The biopsychosocial-digital continuum of foot orthosis practice and research: the VALUATOR model

Foot orthoses (FO) have been used for decades to treat foot and lower limb pathologies (e.g. knee and hip) [1‐10]. Rigorous and expert analysis of the available evidence documenting the relative benefits and risks of foot orthosis therapy has led to the development of national and international practice guidelines [11‐15]. The ultimate goal of these guidelines is to improve the effectiveness of care, optimize patient outcome and strive for the most cost-effective healthcare provision. To align practice with the available evidence, most of these guidelines adopted a therapeutic efficacy classification system, which typically comprises the following recommendations/hierarchy: 1) medically indicated and essential, 2) useful, 3) adjunctive, 4) not useful [11]. Ultimately, however, the decision to provide orthoses or not, occurs within the framework of evidence based practice (EBP) and involves the integration of the best available evidence with clinical knowledge and expertise, while considering patients’ unique needs and personal preferences.

Whilst practitioners are generally positive towards evidence based foot orthosis practice, considerable issues and uncertainties remain. For example, for what purpose or clinical endpoint are orthoses useful, how does cost and quality of the orthoses matter, and how do the nature and model of services that deliver orthoses impact outcomes [16‐19]. Efficacy of foot orthoses has been subject to protracted debate and a number of factors associated with variable efficacy have been proposed [11, 19] including: i) the evolution of orthoses materials and associated technologies [20‐23], ii) application by a wide range of practitioners [24, 25]; poor knowledge transfer between research and practice [26‐28]; and increasing interest by consumers and other purchasers and dispensing by unqualified vendors [11].

Another factor that may contribute to the variable outcomes is a focus on the orthotic device as the key aspect of the intervention, and failure to contextualise orthoses within a wider person-centered approach. This is evident in some scientific studies/clinical trials where an association between an orthosis feature and biomechanical effect is assumed to be associated with the intended clinical, or personal, outcome. Some may criticize scientific studies for their use of prefabricated FO since customisation of a foot orthosis’ shape is central to many assumptions underlying orthosis theories. It is reasonable to assume that some degree of customization is required for a number of clinical conditions, especially where foot shape is affected or tissue viability impaired. However, research has demonstrated that there is not always a clear predictable difference between custom and non-custom foot orthoses in regards to clinical effectiveness [29].

The selection or customisation of orthoses is often driven exclusively by the need to affect neuro-biomechanical or biological input and outcome variables. For example, altering plantar pressure using a specific geometric orthotic feature, or using orthoses to stimulating plantar cutaneous receptors and thereafter affect postural control. This reflects an exclusively biological, or biophysical, basis to practice where defined inputs lead to defined outputs, and do so repeatedly. Most concepts underpinning foot orthosis practice assume orthoses provide a dose of a specific neuro-biomechanical stimulus that targets one or multiple neuro-biomechanical response(s). Well-known models and theories emerging from this dosage-response perspective include: 1) Tissue Stress Theory [30], 2) Zone of Optimal Stress theory (ZOOS) [31], 3) Root Model [32], 4) Rotational Equilibrium Theory of Foot Function [33], 5) The foot as sensory organ model [34, 35], to name a few. In addition, national guidelines may reflect this dominance of a biophysical definition of foot orthoses [11].

More recently, however, the potential for orthoses to achieve a therapeutic effect by providing a psychological as well biomechanical stimulus has started to receive attention [36]. In addition to softening of the strictly biological model, it is reasonable to assume that the design of foot orthoses and outcomes are influenced by factors such as the work or physical activity environment of the patient, the presence of kinesiophobia, the mental status of the patient, the type of pain, the absence of pain sensation, the patient’s (or payers) financial situation, to name a few. As a consequence, there is no holistic model upon which foot orthosis practice is taught, practised nor investigated. Encompassing these factors into paradigms of orthotic practice aligns well with the concept of ‘value based healthcare’ [37, 38], however, the latter has only been embraced in a very small number of countries (e.g. United Kingdom, United States, Germany). This paper aims, for the first time, to introduce a conceptual model of foot orthosis practice that embraces a broader range of factors that are pertinent in practice and reflect contemporary health service behaviours and values. The Value Based Foot Orthosis Practice (VALUATOR) model is targeted at students, lecturers, scientists and practitioners.

