Emerging Concepts In Podiatric Biomechanics

By Kevin A. Kirby, DPM, MS

    The world of podiatric biomechanics is very different now than when Merton Root, DPM, created the first Department of Podiatric Biomechanics at the California College of Chiropody in San Francisco in 1966.1 During those exciting early years of development within the new subspecialty of “podiatric biomechanics,” Dr. Root and his podiatric colleagues created a classification system, based on the subtalar joint (STJ) neutral position, that remains to this day the most complete method by which to classify the structure of the foot and lower extremity.1,2     During that same period of seminal intellectual growth, Dr. Root also developed a new type of thermoplastic foot orthosis, popularly called the Root functional orthosis, which has served as the basis for the modern custom foot orthosis that is currently in wide use in multiple medical specialties.3,4     Largely due to the intellectual seeds that Dr. Root and coworkers planted on the West Coast nearly 40 years ago with Dr. Root’s pioneering work in foot biomechanics and custom foot orthoses, there has been a significant increase in the use of custom foot orthoses by foot health professionals for the treatment of foot and lower extremity pathologies.1 In addition, over the past 40 years, there has been a literal explosion in the amount, sophistication and quality of foot and lower extremity biomechanics research within the worldwide podiatric, medical and biomechanics literature.     Due to this ever increasing body of fascinating scientific research by multiple clinical scientific disciplines, it is understandably very difficult for the busy podiatrist to remain abreast of new theories, technologies and therapeutic techniques that are being considered by experts within the scientific discipline of biomechanics. Accordingly, let us take a closer look at the latest available technologies, theories and therapeutic advances within the international biomechanics community in order to facilitate a better understanding of emerging concepts in podiatric biomechanics.

How Modern Technological Advances Have Reinvented Gait Analysis

    Today’s modern foot and lower extremity biomechanics research laboratories have technologies and equipment that researchers from Dr. Root’s early years of investigation could only dream of. These computer-based technologies enable one to precisely determine the forces and moments (i.e. kinetics), and the movement patterns (i.e. kinematics) of the foot and lower extremity during nearly any type of weightbearing activity.     This knowledge allows clinicians more accurate measurement and more rapid analysis of both the external and internal loading forces that act upon on the structural components of the human locomotor system.5-8     The time savings for today’s researcher, when compared to the early biomechanics researchers, is probably the most remarkable aspect of this research revolution. For example, when researchers attempted to apply mechanical analysis to gait at the turn of the 20th century, they were limited by the fact that it took over 1,000 hours to process the data generated from a single step.9 Today, we can perform this same analysis, using the technology and computing power available in most modern biomechanics labs, with far more accuracy and it often takes less than a minute to perform.     The force plate, pressure mat and pressure insole are the main technologies that allow one to determine the kinetics of the foot and lower extremity during weightbearing activities in the modern biomechanics lab and in the clinical setting. The force plate is a rigid plate- type device that can be mounted in the floor of the biomechanics lab. It facilitates the precise determination of the magnitude and three-dimensional location of the ground reaction force vector while the patient stands, walks, runs or performs other activities over the plate.     The pressure mat is similar to the force plate in appearance. However, with its thousands of thin, force-sensing elements, it allows clinicians to determine the pressures acting at multiple discrete locations on the plantar foot simultaneously every few milliseconds during weightbearing activities.     The pressure insole is basically a pressure mat that is specifically designed to fit inside a shoe so one can determine the pressures acting on the plantar foot within the shoe environment during gait. The pressure insole is especially useful clinically since it allows clinicians to determine plantar pressure effects of various foot orthosis designs or different shoe types for the patient.5,7,8 Both pressure mats and pressure insoles have found practical application in many podiatrists’ offices in diagnosing pathologies and designing more effective mechanical therapies for the injured patient.     In the biomechanics research lab, the strain gauge is another technology that has produced significant increases in the understanding of the internal forces within the tissues of the foot and lower extremity. These devices, some of which are less than an inch in length, allow the microscopic measurement of changes in the length of bones, ligaments and tendons within the foot and lower extremity during simulated loading situations of cadaver specimens or weightbearing activities in live patients. When clinicians combine the tiny deformations measured by the strain gauge with the known mechanical properties of the tissue of interest, they may determine the stress and/or force within that tissue.5     This “internal strain data” has recently provided researchers with invaluable information regarding how external forces acting on the foot and lower extremity may be converted into internal deformations within the bone, ligament and tendon of the body.10-17 This research will ultimately allow us to better understand injury mechanisms within the human locomotor apparatus and will therefore allow us to design better treatment methods for patients with mechanically-based pathologies.

