pressure and the diabetic foot: clinical science and offloading techniques

Pressure and the diabetic foot: clinical science and offloadingtechniques

Andrew J. M. Boulton, M.D., F.R.C.P.Department of Medicine, Division of Endocrinology, University of Miami, PO Box 016960 (D-110), Miami, Florida 33101, USA

and Department of Medicine, University of Manchester, Manchester Royal Infirmary, Manchester, United Kingdom

Abstract

Diabetic foot ulceration is a common, yet in many cases an eminently preventable, complication that affects 1 in 20 patients withdiabetes. Risk factors for ulceration include insensitivity (secondary to somatic neuropathy), high foot pressures, callus formation (aconsequence of sympathetic neuropathy and high foot pressures), deformities (such as claw feet, prominent metatarsal heads, etc.),peripheral vascular disease, and most importantly, a past history of ulceration. None of these factors alone causes ulceration; thus, earlyidentification and amelioration of these factors is a primary aim in foot ulcer prevention. A number of therapeutic approaches may helpreduce ulcer incidence: these include therapeutic footwear, hosiery, and, potentially, liquid silicone injected under high-pressure areas. Inthe management of neuropathic ulcers, pressure relief is of the utmost importance, and total contact casting remains the “gold standard”means of achieving such pressure redistribution. The successful management of diabetic foot ulceration depends on a team approach,remembering that ulcers should heal if (1) the arterial circulation is intact, (2) pressure relief is achieved and maintained over the ulcer, and(3) infection is appropriately treated. © 2004 Excerpta Medica, Inc. All rights reserved.

I marvel that society would pay a surgeon a large sum ofmoney to remove a person’s leg—but nothing to save it.

—George Bernard Shaw

The aims in treating diabetic foot problems should primarilybe concerned with limb preservation when at all possible. Inthis regard, pressure applied to the foot is vitally importantat every stage in the causation and management of diabeticneuropathic foot lesions: that is, in primary and secondaryprevention, as well as in the treatment of active ulcers andacute Charcot neuroarthropathy. The need for a better un-derstanding and treatment of diabetic foot ulceration isstrongly supported by the literature as well as by depressingstatistics: trends in amputations show no overall signs ofimprovement [1–3], and foot ulceration is the most commonreason for hospitalization of diabetic subjects in many coun-tries [4]. In this review, the main contributory factors in theetiopathogenesis of diabetic foot ulceration will be consid-ered, with special emphasis on the role of foot pressureabnormalities. Potential approaches for the reduction orredistribution of pressure will be discussed. Finally, theneed for and methodology of pressure reduction as part of

the treatment of neuropathic foot ulceration will be consid-ered.

The pathway to diabetic foot ulceration

Coming events cast their shadows before.—Thomas Campbell

The problems related to the pathogenesis of diabetic footulceration are summarized by the words of the Scottish poetThomas Campbell. Diabetic foot ulceration is very com-mon, affecting 1 in 20 patients with diabetes at some timeduring their lives. The “coming event” of a foot ulcer can inmany cases be predicted by identifying the “shadows” orwarning signs that appear in many patients with diabetes athigh risk of ulceration. Foot ulceration invariably occurs asa consequence of an interaction between environmentalhazards and specific pathologies in the lower extremities ofthese patients. Peripheral vascular disease is of course muchmore common in this population, but in terms of prevention,it is the neuropathic or neuroischemic ulcer that provides themost scope for prevention.

