‘Offloading’ is used as a generic term when dealing with the problem of pressure, friction and shear in the treatment rationales associated with the spectrum of challenges facing patients with diabetic foot disease.
The dictionary states, among other descriptions, that ‘offloading’ is a verb, meaning “to get rid of”, while synonyms suggest “take off” or “transfer”. However, do these descriptions truly identify the actions we require of orthoses, devices or casts?
This article will set out to describe commonly used techniques and modalities, and evaluate what their potential is in terms of delivering offloading. It may also be the case that clinicians are required to review the overarching description of offloading to enable healthcare professionals to prescribe the most effective solutions, based on the outcomes and activities they wish to achieve for individual patients.
At present, the use of techniques, orthoses and casts is usually dependent on the experiences of the clinicians and on historical practice. Figure 1 shows a range of options available and their effectiveness in reducing peak pressure (Cavanagh and Bus, 2010).
What are pressure, friction and shear? Pressure is a linear force exerted on a surface per unit area; friction is a non-linear force, surface resistance to relative motion; shear is a non-linear force that will occur when a force parallel to the plane causes one plane to slip against the other causing deformity of the surface.
Understanding the implications of these definitions can be challenging and, therefore, it is fundamental that the expectations of the capabilities of orthoses and casts are clearly stated and understood. 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 tissues slide under superficial tissue. Therefore, is offloading the correct description?
Dr Paul Brand (1966) and Melvin H. Jahss (1991) introduced the theory of load redistribution, which describes the decrease in pressure due to the increase in weight-bearing surface area. The principle is that by increasing the surface area over which the plantar force is applied, this will, therefore, reduce the plantar pressure. This is sometimes referred to as the ‘snowshoe principle’.
Shaw et al (1997) introduced the theory of ‘load sharing’. Shaw described the decrease in plantar pressure in Total Contact Casting (TCC) due to the proximal portion of the TCC bearing a proportion of the load. This theory was further supported by Leibner et al (2006).
Both of these theories are, therefore, sensible and probably coexist if the principles in the application of casts below the knee or in devices made irremovable below the knee are adhered to. The use of footwear and ankle level devices, therefore, can only load redistribute and not load share.
Footwear and devices for load redistribution
Foot disease in diabetes is a diverse spectrum. To discuss options in this section, reference will be made to footwear and devices that are used for healing neuropathic and neuroischaemic wounds on the plantar, dorsal and margins of the foot. Professor Ernst Chantelau in Germany in the 1990s studied the use of half shoes, while RL Needleman (1997) found that, by using a rocker system, he could take the weight and pressure off of the forefoot, however, this then placed the mid and hind foot in a position where these areas had increased pressure and, in this sense, did not load redistribute but load transfer. Forefoot relief shoes have subsequently been designed with full foot plates to overcome the issues associated with load distribution and to offer more protection to the exposed distal sections of a half shoe.
The practical application of wound healing shoes that redistribute pressure centre around the management of the biomechanics and the dynamics of pressure over time. In the past, the focus centred around Newtonian mechanics (Root, 1994; Kirby, 2001). The rationale was to create a platform using either corrective or accommodative forces, that would enable a stable, three-beam structure of the foot to take loading at 33% on the medial beam, 17% on the lateral beam and 50% on the hind foot.
For this to be achieved, the foot orthoses section of the shoe should have the capability of functioning in conjunction with the upper to provide stable grip of the foot to enable the foot to be held in the correct position without the upper creating a torque and subsequently translation and drift of the foot over the foot orthoses. The upper should be constructed of a material that is proportional to the mass of the patient and the retaining strap should be at the level of the talus and be capable of holding the heel secure with the angle of the talus to the distal posterior edge of the heel being between 42° and 45°. Furthermore, the position of the rocker must be behind the metatarsal heads to enable the shortening of the lever arm to facilitate the simulation of dorsiflexion.
