(4) transferring load from regions thought to be at risk, to regions , such as the mid-metatarsal shaft area or the arch , that are able to accommodate more load; and (5) changing the patient’s gait and the motion of the foot during walking by making the shoe rigid.
All of the above changes must be accomplished in a way that does not compromise the stability of the patient during gait and, most importantly, in a manner that is acceptable to the patient from an aesthetic standpoint (see below for a discussion on patient compliance).
Comfort, usually a dominant factor in everyday footwear, can often not be perceived by the neuropathic patient.
The terms ‘insole’ and ‘orthosis’ are often used interchangeably, although an orthosis usually implies that some custom attempt to alter the load distribution has been implemented.
As we have discussed elsewhere,26 insoles for patients with loss of sensation should usually be ‘accommodative’ rather than ‘corrective’.
The ‘functional orthotics’ that are often used to correct foot alignment , in sports medicine, for example , are typically not appropriate for neuropathic patients, because they tend to increase loading in order to change foot function.
Cushioning ‘Cushioning’ is a rather vague term that has both static and dynamic connotations.18 The most useful visualisation of cushioning in footwear is to contrast the interface conditions of the foot of a patient standing on a rigid flat surface with those of the same patient standing on a soft foam mat.
The foam mat ‘cushions’ the foot by providing a surface that accommodates to the contours of the bony prominences, applying load to surfaces that were not previously loaded.
The ‘stock’ insoles delivered with most therapeutic shoes are meant only as fillers and usually need to be replaced before footwear is provided to the patient.
Significant cushioning can be derived from flat insoles, often up to 10 mm (3/8 in.) thick, if there is room in the shoe.
A completely custom-moulded insole is the extreme of this approach to reduction in plantar pressure.
Such an insole is typically made based on an impression of the foot (either a plaster cast, a foam box or, more recently, a digital scan; see below).
Although a custom-moulded insole can be effective in reducing pressure, it will usually require some of the load-relief and load-redistribution functions described below to achieve optimal efficacy.
It is easy to underestimate the pressure that can exist underneath the foot during walking, as well as the consequent compression of soft insole materials that essentially ‘bottom-out’ such that they no longer offer significant cushioning.
This implies that both thickness and stiffness are important in insoles designed to provide cushioning.
If the insole can be significantly compressed (by more than 50%) by pinching it between the thumb and finger, then it is probably too soft for effective cushioning of the foot.
Insoles also lose the ability to provide cushioning with repeated use.27,28 The practitioner should examine insoles at every visit to see if replacement is required.
Telltale deep impressions under bony prominences where the material may be extremely thin are signs that the insole should be replaced.
Dispensing several pairs of insoles with a pair of new shoes is recommended if it is economically feasible.
Load Relief An alternative strategy to address high pressure at bony prominences is to provide load relief. Load reduction has traditionally been accomplished by using metatarsal pads and bars (Fig- ure 28.2), which are intended to elevate one or more MTHs. Such pads have an asymmetrical
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(a)
(b)
Figure 28.2 Interventions to relieve high pressures under metatarsal heads by transferring load to other regions. (a) Metatarsal pad; (b) customised metatarsal bar; (c) schematic diagram of rigid rocker shoe with a plug under a metatarsal head region and metatarsal pad.
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(c)
Figure 28.2 (Continued)
section in the sagittal plane and the exact location of the apex in relation to the MTH is critical to their effectiveness.
Studies have shown, for example, that a 5-mm difference in positioning can markedly affect the load relief provided20 and that one standard position is not best for all feet.
Some placements have been shown to result in minimal reduction in pressure29 or even to increase in pressure at the target site.30 This variation in response makes effective prescription difficult, especially since determining the exact location of the plantar prominence on an insole can be problematic.
In general, the closer the apex is placed to the MTH, without being under- neath it, the more effective it is likely to be.
Pads are available in different heights (typically between 1/4 and 3/8 in. (6 and 10 mm)), and the higher the pad that can be accommodated in a given shoe, the better.
They usually have an adhesive to allow attachment atop an existing insole.