Diabetic neuropathy

Diabetic neuropathies are among the most common ofall diabetic complications, appearing in up to 50% of older

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The American Journal of Surgery 187 (Suppl to May 2004) 17S–24S

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patients with type 2 diabetes [5,6]. Of the various neuro-pathic subgroups, it is distal sensory polyneuropathy andperipheral autonomic dysfunction that are most associatedwith risk of foot problems. Distal sensory polyneuropathy isextremely variable in its presentation, ranging from theseverely painful symptomatic variety at one extreme to thecompletely painless variety at the other, which may presentwith insensitive foot ulceration. Indeed, up to 50% of pa-tients with neuropathy never experience symptoms, so ab-sence of neuropathic symptomatology must never be asso-ciated with absence of risk of foot ulceration (Fig. 1). The“at-risk” foot can be identified only by a careful clinical

examination for loss of sensation to large and small fiberfunctions (for example, vibration and pinprick sensation)and absence of ankle reflexes. A large community-basedproject in northwest England, the Northwest Diabetes FootCare Study [7], has recently confirmed in a prospectivefollow-up of almost 10,000 patients that a simple clinicalexamination, such as that noted above, is highly predictiveof risk of foot ulcers. Those with a moderate or severedeficit on clinical examination had a 6-fold increased risk ofdeveloping ulceration, compared with those with normalsensory function [7]. Peripheral autonomic dysfunction re-sults in increased peripheral blood flow in the absence of

Fig. 1. The dangers of insensitivity. This patient had marked sensory loss with loss of pain and large fiber sensation. He also had a Charcot joint at thecuneiform-metatarsal area of the left foot. One evening he fell asleep in front of the fire and woke to the smell of burning: the burning was that of his ownflesh, which had gone completely unnoticed because of the sensory loss.

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large vessel obstructive vascular disease, together with dryskin because of the absence of sweating. Thus, the warm,dry, insensitive foot is at high risk of ulceration.

The strong association between peripheral neuropathyand foot ulceration has been recognized for many years [8],but it was not until the last decade that this was confirmedin prospective studies. A single-center study demonstratedthat neuropathic patients had a 7-fold annual increase in therisk of ulceration, and this earlier report was confirmed in alarger multicenter study from Europe and North America,which showed that those with neuropathy had a 7% annualrisk of developing foot ulceration [9,10].

Foot pressure abnormalities in diabetes

The insensitive diabetic neuropathic foot does not ulcer-ate spontaneously: traumatic ulcers result as a consequenceof trauma to the insensitive foot, as in the patient whopurchases shoes of an insufficient size and fails to experi-ence any discomfort while wearing them. This constantlymaintained pressure leads to breakdown of the skin at thesite of highest pressure, usually the lateral border of the fifthor first metatarsal head.

In contrast, pressure ulcers occur as a result of pressurethat would not normally cause ulceration, but which, be-cause of intrinsic abnormalities of the neuropathic foot,leads to plantar ulceration when applied repetitively. Ab-normalities of pressures and loads under the diabetic footare very common, as has been shown in several cross-sectional studies [11,12]. Thus, the combination of insensi-tivity, abnormally high foot pressures, and repetitive stressfrom, for example, walking may lead to breakdown underhigh-pressure areas, such as the metatarsal heads. In a pro-spective study, Veves et al [13] observed a 28% incidenceof ulceration in neuropathic feet with high plantar pressuresduring a 2.5-year follow-up period; in contrast, no ulcersdeveloped in patients with normal pressure. Thus, biome-chanics, the branch of science concerned with the conse-quences of forces applied to living tissues, is clearly rele-vant to diabetic foot disease because the majority ofneuropathic foot ulcers result from mechanical stress that isnot perceived by the patients. The repetitive application ofhigh pressures to the same soft tissue sites overlying a bonyprominence in the absence of protective sensation is be-lieved to cause tissue damage, which begins deep and ini-tially may not be visible on examination. However, callustissue frequently forms at the surface, which is a cutaneousmarker for high pressure. Before ulceration, hemorrhageinto the callus occurs; this should be recognized as a “preul-cerative” lesion [14] (Fig. 2). In a separate observationalstudy, Murray et al [15] reported that the increased risk ofulceration in an insensitive foot with a callus was 77-fold,whereas in the prospective follow-up, ulceration occurredonly at sites of callus.

