Biomechanical Theories and Foot Orthotics:
1,000 Recipes For The Same Dish
Development of biomechanical theory associated with the feet began to escalate in the 1970’s when Root et al created criteria to help label and what was believed to be biomechanical disorders of the foot. Since then a number of researchers have added to, subtracted from and completely disregarded Root Theory to produce and publish their own thoughts and theories around foot function and how it can be altered or improved to allow for better results in regards to patient pain and orthotic tolerance. In today’s article we look into a number of biomechanical theories which underpin much of the orthotic assessment and prescription in allied health.
Root Theory was established by Root et al in 1971 through 1978 with primary focus on labelling what was considered pathological biomechanics of the foot. Much of what Root discussed in these initial papers has been widely misquoted or misinterpreted over the years by varying researchers in all fields. Root defined the neutral position of the subtalar joint as neither pronated nor supinated. Root established 6 rules or categories in which functions of the foot could be categorized into and based on the normal or abnormal findings, an orthotic device may be created to address the highlighted issues, they were as follows:
1. Bisectional of the lower heel in vertical
2. The Subtalar Axis is neither pronated nor supinated
3. The MTP joint line is perpendicular to the bisection of the heel
4. Metatarsal joint is in maximum pronation
5. The foot is rotated outward with an average angle or 7 – 10deg
6. Ankle joint allows dorsiflexion a minimum of 10deg
Much of the Root Theory continues to underpin how orthotics are prescribed today. A negative side to Root Theory is the fact that none of the assessed items are completed in a functioning matter. Recent research has called into question whether there is a positive or even correlating relationship between the static posture of the foot and the dynamic function of the foot. To that point literature has also described conflicting clinical reliability is locating subtalar neutral position and reproducing results over multiple individuals.
Since the introduction of the Root Theory, a number of additional assessments and adjunct theories which look to contribute to the overall clinical outcomes has been described. One such assessment is the Foot Posture Index, which found that majority of the population actually have a slightly pronated foot and function in a non-problematic way. This also identified the fact that being in a subtalar neutral position does not necessarily lend to optimum biomechanics and as such other properties must be at play in the contribution to an individual’s pain or injury prospects. This opened the gates for further investigation and hunger to find out why foot biomechanic alteration was an effect way to reduce peoples pain.
Tissue Stress Model
The Tissue Stress Model was initiated by Kirby as he assessed moment occurring across the STJ and the influences of these stressors on anatomical structures. This ideology was then furthered too, as Fuller looked to include the function of the windlass mechanism and centre of pressure (COP) to the model to help identify pathology in relation to mechanical occurrences around the foot. The model is focused on kinematic assessment as opposed to Root Theory which is largely based around kinetics. The model can be related back to the load deformation curve which is divided into two zones, namely, Elastic Region and Plastic Region. Daily stressor commonly occur in the Elastic Region and injury is absent from the give and take hypothesis. When the tissue limits are pushed and micro-failure becomes apparent, Plastic Region entry risk greatly increases. Each individual has their own unique time in which this may occur.
The Tissue Stress Theory suggests that pronation and supination do not produce injury, however, the biomechanical opposition to these movements does. In other words, if a structure which is designed to limit or slow pronation is not capable of doing so at the frequency, intensity or time of that action it is most likely to become injured. McPoil and Hunt established a 4-stage protocol associated with the Tissue Stress Model;
STAGE 1 – identify anatomical areas of concern
STAGE 2 – Apply controlled stress to the area of concern (palpation)
STAGE 3 – Determine if pathology is mechanical
STAGE 4 – The prescription of activity modification, off-loading, tissue manipulation and medications
The co-siding treatment or management of conditions under the Tissue Stress Model is based on the reduction of patient symptoms, reducing the pain cycle and as an adjunct conditioning the tissue to perform it purpose effectively.
Sagittal Plane Model
Dananberg, put forward the sagittal plane model in lower limb biomechanics in 1986. He describes the foot as a pivotal structure which rocks forward from the heel, creating adequate extension from the hip, leading through the propulsive phase of gait. The sagittal plane model is one of the more subjective functional orthotic prescription biomechanical theories (that was a mouthful…)
He argues that complete dorsiflexion moment of the hallux cannot occur to produce propulsion if a hallux limitus exists. As a result, compensatory movements occur through the rest of the lower limb.
Dananberg, suggests that the prescription process is very much an educated trial and error approach, with the occasional use of gait analysis technology or in-shoe pressure equipment. However, this approach has been adopted quite well by a multitude of allied health professions given its likeness to the proposed biomechanical moment of triple extension during allowing for best propulsion. The benefits to a subjective approach is the fact that most orthotic devices are individualized to that specific patient and their presenting complaint.
