by Robert D. Phillips, DPM1
1 – Orlando Veterans Administration Medical Center
I read with interest in the March issue the article by Glaser and Fleming, “Foot Posture Biomechanics and MASS Theory.” If this article had appeared in a non-refereed magazine, I probably would not have said much, however since The Foot and Ankle Online Journal purports to be a refereed journal, with peer review of submitted articles, I am going to have to chide the reviewers for allowing an article with such sloppy scholarliness slip by them. The article with riddled with gross misstatements about what Dr. Merton Root taught, poor interpretation of the limited literature it presented as references, and poor thought logic and reasoning, and statements of authoritative opinion that have less research to back them up than the opinions of Root they supposedly tear down.
Let’s first of all look at some of the outright misstatements about the theories of Merton L. Root
- In the very first paragraph, the authors state, “He [Root] discovered that by placing the patient prone while holding the off weight bearing foot in a palpated, “neutral” position, it was observed that most heels were inverted; rearfoot varus.”
Nowhere in the cited reference does Root make such a statement, though he does say that rearfoot varus is one of the common causes of excessive subtalar joint pronation (p.298) and also that compensation for rearfoot varus did not usually produce highly pathological conditions as the subtalar joint would still resupinate after heel off (p 313) .
Now, it is true that some of Root’s contemporaries have taught that most people have rearfoot varus. This particular writer has heard many times such theory from colleagues, e.g. Dr. Chris Smith, however Root himself did not did not teach this. Phillips and Phillips (1983) found in the average patient in their series had 1.5° of subtalar varus . McPoil et al. (1988) reported that 84% of young females had subtalar varus, with 41% less than 4° and 41% between 4°-8° . On the other hand Åström and Arvidson (1995) reported the average person had 2° of subtalar valgus .
It should be pointed out that Root never believed that the average person represented the normal foot. A full and most scholarly discussion of Root’s concept of normal can be found in the historical treatise by Lee of the Root concepts (2001) .
- The authors go on to state, “Root recommended taking 17 measurements called the Static Biomechanical Exam ”.
Root did not call the examination “the Static Biomechanical Exam”. He called the form, “Biomechanical Exam of Lower Extremities.” The exam is broken into 7 basic areas, the metatarsus, the midfoot, the rearfoot, the ankle joint, the lower leg, the hip, and a static functional test. The word ‘static’ could possibly be applied to three fixed measurements, i.e. the malleolar torsion, the forefoot to rearfoot relationship and the tibial angulation with the ground. The others are ranges of motion wherein the joints have capability to function. Therefore we could state that part of the exam is static, part is functional ranges and part is measure of function. In addition to taking goniometric measurements, he also advocated extensive and complete muscle testing.
- The authors state, “Treatment was aimed at correcting what was viewed as a frontal plane deformity with frontal plane correction of the rearfoot and forefoot, called posts, designed to encourage the foot into a more neutral rotational position around the subtalar joint (STJ) axis.”
There are several errors in this one sentence. First of all treatment was aimed at stabilizing the midtarsal joint by capturing the plantar foot shape in a nonweightbearing state, and using a “rigid” material to push the midtarsal joint toward this state. The stable state of the midtarsal joint is based on the twisted plate theory, first advocated by Steindler (1929) . This twisted plate theory of function was rejected by Schuster (1976)  The orthotic did not correct the forefoot or rearfoot, it merely tried to support either the medial or lateral side of the forefoot off the floor when the subtalar joint was in neutral. This support platform is called the forefoot post. The rearfoot post was added much later to make the orthotic more stable in the shoe, not to make any corrections.
The authors try to confuse the reader with their last phrase, utilizing terminology that is redundant jargon. All researchers of subtalar joint motion papers have maintained that there is an axis for the joint to move around. Variations in methodology have disagreed as to whether the motion is a strict axis motion , a moving joint axis  or a helical motion . Whichever it is, it is unknown what a “neutral rotational position is.” Root did state that a majority (not all) orthotics should push the subtalar joint closer to its neutral position, not because the heel was more stable, but because the midtarsal joint is more stable when the subtalar joint is in neutral position.
- The authors state, “Neutral position, which Root defined as “neither pronated nor supinated”, is simply a rotational position around a singular axis; the subtalar joint axis. Pronation and supination are defined in both the open and closed chains as rotations around this singular axis.”
