A New Model for Foot Orthoses
Human feet are uniquely different from those of other animals. They provide a base for stable and efficient bipedal gait freeing the hands for the manipulation of the environment with tools. It could be argued that the genetic mutations that produced the structure of the human foot began our evolution to become Homo Sapiens, the dominant species on this planet.
We have become so successful at manipulating our environment that we have replaced the natural walking surfaces, on which we evolved and lived for millennia, with hard, flat, level ones that our feet must pronate and flatten to adapt to.
The labeling of “flat feet” as a physical deformity was proposed in the late 19th century and, in 1907, Dr. Royal Whitman recommended contoured steel arch supports for its correction. These Whitman Plates were based on the notion that the anatomy of the feet that had served us so well in the past was defective and needed static support. This illogical assumption still persists when, in reality, "fallen arches" are simply the postural adaption of the foot to flat, hard surfaces beneath it.
In the 1960’s American Podiatrist Dr. Merton Root and colleagues adapted the principles of biomechanics to the prescription and fabrication of arch supports and coined the term Foot Orthoses. His theoretical model, the ‘Podiatric Model’, was so well described and presented that it became accepted as scientifically valid.
The Podiatric Model describes structural “deformities” of the feet and legs, which result in misalignment and mechanically inefficient gait on flat level surfaces. It proposes a method of taking a plaster cast of the foot (in a position called the Neutral Subtalar Position) and measurements of the deviation of the rearfoot and forefoot from a theoretically “normal” alignment. This plaster replica of the foot and the measurements are then used in the fabrication of hard orthotics. In theory these "Balanced" Orthoses should improve the biomechanical function of the foot and leg during gait.
Research into the efficacy and method of action of foot orthoses indicates that, although they are effective in treating many musculoskeletal conditions, the Podiatric Model is not scientifically validated. Some authors have suggested that a new model is required to better explain the effects of foot orthoses.
A new model (The Formthotics™ Medical System) is proposed. It includes the biomechanical concepts of the Podiatric Model but expands on them in the light of theoretical and scientific information that was not available when Dr. Root developed his theories.
This new model is an application of Panjabi’s Model of Spinal Stability. It describes how three subsystems (Static, Active and Neural) interact to provide joint stability. By this model, excessive subtalar pronation represents "instability" rather than "deformity". The function of the Static Subsystem (bones, ligaments, capsules and tendons) is mechanical and has been well described by Root and others. The functions of the Active Subsystem (muscles that act on and stabilize the joints) and Neural Subsystem (neuroreceptors and CNS reflexes, etc.) are not as well understood or utilized. It is postulated that Foot Orthoses can and should improve the function of all three subsystems and therefore careful assessment of the way the subsystems interrelate is essential when examining the foot and leg and prescribing functional foot orthoses.
This new model attributes dysfunction of the foot and leg to the walking surfaces and footwear of our modern civilization rather than to anatomical deformity.
Table 1. The differences between the two models.
|
The Podiatric Model |
The Formthotics™ System Model |
|
Abnormal Anatomy |
Abnormal Environment |
|
Structural Deformities |
Dysfunction of Normal Structures |
|
Correction of Deformities |
Improvement of Function |
|
Prescriptive Model |
Diagnostic-Therapeutic Model |
|
Measurements |
Functional Tests |
|
Lab Device |
Clinical Device |
|
Balanced Orthotic theory |
Total Contact Theory with Posts |
|
Device determines results |
Response determines results |
|
Biomechanical |
Biomechanical and Neuro-motor |
|
Control |
Functional Adaptation |
|
Joint Restriction |
Joint Mobilization |
|
Motion Control |
Motor Pattern Alteration |
|
Hard Materials |
Softer materials |
|
Rigid Devices |
Flexible Devices |
|
Permanent |
Therapeutic |
|
Uncomfortable |
Comfortable |
|
Therapist Centered |
Patient Centered |
|
Theoretical |
Evidence Based |
Read on: Functional Assessment: 6 Tests
Functional Assessment: 6 TestsTM
Test 1: The Subtalar Motion Test assesses biomechanics and joint function.
