How to Alter Socks for Transmetatarsal Amputation
Photo by Dino Petrocelli (www.dinopetrocelliphoto.com).
Patients with diabetes who have undergone partial foot amputation are likely to be those most vulnerable to reulceration. Footwear, foot orthoses, partial foot prostheses, and ankle foot orthoses can help reduce that risk while improving function.
By Erick Janisse, CPed, CO, and Dennis Janisse, CPed
O&P professionals care for many patients with diabetes. Amputations in those patients are unfortunately a far too common outcome.1-7 The roles of the pedorthist, orthotist, and prosthetist should not be undervalued in the prevention of diabetic foot complications and in returning the patient to a normal, active, and productive lifestyle after an amputation. The O&P professional's goals when working with partial foot amputees are to restore stability and function that have been lost due to an amputation, facilitate energy-efficient gait, maintain support, and prevent further complications.8
A pedorthist can help prevent ulcerations and amputations by providing appropriate footwear and custom made foot orthoses. The pedorthist also utilizes modalities like partial foot prostheses and shoe modifications to help protect the residual foot after an amputation. Ill-fitting shoes are a significant cause of skin trauma that precedes diabetic foot ulcers.9 Areas of high plantar pressure and shear – two factors that can lead to diabetic skin ulcerations – are issues that can be addressed with custom foot orthoses.
Through use of lower limb orthoses, the orthotist helps restore functional gait after amputation.8,10 Ankle foot orthoses can be utilized to replace the lost lever arm of a transmetatarsal or hallux amputation.8,10 They may also be used as offloading devices to decrease pressure on the plantar surface of the residual foot.
While the prosthetist often fits lower limb prostheses for transtibial amputations, he or she also contributes to the care of partial foot amputations – especially in the cases of a Chopart's or Syme's amputation.
With modern pedorthic, orthotic and prosthetic techniques and devices, partial foot amputees are often able to return to a fully functional lifestyle.
Proper shoe selection and fit
Proper shoe selection and shoe is important. Footwear plays a vital role in the prevention of skin breakdown and subsequent infection, in preventing amputations, and in the care of the residual foot after amputation.11-15
Shoe selection is based primarily on function. But it stands to reason that a patient will be less likely to use the proper footgear if they do not like its appearance.
The functions of the shoe are to:
- Protect the residual foot
- Maintain foot position inside the shoe and reduce shear
- Contribute to restoration of normal gait
- Accommodate a partial foot prosthesis, foot orthosis, or AFO14
If a partial foot amputee has been diagnosed with sensory neuropathy, the upper portion of their shoe should be made of a material that is moldable, stretchable and breathable.14 The interior lining of the shoe is equally important. Shoes are readily available that are lined with materials that wick moisture away from the skin and/or have antibacterial properties.
High top shoes work well for patients with transmetatarsal, Lisfranc's, and Chopart's amputations as they allow more of the shoe to interface with the foot and ankle, enabling the shoe to gain better purchase on the foot and leg.10 Slip-on dress shoes and loafers should be avoided as they tend to be tight and restricting. Shoes for patients with a partial foot amputation require some sort of closure system like laces or Velcro. An in-depth shoe – one that's constructed with additional room and a removable insole16 – is preferable when an AFO, prosthesis or foot orthosis is used.
Shoes come in countless styles and shapes. Finding a shoe that is perfectly matched to the patient, their feet, and their needs requires the skills of a qualified practitioner. Since there is little consistency in shoe sizing among manufacturers, it is almost impossible for the consumer to select a properly-fitting shoe without guidance.17
Some shoe styles are available in true widths, which means the base of the shoe is proportionally wider as the widths increase. This is not the case, however, with many commercial shoes. Only a shoe fitter with a strong working knowledge of their inventory can guide a patient to an appropriate shoe.
A partial amputation foot can be challenging to fit properly. Shoes are designed so that the widest part of the foot rests in the widest part of the shoe. In many cases a partial foot amputation changes which area of the foot is the widest. This may require mis-mating of shoe pairs, with a wider, shorter shoe on the affected side.