The authors are very imprecise in their discussion of Root theory, and in doing so they muddy the reader’s minds as to what Root stated. Root stated that there is a neutral position of the subtalar joint. He also stated that there were neutral positions of other foot joints, including the hip joint, the ankle joint and the first ray. He discussed pronation around the subtalar joint during gait, as well as pronation around the oblique axis of the midtarsal joint and the long axis of the midtarsal joint. It is important to realize that pronation of the subtalar joint in closed kinetic chain is passively accompanied by pronation around the oblique axis of the midtarsal joint and supination around the long axis of the midtarsal joint. These three motions are seen as a single motion, which Nester described as motion around a single axis that moves (1997) .
- The authors make the following statements regarding what Root taught about how many axes there are in the foot: “The extreme of single axis theory is to imagine that the foot only has one axis and consider the foot as just two rigid bodies teetering around this singular axis. This model concerns itself with the distribution of kinetic forces and their perpendicular distance to this one axis. This describes the Subtalar Axis Location and Rotational Equilibrium (SALRE) theory of Kevin Kirby, DPM.” … “The basic difference between single axis models, such as the STJ Neutral Model, and a postural model is that single axis models, by definition, ignore the rest of the foot” …. “The basic difference between single axis models, such as the STJ Neutral Model, and a postural model is that single axis models, by definition, ignore the rest of the foot. You can find STJ neutral in a broad range of foot postures both in the open and closed kinetic chain.”
These statements are a gross misrepresentation of both Root and Kirby. As noted above, Root never described motion of the foot as occurring around a single axis. Root described motion of the foot as occurring around multiple joint axis, including motion around the subtalar joint, the midtarsal joint, the ankle joint, the first and fifth ray joints and motion around the metatarsophalangeal joints. Kirby likewise never said that the foot rotates around one axis, though he most often writes and talks about the subtalar joint axis, so that the casual reader or listener may believe that he believes there is only one axis of motion for the entire foot.
- The authors somehow believe they have discovered something unique with these statements. “Posture is simply stepping back and looking at the foot as a whole and observing the way elevation of the longitudinal arches causes bones to nest into each other in a more closed pack position. Paul Jones attributes this to a generalized spiral twisting of the forefoot on the rearfoot, The Wring Theory . Sarrafian described the frontal plane forefoot to rearfoot relationship as a twisted plate. All of these models are posture based . Posture is the All Axis Model.”
Root was very much a twisted plate theorist, though he may not have ever used those exact words. Forefoot varus had been described in the literature before Root, however this writer has not found any previous author who described forefoot valgus . Root was the one who proposed that a pathology of the forefoot to the rearfoot could be only be diagnosed if the entire range of midtarsal joint motion had been utilized in the pronation direction. Posture of the foot in static stance and also function of the foot was partially dictated by these forefoot to rearfoot relationships and available motions.
- The authors then make the following statement: “The small amount of STJ rotation is where Merton Root and Kevin Kirby concentrated their attention . According to Root’s own measurements the total range of STJ rotation in ideal gait is only six degrees (+2 to -4).”
First of all, Root never took measurements of the foot motion in ideal gait, though he did measure range of motion available for the subtalar joint to move within and also the static subtalar joint position in stance. As to gait, Root looked to the best literature of his time for information about the range of motion of the subtalar joint during gait [13,14]. Since then, multiple authors have shown that Root’s proposal of the range of subtalar joint motion that the foot utilizes during gait is basically correct [15-19]. All authors, including Root, have agreed that subtalar joint motion when the heel is on the ground is small, but that once the heel comes off the ground, during propulsion, it is significantly more.
- The authors make the following statements: “Traditional orthotics based on the single axis models tend to be rather low in posture. The cast is taken in a partially pronated position and then the arch is further lowered to varying degrees to make the orthotic tolerable. Filling in, or lowering the arch of the orthotic, is often called ‘cast correction’ even though it divorces the geometry of the foot from the geometry of the orthoses and allows for greater postural collapse before the orthotic is contacted by the arch.”