Manual Therapy Techniques are used to assess joint motion and mobilize any restrictions. Range of motion, quality of motion, end feel and restriction of motion are examined and restrictions are mobilized. The alignment of the leg, rearfoot and forefoot is assessed as per the Root method. Angles can be measured but may not predict the functional effect of the orthoses. The range of Pronation and Supination and the Neutral Subtalar Position are identified. Deviations from the mechanically ideal alignment are noted. The foot is examined in stance to determine the difference between its relaxed posture and the Neutral Calcaneal Stance alignment.
Test 2: The Alignment Test assesses Closed Kinetic Chain Mechanics.
Biomechanical principles are used to assess skeletal alignment. The patient is examined in stance, from both the front and the back, and “misalignments” from the Theoretically Ideal are noted. The examination is from head to toe. There are numerous reasons for asymmetry or “misalignment” of musculoskeletal structures. They are frequently related to dysfunction of the foot and leg. However, apparent mechanical “deformity” in the foot may be a compensation for joint and/or soft tissue restrictions in structures of the spine, pelvis, hips or knees. The assessment is done in Resting and Neutral Calcaneal Stance.
Test 3: The Supination Resistance Test relates to the Subtalar Joint Axis.
The examiner manually supinates the foot and grades the force required from 1 to 5. The force required to supinate the foot is related to the axis of the subtalar joint and varies significantly from person to person. In gait this force is generated by muscles in the foot and leg. The higher the supination resistance, the greater will be the loading on these muscles and associated structures and the more likely that overuse symptoms will occur. Orthoses that reduce supination resistance decrease this loading and therefore can relieve or prevent symptoms. The therapist should be familiar with the anatomy, enervation and function of all muscles of the foot and leg.
Test 4: Jack’s Test relates to the Windlass Mechanism.
This clinical test assesses the tension in the plantar fascia associated with heel lift and propulsion. The way in which the terminal slips of the plantar fascia wind around the metatarsal heads has been called the Windlass Mechanism (Hicks, 1954). The Subtalar joint must supinate in order for the toes to dorsiflex.
People vary greatly in the force required to dorsiflex the great toe to “activate the windlass mechanism”. The examiner manually dorsiflexes the great toe until the foot supinates and grades the force required from 1 to 5. Orthoses that decrease this force reduce strain in the Plantar Fascia and improve the efficiency of gait.
Test 5: The Balance Test relates to Proprioception and Postural Stability.
The patient stands on one leg, establishes balance and then closes the eyes. The arch rises and the foot seeks a “Functional Neutral Position”. It then oscillates about this position. Balance is eventually lost either medially or laterally. The examiner observes the stability of balance, the duration of balance, the “Functional Neutral Position” and the loss of balance (medial or lateral). The Orthoses should improve the stability and duration of balance but should not induce lateral instability.
Test 6: The Forefoot Stability Test relates to Propulsion.
Efficient Propulsion requires a stable base. The patient is instructed to stand on one foot and then rise up onto the toes. The examiner observes the ease of rising onto the toes, the stability and the tendency to inversion instability. The Orthoses should improve stability and prevent inversion instability.
Read on: Interpreting the 6 Tests
Interpreting the 6 tests
The 6 Tests™ provide a means of assessing the function of the three subsystems that interact to determine the dynamic stability of the foot and leg.
Tests 1 and 2 relate to the Passive Subsystem.
Tests 3 and 4 relate to the Active Subsystem.
Tests 5 and 6 relate to the Neural Subsystem.
The findings of the 6 tests™ may seem to be inconsistent. Some patients have more obvious problems of biomechanical alignment (tests 1 and 2), other have more obvious overloading of the active subsystem (tests 3 and 4) and still others show evidence of Neural Dysfunction (tests 5 and 6) with poor balance and stability. Understanding these differences is very useful in choosing the most appropriate device for the individual patient and problem being treated.
