As the foundation of our closed chain posture, the foot must survive a lifetime of ballistic, static and repetitive stress while providing support, leverage, movement and function to accomplish our weightbearing tasks effectively and efficiently.

Variables in foot type, body weight, health and fitness state, the unyielding ground and unyielding shoe boxes and the lack of a teachable, researchable paradigm for diagnosis and care, reduce the body of valid intrapersonal evidence (EBM) that would guide us in diagnosing and treating foot pathology interpersonally. This has led to a lack of consensus for evidenced based practice (EBP) of biomechanics.

The Physical Stress Theory (PST)¹

The tissues of the weighted foot accept stress the instant it applies force against the ground, because of the ground reactive force (GRF) that Newton’s Laws govern. In resting stance, on both a micro (Tissue Stress) and macro (Physical Stress) level, bones jam into one another, tendons and ligaments pull at their insertions and soft tissues get squeezed and crushed as pathologic kinetic moments develop before any actual movement or work is accomplished.

Tissue and physical stress builds universally as the weighted foot begins to move or perform tasks such as walking, moving side to side or going up on toes to reach an object. As movement begins, ligaments and tendons exert stressful forces on the tissues and themselves, keeping feet supported and capable of kinematic function.

Every tissue has a threshold for tolerating stress beyond which they injure or become painful, called the Pain and Injury Threshold or PIT. Defining and monitoring PIT is difficult, since that process itself is often invasive, difficult to reproduce and adds tissue stress when performed.

Daily life causes subclinical injury to tissues that are repaired internally before producing pain, injury, deformation or performance and quality of life issues. However, when stress is too strong or repetitive, subclinical events cannot repair fast enough, and injury and pain become noticeable or tissues begin to deform or weaken requiring changes in lifestyle or some form of treatment in order to repair. Left untreated, these stresses result in permanent progressive injury and deformity, as well as reduced performance and quality of life issues.

PST suggests that the way to attempt to treat pathologies and pain is to reduce the stress on the tissues by one of four methods (Table I).

Multiple paradigms exist offering a variety of diagnostic and treatment protocols for biomechanical care. However, none give practitioners a proven, evidence based menu of care to incorporate into Evidence Based Practice.

Variability Factors

Steven Hawkings stated, “the goal of science is to have theory match observation”, but because we are applying the laws of physics, engineering, architecture and mechanics to a reactive, thinking and compensating subject that is capable of “defying” their primary tenets, it becomes the responsibility of each practitioner to factor in secondary tenets in order to maximize custom care.

For example, if we drop a bar of gold and an apple of equal mass from a tree at the same height, they obey Newton’s Primary Law of gravity, hitting the ground at the same time.  However, if we add a third object of equal mass in the form of a bird, it may never even hit the ground.  In order to apply Newton’s Second Law to this living object, we must utilize secondary laws such as draft, lift and aerodynamic kinetics to explain why this result didn’t match our theory.  Biologically, the primary Newtonian law wasn’t obeyed.        

Biomechanists are either practicing orthodox methods or are converting to one of the newer paradigms in search of one that can replace or upgrade the current standard. The problem in making the selection for the mainstream practitioner is that there are too many variables in play, obscuring the primary rules of any paradigm. These variables can be divided into three intrapersonal categories. There are variations within the subjects being treated. There are variations between the practitioners of biomechanics and there are variations that exist within the laboratories filling the prescriptions of these practitioners for fabrication of their custom foot orthotics. These variations come into play at all levels of EBM and EBP (Table II).

Until a paradigm surfaces that holds the practitioners and the laboratories to standards of practice, biomechanics can best be examined and applied on a single practitioner level, utilizing a single fabrication source, on a single case basis.

In summary, biomechanics currently is an interpersonal science that works best in the hands of single dedicated, well trained and practiced individuals utilizing a single fabrication source. Although, like all sciences, biomechanics is impacted by research statistics and diagnostic and treatment protocols involving multiple subjects, the validity and applicability of existing biomechanical evidence loses weight when applied to isolated practitioner-patient encounters, due to the error created by coexisting variability factors.

Subtalar Joint Neutral Theory – Root²

The model utilized most often in biomechanics is the Subtalar Joint Neutral Theory developed by Merton Root, D.P.M. in the seventies. The theory relies on an orthotic shell produced from a negative cast taken in a tried, tested and accepted optimal functional position called Subtalar Joint Neutral (STJN) Position. It is “the position of the rearfoot neither pronated, nor supinated”. Additional wedges or posts and other modifications are then prescribed to produce the final product, depending on rearfoot and subsequent rearfoot to forefoot relationship examination.

