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- Author or Editor: Kevin A. Kirby x
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A new theory of foot function based on the spatial location of the subtalar joint axis in relation to the weightbearing structures of the plantar foot is proposed. The theory relies on the concept of subtalar joint rotational equilibrium to explain how externally generated forces, such as ground reaction force, and internally generated forces, such as ligamentous and tendon tensile forces and joint compression forces, affect the mechanical behavior of the foot and lower extremity. The biomechanical effect of variations among individuals in the spatial location of the subtalar joint axis are explored, along with their clinical consequences, to offer an additional theory of foot function, which may improve on existing podiatric biomechanics theory. (J Am Podiatr Med Assoc 91(9): 465-487, 2001)
The Subtalar Joint Axis Locator
A Preliminary Report
A new clinical device, the subtalar joint axis locator, was created to track the three-dimensional location of the subtalar joint axis during weightbearing movements of the foot. The assumption was that if the anterior exit point of the subtalar joint axis is stationary relative to the dorsal aspect of the talar neck, then, by performing radiographs of the feet with the subtalar joint axis locator in place on the foot, the ability of the locator to track rotations and translations of the talar neck and thus the subtalar joint axis in space could be approximated. In this preliminary study of two adults, the subtalar joint axis locator accurately tracked the talar neck position during weightbearing rotational motions of the subtalar joint. The device was also used in a series of subjects to determine its dynamic capabilities. It is possible, then, that the subtalar joint axis locator can reliably track the spatial location of the subtalar joint axis during weightbearing movements of the foot. (J Am Podiatr Med Assoc 96(3): 212–219, 2006)
In-Shoe Pressure Measurement and Foot Orthosis Research
A Giant Leap Forward or a Step Too Far?
Foot orthoses are believed to exert their therapeutic effect on the human locomotor apparatus by altering the location, magnitude, and temporal patterns of ground reaction forces acting on the plantar foot during weightbearing activities. In-shoe pressure-measurement systems are increasingly being used by clinicians and researchers to assess kinetic changes at the foot-orthosis interface to better understand the function of foot orthoses and to derive more efficacious treatments for many painful foot and lower-extremity abnormalities. This article explores how the inherent three-dimensional surface topography and load-deformation characteristics of foot orthoses may challenge the validity, reliability, and clinical usefulness of the data obtained from in-shoe pressure-measurement systems in the context of foot orthotic therapy and research. The inability of in-shoe pressure-measurement systems to measure shearing forces beneath the foot, the required bending of the flat two-dimensional sensor insole to fit the pressure insole to the three-dimensional curves of the orthosis, the subsequent unbending of the sensor insole to display it on a computer monitor, and variations in the load-deformation characteristics of orthoses are all sources of potential error in examination of the kinetic effects of foot orthoses. Consequently, caution is required when interpreting the results of orthotic research that has used in-shoe pressure insole technology. The limitations of the technology should also be given due respect when in-shoe pressure measurement is used to make clinical decisions and prescribe custom foot orthoses for patients. (J Am Podiatr Med Assoc 100(6): 518–529, 2010)
The reliability of biomechanical measurements of the lower extremities, as they are commonly used in podiatric practice, was quantified by means of intraclass correlation coefficients (ICCs). This was done not only to evaluate interrater and intrarater reliability but also to provide an estimate for the accuracy of the measurements. The measurement protocol involved 30 asymptomatic subjects and five raters of varying experience. Each subject was measured twice by the same rater, with the retest immediately following the test. The study demonstrated that the interrater ICCs were quite low (≤0.51), except for the measurements of relaxed calcaneal stance position and forefoot varus (both 0.61 and 0.62 for left and right, respectively). However, the intrarater ICCs were relatively high (>0.8) for most raters and measurement variables. Measurement accuracy was moderate between raters. (J Am Podiatr Med Assoc 92(6): 317-326, 2002)
Background: Research on foot orthoses has shown that their effect on the kinematics of the rearfoot is variable, with no consistent patterns of changes being demonstrated. It has also been hypothesized that the mechanical effect of foot orthoses could be subject specific. The purpose of our study was to determine if maximally pronated feet have a different response to frontal plane wedging of foot orthoses than do nonmaximally pronated feet during static stance.
Methods: One hundred six feet of 53 healthy asymptomatic subjects were divided into two groups (maximally pronated and nonmaximally pronated) on the basis of their subtalar joint rotational position during relaxed bipedal stance. Functional foot orthoses were constructed for each subject and the relaxed calcaneal stance position was measured while standing on five separate frontal plane orthosis wedging conditions, 10° valgus, 5° valgus, no wedging, 5° varus, and 10° varus, to assess changes in calcaneal position.
Results: Relative to the no-wedging condition, there were statistically significant differences (P < .05) in calcaneal position between the maximally pronated and the nonmaximally pronated feet with the 10° valgus and the 10° varus wedging conditions. No significant differences in calcaneal position were found with the 5° varus and the 5° valgus wedging conditions.
Conclusions: Our study shows that the response to foot orthoses is variable between individuals. Maximally pronated subjects do not exhibit the same response to frontal plane wedging of foot orthoses as do nonmaximally pronated with 10° wedging. Intrinsic biomechanical factors such as subtalar joint position may influence the response to foot orthoses. (J Am Podiatr Med Assoc 99(1): 13–19, 2009)
The mechanical effects of genu valgum and varum deformities on the subtalar joint were investigated. First, a theoretical model of the forces within the foot and lower extremity during relaxed bipedal stance was developed predicting the rotational effect on the subtalar joint due to genu valgum and varum deformities. Second, a kinetic gait study was performed involving 15 subjects who walked with simulated genu valgum and genu varum over a force plate and a plantar pressure mat to determine the changes in the ground reaction force vector within the frontal plane and the changes in the center-of-pressure location on the plantar foot. These results predicted that a genu varum deformity would tend to cause a subtalar pronation moment to increase or a supination moment to decrease during the contact and propulsion phases of walking. With genu valgum, it was determined that during the contact phase a subtalar pronation moment would increase, whereas in the early propulsive phase, a subtalar supination moment would increase or a pronation moment would decrease. However, the current inability to track the spatial position of the subtalar joint axis makes it difficult to determine the absolute direction and magnitudes of the subtalar joint moments. (J Am Podiatr Med Assoc 95(6): 531–541, 2005)
Background: The scientific evidence behind the mechanical function of foot orthoses is still controversial. Research studies that have investigated the kinematic effect of foot orthoses on the lower extremity have shown variable results, with orthoses causing either no significant change or a small significant change in foot kinematics.
Methods: The right limbs of 12 healthy asymptomatic individuals were studied in three walking conditions: barefoot, with a 7° rearfoot varus wedge, and with a 7° rearfoot valgus wedge. Kinematic and kinetic variables measured were the foot progression angle, the peak internal tibial rotation angle, and net ankle inversion moments during the stance phase in the three conditions.
Results: There were statistically significant differences in the foot progression angle between the barefoot and varus wedge conditions and between the varus and valgus wedge conditions. There were no significant changes in peak internal tibial rotation among the three conditions tested. However, rearfoot varus wedges significantly reduced net ankle inversion moments compared with barefoot and rearfoot valgus wedges.
Conclusions: These results support the idea that foot orthoses work by methods other than by changing kinematic parameters. The present study supports the concept that foot orthoses work primarily by altering kinetics, with their effects on kinematics being secondary. (J Am Podiatr Med Assoc 99(5): 415–421, 2009)