What do the authors mean that the cast is taken in a partially pronated position? The Root technique takes a typical cast with the subtalar joint in neutral position and the midtarsal joint in its fully pronated position. This was the truly innovative idea that Root proposed, that the orthotic should have a supinatory effect on the subtalar joint and a pronatory effect on the midtarsal joint . I haven’t read of anybody advocating taking a partially pronated cast. It is true that many orthotic laboratories excessively lower the arches of the orthotic casts they receive. This is not “cast correction” as the authors maintain.
I recommend to the authors the following text by Dr. Root: “Plaster modifications for the standard functional orthosis consist of the balance platforms beneath the first and fifth metatarsal heads, a filler between these platforms, a lateral expansion, and a medial arch filler….” “The lateral expansion of plaster is designed to accommodate the slight bulging of soft tissue all along the lateral side of the foot and around the lateral and posterior aspect of the heel. This prevents the orthosis from pinching this soft tissue, which occurs in a significant percentage of feet if a lateral expansion is not used”…. “The plaster medial arch filler is designed to flare the medial edge of the orthosis away from the medial arch of the foot to prevent the edge of the orthosis from cutting into the foot. It initially was used only on feet with a fairly large angle of forefoot adductus because such feet have a sharp angle in the medial arch in the area of the midtarsal and tarso-metatrasal joint. It was not possible to train technicians or new practitioners to recognize when this medial arch filler would be necessary. As a result, a filler was standardized that could be used on any foot without interfering with function of the orthosis. The medial arch filler should extend form about mid-shaft of the first metatarsal and no farther posteriorly than the midtarsal joint. The filler should extend laterally in the arch of the foot to a line slightly lateral to where the medial edge of the finished orthosis will sit when placed on the cast” .
We can see from Root’s own description, the medial expansion was not intended to lower the medial arch but rather to flare only the very medial aspect of the medial edge away from the soft tissue. Many laboratories use the medial expansion as an accommodation for practitioners who send a tremendous number of casts taken with the long axis of the midtarsal joint supinated, or the first ray dorsiflexed or the lateral column plantarflexed. This is a business decision by these companies to accept casts that are of poor quality and then produce a device that does not hurt the patient. It does reflect on the poor practices of a great many clinicians in this country.
This writer has also made some observations about the advantages of the true Root orthotic flaring the orthotic away from the most medial edge of the arch of the foot. One of these is that the orthotic must allow normal pronation to occur during the contact period of gait. Second is that the lateral column is more flexible than the medial column. Therefore when the orthotic is made of uniform thickness, the lateral column of the orthotic flexes more than the medial column, which supinates the long axis of the midtarsal joint and makes the orthotic more uncomfortable on the medial side. If the clinician makes the lateral column thicker than the medial column, then equal flexes of both columns can be achieved and the orthotic lab does not have to artificially lower the medial arch. Again it should be emphasized that the Root orthotic is not an arch support — it is a dynamic torsional device, intended to provide an inversion force on the rearfoot, with support under the sustentaculum tali, and to evert the forefoot against the rearfoot.
Let’s look at some of the poor representation of the literature used to support the authors’ contentions that the theories of Root should be discarded.
- The authors state the following: “Root et al, called Royal Whitman’s observation the phenomena of midtarsal locking and unlocking and attributed it to Elftman’s theory, that the talonavicular and calcaneocuboid axis deviated as the foot supinated . Thus, this decreased the range of motion and parallelism of the axes, results in increased range of motion.”
The authors correctly state that in 1971, at the publication of their first book, Root et al did believe in the “locking” position mechanism proposed by Elftman, however by the time of their 1977 seminal publication, Root et al. had discarded the theory of Elftman as why the midtarsal joint had a smaller range of motion when the subtalar joint was in a supinated position than when it was in a pronated position. The authors would find a detailed account of first the acceptance and then the rejection of the Manter theory by Root in the exhaustive work by Lee on the history of Root’s ideas (2001) .
- The authors make the following argument: “The STJ axis exits the foot at the same point; the momentum down the leg similarly point; the momentum down the leg similarly passes its force vector down the center of the dome of the talus thereby intersecting the STJ axis.. The ground reactive force enters the foot ideally on the plantar posterior lateral aspect of the heel asses its force vector down the center of the dome of the talus thereby intersecting the STJ axis. The STJ axis is placed in an orientation that passes through the major forces entering the foot at heel contact, other than the force of friction which is horizontal and causes the forward roll of the calcaneus.”
