Diabetic Foot

Introduction

• Diabetes is a systemic disease that affects the vascular, nervous, integumentary & immune systems

Epidemiology

• At any one time, 3-4% of patients with DM have a foot ulcer. 15% develop a foot ulcer over their lifetime
• When a foot ulcer develops the chance of going on to amputation ↑ 8 fold

Pathophysiology

Nervous system

• Diabetic neuropathy is most common in patients with poor glycaemic control. All branches of the nervous system are affected. Both myelinated & non-myelinated fibres are involved
• Sensory neuropathy
  • predominates
  • appearing distally in a glove & stocking distribution & progressing proximally
  • Loss of sensation leads to an inability to respond to mechanical stress
  • major cause of tissue breakdown in the insensate foot is shear force.
    • ultimate tensile strength of normal plantar skin is 1200 lb/sq inch. In normal walking average pressures do not exceed 75 lb/sq inch in a barefoot person, 50 lb/sq inch in a person in leather soled shoes, & 25 lb/sq inch in a person using padded, molded inserts.
    • Pressures as low as 20 lb/sq inch at 10 000 reps/day progressive inflammation develops by day 3 & true ulceration by day 8. Normal people feel pain early in the inflammatory phase & remove the foot, but insensate patients are not able to do this
    • Pressures as low as 1-2lb/sq inch can block capillary flow, resulting in ischaemia, & these pressures are easily achieved in poor fitting shoes
• Autonomic dysfunction
  • results in a ↓ in sweating. This results in dry, cracked skin. Cracking may allow bacteria to penetrate the skin
• Neuropathic skin
  • tends to produce excessive keratin. Hard callus can form which damages underlying tissues by causing local ↑ in pressure
• Motor loss
  • less common & can affect both the nerve & the motor endplate. Motor loss affects the distal motor nerves most, leading to atrophy of the small muscles of the foot, which becomes manifest as claw toes, with high arches. This results in abnormal pressure distribution which predisposes to plantar ulceration under the prominent metatarsal heads, & corns over the prominent PIPJ

Vascular system

• Atherosclerosis
  • is more common in diabetics, occurs earlier, & affects a higher percentage of women
  • involvement is more diffuse, more often bilateral & more rapidly progressive
  • Because the disease is more rapidly progressive, there is less time to develop collateral circulation
• Disease affects both small & large vessels.
  • basement membrane is thicker in the capillaries
  • large vessels are often occluded at sites of bifurcation or passage through a hiatus, thus at the aortic bifurcation, iliac bifurcation, femoral artery bifurcation & passage through the adductor hiatus. The arteries become calcified & non compressible
  • Note that the blood supply necessary to allow healing of an ulcer is greater than that needed to maintain intact skin

Immune system

• Diabetics have altered white cell function, with impaired chemotaxis
• They have altered fibroblastic function & ↓ collagen production & strength, which results in impaired wound healing

Osteoporosis

• Generalized osteoporosis of the feet may predispose to insufficiency fractures around the ankle or in the metatarsals

Evaluation

The skin reflects the functional level of circulation & sensation.

Sensation

• Qualitative measures of skin sensation
  • light touch & pin prick sensation
  • two point discrimination
  • proprio-ception
• Quantitative measures
  • Semmes Weinstein monofilaments
    • Semmes Weinstein monofilament 5.07 (represents 10g of pressure) will predict most accurately the likelihood of a patient developing an ulcer
    • 90% of patients that can feel this monofilament will not develop ulcers
    • However, if the patient has previously had an ulcer, this is the best predictor of the likelihood of developing another
  • Biothesiometer
    • a device that delivers a measured reproducible vibratory stimulus, but has problems with inconstant applied pressures

Vascular

• Qualitative measures
  • evaluation of the skin temperature
  • capillary refill
  • pulses
  • hair & nail growth
• Quantitative measures
  • measurement of the ankle brachial indices, s
  • skin perfusion pressure
  • transcutaneous oxygen diffusion

1. Arterial Doppler measurements
   1. If a patient obviously has ischaemia but the ABI (ankle to brachial arterial index) is 1.0 or higher this suggests stiffening of the vessel wall by calcification rather than good flow. There should be a level of 0.45 or greater
   2. Doppler flow study will also show pulsatile flow tracings & not the normal triphasic wave forms seen in non calcified vessels
2. Skin perfusion pressure can be measured using a photodetector pressed against the skin by a manometer. The pressure is read at the time of skin reddening as the pressure is released. Pressures of 31-40mmHg have been associated with an 85% rate of healing
3. Transcutaneous PO2 measurements are probably the best measurement of healing potential. They are performed at standardized points on the foot using a neonatal sats probe. Prior to performing the test the skin should be heated to 45° for 10 minutes
   1. Levels below 20mmHg indicate a poor healing potential (less than 50%)
   2. Levels from 20-29 mm Hg have a 75% healing rate
   3. Levels greater than 30mmHg have a 92% healing rate
   4. Given a PO2 of 35mmHg at mid foot a transmetatarsal ampu-tation should heal
   5. TcPO2 level is adversely affected by cellulitis & oedema