A number of methodologies are available to assess plan-tar pressures: these devices may assess barefoot or in-shoe

plantar pressures, and all usually require the use of a com-puter together with software. Optical systems includethe optical pedobarograph (Fig. 3), whereas the majorityof methodologies use a matrix of transducers [16,17]. Allthese systems are necessarily expensive and require trainedpersonnel and often a gait laboratory. More recently, a semi-quantitative estimation of pressure distribution known asPodoTrack (A. Algeo Ltd., Liverpool, UK) or PressureStat(FootLogic, New York, NY) has been reported. This isbased on the original Harris mat technique, and comprises a3-layer sandwich on which the patient’s foot leaves animpression in different shades of gray. Thus, this is a por-table, inexpensive, and disposable method for semiquanti-tative assessment of pressure under the foot. After walkingacross this mat, plantar pressure can be assessed by visualcomparison between the grayness of the foot print and acalibration card [17]. In a comparative study with the pe-dobarograph, the PodoTrack identified all high-pressure ar-eas, suggesting that this could be a useful screening tool toidentify areas at risk of ulceration in patients with diabetes[18].

More recently, techniques have been developed to assessthe depth of subcutaneous tissue under high-pressure sites,such as the metatarsal heads. One such technique usesultrasound to determine depth while standing [19]. In a largecross-sectional study, Abouaesha et al [20], using the Plan-scan (Toshiba Medical Systems Europe, Zoetermeer, Neth-erlands) ultrasound technique, demonstrated a significantinverse correlation between peak plantar pressures and plan-tar tissue thickness at all metatarsal heads. Thus, high pres-sures are associated with reduced subcutaneous tissuedepth, and the simple assessment of subcutaneous tissuedepth by ultrasound may be an alternative method of iden-tifying areas at high risk of ulceration.

Other risk factors for foot ulceration

Several studies have confirmed that foot ulceration ismost common in those patients with a past history of ulcer-ation, amputation, or Charcot neuroarthropathy. This riskmay be as high as 50% of patients with new foot ulcers whohave a past history of similar problems [8].

Patients with other complications of diabetes, includingretinopathy and/or renal impairment, are at increased risk ofulceration. In contrast, patients from certain racial or ethnicbackgrounds, including Indian subcontinent Asians, seem tobe at reduced risk of ulceration. This may be related tobetter foot care in certain religious groups, but it is notrelated to geographical differences [8]. Finally, any defor-mity occurring in the diabetic foot, such as prominence ofmetatarsal heads, clawing of toes, Charcot deformities, orhallux valgus, increases ulcer risk.

An understanding of all these contributory factors thatultimately result in foot ulceration is likely to help in theplanning of effective preventive strategies. In a 2-center(US/UK) study assessing causal pathways to foot ulceration

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[21], neuropathy was the most common component cause inthe pathway. According to this study, the most commoncausal pathway leading to ulceration involved a combina-tion of neuropathy (resulting in insensitivity), deformity(such as claw toes or prominent metatarsal heads), andtrauma (most commonly resulting from inappropriate foot-wear; Fig. 4).

Prevention of foot ulceration

A number of therapeutic approaches to “offload” or re-distribute high pressure have been reported in the last 10years.

Therapeutic footwear

The use of therapeutic footwear is certainly not a newapproach: examples of footwear worn by Roman soldierscan be seen at Hadrian’s Wall Museum in northern England.Although it is well recognized that inappropriate footwear isa very common component cause in the pathway to footulceration, until recently, footwear design has been more ofan art than a science. In a systematic review of foot ulcerprevention for patients with type 2 diabetes, Mason et al[22] found only 1 controlled study that had assessed theefficacy of therapeutic shoes to reduce ulcer incidence. In arandomized study of 69 patients with previous ulcerations,

Fig. 2. Preulcerative lesion. A typical neuropathic foot with prominent metatarsal heads and marked callus formation of the fifth metatarsal head and bleedinginto the callus. This was removed, and the patient had a superficial grade 1A ulcer under this callus.