The authors seem to be oblivious to the paper by Phillips and Lidtke (1992) that shows that the subtalar joint axis does not exit the foot at the posterior-lateral-inferior edge of the calcaneal fat pad, but instead intersects the posterior calcaneus between 4-5 cm above the inferior edge, and it intersects the ground approximately 5 cm posterior to the heel. This means that the actual point of contact at the initiation of the gait cycle is actually lateral to the subtalar joint axis. What the authors also fail to realize is that there is a significant shear force, directed laterally during contact, created by the internal rotation of the leg. While this shear force may be only about 10% of the vertical force, it has between 5-10 times the lever arm with the subtalar joint axis, so that it has an angle that is more perpendicular to the subtalar joint axis than the vertical ground force. This means that the horizontal, laterally directed shear force produces at least 50% of the total pronation torque around the subtalar joint axis. The authors try to confuse the reader with their description of what the shear forces do. The posterior shear force rolls the calcaneus forward at the ankle joint, not the subtalar joint. Readers will find a detailed description of what how the vertical, the medial-lateral and the anterior-posterior ground forces affect each joint of the lower extremity, from the hip to the toe through the entire gait cycle, in the chapter on biomechanics by this writer in the text, Principles and Practices of Podiatric Medicine (2007) .
- The authors make the following statement: “Tom McPoil’s Tissue Stress Theory states that when microtrauma occurs faster than a person’s ability to heal, they experience a symptom. During the last few degrees of postural collapse tissue stresses are highest. Microtrauma occurring in this zone of foot posture causes symptoms.”
Those who claim to be “Tissue-Stress Theory” advocates, fail to recognize that Root was also a tissue stress advocate. Just a couple of quotes from his major work demonstrate this. For example on page 229 we find, “The everted calcaneal position, which results from pronation that compensates a forefoot varus deformity, causes … a significant everting rotary moment that causes further pronation of the subtalar joint … The inherent arch structure of the foot begins to collapse, and ligamentous stretching and strain ensues. The entire rearfoot becomes unstable.” Later on page 326 we find, “The weightbearing forces move the joint either beyond its normal range of motion, or in a direction other than its normal plane of motion. In either event, the ligaments are immediately placed under tension. Since ligaments are elastic, they continue to elongate as long as the subluxing force is unresisted, and the articular surfaces separate slightly or may even dislocate with time.”
This writer often personally heard Root say that one only had block the last 1°-2° of subtalar joint pronation to alleviate a patient’s symptoms. Certainly a great many papers have shown that a great many “Root-type” orthotics only prevent about 2°-3° of calcaneal eversion [24-27]. Therefore it is evident that the majority of the studies on the “Root” orthotics are documenting a marked decrease in symptoms with only small changes in kinematics, which supports the idea that symptoms are caused by plastic deformation of ligaments. McPoil should not be considered to have introduced a new theory of biomechanics, but instead to give added definition and clarity to the basic Root principles.
- The authors contend that different types of activities require different orthotics with the following argument: “Momentum (mass times velocity) is the third factor that affects the magnitude of the downward force of the body. Running over a force plate produces more impact force than walking. Therefore, we must consider a range of forces to resist called, ADL or activities of daily living, and calibrate the orthotic to deliver an equal and opposite range. Athletes may have a different range of forces, these can be referred to as training or competing ranges, which are much higher. A power lifter, for example, may want an orthotic calibrated to resist his entire weight plus the weight he is deadlifting or squatting. That same athlete will need a different pair of orthotics for his ADL.”
This paragraph assumes that all ADLs require the same amount of pronation or supination. If this were true, then the authors’ argument would be correct, however we know that when running, the foot has to pronate more . Therefore to make an orthotic thicker for running, would limit the runner to less than the desired amount of pronation. The Root orthotic, on the other hand, says that with increasing vertical braking force of the ground, such as what the runner encounters with every foot strike, the orthotic will allow greater amounts of pronation. Just as the Modulus of Elasticity is the same for the foot when it is walking as when it is running (assuming a nonviscoelastic model), so the same orthotic can often be used for many different activities, as it will flex more when running, thus allowing for more pronation, than when walking.
- The following statement about practitioner testing the patient is made by the authors: “Foot flexibility can be measured in different ways. One way to grade foot flexibility is to rotate the forefoot around the fifth metatarsal. This is called the Gib Test or forefoot flexibility Forefoot Flexibility Test. The foot can be graded from one to five .”