A balanced orthotic of hard material provides strict biomechanical control but may not enhance neuro-motor function as well as a more flexible total contact device. Patients with symptoms of overuse of the supinating muscles should have devices that reduce supination resistance and activate the Windlass Mechanism. A more flexible total contact orthosis, with posts if required, has more effect on Neuro-Motor Facilitation and proprioception for patients with poor balance or recurring ankle sprains.
Flexible, total contact orthoses can be as effective as harder balanced devices and are usually less expensive. It is practical to begin with more simple and less expensive devices and then move on to harder, more technical ones if the problem requires them.
Read on: Treatment Method: 6 Steps
Treatment Method: 6 StepsTM
Step 1. A shell is custom formed to the shape of the sole of the foot.
The effect of so forming the device is to create a Total Contact Foot Orthosis. This is best done by thermoforming a suitable material directly to the foot and in the footwear that will be worn. Neutral Subtalar position is approximated by having the centre of the patella aligned over the second toe. Total Contacts Orthoses provide Neuro Motor Facilitation and Mechanical Control.
Step 2. The Break-in Period.
The effect of a therapy is related to the adaptation of the patient’s body to the therapy. The patient uses the device in their shoes from 3 to 7 days. During this time the patient’s body adapts to the devices. The devices are slightly deformed, in accordance with the patterns of activity of the patient, to adapt to the patient’s dynamics. This break-in period is important and should not be omitted.
Step 3. The rearfoot is adjusted by Functional Posting
Wedges are used as a means of applying a force to the rear-foot (as in using a wedge to split a log of wood). The 6 Tests™ are repeated as a wedge is inserted to various depths under the medial and/or lateral aspects of the rearfoot portion of the device. Commonly, a medial or Varus wedge will yield the most improvement in alignment, balance and supination resistance. The improvement in function should be verified by the patient.
Step 4. The forefoot is adjusted by Functional Posting.
Wedges are used as a means of applying a force to the fore-foot. The depth of insertion and angle of the wedge determine the force that is applied. The 6 Tests are repeated as a wedge is inserted to various depths under the medial or lateral forefoot of the device (generally just proximal to the MTP joints). Commonly, a lateral (or forefoot valgus) post will yield the most improvement in Jacks Test and the Forefoot Stability Test. The improvement of function should be verified by the patient.
Step 5. In Shoe Testing.
The technology and structural characteristics of footwear effect foot and leg function. The functional effects of the devices should be assessed with the patient wearing them in the shoes. Orthotic Therapy may fail if there is conflict between the design of the devices and the structure of the shoe. The Supination Resistance Test and Jack's Test may be difficult to perform with the shoes on but the other tests can be done. Changing shoes can alter the functional effect of the devices.
Step 6. Ongoing adjustment
The patient's adaptation and functional response to orthotics varies over time and is effected by other therapies employed. The devices should be checked and modified at 3 to 6 month intervals. Modifications include the addition or reduction of wedges and also sometimes reforming the device. The end point of therapy occurs when the patient is able to perform his/her desired daily activities without pain or unacceptable restrictions of mobility.
Read on: The Duration of Orthotic Therapy
The Duration of Orthotic Therapy
According to the Podiatric Model, Foot Orthoses are corrective devices that should be worn for life. However, no valid studies have been done to clarify or validate this or to determine the long-term effects of wearing them.
If the basic premises of the FormthoticsTM Medical System Model are true then the adverse effects of modern civilization will continue to impair foot and leg function if the Formthotics orthoses are not worn. Patients can justifiably be told to wear the devices for life to maintain healthy foot and leg function and “protect” themselves from the effects of our modern environment.
In reality, the patient will decide whether to continue using the orthoses after their problem has resolved or to stop wearing them altogether. If the devices are comfortable and work well with the shoes people will prefer to keep using them.
Formthotics orthoses wear out, are lost or otherwise need replacement. A top cover of leather or other suitable material will prolong their useful life. Over time, the Orthoses may have to be replaced but, by then, Lower Extremity function may well have changed to necessitate re-assessment and new Formthotics.
If and when a patient does need hard and durable devices for biomechanical control, Formthotics should be used as an interim measure and to provide a useful template for the design of the hard custom devices.
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