pedobarograph in the assessment of pressures under the diabetic foot.
Diabet Med 1999;16:154, 159. 42.
Pinzur MS, Slovenkai MP, Trepman E, et al.
Guidelines for diabetic footcare.
Foot Ankle Int 2005;26:113, 119. 43.
Veves A, Murray HJ, Young MJ, Boulton AJM.
The risk of foot ulceration in diabetic patients with high foot pressure: a prospective study.
Diabetologia 1992;35:660, 663. 44.
Pataky Z, Assal JP, Conne P, et al.
Plantar pressure distribution in type 2 diabetic patients without peripheral neuropathy and peripheral vascular disease.
Diabet Med 2005;22:762, 767.
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6 What the Practising Clinician Should Know About Foot Biomechanics Peter R. Cavanagh and Jan S. Ulbrecht
INTRODUCTION Biomechanics is a branch of the life sciences concerned with the consequences of forces applied to living tissues.
This field is clearly relevant to diabetic foot disease, since the majority of foot ulcers result from mechanical stress, which, because of loss of protective sensation (LOPS) to pain,1 is not perceived by the patient.
The relevance of biomechanics to the practising clinician who treats diabetic foot problems can be stated very clearly: many of the recalcitrant diabetic foot ulcers that fail to heal in a typical practice fail not because of medical issues, in which the clinician is well versed (infection, impaired immunity, vascular disease, etc.), but because of simple biomechanical issues, which were often not discussed during medical training.
Thus, a few minutes spent becoming familiar with those biomechanical issues will pay considerable dividends in improved patient care.
Biomechanical considerations are important in all three phases of care of the diabetic foot: primary prevention, healing foot ulcers and secondary prevention (prevention of ulcer recurrence).
This chapter discusses several very practical concepts that can be applied to diabetic feet; it does not address the more quantitative areas of biomechanics (such as tissue property charac- terisation and modelling).
It should also be pointed out that there is an entire field of foot biome- chanics, which is concerned with ‘balancing’ structural abnormalities in non-neuropathic feet.
The types of interventions typically used by practitioners of that field (such as rigid ‘corrective’ orthoses) are not relevant to our present discussion.
Most of this chapter will concern itself with the most common diabetic foot ulcer, the neuropathic plantar ulcer.
Skin breakdown due to pen- etrating injuries, burns and dorsal surface injuries due to ill-fitting footwear are all also common, but will be addressed only briefly (see Chapter 28 for a more complete discussion of footwear).
STRESS AND STRESS CONCENTRATION Because force and stress (force divided by the arrea over which it is applied , which we shall call pressure) cannot be seen without the aid of specialised instruments, it is easy to overlook
The Foot in Diabetes, 4th Edition. Edited by Andrew J.M. Boulton, Peter R. Cavanagh and Gerry Rayman. C 2006 John Wiley & Sons, Ltd. ISBN: 0-470-01504-7
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STRESS AND STRESS CONCENTRATION 69
Figure 6.1 Postero-medial view of a peak pressure distribution (a) measured during barefoot walking, under the foot, and (b) of a patient with a prior ulcer at a prominent second MTH (MTH2).
the dramatic concentrations of load that can occur at bony prominences on the plantar aspect of the foot.
In the single-limb support phase of gait, the total force under the foot will always be approximately 110% of body weight (the extra 10% comes from the ‘inertial’ component as the body decelerates and accelerates throughout the gait cycle).
Since a typical man’s size-10 foot has a total area of approximately 130 cm2 , the average pressure under the foot of a 100-kg person would be 0.77 kg/cm2 (force/area) or, stated in the more usual units, approximately 75 kPa (kilopascals).
Figure 6.1 shows an actual pressure distribution measured during barefoot
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70 WHAT THE PRACTISING CLINICIAN SHOULD KNOW
walking under the foot of a patient who had a prior ulcer at a prominent metatarsal head (MTH).
The actual peak pressure is almost 15 times greater than if calculated as above, using the simple force/area argument.
In units that might be easier to comprehend, this peak pressure under this patient’s foot is approximately 160 pounds per square inch or 11.2 kg/cm2 .