Understanding The Potential Benefits Of Finite Element Analysis

    Since it is difficult for researchers to receive medical ethics approval for the surgical implantation of instrumentation into live subjects to gain more accurate information on the forces, stresses and strains acting within the foot and lower extremity, many researchers have turned to a computer modeling technique called finite element analysis (FEA), which allows them to obtain this valuable information noninvasively.18     Finite element analysis originated with the work of Courant in 1943 and has been used in science and engineering applications for decades to determine the stresses and strains within the component structures of machines, buildings and bridges.19 However, it has only been within the past few years that biomechanics researchers have increasingly applied this powerful computer modeling technique to produce accurate two-dimensional and three-dimensional models of the foot and lower extremity.     When researchers combine these models with the known mechanical characteristics of the structural components of the foot and lower extremity, the internal stresses and strains within these structures under various loading situations may be calculated.20-27     For example, once the “mesh” of each structure of the foot and/or lower extremity has been constructed by the computer, finite element analysis then allows the computer to calculate how different loading conditions (e.g. changes in body weight, activity type, orthosis construction or shoe design) mechanically affect the stresses and strains acting within any section of any of the bones, ligaments or tendons of the foot and lower extremity.     Researchers will likely utilize the noninvasive nature and relatively unlimited experimental potential of finite element analysis with increasing frequency in the coming years to determine which mechanical therapeutic techniques may best reduce the pathologic internal forces acting within the foot and lower extremity.     Indeed, this knowledge may go a long way toward preventing and healing mechanically-based injuries among our patients.

Examining The Debate Over Subtalar Joint Neutral Theory

    As mentioned earlier, Root, et. al., proposed theories over 40 years ago as to how an ideal or “normal” foot should be structured, how it should function during gait and how one should treat the abnormal foot with foot orthoses. They theorized that since the normal foot should function around the STJ neutral position with the midtarsal joints “locked” in the maximally pronated position, foot orthoses had to accomplish three key functions in order to be therapeutically effective. According to these authors, effective foot orthoses:     • cause the STJ to function around its desired neutral position;     • prevent “compensation” for foot and lower extremity deformities; and     • cause a “locking of the midtarsal joint.”28     The theory that the feet should function around the STJ neutral position has been the topic of considerable debate within the medical literature for the past few years.29-36 Unfortunately, there is no research evidence to date to support the claims by Root, et. al., that the foot must function around the STJ neutral position in order to remain injury-free or function normally. However, there is considerable research evidence that suggests that treating patients with foot orthoses cast with the foot in the STJ neutral position does, in fact, make their symptoms improve.37-53     Due to this lack of research evidence to validate the STJ neutral theory, researchers have proposed alternative theories of foot function to explain the known mechanical behavior of the human foot and the findings from other researchers. Clinicians should peruse the excellent review articles by Lee and Payne for further reading on the various theories of foot function.1,2,36,54

