Flexible flatfoot: Foot orthoses and outcomes
Many studies have shown an effect of foot orthoses on biomechanical function in patients with flexible flatfoot, but few studies have focused on patient-centered outcomes. There is a need to investigate the effect of foot orthoses on pain and fatigue in these individuals over time.
By Sarah A. Curran, PhD
Flatfoot is a well-recognized condition among health professionals who treat foot and ankle problems. It is characterized by rearfoot eversion and a reduction in the height of the medial longitudinal arch.1-3 While the actual term “flatfoot” appears to be universally used by clinicians and patients,4 other terms are often used within the medical literature, including “pes planus,” “calcaneovalgus,” and “postural valgus hindfoot.”5-7
Flatfoot can present in a rigid or flexible form, with the former being congenital and affecting less than 1% of the population. In contrast, a flexible flatfoot is an acquired deformity that affects up to 23% of the adult population2,8,9 and is referred to by some authors as a “hypermobile” foot.10-12 From a biomechanical point of view, a flexible flatfoot fails to form the stable base required during propulsion, fails to achieve the dynamic stability that is critical to resist the potentially damaging effects of ground reaction forces, and has an inability to attenuate and adapt to the supporting surface.12-15 Consistent with the adage “form follows function,” the long-term effects of the compromised mechanical function of a flexible flat foot may include alterations to ligaments, tendons, muscles, and joints. Clinically, this may present as changes to the shape of the foot, loss of motion, and a predisposition to osteoarthritis.1-3
Figure 1. Left: The modified foot orthosis. Right: University of California Berkeley Laboratory (UCBL) orthosis. Images courtesy of Prosthetics and Orthotics International, SAGE Publishers Ltd, London, UK.44
Some clinicians consider a flexible flatfoot a clinical problem only when symptoms become present.7,16-18 Many would argue that just because a foot has a reduced or absent medial longitudinal arch does not necessarily imply a pathological condition. This opinion is supported by the many individuals who present with flatfoot and have congruent joints and normal lower limb function.10,11Nevertheless, many consider a flexible flatfoot a deviation from the “norm” and suggest that it may predispose individuals to greater stress (impact forces) on a range of tissues, resulting in a lower physiological and biomechanical threshold of the lower limb tissues. This perspective is consistent with the historical tendency for military candidates to fail recruitment on the basis of their flat feet, with Ilfeld in the 19th century observing up to a 40% discharge from the US Army of personnel who presented with lower limb pain and flat feet.19 Although evidence suggests that various musculoskeletal pathologies—including osteoarthritis, plantar fasciitis, Achilles and tibialis posterior tendinopathy, patellofemoral joint pain, and hip and lower back pain9,10,13,20-22—are linked with flexible flatfoot, the exact nature of the relationship remains unclear.
Although the cause of flexible flatfoot remains uncertain, as with many musculoskeletal conditions involving the lower limb, it is likely to be multifactorial in nature and involve a combination of altered kinematics (joint function), kinetics (forces), and muscle function.1-3,6,10,11,13,15,16 Although extreme and nonresponsive cases can be treated surgically,3,23-24 the most common approach to management of flexible flatfoot is conservative. Awareness of a flexible flatfoot or pain proximal to the foot can cause natural anxiety in some patients, but the symptom of fatigue is often the key feature that prompts them to seek advice and treatment from a health professional.4,25,26 However, the literature suggests a clear trend aimed at prevention, which in turn has led to the provision of conservative management prior to symptoms occurring. This is reflected in a range of studies8,10,14,16 that present data on healthy (pain-free) individuals with flexible flatfoot based on the assumption that structural change will predict symptomatic response.
Foot orthoses and functional influence
Although a number of methods are employed to control abnormal foot function, including bracing and taping, foot orthoses have been perhaps most commonly used to treat a variety of lower extremity musculoskeletal pathologies, and their use in patients presenting with flexible flatfoot is no exception.3-5,15,20 A foot orthosis is thought to provide support to the foot and reduce symptoms.27,28 Foot orthosis designs can range from a simple flat or mildly contoured device to a fully customized device manufactured from a cast or other impression, using technologies such as CAD-CAM and 3D printing.
Although the actual mechanism of action of foot orthoses continues to be debated, many authors suggest the devices influence the kinematics of the foot and lower limb, and in particular rearfoot (subtalar joint) eversion. However, to date, studies that have assessed the effect of foot orthoses have shown only small and generally nonsignificant changes in rearfoot eversion.29-32 Some authors have used plantar pressure measurement to better understand the nature of the relationship between flexible flatfoot, foot function, foot posture, and foot orthoses.
In a study33 of 22 asymptomatic adults with overpronation, Redmond and colleagues used an in-shoe system to assess plantar pressures under three conditions: shoe only (thin athletic footwear), shoe plus noncast insole (6° varus medial post), and shoe plus modified Root orthosis. Foot function with the noncast orthosis differed only minimally from the shoe-only condition. The modified Root orthosis was associated with reductions in heel forces and pressures and in medial and lateral forefoot pressures. These effects were accompanied by significant increases in loading forces at the midfoot (increases of 54% and 20% for force time integral and maximum force, respectively). This was coupled with a longer contact time in this area with the modified Root orthosis. While this increases the load distribution across the foot to what is considered a nonweight-bearing area (in a normal foot), it does in essence create a functioning arch support.33
Given that center of pressure (CoP) measures have the potential to determine the progression pathway of the foot and its stability, it may be a useful parameter in determining normal and abnormal function. During walking in healthy subjects, the CoP invariably deviates mediolaterally over the plantar surface of the foot and, while it is unclear if this deviation is consistent across various foot postures, a number of studies have shown that the more pronated the foot, the higher the deviation of the CoP medially.34,35 Foot orthoses are thought to influence the CoP pathway and therefore alter the direction of forces applied to the lower limb. There is, however, limited information to support this assumption, with only a handful of studies exploring the role of the CoP in flexible flatfoot and the use of various forms of foot orthoses, each of which appear to show minimal changes in CoP displacement.36-38 Perhaps more important is that they fail to link the influence of the foot orthoses with patient outcomes in terms of reduction of signs and symptoms.
