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I would like to raise some comments regarding the paper ‘Foot orthoses for plantar heel pain: a systematic review and meta-analysis.’ Whittaker et al, 2018, Br J Sports Med. 52(6): 322-328, and the editorial ‘Foot orthoses research: identifying limitations to improve translation to clinical knowledge and practice’, Griffiths & Spooner 52(6) in the same edition.
I would like to applaud the intention of the paper by Whittaker et al, to try and establish a conclusion to the question of foot orthoses efficiency in treating heel pain symptoms. This same praise I also give to the editorial in bring into the discussion the potential issue of the validity of random control trials as a research method to test foot orthoses efficiency. However, some key issues with the paper need exploring beyond the issues raised in the editorial in regarding how orthoses may work.
There are several key issues with the paper by Whittaker et al, which overall is a noble attempt to make sense of the present research on using orthoses for plantar heel pain. The issues are; plantar heel pain is a symptom not a diagnosis (therefore some of these studies may include multiple conditions); the studies are subject to potential bias (addresses by the authors); the studies do not compare like with like studies and seemed to have been shoe horned together to achieve a conclusion; and finally, foot orthoses do not logically conform to randomised control trials. Despite the best efforts of th...
There are several key issues with the paper by Whittaker et al, which overall is a noble attempt to make sense of the present research on using orthoses for plantar heel pain. The issues are; plantar heel pain is a symptom not a diagnosis (therefore some of these studies may include multiple conditions); the studies are subject to potential bias (addresses by the authors); the studies do not compare like with like studies and seemed to have been shoe horned together to achieve a conclusion; and finally, foot orthoses do not logically conform to randomised control trials. Despite the best efforts of the researchers to correlate the results to present a conclusion, reading the paper only shows that the paper was challenging statistically to produce. The studies so far published on this subject are so full of issues that any attempt to achieve coherence and a conclusion is potentially meaningless no matter how many attempts at statistical manipulation are thrown at the problem.
Despite this statistical need to achieve a result from poor data, the main problem must be that no attempt was given to provide the symptom of plantar heel pain a diagnosis in some of the studies involved in the review. This immediately invalidates the whole premise that these studies be included. We can hardly expect the same treatment to work for plantar fasciopathy, porta pedis nerve entrapment, Baxter’s neuroma or osteoarthritis of the subtalar joint. Each can cause plantar heel pain, but the mechanism of injury is different. Randomly sticking in insoles will give random results, perhaps the only conclusion we can take from these types of papers.
The editorial by Griffiths and Spooner discusses some of the issues in regards foot orthoses research used in random control trials. The authors of the editorial discuss the way the foot and the orthosis must interplay within the fundamentals of the laws of mechanics. These are principles of mechanics not unique to orthoses, but for anything that creates an interface between the foot and the ground. This is why a sham (placebo) orthosis in a study has potentially the same chance of influencing biomechanics as a so-called ‘real orthosis’. The difference in effects between the sham and the orthosis under test will be dependant on the difference between the shape and materials used in the two insole types.
The manufacture technique to produce the orthosis is probably irrelevant (188.8.131.52). Sadly some studies have failed to establish significant difference between insole types, picking a sham that is very close in design and material to the orthoses under test. Others studies pick custom orthoses of a much more robust design compared to the preform comparison orthosis (5.6). Bias is a serious problem in many studies.
The authors of the editorial claim that foot orthoses “can only exert their effect via placebo effect and/or directly modifying ground reaction forces at the foot-orthosis interface”. Kinematic and realignment effects are dismissed as “unlikely”. This statement is difficult to substantiate when compared to a wealth of published-research where much data on kinematic changes are reported (primarily in the foot) with foot orthoses (184.108.40.206.10.11.12). Similar kinematics changes occur when footwear and lack of footwear are compared (13.14).
Different kinematic effects have also been shown with different prescription additions on orthoses (15.16), which indicates how important individual prescription addition is to get a particular desired effect. However, data in regards to foot orthoses producing kinematic changes in sports related studies are less conclusive (17). This is likely to be in part because the contact time for the foot in running is reduced to around 40% (dependant on running pace) resulting in a significant float phase where neither foot is in ground contact. During float phase the orthosis has no mechanical influence except on increasing the foot segment mass in swing. Orthoses cannot redirect forces if there is no ground reaction force to interact with. Other issues such as preferred movement pathways (18) might over ride the effect of the orthosis or shoes especially in studies conducted on healthy asymptomatic runners.