A Closer Look At The Effects Of STJ Axis Location And Rotational Equilibrium

    In 1987, Kirby proposed that the spatial location of the STJ axis had a significant mechanical effect on the function of the foot since its abnormal spatial location relative to the plantar foot significantly alters the magnitudes and direction of rotational forces (i.e. moments) acting across the STJ axis.55 In a subsequent paper in 1989, Kirby applied the physics principle of rotational equilibrium to the concept of STJ axis spatial location to provide a mechanically coherent explanation for the varying mechanical behavior of maximally pronated feet and to explain the etiology of certain clinical pathologies such as sinus tarsi syndrome.56     In 1992, Kirby and Green proposed a mechanical explanation for the increased anti-pronation effects of the Blake Inverted Orthosis in the pediatric flatfoot deformity. They stated that the inverted heel cup shape of the orthosis caused an increase in medially positioned orthosis reaction force (ORF) and a decrease in laterally positioned ORF.4,57-59 During the same year, Kirby introduced the medial heel skive orthosis technique as a new positive cast correction method. This technique introduced a varying magnitude of inverted heel cup contour in order to enhance the pronation controlling ability of the orthosis so one could facilitate improved treatment of conditions such as posterior tibial dysfunction.60     In 1999, Fuller described the mechanical importance of the STJ axis location relative to the center of pressure on the plantar foot in relation to how abnormal STJ pronation and supination moments may cause certain foot and lower extremity pathologies.61 In 2001, Kirby expanded on his previously published concepts to offer a new theory of foot function, the Subtalar Joint Axis Location and Rotational Equilibrium Theory of Foot Function.62 Other researchers have also been very interested in the importance of the STJ axis in the biomechanical function of the foot and lower extremity.63-97

How The Tissue Stress Theory Has Evolved Over The Years

    In regard to treating mechanically-based pathologies in podiatric biomechanics, the measurement system (based upon STJ neutral position) proposed by Root, et. al., may not be the ideal reference system by which to prescribe custom foot orthoses. Even though the STJ neutral theory had been taught for decades at many podiatric medical colleges, researchers have suggested, as early as 16 years ago, that there are significant problems with the STJ neutral theory.98 Since that time, new ideas have begun to emerge as to how the podiatrist should design the optimal prescription foot orthosis for his or her patient’s pathology and abnormal gait pattern.2,54,99     The treatment paradigm that seems to be gaining the most attention from experts in podiatric biomechanics is commonly called the “Tissue Stress Theory.”     This theory first emerged in the podiatric literature in 1992 and emphasized placing more importance on the pathological stresses on an injured tissue than the apparent “deformities” of the foot and lower extremity when attempting to determine an appropriate mechanical foot therapy. The proponents of the tissue stress theory encouraged the podiatrist to “think like an engineer” and use basic modeling techniques to determine how best to treat injuries with prescription orthoses.98 Similar to how an engineer may use models of a bridge to determine the stresses within its structural components to optimize its design, the podiatrist can use basic models of the forces acting on the foot to make intelligent predictions regarding which structural components of the foot are under tensile, compression and/or torsional loading stresses during weightbearing activities so one can facilitate better design of foot orthoses.     In 1995, McPoil and Hunt first coined the term “tissue stress model” to promote their idea that clinicians should direct mechanical foot therapy toward resolving tissue stress in order to reduce the magnitude of pathological forces on the specific tissues that are injured.100 McPoil and Hunt stated that the tissue stress model serves “as the basis for developing an examination and management paradigm for treating individuals with foot disorders.”100 They also felt that the current measurement techniques that have been used for years for the foot and lower extremity were unreliable. According to those authors, using the tissue stress model would allow the clinician to more intelligently and accurately direct the prescription of the foot orthosis toward the specific anatomical structure that needs to have a reduction in the magnitude of stress as opposed to directing the orthosis prescription to a foot deformity that may not have been reliably measured.     Following up on McPoil and Hunt’s theory, Fuller described how computerized gait evaluation and modeling techniques of the foot and lower extremity may help predict the stress in specific structural components within the foot and lower extremity as a way to guide foot orthosis treatment.101 In later published works and in a soon to be published book chapter, Fuller and Kirby have expanded on their concepts of how one can utilize modeling techniques and other basic engineering concepts along with the STJ equilibrium and tissue stress theories to produce a comprehensive model for the treatment of mechanically-based pathology of the foot and lower extremity.98,102,103

What About The Sagittal Plane Facilitation Of Motion Theory?