The reference is to the primary author’s own article in a non-refereed magazine article. Inspection of this article shows nowhere in it is there anything about the Gib Test or the Forefoot Flexibility Test. A search of the NHI library likewise turns up no article that discusses the Gib Test. The only place that a practitioner can learn anything about this test is a Youtube video .
- In regards to some limited research that has been done with the MASS orthotic, the authors state the following: “Higby measured the force distribution on the metatarsal heads at toe off . What are these forces? Initially, MASS posture orthotics transferred 44% more force to the first metatarsal head at toe off than neutral position orthotics with posts. At six weeks this difference grew to 61% (p=.006) . This means that when the arch is raised, the first ray not only comes down and lateral, but additionally increases its purchase.”
First the editors should have noticed that reference 23 and reference 24 are the same, to a paper by Hodgson, Tis, Cobb, McCarthy and Higbie (not Higby) . So let’s look at the paper quoted by Hodgson. Glaser and Fleming have totally misrepresented the paper. The paper looked at two groups, both with greater than 7° forefoot varus. One was assigned to be treated with a 3/16” polypropylene orthotic manufactured by PAL Labs of Pekin, Ill., and the other by Sole Orthotic Lab. The “Root” orthotic in this study differed from classic Root techniques in that cast was taken using a prone non-weightbearing casting technique, and the material was more flexible than what Root advocated.
When one looks at the two groups, we see some glaring differences before wearing orthotics. The “Root” group showed an initial condition in which the first metatarsal head was bearing 71 KPa less pressure than the central metatarsal head area, and the SOLE group had an initial difference of the first metatarsal head having 41 KPa less pressure than the central metatarsal head area. This is a very significant difference showing that the Root group had significantly more hypermobility of the first ray than the SOLE group. This is an interesting statistical aberration, where random assignment to two groups does not always produce two groups of equality.
When we look at the end results (6 weeks of wearing the orthotics), the hypermobility of the first ray had not changed with either group. The first group still shows the first metatarsal head area to be averaging 73 KPa less than the central metatarsal head area, and the second group still shows the first metatarsal head averaging 40 KPa less than the central metatarsal head area. Therefore the claims by the authors that the SOLE orthotic increased the pressure under the first metatarsal more than the Root orthotic is not an accurate interpretation of the data.
There is an assumption here that the more force one produces under the first metatarsal head, the better the foot is functioning. Actually the Hodgson study is measuring pressure, not force, and in this writer’s eyes, the average pressure under the metatarsal heads over the entire gait cycle should be equal for all five.
Finally the following examples are ways that authors make outlandish assumptions, demonstrate poor reasoning, and write what can be best called “mechano-babble”.
- The authors make the following statement: “I propose that the locking mechanism of the midfoot is multifaceted. When the talar head is directly on top to the anterior facet, sagittal plane motion between the talus and calcaneus is blocked. Thus, when the gastroc-soleus complex fires, rotation occurs at the ankle joint.”
Not quite sure why the personal pronoun is utilized at the beginning of the article. Who is the “I”, Dr. Glaser or Dr. Fleming? This is just one of many editorial errors in the article. Nevertheless, the assumption that the anterior facet blocks sagittal plane motion between the talus and the calcaneus is faulty reasoning. First of all, a great many people show the anterior and middle and anterior facet to be one continuous surface [31-33]. The authors fail to mention how the middle facet plays into the equation in blocking subtalar joint motion. It is well recognized that the middle facet is larger than the anterior facet, so why doesn’t the middle facet block sagittal plane motion, especially since it is further from the subtalar joint axis than the anterior facet? And what about people with no anterior facet, is there anything blocking subtalar joint motion ?
Second of all, the subtalar joint moves around an axis. It is true that that the facet morphology determines the direction of the subtalar joint axis. It is important to remember that when we talk about subtalar joint motion being tri-planar, we are not talking about three different motions, we are only saying that angular motion can be measured in three different planes. Therefore, the more horizontal the facets are with the ground, the greater will be the angular displacement measured in the transverse plane rather than in the sagittal or frontal plane. In other words, the subtalar joint axis will be more vertical, and therefore the lower will be the torque around subtalar joint axis exerted by vertical ground forces, and the greater will be the torque exerted by shear ground forces. Since there are strong transverse plane rotational forces occurring in the lower leg during gait, no one can say that a subtalar joint axis that is more vertical will demonstrate less total motion, though a person that has only the capability to measure the frontal plane component of motion may erroneously conclude that less motion may be occurring .
