et al.⁸⁷ demonstrated a significant reduction of thermal damage and scar formation compared with conventional lasers and scalpels.⁸⁷ In principle, the shorter pulse duration mitigates the destructive photochemical and photothermal effects associated with conventional devices with longer pulse widths.⁸⁷ This highlights the role that novel surgical modalities might have in minimizing scar formation. However, this study was performed in a rodent model, whose skin and wound healing properties differ from those of humans.⁸⁸

Wnts are glycoproteins that are important in embryonic and adult tissue maintenance.⁹⁰,⁹¹,⁹²,⁹³ Depending on the context, Wnt ligands signal through either the canonical (mediated through β-catenin) or the noncanonical (independently of β-catenin) Wnt signaling pathway.⁹²,⁹⁴ The focus here will be on the canonical Wnt pathway because it plays an important role in skin development and in various aspects of cutaneous wound repair.²,⁸⁹,⁹⁵,⁹⁶,⁹⁷ Various Wnt glycoproteins (Wnt 1, 3, 4, 5a, and 10b) can be found in cutaneous wounds in the early phase of healing and are present up to 7 days after injury.⁹⁸,⁹⁹

In this signaling pathway, Wnt binds to the membrane receptor Frizzled and the coreceptor low-density lipoprotein receptor-related protein 5/6 (LRP5/6). Once bound, the protein Dishevelled is activated. Dishevelled inhibits a protein complex—consisting of glycogen synthase kinase 3β (GSK-3β), adenomatous polyposis coli protein (APC), casein kinase I, and axin 1—that is responsible for the ubiquitin-mediated degradation of β-catenin. Ultimately Wnt leads to an intracellular accumulation of β-catenin.⁹¹,⁹²,⁹³ β-catenin itself is a transcriptional coactivator (bound to T-cell factors; Tcfs) for genes that encode important proteins for wound healing such as fibronectin,⁹⁰ but it also functions as a structural protein in the adherens junction that mediates cell-cell contacts⁹¹,¹⁰⁰ (see Fig. 9-1).

The Wnt pathway participates in the dermal accumulation of β-catenin, but it does not seem to be a key mechanism of maintaining it during wound healing.¹⁰¹,¹⁰² β-catenin levels can be elevated independently of the Wnt pathway by TGF-β1, a growth factor excreted during early wound healing,¹⁰³,¹⁰⁴,¹⁰⁵ as well as by fibronectin, an ECM component, through a β1-integrin-mediated GSK-3β-dependent pathway.¹⁰¹ β-catenin and its target genes are elevated in dermal fibroblasts during the proliferation phase of wound healing, and return to baseline levels during the remodeling phase.¹⁰⁶,¹⁰⁷ In humans the duration and level of β-catenin activity correlate with wound size¹⁰⁸,¹⁰⁹ and the number of macrophage cells present in the wound.¹¹⁰ In an uncharacterized process, the Wnt pathway can reprogram or endow other epidermal cells. Ito et al.⁹⁷ showed that in large wounds the Wnt pathway pushes epithelial cells toward de novo folliculogenesis.

β-catenin is essential for adequate wound repair, though excessive expression is found in human hypertrophic scars and keloids.¹⁰⁷,¹⁰⁸ A high amount and activity of β-catenin leads to an enlarged and hypercellular dermal compartment with an increased dermal collagen deposition, scarring, and myofibroblast formation.¹⁰³,¹¹¹,¹¹² Genetic mutations that cause an unphysiologic stabilization of β-catenin lead to aggressive fibromatosis (desmoid tumor).¹¹³ Interestingly the Wnt/β-catenin pathway also seems to have an inhibitory effect: it decreases the reepithelialization after wounding. A nuclear accumulation of β-catenin has been found in the edge of chronic ulcers, and in vitro pharmacologic stabilization of β-catenin inhibited the migration of keratinocytes.¹¹⁴

