Neuropathic Ulcer

Diabetic foot ulcer is a breakdown of the skin and sometimes deeper tissues of the foot that leads to sore formation. It is thought to occur due to abnormal pressure or mechanical stress chronically applied to the foot, usually with concomitant predisposing conditions such as peripheral sensory neuropathy, peripheral motor neuropathy, autonomic neuropathy or peripheral arterial disease. It is a major complication of diabetes mellitus, and it is a type of diabetic foot disease. Secondary complications to the ulcer, such as infection of the skin or subcutaneous tissue, bone infection, gangrene or sepsis are possible, often leading to amputation.

A key feature of wound healing is stepwise repair of lost extracellular matrix (ECM), the largest component of the dermal skin layer. However, in some cases, physiological insult or disorder - in this case, diabetes mellitus - impedes the wound healing process. In diabetic wounds, the inflammatory phase of the healing process is prolonged, delaying the formation of mature granulation tissue and reducing the healing wound's tensile strength.

Treatment of diabetic foot ulcers includes blood sugar control, removal of dead tissue from the wound, wound dressings, and removing pressure from the wound through techniques such as total contact casting. Surgery in some cases may improve outcomes. Hyperbaric oxygen therapy may also help but is expensive.

34% of people with diabetes develop a diabetic foot ulcer during their lifetime, and 84% of all diabetes-related lower-leg amputations are associated with or result from diabetic foot ulcers.

Risk factors

Risk factors implicated in the development of diabetic foot ulcers are infection, older age, diabetic neuropathy, peripheral vascular disease, cigarette smoking, poor glycemic control, previous foot ulcerations or amputations, and ischemia of small and large blood vessels. Prior history of foot  disease, foot deformities that produce abnormally high forces of pressure, callus at pressure areas  renal failure, oedema, impaired ability to look after personal care (e.g. visual impairment) are further risk factors for diabetic foot ulcer.

People with diabetes often develop diabetic neuropathy due to several metabolic and neurovascular factors. Peripheral neuropathy causes loss of pain or feeling in the toes, feet, legs, and arms due to distal nerve damage and low blood flow. Autonomic neuropathy causes Sudomotor dysfunction and dryness of the skin. Blisters and sores may appear on numb areas of the feet and legs, such as metatarsophalangeal joints and the heel region, as a result of pressure or injury which may go unnoticed and eventually become a portal of entry for bacteria and infection.

Pathophysiology

Extracellular matrix

Extra cellular matrix (or "ECM") is the external structural framework that cells attach to in multicellular organisms. The dermis lies below the epidermis, and these two layers are collectively known as the skin. Dermal skin is primarily a combination of fibroblasts growing in this matrix. The specific species of ECM of connective tissues often differ chemically, but collagen generally forms the bulk of the structure.

Through the interaction of a cell with its extracellular matrix (transmitted through the anchoring molecules classed as integrins) there forms a continuous association between the cell interior, cell membrane and its extracellular matrix components that helps drive various cellular events in a regulated fashion. Wound healing is a localized event involving the reaction of cells to the damage sustained.

The cells break down damaged ECM and replace it, generally increasing in number to react to the harm. The process is activated, though perhaps not exclusively, by cells responding to fragments of damaged ECM, and the repairs are made by reassembling the matrix by cells growing on and through it. Because of this, extracellular matrix is often considered as a 'conductor of the wound healing symphony'. In the Inflammatory phase, neutrophils and macrophages recruit and activate fibroblasts which in subsequent granulation phase migrate into the wound, laying down new collagen of the subtypes I and III.

In the initial events of wound healing, collagen III predominates in the granulation tissue which later on in remodeling phase gets replaced by collagen I giving additional tensile strength to the healing tissue. It is evident from the known collagen assembly that the tensile strength is basically due to fibrillar arrangement of collagen molecules, which self-assemble into microfibrils in a longitudinal as well as lateral manner producing extra strength and stability to the collagen assembly. Metabolically altered collagen is known to be highly inflexible and prone to break down, particularly over pressure areas. Fibronectin is the major glycoprotein secreted by fibroblasts during initial synthesis of extracellular matrix proteins. It serves important functions, being a chemo-attractant for macrophages, fibroblasts and endothelial cells.

