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Science of Micro-Autografting

Split thickness skin grafting (STSG) is the gold standard technique for closing acute wounds following surgery, trauma, severe burns or chronic wounds that fail to heal (1-3). Without STSG wounds heal from the wound margin with contraction assisting in closure. Skin grafting has been to date restricted to trained surgeons operating in a theatre environment, using specialised instruments and general anaesthesia. In acute burns where lack of donor sites limit closure, skin grafts are often meshed through a specialised instrument (See: Humeca Mesher) or processed through a Meek technique that expands the skin graft to cover a larger area (see: Meek Technique(4,5 19-21). Techniques to replace STSG have included the application of cultured keratinocytes and epidermal grafting. Cultured allograft keratinocytes require a specialised laboratory, are expensive to produce with a short shelf life (6-9). Epidermal grafting harvests the epidermis usually in the form of a blister roof. Recent technology has seen automated versions of blister roof grafting which is expensive. Both techniques have the limitation of only containing epidermal keratinocytes, without dermal cells such as fibroblasts that are necessary for complete healing and true skin integrity.

Complete

Xpansion minced skin technology creates a complete split thickness skin graft providing the right cells types in the correct cell populations directly to the wound. As the STSG is from the patient’s own skin it is safe with not risk of rejection (10).

Edge growth & cell-signalling 

The mincing of the graft before application does more than expand the skin to a 1:100 ratio (24). Smaller pieces create more edges where growth occurs. Cells at the edge undergo profound phenotypic changes and acquire the ability to migrate rapidly (11,12). The more edges the faster the wound closure (19-21, 25). Skin pieces in a ‘free edge’ state rapidly secrete a series of chemicals that initiate wound healing. These cell signalling chemicals encourage cell proliferation & migration and stimulate angiogenesis and dermal remodelling. The cell signalling process from the graft can kick start the healing for an unresponsive wound (13-14, 25). The correct quantities of fibroblasts and keratinocytes from a complete STSG ensure a durable skin with reduced contraction (15,16, 25).

Pigmentation

Of note in a STSG are melanocytes which are difficult to culture and sensitive to stress from other methods including enzymes. A minced STSG preserves the melanocytes and will result in normal pigmentation of the new skin. Melanocyte transplantation such as Xpansion is a promising treatment of hyper-pigmentation & hypo-pigmentation conditions such as Vitiligo (17-18).

Cell Orientation

The Xpansion technique results in graft pieces that are applied to a wound in many orientations. The Meek technique seeks to orient the STSG correctly by applying the graft to a gauze and stapling to the wound (19-21). In 2002, Svensjo et al described a technique where it was not necessary to orient the minced skin pieces in any specific direction (22). Zuhaili et al showed that in a moist environment, the micro-autografts will propagate from the wound bed to the wound surface. Epidermal proliferation will occur from the borders, appendages, and basal layer regardless of particle orientation (23).

Chronic Wounds

A Pre-clinical study in a porcine model of the micrograft method was performed in 2011. It demonstrated transplantation of micrografts in a 1:100 expansion ratio resulted in complete epithelialization of both healthy and diabetic wounds within 14 days. In comparison, non-transplanted wounds showed 62 percent re-epithelialization in healthy pigs and 49 percent in diabetic pigs at the corresponding time point (24).

Unlike a meshed STSG Xpansion minced grafts produce small islands of skin onto the wound. The lack of continuity of these islands may limit infection problems to small areas, rather than spreading to the whole graft (19-20). Minced grafts would likewise demonstrate a better take rate, as per Meek islands, in challenging areas and in low quality wound beds (20).

Pertusi G et al showed the significant promise of the application of minced skin onto a chronic wound when he compared the release of cytokines from a minced graft compared to a whole sample: The minced sample, which could behave like the skin fragments used in vivo in the autologous minced micrografts technique, expressed higher levels of tumor necrosis factor-α, interleukin-1α, platelet-derived growth factor, and basic fibroblast growth factor, and lower levels of interleukin-6, monocyte chemoattractant protein-1, growth related oncogene-α, and vascular endothelial growth factor compared with the whole sample. In conclusion, mincing of healthy skin may allow appropriate regulation of the inflammatory phase of wound healing and could induce overexpression of some growth factors, which facilitates the proliferative phase of healing (25).