Load relief can also be achieved at a plantar prominence by altering the material properties or configuration of the insole directly under the prominence.
Historically, some concern has been expressed regarding annular pads that remove pressure on one area and redistribute it to the immediate surrounding areas.31 However, our own experiments have shown that a pad appropriately contoured across all the MTHs with an annular aperture under one MTH can relieve load without causing unacceptable local increases in pressures.
Another possibility is to excavate a ‘well’ in the insole (and sometimes also in the midsole) directly under the prominence.
This well is then filled with a soft compressible material that will reduce pressure at the MTH.
The edges of the well must be designed such that they do not cause a local increase in pressure, and the best way to do this is to extend and taper the posterior border.32 It is also important not to fill the well with incompressible material (such as silicone) because this may result in an increase in plantar pressure.
Load Redistribution This strategy involves changing the interface between the foot and the shoe so that load is transferred away from areas at risk for ulceration or tissue damage. The medial longitudinal arch of the foot is the preferred region to receive transferred load because of its broad area;
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thus, so-called ‘arch supports’ or ‘scaphoid pads’ are a feature of many insoles or orthoses. Studies have shown that raising the height of an insole in the arch area can relieve areas such as the heel, the first MTH and the hallux.19 The premise of this intervention is that the arch can accept the added load without risk of injury. Little is known about the effects of such interventions on the lateral border of the foot, which has the potential to be additionally loaded because the foot tends to roll outward off the elevated arch.
Rigid Outsoles (Rocker and Roller Shoes)
All of the above interventions are designed to be placed between the foot and the shoe, but important reductions in forefoot pressure can be obtained by making the shoes rigid and contouring the outsole.
The patients walk somewhat differently than they would in flexible shoes, and some practice is required to use the shoes comfortably and successfully.
This type of modification can be performed on almost any shoe by a shoemaker who inserts a rigid plate (often of carbon fibre) into the midsole.
The two basic designs are the ‘rocker’ , where there is an abrupt transition between front and back of the sole , and the roller , where the transition is made by a smooth curve (Figure 28.2(c)).
Some shoe manufacturers offer ‘off-the-shelf’ rocker shoes.
As discussed above, rigid shoes can be remarkably effective, reducing peak plantar pressure in the forefoot by 20, 50%.
Studies have shown that their effectiveness depends on specification of appropriate design parameters for rocker height and for position of the rocker axis (and probably the orientation of the axis)23,25 and that different individuals respond to the same shoe design in different ways.
A rule of thumb from these studies is to place the axis of the rocker at approximately 55, 60% of shoe length forward from the heel and to give the platform on which the foot rests a minimum amount of toe spring (‘turn-up’ at toe).
Even though such a configuration will not be optimal for all subjects, it will almost certainly result in reduced plantar pressure at the MTHs.
Rigid shoes appear to reduce pressure because they allow the patient to ambulate without extending the toes (at the metatarso-phalangeal joints).
This change probably prevents pro- longed load bearing on the fragile tissues underlying the MTHs.
Patients should be encouraged to walk more slowly and to take shorter strides than normal in rocker shoes, since this will prevent load bearing on the flattened surface of the rocker.
CHOOSING THE APPROPRIATE FOOTWEAR FOR THE PATIENT In this section we will give guidance to the practitioner who needs to make decisions regarding footwear for patients who may range from those who are recently diagnosed with diabetes and are concerned about their feet to those who have experienced recurrent ulcers in the past and are at high risk for amputation.
Ideally, decisions regarding the complexity (and, therefore, the expense) of footwear that a patient should receive would be based on a detailed biomechanical examination using the tools described above to measure pressure distribution.
However, such equipment is still beyond the financial reach of all but a few specialised centres, and footwear prescription decisions must be based on surrogate measures of risk.
Two such indicators are the extent of deformity and the presence of callus on the patient’s foot; the assumption is that more of either factor will lead to (or reflect) higher pressures and greater risk.
Risk of ulcer recurrence is high; thus, patients with prior ulcers are also at higher risk.