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The use of wound healing casts, Böhler’s iron and the biomechanics of offloading

Total contact casting is the ‘gold standard’ form of pressure offloading in people with diabetic foot ulcers. Hybrid versions of total contact casts, also known as wound healing casts, are increasingly being used in practice. A Böhler’s iron can be used in conjunction with a cast to provide axial offloading where appropriate. Studies have proven the efficacy of both interventions but there is a lack of studies comparing the benefits and drawbacks of different cast systems. Currently cast use is limited as many staff lack the biomechanical knowledge and skills to correctly apply and manage cast systems. As a result, despite proven benefits, casting is rarely used in practice. This article reviews the current forms of casting, its indications and contraindications and the basic biomechanics involved in offloading to encourage healing.

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Total contact casting has proven an effective method of healing plantar ulcers in patients with neuropathy. Total contact casts (TCCs; Figure 1), are specially designed to take weight off the foot in patients with diabetic foot ulcers, and have proven very effective in the treatment of this condition.

Despite it being the ‘gold standard’ management, this defining method of casting is rarely adhered to due to the specialised skills required to apply and manage the casting system. This was evident in the prospective study of 14 European centres, where casts were applied to only 35% of patients with either a forefoot or midfoot ulcer (Prompers et al, 2008). Only 18% of the prescribed casts were TCCs. It was therefore concluded that casting is underutilised in Europe as a result of a ‘lack of qualified staff’. The statistics in the United States were indifferent, with an even lower percentage of TCC use in foot clinics (Wu et al, 2008).

Hybrid versions of total contact casts
Hybrid versions of TCCs described as ‘wound healing’ casts are being applied with more padding to offer greater protection to vulnerable tissue. These casts vary from slipper to ankle and below knee. The method of application is similar to that of a TCC. The elements of padding utilising felt and wool as an interface to the casting material reduce damaging shear, but may reduce the axial offloading in below-knee casts due to lack of circumferential containment.

Several other forms of the TCCs have been developed over time with the aim of expanding its use through the simplification of application techniques and reduction in the number of complications arising from the original TCC. One of the main examples is the instant TCC, which comprises of a removable cast walker encased in a plaster or fibreglass wrap or, in some cases, a cohesive bandage. The healing rates are similar to the typical TCC (Armstrong et al, 2005). Simpler casting modalities remain questionable, however in terms of axial offloading and circumferential containment.

Böhler’s iron with casts for offloading
The use of a Böhler’s iron in conjunction with a cast will provide axial offloading. Axial load is defined as “a force with its resultant passing through the centroid of a particular section and being perpendicular to the plane of the section” (Study Stack, 2017).

Dr Lorenz Böhler was an Austrian surgeon specialising in fracture management in the early part of the 20th century. He devised a below-knee offloading device bearing his name in 1932. This was initially a stirrup type device attached to a plaster of Paris below-knee cast. This was a shorter version of the patten-ended calliper as described by Thomas (Rose, 1986). In the case of the patten-ended calliper, the weight of the limb is suspended between the ischial tuberosity and the patten end. In the case of Böhler’s iron, the lower limb is suspended from the fibular head through circumferential containment to a similar patten end.

Studies investigating the efficacy of Böhler’s iron have included variations of the iron but the general concept of the stirrup design with side supports and a rubber sole plate remains. The distal gap between the cast and the sole plate plays a pivotal role in axial offloading. A significant reduction of forces through the foot in was evidenced in a study comparing Beagle Böhler’s iron to a classic below-knee cast (Berwin et al, 2015). However, this study was carried out on healthy individuals and perceived the advantage of such device mainly for fractures. The most recent study used a different version of Böhler’s iron but proved its efficacy in addressing wound exudate and healing of diabetic foot ulcers (Saikia et al, 2016). No complications were reported from the use of this type of offloading device.

Indications and contraindications
TCCs and hybrid or wound healing casts are indicated for superficial plantar ulcers (University of Texas classification A1.2.3) (James, 2008). Casting is contraindicated in deep foot ulcers where abscess, osteomyelitis or other deep infection is present. In addition, critical limb ischaemia is also perceived as a contraindication to the use of casting as a treatment measure. It is the authors’ practical experience that by offloading using a Böhler’s iron that is bivalved for dressing changes, less superficial ulcers can be treated. The authors would not cast using the iron for critical limb ischaemia, however, a few casts have been applied with the iron with osteomylitis present for a limited period of time, in order to prevent catastrophic bone collapse.

Exudate: a challenge
An understanding of the mechanisms that result in the production of exudate is important across all disciplines involved in casting and wound healing. Exudate is derived from fluid that leaks out of capillaries and into body tissues at a rate that is determined by the permeability of the capillaries and the hydrostatic and osmotic pressures across the capillary walls. The relationship between the factors that determine how much fluid leaks out is known as Starling’s hypothesis. In a healing wound, the amount of exudate produced will reduce over time; however in non-healing wounds exudate production may continue and may remain excessive due to inflammatory processes. A moist environment is necessary for optimal wound healing. Conditions of extreme wetness, in contrast, will adversely affect the healing outcome. It is the authors’ experience that using the Bohler’s Iron reduces exudate levels and, therefore, helps to maintain the cast integrity. Exudate may pose a challenge to the integrity of the cast as its presence is often damaging to the felt and padding.