Pressures under the foot during running and turning while walking can be 40% greater than those encountered during walking.2 Another important mechanical quantity that has the potential to damage tissue is shear stress, which results from the forces exerted parallel to the skin and tends to cause a tear.3 Because only a few rudimentary measurements of shear stress have been made,4 we have no clear idea of the role of shear stress in foot injury, although most authorities believe that it is a significant factor.
NEUROPATHY AND HIGH PRESSURE , THE KEY COMBINATION As discussed elsewhere in this volume (Chapters 3 and 5), peripheral neuropathy results in what has been called a ‘loss of protective sensation’ (LOPS).
The loss of sensation to touch, temperature, pain and deep pressure can be so dense that patients, without being aware of it, can allow objects to penetrate completely through the foot from plantar surface to the dorsum, or they can burn their feet with hot water, etc.
However, most injuries or ulcers in patients with diabetes and LOPS occur at sites of high plantar pressure.
High pressures, such as those shown in Figure 6.1, are not usually seen in healthy feet and would result in pain during ambulation for an individual with adequate sen- sation.
For example, patients with bony deformities from rheumatoid arthritis can experience such pressures5 without ulceration because they either adjust their gait to avoid bearing load on a prominent and painful area and/or choose footwear that will reduce the pressure (see below).
However, the repetitive application of high pressures to the same soft tissue overlying a bony prominence in the setting of LOPS is believed to cause tissue damage that begins deep (close to the bone).6 Callus frequently forms on the skin surface.
When a patient presents with callus exhibiting a shadowy dark base to visual examination, this is usually an indication that there is a deep ulcer causing haemorrhage into the callus.7 This ‘pre-ulcer’ will then usually develop into an ulcer, with further walking.
Thus, high pressure alone is not sufficient for plantar ulceration, and neither is neuropathy , it is the combination of the two that provides the necessary and sufficient conditions for ulceration.
Since most clinicians will encounter neuropathic diabetic patients who are hospitalised for non-foot-related complaints, it is important to mention here that low pressure applied for long periods of time to feet with LOPS can also cause devastating lesions.
The most common manifestation is deep bilateral pressure ulcers, often penetrating to tendon and bone, on the heels of patients who have been bedridden for a period of time.
A similar result can occur in just a few hours in patients who have been lying supine during a surgical procedure.
In both situations, the ulcers are entirely iatrogenic, caused by failure of the clinician to insist on load relief for neuropathic patients, and the failure of the nursing staff to either recognise or act on the knowledge that the patient was neuropathic.
MECHANISMS FOR ELEVATED PRESSURE Over time, people with diabetes can develop areas of abnormally high pressure under the foot during weight-bearing activities, and this can result from a number of intrinsic, extrinsic and
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MECHANISMS FOR ELEVATED PRESSURE 71
Table 6.1 Biomechanical factors that can lead to elevated plantar pressure under the foot during walking
Intrinsic Extrinsic Behavioural
Foot architecture Poor footwear Walking without shoes Long second toe Tight or loose shoes High arch Shoes with hard soles Soft tissue alterations Accidents and incidents Poor choice of shoes Callus Glycosylation (presumed) Migration of tissue Thin tissue Limited joint mobility Prior surgery Inadequate callus care Foot deformity Walking patterns Claw toes Hallux valgus Charcot fracture
behavioural factors (Table 6.1).
According to Edmonds et al.,8 most neuropathic ulcers occur on the toes (39%), the hallux (30%) and the MTHs (24%); these areas, therefore, are of principal concern in both understanding the causes of elevated pressure and how intervention might be accomplished successfully.
There is some debate about the critical magnitude of plantar pressure that is required for tissue damage.
Veves et al.9 believe that a value of over 1000 kPa during barefoot walking is required, but other studies report ulceration at values below 500 kPa.
Armstrong et al.10 have suggested that a threshold of 700 kPa is the best compromise between sensitivity and specificity.
It is, however, likely that each patient’s threshold is different and that the more active a patient is, the less pressure is needed at each step to cause ulceration.
Also, since most studies have measured barefoot pressure, the footwear chosen by an individual patient can clearly make the difference between ulceration and no ulceration.
It is likely that the pressure between the foot and the shoe is the most important variable to minimise.11