The issue in sport is different to orthopaedics, in that most commonly in sportspeople, issues of mal-alignment and joint dysfunction are usually small. Sports medicine is largely about treating the ‘fit and well’, pushing their tissues to fatigue. In sport poor lower limb function or mal-alignment of a significant nature enough to significantly influence energetics, is rare. Such dysfunction would prevent a patient from being normally active. In orthopaedics serious postural mal-alignments that cause, or are a cause of dysfunction, are commonly found in patient contacts, and here changes with foot orthoses are likely to be more significant.
Griffiths & Spooner’s editorial proposes that the effects of orthoses are most likely to be due to kinetic changes within the tissues. This leads to the suggestion that for research, orthoses must be chosen that equally change the tissue stresses within specific tissues in different individuals, rather than picking the same prescription.
Sadly as we have no way of routinely assessing internal body kinetics within tissues dynamically, this suggested research pathway is not yet readily open to us. If possible, such an approach would also raise complications in that individual variance in morphology and body tissue composition would still make subject selection as well as prescription selection quite challenging. Orthoses requirements to achieve the same outcome could be quite different, unless subjects were match on many physiological and anatomical similarities. This would also make random control trials almost impossible.
There are other ways by which foot orthoses can be implied to change tissue kinetics. Changes in muscle activity in the presence of foot orthoses would imply that tissue kinetics has changed. Such evidence exists if not strongly (19), although evidence exists of rocker shoes altering muscle activity (20.21). As stated before orthoses and shoes work on the same interface.
Foot orthoses may also be able to initiate a neuro-mechanical effect. Something touching the body will create a reaction, possibly just in reaction to avoid the object as occurs when we feel a small stone in our shoe. What ever induces changes in muscle activity is likely to change kinematics, energetics as well as internal and external kinetics.
Again it must be understood that a so-called ‘sham orthosis’ has the potential to alter biomechanics and energetics. This single fact seriously questions whether random control trials using a so-called ‘sham orthosis’ are suitable for testing so called ‘real foot orthosis’ efficiency.
Another consideration is that poor biomechanics is primarily more than just forces being applied to the body in the wrong direction. Joint and muscle dysfunction are intrinsically linked. As a consequence, outside of the research world, foot orthoses are rarely used in isolation. If exercises, mobilisation, manipulation, shoe selection and foot orthoses all produce statistically arguable benefit when studied individually, there might yet be a significant effect when each are combined appropriately together. There is a challenge for research to look at treatment protocols rather than one treatment at a time!
Where the possibility of merely a placebo effect achieved by foot orthoses can firmly be challenged is in papers like Halstead et al (22), which looks at changes in pathology over time with foot orthoses on MRI and is the kind of randomised control trial that is more suitable for foot orthoses. More of these studies on diagnostic image changes produced by foot orthoses are required if we are ever going to learn more specifically which prescription variances are required to reduce stress and therefore strain on specific musculoskeletal tissues and their pathologies. Whether the outcome is good or bad, will still give us a far greater insight into the orthoses ability to change tissue stresses.
The reason good foot orthoses research is limited is more a result of most studies not knowing what effects they wished achieve to resolve particular pathologies. Especially so in studies where symptoms, rather than pathology is chosen. These ‘chosen’ pathologies need to be very specific, for in just the case of Achilles tendinopathy the mechanical causes are multiple, and specific areas within the tendon function differently (23.24.25).
To compare effects of a foot orthosis prescription in a manner similar to a random control trial of a pharmaceutical would need a large scale study performed on subjects of similar mass, strength, tissue age, morphology, and limb segment lengths. Subjects would need to have the same pathology and mechanism of injury with the same level of tissue damage. I would also suggest that treatment orthoses were tested for effects on energetics too to test comparable effects on mechanical efficiency. Any other type of random control study risk being like testing a drug for its effect on abdominal pain, regardless of the cause. Sadly most foot orthoses studies published at present make establishing truth within them extremely difficult and the construction of coherent random control reviews almost impossible.
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