We represent VALUATOR with a visual metaphor (Fig. 1) representing a value based healthcare approach [37, 39‐41]. Within the model, foot orthosis design (i.e. the geometric and material properties of a foot orthosis) and clinical value is considered along a bio-psycho-social-digital continuum reflecting the nature of FO practice. The model helps to contextualise the variable outcomes that are observed in research and practice. The model is built upon 6 key areas that are discussed here below.

The aim of the current manuscript was to introduce a conceptual model of foot orthosis practice which may be used by students, lecturers, scientists and therapists around the globe. The model uses a visual metaphor to introduce a value based healthcare approach which projects orthotic practice and the clinical value of foot orthoses along a bio-psycho-social-digital continuum.

The biological component of the VALUATOR model assumes that clinical symptoms are the result of loads acting upon foot tissues and that orthoses adjust these loads and thereby affect symptoms [63]. Adjusting loads applied to the foot is a complex phenomenon not least because the foot orthosis has four functional surfaces [63] and the foot, orthosis and footwear have different static and dynamic properties. The priority is often the upper orthotic surface that contacts the foot and the extent to which it should reflect the static shape of the foot has been the topic of much debate. The two sides of the orthosis interact with the shoe upper, and the base with the shoe sole. It is through these three sides that forces are transferred through the shoe to the foot, and thus all four sides affect the effect of the orthosis. of the four sides of an orthosis are rarely debated thus far and this points at the need for work on this respect to improve the biological components in the model.

That said, the association between specific orthotic features and predictable biomechanical responses (a dose – response model) and outcomes of value is unproven. This is unlikely given the multi factorial nature of most foot problems. Indeed, evidence points not to specific orthosis design features having a predictable outcome, but rather for specific effects (e.g. pressure reduction) achieving clinical aims (reduced ulceration risk), regardless of the orthosis feature used to achieve it. Whilst a biomechanical lens has thus far dominated the adoption of a dosage-response model in orthotic practice, the VALUATOR model goes beyond this and proposes a broader view that better reflects the evidence and practical experience too.

The digital component of the bio-psycho-social-digital continuum has only been moderately illustrated so far. The factors which influence the process of foot shape capture (e.g. plaster of paris, foam impression box, digital scan), orthoses design (shape and material choices, specified in digital design software or by written prescription) and orthosis manufacture (hand-made versus computer aided manufacture) vary across professional disciplines, industry, educational and research contexts. Debates in the literature compare these digital and manual processes, focussing on levels of accuracy and repeatability, but this assumes these are critical to the achievement of the intended outcomes and thus deriving value. This is unproven and whilst it is reasonable to assume that digital workflows will support transition of orthotic practice from artisan art to a clinical speciality with appropriate medical device production quality control [22], much of this requires further research.

The VALUATOR model embraces the three-domain classification system described by McLeod et al. [59]. Using this highlights the fact that current foot orthosis practice often relies upon surrogate measures, and too much reliance on these is an ongoing risk. By contrast, greater focus on biomarkers might facilitate greater understanding of mechanobiology and foot pathology. This also reflects the fact that by evolving the conceptional basis of orthosis practice through greater incorporation of non-biological elements, we are not diminishing their importance. Rather, we are seeking to contextualise them in the context of other evidence and practice.

The majority of the quality measures identified as examples within VALUATOR have not yet been validated in the context of foot related disorders. This raises questions about the viability of the proposed model, although at this initial stage, the conceptual basis for the VALUATOR model is not sensitive to the specific measures it advocates. This also reflects the fact that any model is a platform for posing unanswered questions that research should seek to answer, and thereby allow concepts and practical actions that follow to evolve as increasingly evidence based. Recognising the model as incomplete, needing further work and as a dynamic knowledge base for professional practice, is in itself an evolution from previous models, which have tended to provide conclusive answers to questions about orthotic practice, regardless of the actual evidence underpinning the ideas.

In conclusion, a value based foot orthosis practice model has been proposed which integrates different concepts, viewpoints and quality measures. Whatever promise the current model holds, it is believed that it provides a contemporary viewpoint and a structured conceptual metaphor to support the intellectual debate and teaching among the professions that use foot orthoses.