The optimal arch height for a patient with functional flat foot may depend on the goals of orthotic management. Studies suggest that arch height directly affects excessive motion, but that controlling dynamic balance may require a more tailored approach.
By Stephen D. Perry, PhD, E. Anne Cunningham, Msc, CPed, and Kelly M. Goodwin, BSc
The interaction between the foot and its environment is critical in all forms of gait. During running, the foot provides a flexible landing structure that is adaptable for placing and accepting weight during initial contact. Then during push off, the foot provides a rigid structure to permit the transmission of forces created by the lower leg muscles to propel the body forward. In walking, the forces are much smaller, but now the foot acts as a neuromechanical conduit that provides both sensory information and the transference of mechanical forces to maintain the body’s stability. All of these functions occur in all forms of gait, but the prominence or importance of each of these roles is determined by the type of gait considered. Due to the importance of this type of interface between the foot and its environment, the application of in-shoe foot orthotics is critical to preserve these functions.
Flatfoot deformity or pes planus is the most common foot pathology in patients of all ages.1 The deformity may be associated with discomfort and pain, instability, serious foot, ankle, knee, and lower back joint problems, misalignments, and postural strain. However, individuals with this deformity can also be non-symptomatic. Within pes planus, the functional (sometimes termed “flexible”) flat foot (FFF) is defined as a hypermobile foot with excessive hindfoot valgus and minimal medial-longitudinal arch height when weight bearing (Figure 1A; arch is evident when not weight bearing, Figure 1B).1 Most often, FFF is treated with custom orthotics to aid in arch realignment and to provide stability. The research that will be discussed here will look at two aspects of gait; one will be the kinematics of the lower leg and foot, during running, exhibited by individuals with functional flat foot when an arch support is used. The other will be the impact of these arch supports, worn by the functionally flat footed individuals, on dynamic balance control during walking.
Indications for orthoses
Currently, orthotic prescription is recommended for individuals with symptomatic FFF in order to control the excessive motion of the lower extremity during running. The main reason that orthotics for FFF individuals are targeted to control motion during running involves the higher forces experienced during running, which may cause more motion and injuries/pain.2 Even so, the orthotics recommended could be used in other activities. There is general agreement in the literature with respect to the clinical effectiveness of orthotic intervention among runners. In particular, the use of foot orthotics has been positively associated with patient satisfaction2,3 and pain reduction,3-6 enabling individuals to return to running.2 Currently, researchers are attempting to understand the mechanism by which orthotics produce these encouraging symptomatic reductions. It has been speculated that orthotics may realign the lower extremity, decreasing the excessive motion of the rearfoot and tibia that is typically seen among individuals with FFF.7-9 However, it seems that for every study that indicates a positive mechanical effect of orthotics in reducing excessive motion of the lower extremity,4, 7-10 there is a study reporting that orthotic intervention has no such effect.4,10-13
With regard to dynamic balance control during walking, the only studies that could be located involved cadaveric models and the application of orthotics in static situations. As we age, unintentional falls cause debilitating injuries. Although falls are complex and many factors are involved, footwear and foot problems play a major role in the control of balance to avoid falls.14 Imhauser and colleagues15 quantified and compared the efficacy of orthoses in the treatment of flatfoot deformity of cadaveric models in a static
situation and determined that orthoses stabilize and restore the medial longitudinal arch. Furthermore, Kitaoka et al16 demonstrated a significant improvement in arch alignment and structural alignment of the lower limbs with the use of orthoses in cadavers. Given the limitations of the literature, however, the transferability of the findings has little clinical value as all studies have focused primarily on static conditions.
Our current ongoing research interests include foot function,17,18 footwear,19,20 and orthotic interventions,21,22 including studies of functionally flat footed individuals. These individuals were deemed eligible to participate in these studies if they met predetermined functional flatfoot (FFF) criteria (these criteria were predetermined in consultation with a certified pedorthist and have been reported elsewhere (Cunningham and Perry, submitted)). In addition, all participants completed a screening questionnaire and were excluded from the study if they demonstrated any neurological or physical condition that affected the use of their lower extremity. Ethics approval for these studies was received from our institutional ethics review board.
All participants (running study n=19 and walking study n=10) were functionally flat footed bilaterally with little or no pain. Each subject had both their feet casted in a subtalar neutral position by a pedorthist. The research studies presented here both utilized arch inserts (Figure 2). Each participant’s subtalar arch height was determined by aligning a ruler at the medial edge of both the first metatarsal and heel regions, then measuring the height from the medial edge of the ruler down to the casting along a vertical axis (Figure 3). Arch inserts of 0%, 33%, 66%, and 100% of the subtalar neutral arch height were created for each participant (Figure 4).