Fig. 3. The printer output for 1 footstep recorded from a patient with neuropathic diabetes’ previous foot ulcer under the first metatarsal head region, left foot.Ten sample frames from the footstep are shown on the left from heel strike (top left) to the lift-off phase of the footstep (bottom middle). The composite imagefor the whole footstep is shown (top right), along with a graph of pressure versus time for selected areas of interest (bottom right). Note the very high pressuresfound under the first metatarsal head, particularly toward the end of the footstep.

Fig. 4. Pathways to diabetic foot ulceration. The classic pathways to foot ulcers are demonstrated here, but increasingly patients with postneuropathy andvascular disease are presenting with neuroischemic ulcers.


Uccioli et al [23] reported significantly fewer recurrentulcers in those wearing therapeutic shoes rather than theirnormal footwear. This study therefore suggested importanthealth benefits from appropriately designed footwear. Otherreports have confirmed the importance of footwear as partof a multidisciplinary approach, including regular podiatry,foot clinic visits, and so on [24].

It should also be remembered that the vast majority ofpatients at risk for foot ulcers can be successfully managedeither in over-the-counter sports shoes, such as sneakers, orin extra-depth shoes with prescribed flat or customizedinsoles [14]. Appropriate footwear should provide cushion-ing or relief of pressures at sites of elevated pressure andmight also transfer load from one site to another. For de-tailed discussion of the use of footwear and insoles tooffload high-pressure areas, the reader is referred to detailedreviews by Cavanagh et al [14,17].

Hosiery

Protective padded hosiery has been shown in a prospec-tive study to reduce high foot pressures that are known to beassociated with the genesis of foot ulceration [25]. A sub-sequent longitudinal study conducted in the United Statesconfirmed that such protective hosiery had a high level ofpatient satisfaction when worn with suitable shoes, and thusappears to be an acceptable and inexpensive addition toexisting methods of protecting the high-risk, insensate dia-betic foot [26].

Injected liquid silicone

As noted above, foot ulceration frequently occurs at sitesof higher plantar pressure and callus formation under themetatarsal head region of the insensate neuropathic foot.Indeed, the removal of plantar callus has previously beenshown to reduce abnormally high foot pressure under themetatarsal heads [27] and therefore probably to reduce therisk of ulceration. Previous uncontrolled reports from Cal-ifornia had suggested that the therapeutic use of liquidsilicone injections in the foot to replace the fat padding atcallus sites, corns, and localized painful areas led to areduction in callus formation and foot ulcer occurrence [28].More recently, a randomized controlled trial investigatedthe effectiveness of liquid silicone injections in the diabeticfoot and confirmed that those patients receiving siliconeinjections had significantly increased plantar tissue thick-ness at injected sites compared with the placebo group, aswell as correspondingly significantly decreased plantarpressures. Such benefits were maintained for up to 12months of follow-up. A trend was also noted toward areduction of callus formation in the silicone-treated groupcompared with no change in the placebo group [29]. Furtheranalyses of the results in this study suggest that the patientsmost at risk of ulceration (ie, those with highest foot pres-sures and lowest subcutaneous tissue thickness) are likely to

respond best to treatment of injection of liquid silicone inthe plantar surface of the foot [30]. These preliminary datasuggest the potential efficacy of plantar silicone injections,which appears to be safe and without side effects [29,30].

Management of neuropathic foot ulceration

Offloading of high-pressure areas in the neuropathic footis a key step in the management of ulceration. Many meth-ods have been suggested, and these include bed rest, wheel-chair, crutches, total contact casting (TCC), felted foam,half-shoes, therapeutic shoes, cast walkers, and so on [31].The first 3 on this list theoretically should be very effectivebut rarely work in practice. It must be remembered thatpatients with insensitive feet have lost their warning signal,pain, which ordinarily prevents them from walking on anulcer. The care of the patient with no pain sensation is achallenge for health care professionals, few of whom haveany training in this area. It is difficult for us to understand,for example, that an intelligent patient with an active ulcermay rest in bed for the first day but then take a small step,find that there is no discomfort, and soon get back to normalactivities. It is therefore important to understand that if weare to succeed, we must realize that with loss of pain thereis also diminished motivation in the healing of injuries. Thisis among the main reasons why, of all the items listed asoffloading modalities, it is TCC that is accepted as the “goldstandard.”