The basement membrane that separates the epidermis from the dermal layer and the endothelial basement membrane mainly contains collagen IV that forms a sheet and binds to other extracellular matrix molecules like laminin and proteoglycans. In addition to collagen IV, the epidermal and endothelial basement membrane also contains laminin, perlecan and nidogen. Hyaluronic acid, a pure glycosaminoglycan component, is found in high amounts in damaged or growing tissues. It stimulates cytokine production by macrophages and thus promotes angiogenesis. In normal skin chondroitin sulfate proteoglycan is mainly found in the basement membrane, but in healing wounds they are up-regulated throughout the granulation tissue especially during the second week of wound repair where they provide a temporary matrix with highly hydrative capacity. Binding of growth factors is clearly an important role of perlecan in wound healing and angiogenesis. Poor wound healing in diabetes mellitus may be related to perlecan expression. High levels of glucose can decrease perlecan expression in some cells, probably through transcriptional and post-transcriptional modification. Wound healing phases especially, granulation, re-epithelization and remodelling exhibit controlled turnover of extracellular matrix components.

Altered metabolism

Diabetes mellitus is a metabolic disorder and hence the defects observed in diabetic wound healing are thought to be the result of altered protein and lipid metabolism and thereby abnormal granulation tissue formation. Increased glucose levels in the body end up in uncontrolled covalent bonding of aldose sugars to a protein or lipid without any normal glycosylation enzymes. These stable products then accumulate over the surface of cell membranes, structural proteins and circulating proteins. These products are called advanced glycation endproducts (AGEs) or Amadori products. Formation of AGEs occurs on extracellular matrix proteins with slow turnover rate. AGEs alter the properties of matrix proteins such as collagen, vitronectin, and laminin through AGE-AGE intermolecular covalent bonds or cross-linking. AGE cross-linking on type I collagen and elastin results in increased stiffness. AGEs are also known to increase synthesis of type III collagen that forms the granulation tissue. AGEs on laminin result in reduced binding to type IV collagen in the basement membrane, reduced polymer elongation and reduced binding of heparan sulfate proteoglycan.

;Impaired NO synthesis
:Nitric oxide is known as an important stimulator of cell proliferation, maturation and differentiation. Thus, nitric oxide increases fibroblast proliferation and thereby collagen production in wound healing. Also, L-arginine and nitric oxide are required for proper cross linking of collagen fibers, via proline, to minimize scarring and maximize the tensile strength of healed tissue. Endothelial cell specific nitric oxide synthase (EcNOS) is activated by the pulsatile flow of blood through vessels. Nitric oxide produced by EcNOS, maintains the diameter of blood vessels and proper blood flow to tissues. In addition to this, nitric oxide also regulates angiogenesis, which plays a major role in wound healing. Thus, diabetic patients exhibit reduced ability to generate nitric oxide from L-arginine. Reasons that have been postulated in the literature include accumulation of nitric oxide synthase inhibitor due to high glucose associated kidney dysfunction and reduced production of nitric oxide synthase due to ketoacidosis observed in diabetic patients and pH dependent nature of nitric oxide synthase.

;Structural and functional changes in fibroblasts
:Diabetic ulcer fibroblasts show various morphological differences compared to fibroblasts from age matched controls. Diabetic ulcer fibroblasts are usually large and widely spread in the culture flask compared to the spindle shaped morphology of the fibroblasts in age-matched controls. They often show dilated endoplasmic reticulum, numerous vesicular bodies and lack of microtubular structure in transmission electron microscopy study. Therefore, interpretation of these observations would be that in spite of high protein production and protein turnover in diabetic ulcer fibroblasts, vesicles containing secretory proteins could not travel along the microtubules to release the products outside. Fibroblasts from diabetic ulcer exhibit proliferative impairment that probably contributes to a decreased production of extracellular matrix proteins and delayed wound contraction and impaired wound healing.