Thirty years without quality control over the effect of the variables by practitioners have fostered slack practice habits fueled by profit and time saving incentives. In addition, unacceptable fabrication habits by the orthotic labs driven by a desire to increase profits and save time at the cost of quality left uncensored by the practitioners has diluted Dr. Root’s paradigm, rendering it simplistic and less effective. Similar STJN devices from both custom and over the counter sources are being dispensed by a wider variety of practitioners that now include shoe clerks, pedorthists, ski shops and the local pharmacy scanners.³

In summary, modern STJN biomechanics has taken “the doctor out of the box” and for the most part, in mainstream, the paradigm is practiced unprofessionally.

Subtalar Joint Axis Location and Rotational Equilibrium (SALRE) Theory – Kirby⁴

The Subtaler Axis Location and Rotational Equilibrium (SALRE) Theory of Kevin Kirby, D.P.M., offers an engineering concept that tissue stress can be reduced and maintained subclinical by exerting pronatory and/or supinatory Orthotic Reactive Forces or ORF’s affecting the subtalar joint axis to place it in equilibrium. This theory begins when a patient has pain or injury beyond the PIT. It uses subtalar neutral casting for the most part, because SALRE believes that the shell position is of little import. It relies on the premise that ORF’s can be applied to any orthotic shell, that will counter presenting pathology eliminating pain and healing injury. Its drawbacks are that it overlooks the impact of variability upon primary Engineering and Newtonian Laws when making claims and that it accepts compensatory pathology produced by the ORF’s when following the evidence.

Maximal Arch Subtalar Stabilization or MASS Position Theory⁵ - Glaser

Ed Glaser D.P.M., in his Maximal Arch Subtalar Stabilization or MASS Position Theory, suggests that the way to reduce the demands on the tissues is to position the Vault of The Foot into an OFP high enough to allow for resupination of the foot in function, by creating a full contact orthotic supporting that position. MASS position provides mechanical advantage and improved function to the ligaments and muscle engines of the foot. Its drawback is that it gives no import to applying ORF’s to the foot as a means of reducing tissue stress or improving function.

Sagittal Plane Block Theory⁶ - Dananberg

The Sagittal Plane Block Theory fathered by Howard Dananberg, D.P.M., promotes the concept that the sagittal plane rockers of the foot must be evaluated and treated if their function is reduced in closed chain. It relies on gait analysis and the use of sagittal plane lifts and cutouts to apply counter force to a generic orthotic shell that is then “corrected” by repeat gait evaluation “on the fly”. This theory focuses on first MP Joint functional hallux limitus and ankle equinus as causative. It has OTC applications, but is rarely used as a paradigm for custom orthotics.

Since these theories are either not position oriented (Root, SALRE and Sagittal Block) or they rely on position alone (MASS), they may not be a viable substitute for Rootian biomechanics for mainstream practice.

Part II will present The Foot Centering Theory⁷ (see Figure I) and its Rearfoot and Forefoot SERM-PERM Tests.

Figure I: The Off Centered Arches of The Foot

Biomechanics EBM Part III: The Levels of Evidence in EBM, a logical and coherent argument that may be capable of placing Mainstream podiatry back atop the Biomechanical Pyramid.⁷

References:

1. Mueller MJ, Maluf KS. Tissue adaptation to physical stress: a proposed "Physical Stress Theory" to guide physical therapist practice, education, and research, Phys Ther Apr 2002;82(4):383-403
2. Root M, Orien W, Weed J: Normal and Abnormal Function of the Foot; 1977, Biomechanics Publishing
3. Guldemond A, Leffers P, Sanders A: Casting Methods and Plantar Pressure: Effects of Custom-made Foot Orthoses on Dynamic Plantar Pressure:JAPMA, 96:1:9-18 2006
4. Kirby KA: Subtalar Joint Axis Location and Rotational Equilibrium Theory of Foot Function, JAPMA 91 (9) 465-487 2001
5. Currie S, Bursch D, Glaser E: Active Stance: Orthoses – Functional relevance of the arch, Lower Extremity Review; March 2010: 26-31
6. Dananberg HJ Sagittal plane biomechanics; Journal of the American Podiatric Medical Association, Vol 90, Issue 1 47-50
7. Ezzo J, Bausell B, Moerman DE, Berman B, Hadhazy V (2001). "Reviewing the reviews. How strong is the evidence? How clear are the conclusions?". Int J Technol Assess Health Care 17 (4): 457–466.

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