β-catenin plays a significant role in various cells during the process of wound healing, but its role in macrophages seems to be essential.¹¹⁰ In mice, it has been shown that β-catenin promotes motility and adhesion of macrophages¹¹⁰ by upregulating the expression of cadherins, catenins, AD-AMs (a disintegrin and metalloproteinase), and integrins.¹¹⁰ Macrophages are believed to be an essential mediator of granulation tissue formation after skin trauma.¹¹⁵,¹¹⁶,¹¹⁷,¹¹⁸,¹¹⁹,¹²⁰,¹²¹ These cells express multiple receptors for Wnts,¹¹⁰ and deficient macrophage migration and adhesion has been
associated with poor wound healing.¹²² If macrophages lacked β-catenin, they also showed less TGF-β1 (a cytokine essential for wound healing; see below).¹¹⁰ Furthermore, in the absence of β-catenin, macrophages were not able to achieve a fibroblast-like phenotype.¹¹⁰ Scarless embryonic wound healing takes place in the absence of a fully developed monocyte lineage¹¹⁸ and a less active Wnt signaling pathway.¹²³

Manipulation of the Wnt/β-catenin signaling pathway and β-catenin activity could be a promising therapeutic approach in the treatment of deficient wound healing. Nefopam (5-methyl-1-phenyl-1,3,4,6-tetrahydro-2,5-benzoxazocine), a centrally acting nonopioid analgesic, reduced β-catenin levels to baseline even if the cells were highly stimulated with a Wnt ligand.¹²⁴ These cells showed a decrease in cell proliferation and viability in cell cultures derived from aggressive fibromatosis or hypertrophic wounds without affecting normal fibroblasts.¹²⁴ When this was used daily for 14 days in a murine model, there was a 50% reduction in scar diameter as compared with the control group. However, the exact mechanism of decreasing β-catenin levels and inhibiting fibroblast cell proliferation is unclear.¹²⁴ In a murine model a new laser—the picosecond IR laser—showed the ability to ablate tissue at a monocellular level, which causes less tissue trauma and less activation of the Wnt/β-catenin and TGF-β pathways.⁸⁷ It will be a future challenge to fully understand under which circumstances and in what amount Wnt/β-catenin regulation is beneficial in wound healing.

In addition to the above-described Wnt/β-catenin pathway, various growth factors and cytokines such as fibroblast growth factors (FGFs), vascular endothelium growth factors (VEGFs), and interleukins (IL) have been linked to the process of wound healing; TGF-β is one of the crucial mediators.²,¹²⁵,¹²⁶

The TGF-β proteins consist of three isoforms—TGF-β1, TGF-β2, and TGF-β3—that are secreted by various cell types such as macrophages and fibroblasts.¹²⁶,¹²⁷ Released as inactive precursors and adjacent to TGF-β binding protein, they require activation either through proteases or through conformational changes which are in part caused by cell traction forces of integrins.⁹,¹²⁷ Once activated, they are bound to TGF-β receptors. This initiates the formation of a SMAD complex (intracellular proteins that form a trimer of two receptor-regulated SMADs and one co-SMAD when they are activated by an extracellular stimulus) that acts as a transcription factor for various genes that encode important proteins for wound healing such as collagen (see Fig. 9-2).⁹,¹⁰,¹²⁷,¹²⁸,¹²⁹ TGF-β can also act through non-SMAD-dependent mechanisms including TGF-β-associated kinase I, mitogen-activated protein kinase, and GTPases, but the exact function of this pathway and its effect on wound healing are yet to be elucidated.¹³⁰,¹³¹

The different isomers of the TGF-β superfamily have different effects on wound healing and scar formation (see Table 9-1). TGF-β1 promotes the proliferation of fibroblasts, their synthesis of collagen I and fibronectin as well as wound contraction, but inhibits keratinocyte migration and therefore wound reepithelialization¹³²,¹³³,¹³⁴,¹³⁵,¹³⁶,¹³⁷,¹³⁸. TGF-β3 seems to have antiscarring effects, whereas TGF-β2 is required for hair follicle development.¹¹,¹³⁹,¹⁴⁰ Deficiencies in the TGF-β pathway lead to poor wound healing including a thin disorganized dermis, fewer fibroblasts, fibroblasts with cytoskeletal abnormalities, and a reduction of granulation tissue through reduced ECM deposition.¹³⁶,¹⁴¹,¹⁴²,¹⁴³ Interestingly, a deficiency in the TGF-β pathway also leads to smaller wound areas and less inflammation.¹⁴¹,¹⁴²