Shear and plantar pressure
Excessive shear and high peak plantar pressures are often been implicated as causal agents in the formation of plantar foot ulcers. Therapeutic footwear can decrease weight-bearing pressure and shear forces applied to the skin of the foot.18
Figure 1. Pre-ulcer, foot orthosis with shear-reducing material applied to high coefficient-of-friction areas.
While much attention has been given to areas of high peak pressures as a predictor of foot ulcers, research has revealed that there isn't an appreciable correlation between the two.19-22 Reducing elevated pressure levels is important, but the need to reduce the duration of maximum pressure and shear stresses is key.23
Pre-ulcerative calluses are caused not only by peak pressures, but by frictional shear force.24,25 Tissue breakdown occurs more rapidly when shear is increased.26 Since plantar shear is known to be a factor in the formation of pre-ulcerative calluses, it must also be taken into consideration when discussing diabetic foot ulcers. Excessive shear damages the underlying tissues.27 Peak perpendicular load by itself is not necessarily harmful. Rather, the magnitude of repeated high peak pressures is worrisome because of how they enable and relate to peak friction loads.28-30
The first step in reducing shear inside the shoe is to be sure that the shoe size and shape are appropriate for the foot. If the shoe fits and is secured snugly on the foot, the foot won't shift inside the shoe. Fit is critical since both a loose shoe and a tight shoe can increase shear, friction, and/or pressure on the foot.
Another way to decrease friction and shear is to "lubricate" the surfaces moving against one another by using shear-reducing socks made from an acrylic blend fabric or other fiber that has a low coefficient of friction (COF).31 Traditional cotton socks have a relatively high COF, especially when damp. This "lubrication" can also be accomplished by applying a special shear-reducing material to the interior of the shoe or to a foot orthosis or AFO under areas of high pressure or friction. Footwear and insole materials are also a factor in reducing friction. For example, Plastazote – a traditional topcover used in foot orthoses for diabetic patients – has a relatively high COF against a dry sock (0.57) compared to the friction-reducing material ShearBan (0.16). (Figure 1).
The peak pressure gradient – the spatial change in plantar pressure around the location of peak plantar pressure – is another pressure variable to consider. It has not been as extensively researched as peak plantar pressure, but it may be a strong indicator of pending skin breakdown. Peak pressure gradient is higher in the forefoot than in the heel even when compared with the peak plantar pressure.32 In theory, a well made foot orthosis should be able to reduce peak pressure gradients if it is constructed to truly maintain intimate, total contact with the entire plantar surface of the foot.
Shoe modifications
Figure 2. Shoe with extended shank inserted, and carbon fiber plate, for use with partial foot prosthesis.
Rocker soles are probably the most commonly performed shoe modification, and are especially useful when treating partial foot amputations.14 A rocker sole serves to rock the foot from heel strike to toe-off without bending the foot or shoe. The sole of the shoe is modified to resemble the base of a rocking chair. The basic biomechanical effects of rocker soles are the restoration of lost motion in the foot and ankle due to pain, deformity, stiffness or fusion, resulting in an overall improvement in gait and offloading plantar pressure on some part of the foot.33 The rocker sole is the most effective way to offload the forefoot.34 The rocker sole is also a logical method by which the center of pressure (CoP) can be progressed anteriorly past the distal end of the residual foot in a partial foot amputee.35 Rocker soles may also be used to reduce the duration of maximum plantar pressures on parts of the foot.36,37
There are several types of rocker soles. Selection of the correct shape and type of rocker is based on the foot's individual needs.
Many off-the-shelf walking shoes and running shoes are built with a mild rocker sole. This simple rocker is adequate for a foot that is not at risk of ulceration. Running shoes have been shown to be effective at reducing plantar pressures in the forefoot, providing metatarsal head relief, and gait assistance.38 However, for the patient who has deformity or neuropathy, a custom rocker sole is indicated.
An extended shank is also necessary in most partial foot amputees. The spring steel shank runs from the heel to the toe and is added to replace the toe-off lever arm that is lost due to a hallux or midfoot-level amputation. It also prevents the shoe from bending and causing tissue damage to the residual foot.8 The shank is inserted between the midsole and outsole of the shoe, or better yet, buried in the midsole itself. (Figure 2)
An extended shank is typically used in conjunction with a rocker sole and can make the rocker sole more effective. It helps reduce bending forces through the midfoot and forefoot and strengthens the entire sole and shoe.39
Foot orthoses
A custom-molded foot orthosis can reduce peak plantar pressures in the foot.40-42 Its primary function is pressure redistribution via total contact between the foot orthosis and the foot or residuum. The goal is to decrease areas of high peak pressure.
