The most efficient way to redistribute plantar pressure is by the immediate application of some form of cast. If casting techniques are not available, temporary ready-made shoes with a cushioning insole can be supplied to off-load the site of ulceration. Alternatively, weight-relief shoes can be supplied, and felt padding can also be used. Additional off-loading measures are crutches, wheelchairs and Zimmer frames.
Moulded insoles in bespoke shoes are sometimes used to treat ulcers. However, shoes with insoles are not an efficient way to off-load diabetic foot ulcers. Their main function is to prevent recurrence.
Various casts are available and their use is governed by local experience and expertise. Techniques include:
• Total-contact cast
• Scotchcast boot
The total-contact cast (Fig. 4.5) is an extremely efficient method of redistributing pressure from the plantar sur-
face of the foot. However, it is not without its complications, and should be reserved for patients whose ulcers have not responded to other treatments. It is a close fitting plaster of Paris and fibreglass cast, applied over minimal padding as follows.
The casting procedure is as follows:
• When patients attend the casting clinic, they wear shorts or tracksuit bottoms otherwise they will need to unpick trouser seams
• A layer of stockinette is applied to the patient's lower leg. The length of the stockinette is twice the distance from knee to tips of toes (two-and-a-half times if the leg is plump) and the excess should be gathered up over the knee. When the cast is finished the excess stockinette is brought down to cover the cast and protect the contralateral leg from rubs
• The foot is held in a plantigrade position and any excessive creases in the stockinette around the ankle area are cut out and taped flat
• Small pieces of cast padding are inserted between the toes to keep them apart. The distal end of the stockinette is taped together
• A strip of 5 mm felt padding is applied over the tibial crest and circles of felt are applied over each malleolus and any other bony prominences, which may include the tuberosity of the navicular and the base of the 5th metatarsal
• A double layer of cast padding is applied to the entire lower limb, with three extra layers at the proximal end of the cast and three extra layers over the toes
• Fibreglass tape is applied around the foot and leg, starting at the top of the leg, 3 cm below the cast padding. The layer of tape is rubbed gendy to accommodate the contours of the foot and leg. At least three layers of tape will be needed, and heavy patients will need up to six layers. Patients should be told not to touch wet fibre-glass which can cause skin irritation
• The excess stockinette which was gathered up over the knee when casting commenced is rolled down to cover the outer fibreglass layer. Rubbing the stockinette provides a smooth finish
• Casts should be replaced after 1 week if reduction of oedema renders them loose
• The maximum period a cast should be left on without renewing is 1 month
• An emergency cast removal service should be available for patients if they develop problems with the cast. The most common mistakes made when applying casts are:
• Cast comes too high up the leg, so that when the knee is bent the cast presses on the back of the thigh
• Cast is wrapped too tightly around the toes and border of the forefoot, causing pressure lesions
• Rough fibreglass outside layer is not covered with stockinette and rubs occur on the contralateral limb
• Cast is too lightweight for heavier patients and collapses
• Fibreglass is dented by pressure from finger tips—cast should be handled with the flat of the hands
• Foot is insufficiently dorsiflexed. If cast is applied to a rigid plantar flexed foot there should be compensatory building up of the heel area of the cast.
Advantages of the cast include:
• Redistributes pressure very evenly over the sole; 30% of pressure is transferred further up the leg in a 'coning' effect
• Enforces compliance—the patient cannot remove it
• 'Ball and chain'—patients walk less in a heavy cast
• Reduces oedema
• Ulcers heal very quickly—mean healing time of 6 weeks. Disadvantages of the cast include:
• Cannot be removed so ulcer progress cannot be checked daily
• Heavy and reduces mobility
• Patient may not drive a car in a cast unless fitted with suitable controls and with permission from insurers
• A few patients develop 'cast phobia' and will not wear them. One of our patients borrowed her neighbour's electric saw and removed her cast (but fortunately not her leg). Another used a claw hammer to bash the cast off and sustained trauma to her leg
• Iatrogenic lesions (rubs, pressure sores, infections) may be undetected
• Leg may develop 'cast disease' from prolonged immobilization, e.g. muscle wasting, weakness and osteopenia
• Leg length disparity may cause discomfort and problems with knee, hip and spine (this can be prevented with a shoe raise on the contralateral side)
• Danger of fracture and the development of a Charcot foot when cast discontinued without careful rehabilitation
• Frail patients may suffer falls
• Casts are unsuitable for patients with ulcers or skin eruptions on the leg
• Problems may also arise if patients fail to care for cast.
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