Biomechanics of skin flaps

在皮瓣的设计中，血供方式、皮瓣生理特性以及皮瓣生物力学特性对于皮瓣重建手术的成功具有至关重要的意义。前面两章是讨论皮肤的解剖以及皮瓣的生理学特性，这一章则是讨论对于皮瓣血供以及存活具有重要影响的皮瓣生物力学特性。其中很多特性在手术中我们会不自觉的用到，本章对这些特性做出了一些理论上的总结。

Biomechanical properties of cutaneous tissue

应力-应变被用于描述物质的机械特性，皮肤主要具有三大机械特性：nonlinearity, anisotropy, and viscoelasticity.

• nonlinearity
  
  • During initial deformation, randomly oriented collagen and elastic fibers are stretched in the direction of the applied force. Collagen fibers do not bear a significant burden until the bundle is completely straight in the direction of the applied force. As a result, there is little resistance to initial deformation, and the stress-strain relationship is nearly linear and elastic.
  • As deformation progresses, additional collagen fibers are recruited into the load-carrying role and resistance rises.Region 2 is the strain at which many collagen fibers transition from non–load carrying to a load-carrying role.
  • At high-stress loads (region 3), virtually all the dermal collagen fibers are aligned in the direction of the applied force. At this point,no further deformation is possible because of the inextensible nature of fully oriented collagen. Wholly oriented collagen (region 3) preserves the structural integrity of skin by limiting deformation during accidental stresses
• Anisotropy
  • relaxed skin tension lines(RSTL)
  • lines of maximal extensibility(LME)
  • Langer’s line
• Viscoelasticity, 皮肤在低应力的作用下具有弹性，随着应力增大，皮肤表现出粘弹性，这个时候皮肤的形变是时间和应力的函数。其中两个应用的比较多的和时间相关的特性是皮肤的延展性（creep）和应力松弛特性(extension relaxation)
  • Creep refers to the increase in the length of skin compared with the original length when skin is placed under a constant stress: A small increase in length (strain) occurs as compressed and straightened collagen fibers displace interstitial fluid that is loosely bound to the extracellular matrix (ground substance).
  • Stress relaxation and creep are related. Stress relaxation refers to a decrease in stress that occurs when skin is held under tension at a constant strain. If a skin flap is closed under excessive tension, a certain amount of relaxation occurs as the tissue creeps. Stress relaxation allows large lesions in inelastic regions to be removed with serial excision. It also accounts for the improved vascularity observed in the first 24 hours of flaps closed under tension. These biomechanical properties may save a questionable flap but should not be an integral part of flap design.

Mechanotransduction and skin

宏观方面表现为三大生物力学特性，微观方面则涉及如何实现机械信号转导。

• direct mechanotransduction
  • tensegrity, in which a cell’s cytoskeletal framework is coupled to the extracellular matrix as well as to the cytoskeletal framework of nearby cells, creating a homeostasis of tension from which deviations might be recognized. Cellular adhesion molecules, such as integrins, are key to tensegrity
  • Some cells possess mechanosensitive ion channels that respond to mechanical forces with either the activation or inactivation of an ion flux.One of the most important of these is the mechanosensitive calcium ion channel.
  • Protein deformation is another mechanism of direct mechanotransduction.Protein deformation is the process by which mechanical forces induce conformational changes to a protein, which may unmask new binding motifs or, alternatively, mask previously accessible binding sites.
• indirect mechanotransduction. Indirect mechanotransduction involves a cell-sensing mechanical force leading to the release of proteins or other chemical messengers, which then act on nearby cells to mediate a variety of effects.
  • peripheral nervous system
  • The release of neuropeptides

Surgical application of the biomechanics of skin

• tissue expansion. 皮肤扩张过程中，表皮层能够在扩张后通过细胞增殖恢复原来厚度，但是真皮层厚度的恢复情况不明；have shown that intraoperative tissue expansion increases skin compliance with decreased tension compared with simple undermining.
• wound tension and blood flow. flaps with ample blood flow can withstand the extremes of tension. flaps subjected to excessive wound tesion can necrose because of insufficient blood flow.
• flap design and underminning. Minor variations in flap design can alter the mechanical characteristics of cutaneous tissue and change the force necessary for advancement and wound closure. The edge of a skin flap is advanced both to stretch the skin and to overcome resistance between the dermis and underlying tissues. Most of the benefit from undermining is achieved within the first 1 to 2 cm. Undermining beyond a short distance risks injury to surrounding structures and may compromise blood flow. In broad-based flaps, extensive undermining has been shown to result in an unexplained increase in the force necessary for advancement.