The effects of foot orthoses on achilles tendon loads

Foot orthotics are commonly used to reduce load in the tissues to help manage overuse injuries in runners. They will only help if the injury is in a tissue that foot orthotics can actually reduce the load and the foot orthotic has the actual design features to reduce that load in the specific individual. When it comes to achilles tendinopathy, the use of foot orthotics is a tricky one as I have never been clear on the mechanism of action. I don’t deny that they do work in some runners, but equally they don’t work in others. There is some evidence that they do reduce symptoms in runners. Now we have this study:

Effects of foot orthoses on Achilles tendon load in recreational runners
J. Sinclair, J. Isherwood, P.J. Taylor
Clinical Biomechanics; Available online 8 August 2014
Achilles tendon pathology is a frequently occurring musculoskeletal disorder in runners. Foot orthoses have been shown to reduce the symptoms of pain in runners but their mechanical effects are still not well understood.
This study aimed to examine differences in Achilles tendon load when running with and without orthotic intervention. Twelve male runners ran at 4.0 m.s- 1. Ankle joint moments and Achilles tendon forces were compared when running with and without orthotics.
The results indicate that running with foot orthotics was associated with significant reductions in Achilles tendon load compared to without orthotics. Interpretation: In addition to providing insight into the mechanical effects of orthotics in runners, the current investigation suggests that via reductions in Achilles tendon load, foot orthoses may serve to reduce the incidence of chronic Achilles tendon pathologies in runners.

This study effectively showed that the foot orthoses of the type they used did reduce load in the achilles tendon. Nothing in the methods and anaylsis jumps out at me as being problematic. The only issue I have, and I have this with all foot orthotic studies, is that I ask myself: were the foot orthotic devices used in the study of the type and design that would normally and commonly be used in clinical practice by the more experienced clinicians? More often than not in most studies, the answer is no. In this study they used the Sorbothane Shock stopper – Sorbo Pro (these). The authors gave no rationale for the choice of that device, and it certainly is not used much for achilles tendinopathy in clinical practice, so not sure why they would choose that particular design. The design they used is a very high shock absorbing insole and even though the website claims ‘pronation control’, the design features in the insole to do with that are pretty weak.  A different foot orthotic with different design features may have yield the opposite result or an even better result. This is a big weakness of the study. They should have used something that clinicians would often use in clinical practice to treat achilles tendinopathy. Can we extrapolate this to other ‘foot orthotics’? Of course we can’t until we know how they stack up to the design features of the ones used in this study!

Having said that, yes foot orthotics can be used to reduce load in the achilles tendon which means that have potential to reduce the load in those with achilles tendinopathy, at least in the short to medium term (we can debate long term another day). For example, the soleus muscle via the achilles tendon has a significant lever arm on the medial side of the subtalar joint axis, so the ‘pronation control’ design features can hypothetically reduce how hard the soleus muscle has to work, reducing the load on the achilles, so there is a rationale there and the above study supports that rationale. Then there is a clinical test of the lunge test for angle joint range of motion. Do this test in those with achilles tendinopathy and it is often painful; repeat the test standing on a foot orthotic with the foot inverted and in some the pain is reduced; should they get foot orthotics? – the validity of extrapolating from a static clinical test like this to a clinical outcome is problematic, but it does not stop many clinicians claiming its a good clinical tool.

Should we use foot orthotics for achilles tendinopathy? Its a bit of a lame question, as many already are using them quite successfully in clinical practice. The question is meant, is this enough evidence from a ‘mechanism of action’ study to justify there use?

This reminds me of the keynote address that Dr Peter Cavanagh gave at the, I think, Footwear Biomechanics meeting in Stellenbosch in 2009 (…sorry Peter, if I mixed up meetings!). Peter admonished biomechanists for leaving their work at the lab door and not taking it through to clinical trials. He used examples where a range of different interventions that showed promise in the lab failed to deliver in a clinical outcome study in the field. He also gave similar examples of interventions that were showing promise in the lab (like the study above) and were being widely used in clinical practice based on that rationale from the lab based studies, but had not yet been subjected to clinical trials. … at least that is my interpretation of what Peter was saying.

We do have some clinical trial studies on foot orthotics and achilles tendinopathy. For example, the Donohue et al study which used foot orthotics with very different design features to the above study and showed some very dramatic successful clinical results using those foot orthotics for achilles tendinopathy (and some intriguing kinematic data too … another time); but, and a very big but, there was no control group. The most recent systematic review of the evidence on the use of foot orthotics for achilles tendinopathy showed that the studies are methodological weak and overall, not very effective. It certainly points to the need for more high quality properly controlled studies, especially of foot orthotics with the design features that are commonly used in clinical practice … guess what? There is one going to be published soon. (POSTSCRIPT: here it is)

As always, I go where the evidence takes me until convinced otherwise and while we do have a mechanism by which foot orthotics might work for the treatment of achilles tendinopathy, the clinical trial data is far from compelling.

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