Charcot joints

• occur in less than 1% of diabetic patients
  • (but diabetes is the commonest cause of a neuropathic joint in western countries, leprosy & tertiary syphilis being more common worldwide)
• Affected patients are more likely to be insulin dependent
• 30% of cases are bilateral
• most commonly affected joints are the midfoot (60%) then the hindfoot (30%).
  • In the hindfoot, the typical deformity is in varus, which makes the lateral malleolus prominent
• presentation may be insidious, with a progressive flatfoot deformity, or may be acute, mimicking infection
• The differentiation can be made by elevating the foot above heart level, when the swelling & erythema secondary to the Charcot joint should rapidly settle, & by following the inflammatory markers
• diagnosis can also be helped by using a combination of technetium & indium-111 scans

Treatment

Diabetic foot care

The most important component of diabetic foot care is patient education.

1. Regular inspection of the foot to rule out ulcers/callosities/bruises/cuts
2. Feel the limb for any warmth or heat
3. Hydration of skin
   • 15-20 minute soaks, plain water, room temperature
   • Oiling of skin (lanolin best). This will help to keep the skin hydrated, & callus moist & thereby supple
4. Debridement of callus
   • This should be done daily by the patient & may need to be done fortnightly or monthly by the podiatrist
5. digits need to be put through a range of motion to prevent contracture

• Other pointers in diabetic foot care
  • Adequate shoes – no patient with a neuropathic foot should go without shoes. Grace Warren says the best insole is 150 shore microcellular rubber (MCR), ideally 6mm thick
  • Use white socks; this makes it easier for the patient to see areas of discharge
  • Positional adjustments are necessary

Management of ulceration

• Ulceration is caused by a combination of pressure & neuropathy
• pressure is often applied to a bony prominence
• When neuropathic ulcers are unroofed the underlying granulation tissue is usually healthy
• Ischaemic ulcers often have necrotic tissue in their bases, & patients with ischaemic ulcers need a vascular consultation to see if the limb can be saved. Ischaemic ulcers typically need angioplasty or bypass surgery to achieve healing
• 70-90% of neuropathic ulcers occur in the forefoot
• first step in the management of diabetic foot ulcers is optical medical management of the diabetes with good glycaemic control

• Grade O “ulcers”
  • treated by pressure relief
  • Surgical excision of bony prominences should be considered if non-operative measures are inadequate, assuming the patient has adequate blood supply to allow healing
• Grade 1 & 2 lesions
  • can be treated with a total contact cast
  • Myerson used the total contact cast in the treatment of these lesions with a 90% healing rate at 6 weeks
  • There was however a 30% relapse rate over the 18 month followup period, but 80% of these ulcers healed in a second cast after two weeks of treatment
• Role of hyperbaric oxygen therapy
  • Use is controversial, but has been shown to be beneficial in patients with a positive oxygen challenge (↑ in transcutaneous oxygen tension when breathing 100% oxygen)
• Achilles tenotomy
  • Patients often develop an Achilles contracture which leads to ↑ forefoot pressures. Tenotomy may benefit these patients

Infection

• commonest organisms are S. aureus, Streptococcus species, Enterococcus organisms & S. epidermidis
• Commonly isolated gram negative organisms include Proteus & Pseudomonas
• Anaerobes are cultured in around 1/3
• most specific investigation for osteomyelitis is an Indium 111 labeled white cell scan
• In treating infections, the appropriate antibiotics plus or minus surgery are used
• Areas that are poorly vascularized or necrotic need to be debrided/amputated to the level of viable tissue

Management of Charcot joints

• Stage 1
  • characterized by fragmentation & presents as a fracture, or ligamentous disruption. Treatment should be by elevation for 1-2 days, then total contact casting. Weight bearing is not allowed until the inflammatory phase is over, which takes several months
• Stage II
  • characterized by coalescence. Fragmentation gradually stops on a series of X-rays. The patient is allowed to progress to weight bearing in a total contact cast
• Stage III
  • characterized by bone consolidation & healing. Stages II & III typically last 18-24 months.

Management of calcaneal osteomyelitis

• This depends on adequate blood flow
• If there is adequate blood flow, debridement, IV ABs & total contact cast
• If there is not, below knee amputation. Symes or Boyd amputation is contra-indicated

Management of fractures

• Most common fracture is of the Lisfranc joint
• Fractures should be managed closed if possible with a full contact cast extending to past the toes, until there is demonstration of healing & tissue homeostasis is returning

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