The approach doesn’t look to place the foot in any specific optimum position however, aims to increase the function and efficiency of the propulsive mechanism with the first ray and the proximal structures of the lower limb. This primarily completed through forefoot additions or subtractions in the orthotic itself.
The MASS Theory or Maximum Arch Subtalar Supination Theory, was first proposed by Ed Glaser, in a direct objection to his perception of the issues surrounding the traditional foot biomechanics Root Theory. The biomechanical approach takes much of its basis from the Tissue Stress Model, however, continues to take the position of orthotic foot management and an optimum foot placement more supinated than that described by Root et al. In contrast to Root Theory, MASS Theory uses a multi-axis model and proposes that the midfoot locking action is also multifaceted with an ideology of optimizing the 33 joints in the foot. This is based on Glaser’s case that the talo-navicular joint is an ovoid ball and socket joint that works similar to the acetabulum of the hip. Talar and calcaneus motion is blocked is the sagittal plane when the talar head is directly on top of the anterior facet – thus the gastric-soleus fires a rotation may occur. MASS posture has a number of element listed below:
I) The foot is placed in its highest posture which can be attained in mid-stance.
II) Foot is in adequate supination or approximate a level anterior facet of the STJ
III) If you want to control a motion start at the beginning of that motion
In all honesty I have read through all the MASS articles I could find and have really struggled to understand the concept. In a Biomechanical Boot Camp Seminar hosted by Dr Craig Payne, he highlighted that he will use this method if his traditional approach doesn’t work, then he will try the MASS Theory if he still believes offloading is a requirement.
Many practitioners and researchers have tried and tested each individual theory as well mixing and matching to try and come up with a non-tested theory of their own. For myself Tissue Stress Model is the most relevant, at least to why a breakdown in structures may occur. Physiotherapists have been discussing this concept since the 1970’s, the same time at which the Root Theory was first published. Yes, one concept talks of the reason for tissue breakdown and the other is a theory of how to reduce this impact – but what is taken further by those in the physiotherapy profession is the benefits of not just placing an offloading device under the foot but the rehabilitation of those structures which should be the ultimate goal.
The latest Cochrane Review, Custom-Made Foot Orthoses For The Treatment Of Foot Pain 2008, only a small number of conditions have been found to be positively affected by custom-made orthotics and that evidence which is available has been identified as “limited”. The conditions which have been identified are; Hallux Valgus (bunions), plantar fasciitis, rheumatoid arthritis, juvenile idiopathic arthritis and a painful pes cavus foot type.
However, you may decide to assess and manage your patients in regards to orthotic therapy, it should be agreed upon that continual foot strengthening and mobility exercises is a necessary part in helping the patient reduce reliance on the device and improve their quality of life. The orthotic should only ever be considered a part of the treatment and not the be all and end all of lower limb management. Mobility, strengthening, stress tolerance and education should all be a part of the patient’s management to ensure that a rounded approach is taken in alleviating the issue and not simply offloading as a treatment modality. As we uncover more and more about the body and the influence unmeasurable factors have upon the way function, these methods of assessment will either continue to develop or be written into history.
Until Next Time,
Ball, K., & Afheldt, M. (2000). Evolution of Foot Orthotics – Coherent Theory or Coherent Practice? Journal of Manipulative and Physiological Therapy, 116 – 124.
Glaser, E., & Fleming, D. (2016). Foot Posture Biomechanics and MASS Theory. The Foot and Ankle Online Journal.
Harradine, P., & L, B. (2009). A Review of the Theoretical Unified Approach to Podiatric Biomechanics in Relation to Foot Orthoses Therapy. Journal of the American Podiatric Medical Association, 317 – 325.
Hawke, F., Burns, J., Radford, J., & du Toit, V. (2008). Custom-made Foot Orthoses for the Treatment of Foot Pain (Review). Cochrane Library.
Jamshidi, N., Rostami, M., Najarian, S., Menhaj, M., Saadatnia, M., & Salami, F. (2010). Differences in Centre of Pressure Trajectory Between Normal and Steppage Gait. JRMS, 33 – 40.
Mueller, M., & Maluf, K. (2002). Tissue Adaption to Physical Stress: A Proposed Physical Stress Theory to Guide Physical Therapist Practice, Education and Research. Physical Therapy, 383 – 402.
Petcu, D., & Anca, C. (2012). Foot Function Paradigms. The Romanian Academy, 212 – 217.