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Foot orthoses are employed to treat musculoskeletal injury. Research in this field has increased greatly in the past two decades, with researchers publishing ‘placebo’ and ‘sham’-controlled trials within the foot orthoses literature1 ,2 which appears encouraging. However, there are insufficient predictive models for the myriad of musculoskeletal injuries that are being attempted to be treated, let alone a firm understanding of how the design variables of the foot orthoses employed within such studies are influencing the biomechanics of the injured tissues.
How do foot orthoses work?
Foot orthoses can only exert their therapeutic effects either psychologically (via a placebo effect) and/or by directly modifying reaction forces at the foot–orthosis interface in terms of their magnitudes, location and timing.3 Traditionally, foot orthoses were assumed to exert their primary effects via kinematic change and realigning the skeleton, but this seems unlikely.4 Musculoskeletal injury relates to tissue loading, and clearly, foot orthoses may induce a change in tissue loading without a significant change in foot kinematics.5 Thus, foot orthoses should be assumed to work either via psychological or by kinetic influence (or both).
Considering Newton's first principles, the reaction forces at the foot–orthosis interface will always be participant and task-specific. Thus, even with identical foot orthoses beneath the feet of participants within a trial, the kinetic changes will vary from participant to participant.
Can we learn from the pharmaceutical industry?
When performing drug trials, participant-specific responses will also occur, but the dose of the active ingredient delivered to each participant is always controlled. In foot orthoses research, given the current understanding of their mechanism of action, it may be ideal to aim to deliver the same ‘kinetic dose’ (the active ingredient?) to the target tissue in the treatment group. This has not yet been performed to the authors' knowledge. Taking the Achilles tendon as an example, participants are often given heel raises of identical height within a study to reduce the tensile loading on the tendon. It may be more appropriate to manipulate the heel raise height on an individual basis so that load reduction was identical across the experimental group and minimal/absent in the sham group (if this is possible).
‘Control’ orthoses in research
To negate the potential psychological effects of foot orthoses, researchers have employed what they term either ‘placebo’1 or ‘sham’2 foot orthoses. However, it is highly probable that placing any insert within a shoe will alter the reaction forces at the foot's interface with it.6 ,7 Thus, the term ‘placebo’ is inappropriate.8
If how foot orthoses work is currently poorly understood, how can it be known what constitutes a true ‘sham’ treatment or how ‘sham’ orthoses influence the loading of the participant's tissues? Researchers need to demonstrate that the device that they have inserted into the shoe does not impact on the biomechanics of the locomotor apparatus in such a way that it modifies the loading of the target tissue of the pathology under investigation. Without studies of the kinetic effects of foot orthoses variables, it is virtually impossible to say whether a ‘sham’ device could be having a positive kinetic impact on the pathology within such studies. Most clinicians consider the terms placebo and sham interchangeable and this also needs to be addressed.
Owing to limited time and/or access to journal articles, clinicians rely on sources other than the original work to keep up to date with published data (eg, conferences, magazines, blogs, social media). While all forms of evidence translation to clinical practice are welcomed, it is important to read original sources to critically appraise them and consider the conclusions they make in the context of the chosen study design. Before extrapolating conclusions from research to entire populations (or pathologies) and allowing them to significantly influence their practice, clinicians should scrutinise the research design to ensure conclusions are applicable. For example, the prescribing of identically designed orthoses to an entire cohort does not reflect clinical practice.
Researchers should clearly demonstrate that their control group is indeed a valid control, and it may help achieve this if they provided unpooled raw data (kinematic and kinetic) of the participant-specific responses to foot orthoses in all trials performed, across all groups. If such data are not supplied for the ‘control’ group, then it can potentially be disregarded as a control, and instead, the study could be viewed as a comparison of two different types of orthoses.
Clinicians should consider whether applicable conclusions can be drawn from research in the context of the study design and measures collected. They should always consider whether the ‘sham’ employed was indeed a sham. They should also consider that given the current limitations (the inability to control kinetic variables across participant groups), there is an argument that foot orthoses research is currently no more than a series of n=1 case studies, and perhaps that is the highest level of evidence we can hope to achieve presently.
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