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The Flat Foot – A Closer Look

201102130948.jpg Stuart Currie DC,

Director of Research, Sole Supports.



It would be quite easy to characterize the flat foot as a simple pathological entity however, what is more difficult is a thoughtful analysis of what the literature tells us, and what the everyday clinician should consider in their evaluation and treatment protocols. Although the flat foot appears to be a simple clinical finding upon first glance, delving into the details can provide useful insight into its etiology, pathophysiology and prevalence.

Pes Planus is a general term used to describe the flat-footed condition. It is also referred to as pesplanovalgus, fallen arches, and over pronation.

There is a general consensus that pes planus is a common clinical finding; however, the incidence of pes planus is difficult to ascertain for a variety of factors not the least of which is a consensus on a grading or measureable definition. Foot posture has been correlated with factors such as age, presence of pathology, obesity and gender although the literature is inconclusive.



The pathophysiology of pes planus is widely debated. Adult acquired flatfoot deformity (AAFD) has received attention, with a historical focus on the posterior tibial tendon (PTT) insufficiency or dysfunction as the primary contributing cause1. Information now available also illustrates that PTT dysfunction does not happen in isolation, and ligament involvement (most frequently the spring ligament complex) is extensive in PTT dysfunction2.

Other causes of pes planus that should be considered are: arthridities (both degenerative and inflammatory), congenital abnormalities, trauma, Lisfranc dislocation, rupture of the spring ligament or plantar fascia, and neuropathically induced causes such as Charcot’s arthropathy.

What the literature tells us about the flat foot and how it relates to pathology is telling. Investigations have shown that an increase in plantar surface contact area, associated with pes planus can be a risk factor for the development of overuse injuries3, 4.

It has also been shown that arch structure plays a role in lower extremity kinetics and kinematics5, and that different arch structures lead to different injury patterns in runners.6 From a muscular perspective there is some evidence that pronated feet demonstrate greater activation of inverter musculature and a decreased activation of everter musculature.7 Although there is some lack of agreement between studies evaluating the association between foot posture and injury, it is widely accepted that foot posture, in combination with other factors, influences the risk of injury in sport8.

Having established the role of foot posture in function and injury, the next step is to begin to quantify a pathologic and corrected foot posture, and while on the surface this seems simple there is more to the story that warrants exploration.

Consider that there is currently debate and variation in the definition of a flat foot, and that person-to-person variation can lead to subsets of either flat or high arched feet that operate at different levels of functionality.

One clinical assumption that is often made is that the plantar surface area (footprint) can be used to predict the height of the arch. While this would be clinically ideal due to the ease of measurement, this is a dangerous assumption given that one study revealed that only 27% of the height of the medial longitudinal arch during walking can be predicted from plantar contact area9. This is important because it illustrates a potential disconnect between what is seen as a flat foot (increased plantar contact area) and what happens dynamically during walking.

What all of this literature tells us is that the flat foot requires some thought. Other than simple inspection, there are clinical measures and tools available for the clinician to quantify the pes planus condition and foot posture overall.

The Foot Posture Index (FPI)10 is a tool that can be used to classify foot posture based on several clinical measures of the foot.

The Arch Height Index which is calculated as the ratio of the dorsum of the foot (at 50% of foot length) to the truncated foot length has been shown to have good intra-tester and inter-tester reliability11.

The Longitudinal Arch Angle (LAA) is measured by using the intersection of two vectors, one from the medial malleolus to the navicular tuberosity and the other from the navicular tuberosity to the midpoint of the first metatarsal joint. The LAA, as assessed in static standing, has been shown to be predictive of the arch height at midstance during walking12 notably correlating a static measure with dynamic foot posture.

The Foot Mobility Magnitude (FMM) is another measure showing promise. It takes into account both the vertical and medial-lateral mobility of the midfoot13. Weight bearing and non weight bearing arch height and foot width differentials are also compared; using normative values for this measure in the clinic may give the clinician an additional tool to determine appropriate intervention, and also assist in determining when preventative treatment may be appropriate.


The observation of a flat foot is a good clue that pathology may be on its way and warrants further investigation and quantification by the clinician. Once the clinician has determined that treatment is reasonable various strategies can be applied. These can range from addressing specific tissue pathologies or muscular weaknesses to providing corrective support to the posture of the foot through the use of a foot orthotic. Given the previously mentioned ligamentous stress that can accompany arch collapse, the medial longitudinal arch and its underlying bony architecture becomes an area of focus for an effective strategy aimed at postural restoration of pes planus.


1)   Arai K. Clin Biomech 2007 June;22(5):592-8.

2)   Deland JT. Foot Ankle Int 2005 June;26(6):427-35.

3)   Kaufman KR. Am J Sports Med 1999;27(5):585-93.

4)   Levy JC. Foot Ankle Int 2006 December;27(12):1060-4.

5)   Williams DS. J Appl Biomech 2001;17:153-63.

6)   Williams DS. Clin Biomech 2001;16(4):341-7.

7)   Murley GS. Gait Posture 2009;29(2):172-87.

8)   Murphy DF. Br J Sports Med 2003;37(1):13-29.

9)   McPoil TG. J Am Podiatr Med Assoc 2006;96(6):489-94.

10)         Redmond A. The Foot Posture Index: User Guide and Manual. http://www leeds ac uk/medicine/FASTER/fpi htm 1998.

11)         Williams DS. Phys Ther 2000;80(9):864-71.

12)         McPoil TG. J Am Podiatr Med Assoc 2005;95(2):114-20.

13)         McPoil TG. J Foot Ankle Res 2009;2:6.

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