Intrinsic Factors Certain foot structures predispose an individual to elevated pressures.
Some, like a long second metatarsal (Morton’s toe) and a high arch,12,13 are not diabetes related.
Callus appears to con- centrate pressure rather, as if it were a foreign body under the foot.14 Studies have shown that the presence of callus increases the risk of ulceration by over 11 times.7 Thus, callus should be regu- larly removed.
There are some indications that the properties of the plantar soft tissue may be adversely affected by glycosylation end products, although much remains to be explored in this area.
Palpation of the MTHs in a patient with claw toes often reveals an exquisitely thin layer of soft tissue overlying the bone, which is directly exposed to high pressures during walking and thus easily damaged unless countermeasures are undertaken (see below).
In fact, the lack of adequate thickness of soft tissue under bony prominences has been shown to be an extremely important determinant of elevated pressure in normal subjects.13 Clawing of the toes in diabetic patients appears to result in the plantar fat pads being displaced anteriorly, leaving the condyles
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72 WHAT THE PRACTISING CLINICIAN SHOULD KNOW
(a)
(b)
Figure 6.2 Postero-medial view of a peak pressure distribution (a) measured during barefoot walking showing elevated pressure at the tips of a clawed second toe. Note that the pressures under toe 2 and the hallux are approximately equal. The foot is shown in part (b).
of the MTHs ‘exposed’.
This has been shown to lead to higher than normal plantar pressures.15 The tips of claw toes can themselves be locations of ulcers due to concentrated pressure (Figure 6.2).
Toe deformities (claw toes, hammer toes, hallux valgus) also tend to result in higher pressures.
The range of motion at many joints has been shown to be decreased in patients with diabetes.16 This is not a neuropathic complication, but probably another effect of glycosy- lation whereby the collagen in joint capsules is stiffened by the glycosylation process.
The
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consequence of reduced range of motion at the major joints of the foot and ankle (such as the first metatarso-phalangeal (MTP), sub-talar and talo-crural joints) is likely to be increased plantar pressures under the forefoot.6 The most frequently problematic joint in this regard is the first MTP.17 Invariably, a patient with a neuropathic ulcer under the pad of the hallux will be found to have reduced capacity for dorsiflexion at this joint18 (Figure 6.3).
Figure 6.3 Postero-lateral view of a peak pressure distribution (a) measured during barefoot walking, from a patient with a neuropathic ulcer under the pad of the hallux (b) secondary to a reduced capacity for dorsiflexion at the first MTP joint (Reprinted from Cavanagh PR, Ulbrecht JS, Caputo GM.
Biomechanics of the foot in diabetes mellitus.
In: Bowker JH, Pfiefer M, eds.
The Diabetic Foot 6th edn, Copyright C (2000) Elsevier).
Note that the MTH1 and hallux pressures are approximately equal, although this patient has never experienced an ulcer under MTH2.
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Despite the above emphasis on the forefoot, a number of conditions can cause elevated pressure in other regions of the foot. Charcot fractures of the midfoot20,21 typically result in a ‘rocker bottom’ foot, which bears load principally on the collapsed region of the midfoot (Figure 6.4). Certain surgical procedures that are intended to reduce loads at primary areas of ulceration can have a secondary effect of increasing pressure in other areas. For example,
Figure 6.4 Postero-medial view of a peak pressure distribution (a) under a ‘rocker bottom’ foot and (b) during barefoot walking. Load is principally borne on the collapsed region of the midfoot. Other regions in the rearfoot and forefoot receive almost no load throughout the entire contact phase.
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lengthening of the Achilles tendon, which is sometimes performed following forefoot surgery, can result in what is known as a ‘calcaneus gait’, in which elevated heel pressure occurs during much of the stance phase (Figure 6.5). Removing MTHs because of ulceration in that region can also lead to higher pressures under the remaining MTHs.