Postural stability and balance have also been explored by a number of authors.39-43 It is known that, due to the reduction in the height of the medial longitudinal arch, the center of mass shifts in individuals with a flexible flatfoot. These shifts are thought to normalize once a foot orthosis is placed under the foot.24,28,34 In a recent study, Payehdar and colleagues used a computerized balance system to evaluate the immediate effects of three conditions—shoe only, modified orthosis (characterized by separate walls and sole), and the University of California Berkeley Laboratory (UCBL) orthosis—on postural sway in 20 young adults with flexible pes planus (Figure 1). There were no significant differences among the three conditions for the medial-lateral and anterior-posterior sway directions, but total sway with the UCBL orthosis was significantly lower (13% less) than with the modified orthosis. This may indicate that the UCBL orthosis, which is characterized by higher medial and lateral flanges that cup and support the rearfoot and midfoot has the potential to improve balance in individuals with flexible flatfoot.44 Further research is needed to explore the longer-term effects of foot orthoses on postural sway, and these studies should be supported by CoP measures to facilitate dynamic comparisons during gait.
In a separate study,45 Sheykhi-Dolagh and colleagues explored the influence of three types of foot orthoses on the arch height index and foot mobility in adult patients with flexible pes planus who reported pain and other symptoms in the foot or lower limb approximately one hour after walking. The three orthoses included were soft, semirigid, and rigid. The semirigid foot orthosis was associated with the greatest foot mobility and the lowest mean arch height index, while the rigid orthosis (UCBL), with its extended medial and lateral flanges, was associated with the least mobility and the highest mean arch height index. For some patients, the UCBL design can cause some discomfort; the findings suggest that, in such patients, foot orthoses that facilitate foot mobility (and provide support) should be considered. Furthermore, the authors noted that patients considered the semirigid (polyurethane) foot orthoses the most comfortable, and suggested that this could be related to activity of the intrinsic and extrinsic muscles during walking. This lends support to the “preferred motion pathway model” developed by biomechanist Benno Nigg, PhD, that asserts that for every person there is a pathway of motion associated with the least energy expenditure, and this is associated with perceived comfort; a comfortable or successful orthosis will encourage a better pathway.46 The study by Sheykhi-Dolagh et al,45 however, explored only the static assessment of the foot; dynamic comparisons of foot mobility and the use of various types of foot orthoses in flexible flatfoot need to be explored.
Although there appears to be a range of literature supporting the use of foot orthoses for flexible flatfoot, many of the studies are focused on particular parameters (kinematic relationships, plantar pressures, etc) using individuals who present with no pain or symptoms. The likely reason for this trend in the literature could be due to the clinical popularity of foot orthoses as a preventive intervention and a perceived causal link with musculoskeletal pathologies within the foot, ankle, and lower limb (ie, knee pain, lower back pain, and Achilles tendinopathy). Only Esterman and Pilotto47 and Zammit and Payne48 provide information on pain reduction following the provision of foot orthoses. They noted that results from the Foot Health Status Questionnaire subscale of pain and function represented a significant improvement. However, the time to follow-up for each of these studies was 24 hours and four weeks, respectively, periods that are too short to be clinically relevant given the potentially chronic pain and symptoms some individuals with flexible flatfoot may have.
As stated earlier, neuromuscular fatigue is a common symptom linked with this condition, yet few current studies have explored this.49This perhaps is a current omission since there appears to be a general emerging consensus that fatigue can increase the risk of lower limb injury. The typical progression of neuromuscular fatigue has been related to various factors that center on an increase in joint range of motion, increased loading, and leg stiffness.50, 51 These factors are related to dynamic activities such as running but share a commonality with flexible flatfoot, as patients with flexible flatfoot who lead an active lifestyle, participating in sport and other recreational activities, are therefore at risk of developing overuse injuries in the lower limb.
Therapeutic beneficial response
In spite of the common presence of flexible flatfoot within the clinical setting, and despite the fact that there appears to be a range of foot orthoses prescribed by health professionals in this patient population, the therapeutic value of this intervention continues to remain uncertain. The reason for this variation in study findings is likely due to the various types of foot orthoses prescribed, which makes comparisons between studies difficult. Clear objective evidence is still required, and should focus on patient-centered outcomes such as pain and fatigue. Many studies have explored degrees of foot orthosis posting, but material choices and footwear selection are just as important.
Evidence within the literature on the use of foot orthoses for flexible flatfoot appears limited. Although the functional significance has been explored to an extent, and the complexity of foot and lower extremity function is recognized, there is an explicit need to investigate the role of foot orthoses on pain and fatigue (therapeutic effect) in individuals with flexible flatfoot over a prolonged period of time.
Sarah A. Curran, PhD, is a reader at the Wales Centre for Podiatric Studies, Cardiff School of Health Sciences, Cardiff Metropolitan University, UK.