This will then allow for the change in timing, magnitude and direction, with respect to the foot of the ground reaction force over time (Schaff and Cavanagh, 1990). During the normal gait cycle, the ground reaction force has a point of theoretical application close to the heel at initial contact of the limb in stance phase. This point of application, often termed ‘centre of pressure’ moves forward on the foot as the gait cycle progresses. The position and magnitude of a rocker will have influence from mid stance onwards to toe off on the stance limb. Moving the position of the rocker posteriorly will shorten the stance phase and will not allow the centre of pressure to advance beyond the rocker. Generally, the magnitude of the rocker will be 10°, however, the relative height and pitch of the rocker will also influence the effective timing and duration of the stance phase of gait.
It is fundamentally important for the patient to be given gait training to effectively use a rockered load distribution device as neuropathy damages proprioception and, therefore, has an impact on stability. Failure to do this may create a falls risk, or an increase in forefoot pressure, as the foot will become locked in an equinous position.
Does forefoot footwear redistribution have a place in the clinic? For social, employment and personal reasons, very often, patient compliance dictates a more discrete approach to wound healing, therefore, the use of these devices have a place in the early management of Texas A (not an ulcer), 1 and possibly 2. However, if healing or wound diminution does not take place within the first couple of weeks of use then alternative, potentially more restrictive, forms of redistribution should be considered on an individual basis.
Hind foot lesions to the margins of the calcaneum or the distal posterior border of the heel have traditionally been difficult to heal in a timeous fashion. The literature tends to suggest that these wounds do not respond well to TCC. It has been recognised nationally that prevention is better than treatment, and initiatives such as CPR (Check, Protect, Refer) for feet, which is promoted by the Scottish government and delivered by the Scottish Diabetic Foot Action Group, has the potential to reduce hospital or nursing home acquired lesions. In cases where ulceration has occurred, it is fundamentally important to redistribute the pressure on the affected area both when ambulant and recumbent.
There are important considerations to be made when using a heel pressure redistribution device. If the patient is ambulant, it is important to maintain the foot in a stable configuration that promotes pressure relief and dorsiflexion at heel strike. Devices with a metal posterior upright matched to the patient’s mass will prevent plantar flexion and allow for a more reciprocal gait. These devices (PRAFO) are adjustable and, therefore, gait can be tuned accordingly. Caution must be used when using preformed plastic posterior uprights on ambulation, as deterioration or stressing of the plastic can result in weakness and plantarflexion, resulting in an increase in plantar forefoot pressures. This type of device is better suited to non-ambulant patients that require a window for the heel to be rested in.
Hind foot relief shoes can be used for limited walking as wound healing progresses and protected ambulation within rehabilitation is required. As with the forefoot relief shoes, the construction of the upper is important in securing the foot to mitigate against translation of the foot on the foot orthosis and pistoning anteriorly to posteriorly (Munro et al, 2005).
The casting challenge
Total Contact Casting is the acknowledged gold standard, however, why in the USA “do less than 2% of clinicians utilise this method” (Wu et al, 2008)? In Europe, 35% of plantar ulcers treated in specialist centres in 2008 utilised casting with only half using TCC (Prompers et al, 2008). It is the author’s belief that there are multifactorial reasons for the lack of Total Contact Casting use among healthcare professionals, from lack of training, lack of expertise to lack of funding (Table 1).
At this point, it is necessary to discuss cost and the need to accelerate healing to benefit not only the patient, but the overall health system. The use of devices linked to appropriate referral and choice of the most effective modalities, based on the evidence available, directs practitioners to the use of wound healing casts, load distribution casts and enhanced limb load distribution using Bohler’s irons (Munro and Abdul Hadi, 2017). The integration of plaster room, podiatry and orthotic knowledge would allow this modality of treatment to become universal.
Wound management apart, the management of Charcot neuroarthropathy, which the author believes from personal experience is growing exponentially, would benefit enormously from early intervention by applying effective non-removable load distribution, and aid the fight against transition from pre-fragmentation to fragmentation stage, and possibly reduce the time to coalescence.
The time has come to educate those working with foot disease in diabetes to stop using generic terms and focus on the actions that load distribution and load share can offer. By understanding the limitations and managing expectations based on the mechanics and science currently available, real progress could be made in delivering a nationally agreed set of principles in both wound management and that of Charcot neuroarthropathy. The training, educational, materials and expertise already exist and these just need to be harnessed and managed to deliver sustainable, quality-assured services nationally.