The expression of TGF-β proteins by fibroblasts does not only affect keratinocytes in a unidirectional manner; rather, keratinocytes also have the ability to downregulate TGF-β1 secretion.¹³⁸,¹⁴⁴ Excessive amounts of TGF-β1 lead to hypertrophic scars mediated either through the above-described β-catenin pathway or through connective tissue growth factor (CTGF; CCN2).¹⁰³,¹⁰⁴,¹⁴⁵,¹⁴⁶,¹⁴⁷,¹⁴⁸,¹⁴⁹,¹⁵⁰ Topical
TGF-β1 application on murine wounds causes accelerated healing and reepithelialization.¹³² Interestingly, antagonizing TGF-β can also result in accelerated wound closure and increased keratinocyte proliferation and migration.¹⁴⁰,¹⁵¹,¹⁵² It seems to be of great importance at which exact point the TGF-β pathway is altered, suggesting a complex system of networks in this pathway that are yet to be elucidated.² In scarless embryonic wound healing, the ratio of TGF-β1 to TGF-β3 is lower as compared with what has been found in adults.⁸,¹¹,¹⁵³ Applying TGF-β1 to embryonic wounds (in animal models) promotes scar formation, whereas TGF-β3 reduced it.¹³⁹,¹⁵³ The promising results of wound treatment with tissue growth factor in animal models has led to various clinical trials. Despite positive preliminary data,¹⁵⁴ all of the clinical studies failed to establish a clinical benefit for TGF therapy in wound healing.²,¹⁵⁵,¹⁵⁶,¹⁵⁷,¹⁵⁸,¹⁵⁹ Additionally, TGF also contributes to the formation of hypertrophic scars.¹⁰⁵,¹⁶⁰,¹⁶¹

Today, only a few growth factors are approved for clinical treatment and their indications are very limited. Human recombinant platelet-derived growth factor-BB (PDGF-BB; becaplermin) is used for the treatment of chronic diabetic ulcers on the limbs. It led to a 15% increase in healing compared with standard wound care alone in those patients.¹⁶²,¹⁶³,¹⁶⁴ Health Quality Ontario stated that the efficacy of growth factors and granulocyte colony-stimulating factor (G-CSF) in enhancing the healing of chronic pressure ulcers has not been established yet.¹⁶⁵ PDGF (Regranex; approved by Health Canada for the treatment of diabetic ulcers on the lower extremities) was associated with an increased rate of deaths from cancer.¹⁶⁵ In Japan, FGF-2 is used postoperatively to reduce hypertrophy and widening of scars.2 Next to its antiscarring effects,¹⁶⁶ FGF-2 showed promising results in the treatment of life-threatening, large disruptions of the skin integrity as in severe burns.¹⁶⁷. More sophisticated, well-designed clinical trials will be necessary to evaluate the efficiency of growth factors in the treatment of wounds and scars.¹²⁵,¹⁶⁸

The Hedgehog pathway plays important roles in embryonic skin development, angiogenesis, the regulation of epidermal stem cells, and the development of skin appendices in adults.¹⁶⁹,¹⁷⁰,¹⁷¹,¹⁷²,¹⁷³,¹⁷⁴,¹⁷⁵ Among the three existing Hedgehog proteins—Indian Hedgehog (Ihh), Sonic Hedgehog (Shh), and Desert Hedgehog (Dhh)—Shh seems to be the main contributor to wound healing and scar formation.¹⁶⁹,¹⁷⁶ Shh binds to Protein patched homolog (PTCH)—a transmembrane protein. PTCH is a tumor suppressor that prevents the activation of Gli transcription factors (see Fig. 9-3). Shh removes the inhibition of PTCH and activates Gli transcription factors that promote the encoding of a multitude of different proteins such as cyclin D2, a key regulator of the cell cycle.¹⁶⁹,¹⁷⁶ The exact role of the Hedgehog pathway in wound healing and scar formation is relatively unknown.