    In 1997, Payne and Dananberg described a new theory of foot and lower extremity function called the sagittal plane facilitation of motion model.104 This theory, based on the previous works of Dananberg, relies on the hypothesis that clinicians should direct foot orthosis therapy toward motion enhancement rather than the traditional podiatric biomechanics concept of motion control.105-107     Central to this theory is that any functional restriction or “blockage” of hallux dorsiflexion during propulsion (i.e. functional hallux limitus) will cause alterations in the movement patterns, thereby resulting in abnormal stresses within the foot and lower extremity during gait.108,109 In regard to foot orthoses treatment, the sagittal plane facilitation theory is centered around reestablishing the timing patterns of plantar pressure distribution via computerized in-shoe pressure analysis and restoring normal hallux dorsiflexion during propulsion.     Dananberg and Guiliano demonstrated support for the therapeutic utility of this theory in a 1999 prospective research study involving 32 patients with chronic low back pain. Patients treated with custom foot orthoses, which were designed with the aid of in-shoe pressure analysis, experienced more than twice the improvement in pain and for twice the duration as those who received traditional back pain treatment.110     However, a recently published study by Van Gheluwe, Dananberg and coworkers has demonstrated that only 20 percent of both their normal and functional hallux limitus patients demonstrated maximum navicular drop after heel off (i.e. “retrograde pronation”).111 This finding may indicate that the “retrograde response,” which many believe is due to functional hallux limitus, may in fact be due to additional structural and functional factors within the foot.

What You Should Know About The Preferred Movement Pathway Theory

    In 1999, Nigg, et. al., proposed a theory of orthosis function called the “preferred movement pathway model” and suggested that it should become a new paradigm for movement control of the foot and lower extremity.112 Based on their previous research, they hypothesized that foot orthoses do not function by realigning the skeleton. They noted that foot orthoses alter the input signals into the plantar foot that cause a change in the “muscle tuning” of the lower extremity, thereby producing a change in muscle activity with the goal of dampening soft tissue vibrations within the lower extremity muscles.113,114     Nigg, et. al., proposed that if the foot orthosis allows the joints of the foot and lower extremity to take their preferred movement pathway, that muscle activity will be minimized and if the orthosis counteracts the preferred movement path, that muscle activity will be increased.     Even though the work of Nigg and co-workers has received considerable attention within the international biomechanics community, further research will be necessary regarding this theory as well as the other aforementioned theories in order to support or reject their validity.

In Conclusion

    Early podiatric researchers such as Dr. Root established the foundations of foot and lower extremity biomechanics, and have helped create the exciting foot and lower extremity research environment that now exists within the international biomechanics community. This research revolution, which has been made possible by powerful advances in computer and biomechanics technologies, has not only provided us with increased knowledge but has also allowed researchers to propose and develop alternative theories of foot function and foot orthosis treatment that will either need to be supported or rejected by further meaningful research.     The podiatric physician who continually strives to keep abreast of the latest foot and lower extremity biomechanics technology, theory and research will greatly improve his or her knowledge of the complex function of the foot and lower extremity. As a result, DPMs will become better able to provide improved conservative and surgical treatment outcomes for their patients with mechanically-related pathologies of their foot and lower extremities.     Dr. Kirby is an Adjunct Associate Professor in the Department of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is the Director of Clinical Biomechanics at Precision Intricast Inc.     Editor’s note: For related articles, see “Redefining Biomechanics Of The Foot And Ankle” in the October 2005 issue of Podiatry Today, “Reconsider Biomechanical Causes In Heel Pain Cases” in the November 2002 issue, “Understanding The Impact Of Gait Analysis” in the April 2003 issue or “Comparing Lessons In Biomechanics And The Realities Of Clinical Experience” in the April 2006 issue.     Also be sure to visit the archives at www.podiatrytoday.com.



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