In the running study, athletic tape was used to adhere the arch supports to the plantar surface of the foot, specifically to the medial longitudinal arch. Participants reported that the taping technique did not limit normal movement of the foot. In the walking study, participants were sized and fitted with identically styled laboratory walking shoes (Rockport, World Tour Classic Model; Canton, MA) and custom-sized, flat insoles with the arch inserts adhered to them (Figure 3). In both studies, arch inserts of different heights were worn in random order during experimentation.
The running study had each participant run at a velocity of 2.0 m/s and 3.0 m/s on a treadmill while three-dimensional angular kinematics were recorded using multiple infrared markers placed on the lower leg and foot. Kinematic variables measured included rearfoot angle (frontal plane motion of the foot relative to the leg) and tibial rotation (relative rotation of the lower leg about its long axis). Both measures were represented relative to a static standing trial. Each participant ran with each of the arch inserts adhered under the medial arch during both velocity conditions. All participants were physically active but not competitive runners.
The results from the running study (only data for the 2.0 m/s velocity condition are presented) suggest that as the degree of orthotic intervention (arch insert height) increased, there were significant (p < 0.001) decreases in maximum rearfoot angle and maximum internal tibial rotation angle among this population (Figure 5). However, rate of rearfoot motion and rate of internal tibial rotation were not affected.
Dynamic balance study
The walking study had each participant walk across a series of inclined platforms that simulated uneven surfaces (described by Perry et al23) in order to test dynamic balance control. A 21- marker setup was used to estimate the three-dimensional motion of the center of mass (COM) of the body and the base of support (BOS), defined as the contact surface of the feet. Dynamic balance control was determined by measurement of the lateral stability margin, as defined by the distance (in the transverse plane) between the lateral border of the BOS and the position of the COM during the single support phase of gait (as described in Perry et al18) Again, each participant had each arch height placed on the blank insole and then into the standard footwear.
Increases in arch insert height were associated with demonstrated statistically significant changes in dynamic stability. The greatest improvement happened at the 66% arch height (Figure 6). During the single support phase of gait, subjects wearing the 66% arch height insert exhibited the lowest maximum and highest minimum values for medial-lateral COM-BOS difference (p < 0.04).
The decrease in rearfoot angle (commonly referred to as a good indication of foot pronation24) and internal tibial rotation (which has been shown to have a tight relationship with foot pronation25) with increased orthotic intervention (arch insert height) during running demonstrates the direct link between orthotic height and foot/leg mechanics. However, without an associated significant decrease in the rate of rearfoot angle and rate of internal tibial rotation, the exposure of the lower extremity to quick angular change, which is thought to be a major contributor to injuries, may not be reduced to the extent that had been expected. The results of the walking study indicate that individuals with functional flatfoot experience increased dynamic stability when wearing arch inserts that are 66% of their subtalar neutral arch height.
These findings emphasis that orthotics are effective in reducing the motions of the foot and lower extremity in FFF individuals. They also indicate that an incremental increase in orthotic height does have a direct relationship to how much change will be observed in terms of maximum rearfoot and tibial internal rotation angles.
Additionally, our findings suggest that the more complex area of orthotics and dynamic balance control does not seem to be as straightforward. Rather than a direct relationship, each individual may have an optimal orthotic height that provides optimal dynamic balance control. These two studies suggest the importance of giving consideration to both the benefit of reduced foot and leg motion and the optimization of dynamic control. Either of these mechanisms, excessive motion or a loss of balance, could result in a disabling injury.
Stephen D. Perry, MSc, PhD, is an associate professor in the department of kinesiology & physical education at Wilfrid Laurier University in Waterloo, Ontario, Canada. E. Anne Cunningham, MSc, CPed is a pedorthic intern in Waterloo, Ontario. The running studies were part of her MSc at Wilfrid Laurier University. Kelly M. Goodwin, BSc, MD (candidate) is a medical student at the University of Ottawa. The dynamic balance studies were her BSc senior thesis project at Wilfrid Laurier University.
Acknowledgments: This work was supported by an operating grant from the Canadian Institutes of Health Research (MOP-77772) and equipment was supported by the Canadian Foundation for Innovation, the Ontario Innovation Trust and Wilfrid Laurier University.
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Figure 1: A. Demonstration of arch collapse during weight bearing, B. Evidence of arch formation without weight bearing.
Figure 2: A. Medial view of arch insert, B. Medial-Superior view of arch insert, C. Superior view of arch insert.
Figure 3: Determination of arch height from subtalar neutral foot cast.
Figure 4: Arch inserts placed on custom-fit insoles.
Figure 5: Effect of orthotic intervention on rearfoot motion and internal tibial rotation while running at 2.0 m/s.
Figure 6: Effect of orthotic intervention on maximum and minimum centre of mass-base of support (COM-BOS) in the medial-lateral direction.