Reducing tissue breakdown by alleviating pressure
Tissue breakdown is related to both the magnitude and duration of pressure. Vertical stress destroys healthy tissue through repetitive compression. Shear stress occurs as deep tissue slides under superficial tissue. It is therefore vital to eliminate pressure on an ulcerated area.

By utilising a TCC, the area of weight bearing is increased. This can reduce the effect of peak plantar pressures by 81–92% at the ulcer site (Shaw et al, 1997). Shaw et al were the first to test the theory of a proximal shift of the load in the foot from the forefoot region to the hindfoot using a TCC. In the same study, they expanded this theory to account for the observed load carriage by the walls of the TCC (Shaw et al, 1997). This hypothesis was further investigated and quantified by Leibner et al (2006), who concluded that load sharing occurs when the proximal cast wall carries approximately 30–36% of the loading during ambulation. The force is transferred to the tibia and this is believed to be just as important as redistribution in offloading ulcers.

Biomechanical considerations are therefore important in ensuring the correct positioning of the talocrural, subtalar and midtarsal joints within the cast. The range of motions, flexibility, rigidity and deformities present should be corrected if flexible or accommodated if rigid. Muscle power and proprioception should also be evaluated to ensure that the contralateral limb can cope with the change in dynamics. The positioning of the subtalar and midtarsal joints should result in the redistribution pattern shown in Figure 3. These considerations require specialised skills, as well as knowledge of the biomechanics of the foot during cast application in order to correctly position the limb within the cast (Sinacore and Mueller, 2008). This allows for the achievement of the desired pressure redistribution pattern and correct offloading of the foot to be achieved. This will in turn produce optimal healing rates for patients with diabetic foot ulcers.

Comparison of casts is required
Total contact casting remains the ‘gold standard’ form of offloading the limb for ulcer treatment. Proper documentation and investigation of the hybrid versions and their comparison with the conventional TCC technique is warranted alongside the use and investigation of Böhler’s iron. This will allow for a better assessment of and reduction in any limitations of the offloading devices, as well as acknowledging their advantages and effectiveness in the treatment of diabetic foot ulcers.

Recommendations
Clinical staff dealing with this cohort of patients need to be appropriately trained and equipped with knowledge of the biomechanics of the foot, the pathologies associated with diabetes and the skills required for correct cast application. Following on from the work of Diabetic Foot Consensus group across the UK, chaired by Rachel Berrington, two pilot diabetic foot casting courses were commissioned to mainly reflect the competencies of podiatrists and orthotists with cast room nurses and technicians attending also. The Scottish Diabetes Foot Action Group is currently pursuing endorsement of a recognised set of competencies and cast forms that are suitable for the treatment of wounds and offloading from all health disciplines involved in the application of casting.

The methodology and understanding of the principles outlined in this paper will hopefully aid the development of guidelines that will help standardise practice that delivers sustainable services.

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Armstrong DG, Lavery LA, Wu S, Boulton AJM (2005) Evaluation of removable and irremovable cast walkers in the healing of diabetic foot wounds: A randomized controlled trial. Diabetes Care 28(3): 551–4
Berwin J, Burton T, Taylor J et al (2015) Plantar loading forces while walking in a below-knee cast with an attached loadbearing frame. Foot Ankle Int 36(6): 722–9
James WB (2008) Classification of foot lesions in diabetic patients. In: Bowker JH, Pfeifer MA, eds. Levin and O’Neal’s The Diabetic Foot. 7th edn. Elsevier, London: 221–6
Leibner ED, Brodsky JW, Pollo FE et al (2006) Unloading mechanism in the total contact cast. Foot Ankle Int 27(4): 281–5
Prompers L, Huijberts M, Apelqvist J et al (2008) Delivery of care to diabetic patients with foot ulcers in daily practice: results of the Eurodiale Study, a prospective cohort study. Diabet Med 25(6): 700–7
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Saikia P, Hariharan R, Shankar N et al (2016) Effective and economic offloading of diabetic foot ulcers in India with the Bohler iron plaster cast. Indian J Surg 78(2): 105–11
Shaw J, Hsi W, Ulbrecht J et al (1997) The mechanism of plantar unloading in total contact casts: implications for design and clinical use. Foot Ankle Int 18(12): 809–17
Sinacore DR, Mueller MJ (2008) Total-contact casting in the treatment of neuropathic ulcers. In: Bowker JH, Pfeifer MA, eds. Levin and O’Neal’s The Diabetic Foot. 7th edn. Elsevier, London
Study Stack (2017) Free engineering flashcards. TLTW Principles of Engineering. Available at: www.studystack.com/flashcard-1273754 (accessed 15.05.2017)
Wu SC, Jensen JL, Weber AK et al (2008) Use of pressure offloading devices in diabetic foot ulcers do we practice what we preach? Diabetes Care 31(11): 2118–9

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