;Increased matrix metalloproteinases (MMP) activity
:In order for a wound to heal, extracellular matrix not only needs to be laid down but also must be able to undergo degradation and remodeling to form a mature tissue with appropriate tensile strength. Proteases, namely matrix metalloproteinases are known to degrade almost all the extracellular matrix components. They are known to be involved in fibroblast and keratinocyte migration, tissue re-organization, inflammation and remodeling of the wounded tissue. Due to persistently high concentrations of pro-inflammatory cytokines in diabetic ulcers, MMP activity is known to increase by 30 fold when compared to acute wound healing. MMP-2 and MMP-9 show sustained overexpression in chronic non-healing diabetic ulcers. Balance in the MMP activity is usually achieved by tissue inhibitor of metalloproteinases (TIMP). Rather than absolute concentrations of either two, it is the ratio of MMP and TIMP that maintains the proteolytic balance and this ratio is found to be disturbed in diabetic ulcer. In spite of these findings, the exact mechanism responsible for increased MMP activity in diabetes is not known yet. One possible line of thought considers Transforming growth factor beta (TGF-β) as an active player. Most MMP genes have TGF-β inhibitory element in their promoter regions and thus TGF–β regulates the expression of both MMP and their inhibitor TIMP. In addition to the importance of cell-cell and cell-matrix interactions, all phases of wound healing are controlled by a wide variety of different growth factors and cytokines. To mention precisely, growth factors promote switching of early inflammatory phase to the granulation tissue formation. Decrease in growth factors responsible for tissue repair such as TGF-β is documented in diabetic wounds. Thus, reduced levels of TGFβ in diabetes cases lower down the effect of inhibitory regulatory effect on MMP genes and thus cause MMPs to over express.

Biomechanics

Complications in the diabetic foot and foot-ankle complex are wider and more destructive than expected and may compromise the structure and function of several systems: vascular, nervous, somatosensory, musculoskeletal. Thus, deeper comprehension of the alteration of gait and foot biomechanics in the diabetic foot is of great interest and may play a role in the design and onset of preventive as well as therapeutic actions.

Briefly, the effect of diabetes on the main structures of the foot-ankle complex can be summarised as:
* effects on the skin: skin – and the soft tissues immediately underneath the skin – undergo greater compression and shear loading than usual, thus explaining the onset of tissue damage so deeply correlated to traumatic ulceration processes. Besides this, skin of the diabetic foot loses autonomic nervous control and consequently reduced hydration, making it less elastic and thus more vulnerable to the action of increased mechanical stress;
* effects on tendons and ligaments: protein glycosylation and the resulting collagen abnormalities lead to greater transversal section – i.e. thickening – of tendons and ligaments and a greater coefficient of elasticity. Particularly affected by this process are Plantar Fascia and Achilles Tendon. Both causes lead to increased stiffness of those structures;
* effects on cartilage: similar to what happens to tendons and ligaments, cartilage changes its composition mainly due to the modification of collagen fibers. This increases its stiffness and decreases the range of motion of all joints in the foot and ankle.
* effects on muscles: Diabetes mellitus causes severe damage to nerve conduction, thus causing a worsening in the management of the related muscle fibers. As a consequence, both intrinsic and extrinsic muscles of the foot-ankle complex are damaged in structure (reduction of muscle volume) and function (reduction of muscle strength);
* effects on the peripheral sensory system: impaired nerve conduction has a dramatic effect on the peripheral sensory system since it leads to loss of protective sensation under the sole of the foot. This exposes the diabetic foot to thermal or mechanical trauma, and to the late detection of infection processes or tissue breakdown;
* effects on foot morphology (deformities): due to most of the above alterations, a significant imbalance of peripheral musculature and soft tissue occur in the foot which seriously alters its morphology and determines the onset of foot deformities. Most common deformities of the diabetic foot are represented by a high longitudinal arch (rigid cavus foot), hammer toes and hallux valgus. A completely different morphologic degeneration is represented by neuropathic arthropathy, whose analysis is not part of this discussion.

Diagnosis

Assessment of diabetic foot ulcer includes identifying risk factors such as diabetic peripheral neuropathy, noting that 50 percent of people are asymptomatic, and ruling out other causes of peripheral neuropathy such as alcohol use disorder and spinal injury. Diabetic foot ulcers are often misdiagnosed in patients with undiagnosed skin malignancies, especially high-risk in elderly patients.

The location of the ulcer, its size, shape, depth and whether the tissue is granulating or sloughy needs to be considered. Further considerations include whether there is malodour, condition of the border of the wound and palpable