Figure 6.5 Postero-medial view of peak pressure distributions (a) before and (b) 3 months after surgery that included an osteotomy of the first metatarsal and a lengthening of the Achilles tendon. Note the reduction in forefoot pressures and the increase in the heel peak pressure post-surgically.
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76 WHAT THE PRACTISING CLINICIAN SHOULD KNOW
The effects of motor neuropathy are often underestimated, but a number of studies have shown dramatic atrophy of the intrinsic muscles of the feet (Figure 6.6).22,23 This atrophy alters the biomechanics of the foot and may lead to instability during standing and walking. Postural stability is markedly degraded in neuropathic patients,24 probably because of both sensory and motor neuropathy, and patients need to be warned that they may lose their balance under conditions such as poor lighting or uneven surfaces.25,26
Extrinsic Factors
In terms of the pressures that the soft tissues are exposed to, footwear is the single most important extrinsic determinant of elevated pressure.
While appropriate footwear can be of great benefit in preventing ulcers (see below; Chapter 28), incorrect footwear can actually cause ulceration.27 The two major deficiencies most frequently seen in shoes are incorrect sizing (too loose or too tight) and inadequate cushioning.
Tight shoes can cause ulceration at a number of locations.
Lesions commonly occur over dorsal deformities such as a bunion or a dorso-lateral prominence of the fifth MTH (MTH5).
The tips of the interphalangeal joints on claw or hammer toes are prime at-risk sites, and ulcers in the spaces between the toes can be caused by the toes being crushed together in a shoe with incorrect contours.
Loose shoes, which allow the foot to slip, can also result in ulcers.
The term ‘cushioning’ of the neuropathic foot is usually defined in static terms and can be equated with ‘thickness’ of ‘soft’ material under the foot.
It has been shown that walking in shoes with leather soles is roughly equivalent to walking barefoot, whereas walking in simple sports shoes (trainers) can reduce Plantar pressure by up to 50%, compared to barefoot walking.28 Thus, the wrong choice or prescription of shoes can be devastating for the integrity of the diabetic foot.
Activity Profiles
It is widely believed that barefoot walking is a principal cause of plantar ulceration that is amenable to behavioural intervention.
As mentioned above, we do not know the number of steps and the magnitude of pressure that will exceed an individual’s threshold for ulceration.
However, experience suggests that there are some patients who protect their feet adequately in footwear throughout the day, and yet ulcerate because of just a few steps of barefoot walking, e.g. to urinate during the night.
Thus, at least for some patients, even a few steps of barefoot walking are too many.
Taking showers barefoot is another dangerous behaviour.
Advising the patient to consistently use padded slippers, which can be donned easily, is a simple way to intervene in such cases.
In the last 5 years, technology has been applied to monitor patient activity profiles,29 and this has led to a number of theories about the relationship between what might be called the ‘dose’ of activity30 and the risk of ulceration31−33 (Chapter 25).
Surprisingly, a direct relationship between number of steps per day and risk of ulceration has not emerged from these early studies.
It has been suggested that either a change in level of activity32 or variability in activity31 may be important risk factors for ulceration.
There are some indications that neuropathic patients have altered gait patterns,34 but it is not yet clear whether this results in elevated plantar pressure.
Brand35 hypothesised that
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Figure 6.6 Anatomically referenced T2 images for eight pairs of age- and gender, matched non-diabetic control and neu- ropathic subjects. The remarkable loss of muscle tissue and the fatty infiltration in the neuropathic subject are apparent. (Reproduced with permission from Ref. 23)
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neuropathic gait would be less variable and that this would result in continued application of stress to the same plantar location, but this has not been found to be the case.36 Regardless, patients with LOPS will not consciously alter their gait because they feel no pain developing in high-pressure areas, from too much walking. There is also some evidence that neuropathic patients experience more falls and injuries due to falls than matched non-neuropathic diabetic patients.37,38 Balance34 and limb-position sense39 are also impaired, and these two factors may lead to more frequent traumatic injuries to the feet of neuropathic patients.