The orthosis is constructed using a soft top layer and a firm, supportive base layer. The contours of the plantar surface of the foot are filled with material and then planed flat on the bottom so that when the patient stands on the orthosis the entire plantar surface of the foot is assuming weight bearing responsibility.
A commonly used top layer material for patients with sensory neuropathy is Plastazote. Plastazote – a moldable, static dissipative material – is a nitrogen-charged, closed cell, cross-linked polyethylene foam. Used alone, Plastazote does not have a sufficiently long functional lifespan for use in an ambulatory patient. But when backed with a thin layer of polyurethane foam and/or EVA (ethylene vinyl acetate), it will endure longer under the repetitive stresses of walking. The use of the aforementioned material combinations for foot orthosis fabrication is so common that several manufacturers offer prelaminated sheet stock of them.
The base layer of a total contact foot orthosis should be one that is supportive enough to adequately equalize plantar pressures but is still shock absorbing and easily adjustable. Good base layer materials for the total contact orthosis include EVA or cork with a Shore A durometer of approximately 50-60.
The orthosis should provide at least marginal plantar pressure redistribution and therefore some reduction of pressure under high pressure points. For more extensive offloading, extrinsic posting can be added to reduce pressure in specific spots, such as a metatarsal head or other bony prominence.
Partial foot prostheses
For many surgeons, the main objective in an amputation procedure is to salvage as much functional limb that will heal as possible; in O&P, the goal is to preserve and restore the patient's functional level.8,10 The primary purpose of a partial foot prosthesis in a patient with diabetes is to protect the residual foot, with a secondary aim of restoring normal function and gait.
Figure 3. Partial foot prosthesis.
Equal pressure distribution is especially important in the partial foot patient because peak plantar pressures rise exponentially as weight-bearing surface area decreases – and more often than not, it is an insensate surface area to begin with. The issue of whether these tissues can handle the increased stress is why partial foot prostheses are often used in conjunction with an AFO to transfer the stresses to more proximal normal tissue.43
Like the foot orthoses discussed in the previous section, the partial foot prosthesis is used primarily to help evenly redistribute plantar pressures in the foot, reduce areas of high peak pressure, and decrease shear. Essentially, this is accomplished by fabricating a foot orthosis – in much the same manner as described above – and adding an area of padding just distal to the end of the residual foot and then finishing it with a semi-rigid foam filler to maintain the foot's and the device's position within the shoe. The material combinations are often the same or similar to those used to fabricate the foot orthoses discussed above. (Figure 3)
The loss of the hallux requires some sort of device to replace the lost lever arm for toe-off propulsion. This can be done either via the use of an extended shank or by attaching a full length carbon fiber footplate to the partial foot prosthesis.
Much has been written about the use of silicone and/or acrylic resin partial foot prostheses – especially for Lisfranc's and Chopart's amputations – such as a Chicago boot or a Lange prosthesis that slips over the residual foot, much like a sock or a shoe would.8,10,43-46 It stands to reason that these types of devices provide good cushioning and stability and excellent reduction in shear forces. While they can be difficult to don and doff, they are cosmetically pleasing and some may even be worn sans shoe. Caution should be taken when using these devices in the diabetic population, however, as these devices tend to be hot, make the foot perspire, and don't permit air circulation around the foot, which promotes the growth of bacteria.8
Ultimately, foot amputation is not an admission of failure, but rather a chance to start anew.
Erick Janisse, CO, CPed, is a board certified pedorthist and orthotist and vice president of National Pedorthic Services in Milwaukee, WI. Dennis Janisse, CPed, is president and CEO of National Pedorthic Services and c linical assistant professor in the department of physical medicine and rehabilitation at the Medical College of Wisconsin in Milwaukee.
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How to Alter Socks for Transmetatarsal Amputation
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