Casting

A number of casting techniques have been used in themanagement of neuropathic foot ulceration, with the earlieststudies being performed in patients with leprosy (Hansendisease). However, the use of casting has been supported by2 randomized trials. In the first of these, Mueller et al [32]randomized 40 patients to either traditional treatment orTCC and demonstrated that significantly more ulcers healedwith fewer infections in the TCC group. More recently,Armstrong et al [33] compared TCC with a removable castwalker and a half shoe in a randomized, controlled trial. Theproportion of healing was far greater for those with TCCtreatment, with 90% of patients being healed within 12weeks of casting. During this trial, all patients were pro-vided with an activity monitor, a device that measures thenumber of steps taken over a period of time and records thetime that each step is taken. A further observation from thisstudy was that those treated with TCC were significantlyless active, with 600 daily steps compared with �1,400daily steps taken by those in the 2 other treatment groups.As a consequence of this study, Armstrong et al performeda further observational study in which patients with diabetesand a high risk of recurrent ulceration were dispensed acontinuous activity monitor and a diary to record wheresteps were taken. An interesting finding from this study was


that although 85% of patients wore the approved footwearmost of the time while they were outside the home, only15% continued to wear it at home. Because patients alsotook more steps per day inside their homes, it appears thatfuture preventive education should focus on protection ofthe foot during in-home walking [34]. Indeed, activity-monitoring studies [35] have suggested that we might con-sider “dosing” activity as we dose a drug—that is, restrict-ing mobility during periods of active ulceration and in theearly phases of rehabilitation after foot ulcers have healed.

The contributory factors to the efficacy of TCC treatmentare likely to be, first, that there is pressure redistribution andoffloading of the ulcer area, and second and possibly equallyimportant, that the patient is unable to remove the cast,thereby enforcing compliance and reducing activity levels.A number of other casting techniques are often used andinclude the removable Scotchcast boot (fiberglass castboot; 3M, Maplewood, MN), where a window is cut underthe ulcer area and the cast. The cast gives support up toankle level and may be removed at night. Walking is per-missible with a walking sandal. Such casts have been suc-cessfully used in several centers.

Other modalities

A number of approaches to offloading foot ulcerationhave been described [36] and include a number of differentcasting techniques and also the use of a half-shoe in thehealing of forefoot plantar ulcers in Germany [37]. Theseshoes support only the rear and mid foot, leaving the fore-foot “suspended” in mid air. A number of different orthoticdevices have also been described, as discussed by Cavanaghet al [17]. Most recent is another report from Germany ofthe effects of felt foam on plantar pressures [38]. Theseauthors demonstrated reduced plantar loading at the side ofulceration and emphasized that this technique also enableddaily dressing changes in patients with active ulceration.

Conclusion

In summary, abnormalities of pressure are key contrib-utory factors in the pathogenesis of diabetic neuropathicfoot ulceration in many cases. Early identification and pre-ventive education of high-risk patients is indicated togetherwith further investigations to identify specific high-riskareas. These might vary from the very simple assessment ofpressure using a disposable mat to more complex measuresof loading or subcutaneous tissue depth under the foot.Similarly, strategies to reduce foot pressures range from thesimple, such as appropriate footwear, to the more invasive,which might include in the future injection of liquid siliconeor other substances to increase padding under high-pressureareas. For those with foot ulceration, offloading is pivotal inthe therapeutic strategy. Successful healing should follow if(1) pressure is relieved from neuropathic foot ulcers, (2) the

arterial inflow is adequate, and (3) any infection is treatedby debridement of infected tissue and appropriate antibiotictherapy. Whereas this article has focused on the first of these3 strategies, detailed texts are available and cited for otheraspects of diabetic foot management.

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pressure and the diabetic foot: clinical science and offloading techniques

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