References

1. Franz MG, et al. Wound Healing Society. Guidelines to aid healing of acute wounds by decreasing impediments of healing. Wound Repair Regen. 2008 Nov-Dec;16(6):723-48. 2. Muhart M, et al. Behavior of tissue-engineered skin: a comparison of a living skin equivalent, autograft, and occlusive dressing in human donor sites. Arch Dermatol. 1999 Aug;135(8):913-8. 3. Boggio P, et al. Is there an easier way to autograft skin in chronic leg ulcers? ‘Minced micrografts’, a new technique. J Eur Acad Dermatol Venereol. 2008 Nov;22(10):1168-72. Epub 2008 Apr 10. 4. Polk HC Jr., Adherence of thin skin grafts. Surg Forum. 1966;17:487-9. 5. Vandeput JJ, Tanner JC, Boswick J. Implementation of parameters in the expansion ratio of mesh skin grafts. Plast Reconstr Surg. 1997 Sep;100(3):653-6. 6. Rue LW, 3rd, et al. Wound closure and outcome in extensively burned patients treated with cultured autologous keratinocytes. J Trauma. 1993;34(5):662-8. 7. Myers S, et al. Transplantation of keratinocytes in the treatment of wounds. Am J Surg. 1995;170(1):75-83. 8. Veves A, Falanga V, et al. Graftskin, A Human Skin Equivalent, Is Effective in Management of Non-infected Neuropathic Diabetic Foot Ulcers. Diabetes Care. 2001;24:290-295. 9. Marston WA, et al for the Dermagraft Diabetic Foot Ulcer Study Group. The efficacy and safety of Dermagraft in improving the healing of chronic diabetic foot ulcers. Diabetes Care. 2003;26:1701-1705. 10. Billingham RE, Medawar PB. The technique of free skin grafting in mammals. Journal of Experimental Biology 1951; 385-402. 11. Pastar I, Stojadinovic O, Yin NC, Ramirez H, Nusbaum AG, Sawaya A, Patel SB, Khalid L, Isseroff RR, Tomic-Canic M. Epithelialization in wound healing: A comprehensive review. Advances in Wound Care 2014; 3(7):445-464 12. Block E. R., Klarlund J. K. Wounding sheets of epithelial cells activates the epidermal growth factor receptor through distinct short- and long-range mechanisms. Mol. Biol. Cell. 2008;19:4909–4917. 13. Block Ethan R, Tolino Michael A, Lozana Jennifer S et al Free Edges in Epithelial cell sheets stimulate Epidermal growth factor receptor signaling. Molecular Biology of the Cell 2010 Jul 1; 21(13): 2172–2181. 14. Singer AJ, Clark RAF. Cutaneous wound healing. New England Journal of Medicine 1999;341(10):738-746 15. Sorrell JM, Caplan AI. Fibroblast heterogeneity: more than skin deep. J Cell Sci. 2004;117(Pt 5):667–675 16. Grinnell F. Fibroblasts, myofibroblasts, and wound contraction. J Cell Biol 1994, 124(4): 401-404 17. Hirobe T. Role of keratinocyte-derived factors involved in regulating the proliferation and differentiation of mammalian epidermal melanocytes. Pigment Cell Research 2004; 18:2-12. 18. Yamaguchi Y, Brenner M, Hearing VJ. The regulation of skin pigmentation. Journal of Biological Chemistry 2007; 282(38):27557-27561 19. Medina Abelardo, Rigel Timothy et al. Modified Meek technique for wound coverage in extensive burn injuries. J Burn Care Res 2016;37:305–313 20.  Kreis RW, Mackie DP, Hermans RR, Vloemans AR. Expansion techniques for skin grafts: comparison be- tween mesh and Meek island (sandwich-) grafts. Burns 1994;20(suppl 1):S39–42. 21. Lumenta David B, Kamolz Lars-Peter, et al. Adult burn patients with more than 60% TBSA involved – Meek and other techniques to overcome restricted skin harvest availability – The Viennese concept. J Burn Care Res 2009;30: 231–242 22. Svensjo T, et al. Autologous skin transplantation: comparison of minced skin to other techniques. J Surg Res. 2002;103(1):19-29. 23. Zuhaili Baraa, Aflaki Pejman, et al Meshed Skin Grafts Placed Upside Down Can Take If Desiccation Is Prevented. Plast. Reconstr. Surg. 125: 855, 2010. 24. Hackl Florian, Bergmann Juri, et al Epidermal regeneration by Micrograft transplantation with immediate 100-fold expansion. Plast. Reconstr. Surg. 129: 443e, 2012. 25. Pertusi G, Tiberio R, et al. Selective release of cytokines, chemokines, and growth factors by minced skin in vitro supports the effectiveness of autologous minced micrografts technique for chronic ulcer repair Wound Repair Regen. 2012 Mar-Apr;20(2):178-84. doi: 10.1111/j.1524-475X.2011.00762.x. Epub 2012 Feb 3.