Do Custom Foot Orthoses Harm Flat Feet?

A paper is going to be published in Clinical Biomechanics very shortly that reports evidence that the cross-sectional of at least some plantar intrinsic muscles atrophy significantly after 12 weeks use of custom foot orthoses in people with ‘pes planus’ (1).

The paper reports a significant decrease in the average cross-sectional areas in flexor digitorum brevis (9.6%) and particularly for abductor digiti minimi and abductor hallucis (17%+0.1-0.4), but without a change in EMG activity (1).

The custom foot orthoses were made from cast foam box foot impressions sitting semi-weightbearing using a polypropylene shell, shore 65 EVA rearfoot posts and shore 55 forefoot posts, using a protocol based on the  ‘tissue stress model’ proposed by Eric Fuller of trying to control the centre of pressure CoP trajectory to stimulate balanced muscle function across the foot (2). It is very important to point out this is a model without validation. The authors state they were ‘reducing plantar foot pressures’ (1), something that is only possible if you reduce the mass of the patient, the acceleration of the movement or increase the surface contact area. The aim of a successful orthosis prescription appears to have been to increase pressure through the normally non-weightbearing areas in the midfoot (1).

The selected 18 pes planus group was decided using a foot posture of +4 or a navicular height less than 3.6cm (1). 9 received custom foot orthoses and 9 did not. Orthoses had to be worn for at least 6 hours a day (1). The whole group of subjects had no pathology, injury, symptoms, balance disorders or previous surgery, anywhere in the lower limbs.

The results are interesting in that it shows that providing foot orthoses unnecessarily could be counterproductive. This is true of cancer drugs, but it does not mean that cancer drugs should not be used. It does, however, raise the subject that foot orthoses are probably best utilised with foot rehabilitation. It also draws out the tricky question of when a patient should stop using orthoses. There are issues with the paper regarding the selection of the ‘pes planus’ condition as no normal/abnormal navicular height has been demonstrated (3) and the foot posture is used to identify features of foot pronation (4) not to establish an anatomical pes planus. Whether 4+ on the foot posture indicates a pes planus is up for debate. The prescription protocol is also questionable as it is proposed on an unsubstantiated model of foot function.

Here at HealthyStep_® we have long kept a watch on the research and have well over 50 years of clinical experience between our two podiatrists. We have long argued for the use of effective rehabilitation exercises to be used with and without orthoses and have worked with a large number of superb leading clinicians, researchers and students to develop and research rehabilitation exercises that work in strengthening feet. Pes planus foot types do seem to underuse plantar intrinsics compared to extrinsics when viewed against more normal foot vault profiles (5), possibly making them more at risk of foot muscle atrophy and sarcopenia with age.

Sadly I suspect this article will now start to be quoted for the evidence to show foot orthoses should not be used. Should this happen it is important that the clinician has a response. Foot orthoses have been shown to affect pathology on diagnostic imaging (at least adjustable preform versions have) (6). In pathology, they can work by moving tissue stress from one structure towards uninjured areas within that structure or to other stronger structures using the principles of tissue stress and the laws of mechanics. This is ‘real’ tissue stress based on the mechanical tissue properties and the laws of physics, not a proposed model of foot function. Foot orthoses should not be used in normal feet without pathology or need because they don’t ‘look’ normal. If the feet function without pathology and effectively for the patient’s needs they are functionally ok and don’t need foot orthoses.

If foot muscles are already atrophied or painful, they can be used to reduce the mechanical load on the muscles by reducing motion in the foot, undoubtedly the reason for the reduction in the cross-sectional area reported (1). They also need rehabilitation to hopefully prevent or slow further deterioration, or if possible increase strength, especially id orthoses are provided. For articular and ligament dysfunction they can prove most helpful in relieving symptoms (7). 

Foot orthoses are a treatment, and just like taking antibiotics without an infection, there are dangers to their use, especially long term.

This is the final set of key points:

• If they are used as a prevention of symptoms that come on wearing a certain shoe or during long-distance walking etc and are not used at other times, the foot continues to receive stimulation and only splinting when needed. 
• If there is permanent damage within the musculoskeletal structures to avoid pain and further pathology foot orthoses may be required long term, such as a grade II or III tibialis posterior dysfunction. 
• If they are used to protect tissue during healing and it can heal back to normal strength, or the tissue strength can be improved, with that outcome achieved and the orthoses can be and should be withdrawn. 

This is why preformed orthoses are much better to prescribe than custom. They can easily be modified to continue to be appropriate throughout healing and rehabilitation and they are not so expensive for either the NHS or the private patient to be able to say “you no longer need these devices” and remove them.

We realised this in the 1990’s. The foundations of HealthyStep_® were built on these principles.

References:

1. Protopapas K, Perry SD. (2020). The effect of a 12-week custom foot orthotic intervention on muscle size and muscle activity of the intrinsic foot muscles of young adults during gait termination. Clin. Biomech. https://doi.org/10.1016/jclinbiomech.2020.105063 
2. Fuller EA. (1999). Centre of pressure and its theoretical relationship to foot pathology. J Am Podiatr Med Assoc. 89: 278-291.
3. Rathleff MS, Olesen CG, Moelgaard CM, et al. (2010). Non-linear analysis of the structure of variability in midfoot kinematics. Gait & Posture. 31: 385-390.
4. Redmond AC, Crosbie J, Ouvrier RA. (2006). Development and validation of a novel rating system for scoring standing foot posture. Clinical Biomechanics: 21: 89-98
5. Angin S, Croft G, Mickle KJ, et al (2014). Ultrasound evaluation of foot muscles and plantar fascia in pes planus. Gait & Posture. 40: 48-52
6. Halstead J, Keenan A-m, McGonagle D et al. (2014). An exploration into the effects of foot orthoses on bone marrow lesions associated with mechanical foot pain. Foot Ankle Res. 7(Suppl 2): A1. http://www.jfootankleres.com/content/7/S2/A1
7. Chapman GJ, Halstead J, Redmond AC. (2016). Comparability of off the shelf foot orthoses in the redistribution of forces in midfoot osteoarthritis patients. Gait & Posture. 49: 235-240.