Shh can be found in regenerated hair follicles after wounding, but not in the epidermis or keratinocytes.¹⁷⁷ Nevertheless, if diabetic mice are treated topically with Shh they show better reepithelialization and wound healing. The dermis is thicker, more collagen-rich, and better vascularized.¹⁷⁸ Shh stimulates the proliferation of fibroblasts and promotes the recruitment of bone marrow-derived endothelial progenitor cells and the excretion of VEGF.¹⁷⁸ If the Shh pathway is blocked, mice showed delayed wound healing, reduced granulation tissue formation, and decreased vascularization.¹⁷⁹

The Notch pathway in mammals is activated by the binding of a transmembrane ligand (members of the Delta-like and Jagged families) to one of the four Notch receptors of an adjacent cell.¹⁸⁰,¹⁸¹,¹⁸² This binding causes the liberation of the Notch intracellular domain (NCID) by sequential
proteolytic cleavage. The NCID then translocates to the nucleus where it regulates gene expression (see Fig. 9-4).¹⁸⁰,¹⁸¹,¹⁸²

Notch plays a role in the development and maintenance of the epidermis and blood vessels.¹⁸⁰,¹⁸¹,¹⁸²,¹⁸³ It ensures the correct skin stratification during skin development.¹⁸³,¹⁸⁴,¹⁸⁵

Downregulation of the Notch pathway leads to delayed wound healing, and stimulates increased wound closure in mice.¹⁸⁶ This might be because of an activating effect on the endothelial cells and the fibroblasts, increasing vascularization and collagen deposition.¹⁸⁶,¹⁸⁷,¹⁸⁸ In addition, recent data suggest that the Notch pathway promotes inflammatory activity by enhancing the recruitment of macrophages and the secretion of inflammatory cytokines.¹⁸⁷ However, the role of the Notch pathway in wound healing and scar formation is relatively unknown, as well as its impact on the different cell types contributing to this process (like macrophages, keratinocytes, and fibroblasts).²

Intralesional steroid injection (triamcinolone acetonide suspension) has been considered a mainstay in the prophylaxis and treatment of hypertrophic and keloid scars for decades (see Chapter 10).⁸² It leads to the degeneration of collagen bundles as well as to the inhibition of fibroblast growth.²¹² In a 10-year follow-up study of keloid and hypertrophic scar patients, intralesional triamcinolone acetonide caused a 64% reduction in scar size and a 72% reduction in symptoms. Keloidectomy in combination with β-radiation showed a similar effect on the reduction of scar size (75%), whereas β-radiation alone was inferior to steroid injection and surgery (11% scar size reduction, 55% symptom control).²¹³

Interferons (INFs) are potent inhibitors of human fibroblast collagen production, with INF-γ being more potent than INF-α.²¹⁴ Intralesional injection of INF-γ used as a monotherapy led to a reduction in keloid size in 75% of patients.²¹⁵ As an adjunct to scar excision, INF-α reduced the recurrence rate of keloids from 58% to 19%.²¹⁶ Disadvantages of INF include adverse effects such as flu-like symptoms and high cost.⁸²

Imidazoquinolines (imiquimod and resiquimod) are toll-like receptor (TLR) 7 and 8 agonists that induce key cytokines for wound healing like INF-α.²¹⁴,²¹⁷ Data on the utility of topical application of imiquimod for the prevention of keloid recurrence after excision have been conflicting. The application of 5% imiquimod cream after²¹⁸ or before²¹⁹ surgical removal of keloids resulted in a good cosmesis and no recurrences at 4 and 12 months. However, less successful imiquimod treatment has also been reported. A study evaluating postsurgical administration of 5% imiquimod after surgical therapy of keloids of the trunk had a recurrence rate of 80% (8 out of 10).²²⁰ In addition, pigmentary alteration is a common side effect.²¹⁸