Finally, the authors try to confuse the situation with a statement about rotation around the ankle joint occurring when the gastroc-soleus fires. It is important to remember that when the gastrocnemius-soleus fires, it primarily produces a plantarflexion torque of the calcaneus against the tibia. Since the ankle joint axis is almost perpendicular to the Achilles tendon, the major torque is around the ankle joint. The axis of the subtalar joint has a lower angle with the Achilles and a shorter lever arm, therefore the supination torque is lower around the subtalar joint axis than the ankle. The greater the pitch of the subtalar joint axis with the transverse plane, the less will be the torque exerted by the Achilles on the subtalar joint. Now in closed kinetic chain, when the firing of the gastrocnemius produces a first class lever effect, creating ground reaction force against the metatarsal heads. This ground reaction force creates a strong dorsiflexion torque around the midtarsal joint. This dorsiflexion torque is resisted by the ligaments of the plantar foot. A simple geometrical construct shows that the lower the arch, the greater will be the tension on the plantar ligaments.
- The authors make the following statements: “As the foot goes into further elevation of its posture, there is a zone where, according to Hammel, there is no significant rotation around the STJ axis in any plane . Foot orthoses that attempt to elevate posture into this zone often cause medial longitudinal arch pain as the foot repeatedly drops down to impact the orthotic. Hammel showed that from 25% to 90% of the stance phase of gait, no rotation in any plane occurs between the talus and the calcaneus.”
First of the use of the paper from Hammel to back up this statement is bogus. An examination of the paper shows that cadaver feet were utilized to try to simulate gait and the simulator had no transverse plane simulation of the leg. Without the ability to simulate transverse plane motion of the lower leg, of course they will not be able to pick up the motion of the subtalar joint between 25%-90% of the stance phase of gait. There are many other papers Glaser and Fleming could have picked to discuss the subtalar joint motion that have in vivo multisegment data on all three body planes, including Carson (2001), Hunt (2001), Simon (2006), Stebbins (2006), Leardini (2007), Nester (2007), Pohl (2007), Nester (2014) [36-43]. While all of these authors used slightly different marking systems of the foot when walking, none of them grossly contradicted the original gait cycle motions of the ankle, subtalar or midtarsal joints described by Root and all of these papers, plus many more, contradict the statement of Hammel that Glaser and Fleming rely on.
The statement that subtalar rotation and postural collapse are independent events occurring at different times in the gait cycle is not really a valid statement. Root stated that subtalar joint pronation occurs before the forefoot hits the ground. The moment the lateral side of the forefoot touches the ground, the pronation of the midfoot in the sagittal plane starts to occur. The problem is that the force is transferred gradually to the forefoot from the heel. Thus the dorsiflexion moment across the midfoot increases throughout the midstance period of gait. It is the pronation of the subtalar joint that increases the midtarsal joint range of motion in the pronation direction. Limited data exists in this regard as to the exact mechanism by which the STJ position changes the mobility of the MTJ, and many theories have been proposed [44,45].
At the end of this section in the paper, the readers are still scratching their heads as to what the writers mean by the “Dysfunctional Zone”. It is even more nebulous and ill-defined than Root’s “neutral position.”
- The authors make the following statement: “As foot posture elevates beyond the Dysfunctional Zone the anterior facet of the STJ approaches level in the transverse plane. This allows subtalar rotation to occur. This is where the talar head slides posterior and rotates its six degrees around the STJ axis. The closer the anterior facet is to level, the easier the subtalar rotation occurs and the rearfoot locks in the sagittal plane facilitating efficient propulsion.”
It should be noted that the authors are relying on their own study published in this same edition of the journal on his measurements of facet deviations from each of the planes. Unfortunately, this paper appears at first to have some real data, however a close read of the paper shows that it is mathematical gibberish. The quoted article fails to define the reference coordinate system and what directions are positive and negative. Also the deviation between two planes is determined by the angle between the normals of the two planes. If one says that the angle between a facet and the transverse plane is 10°, it could mean that the facet is tilted forward or it could mean that the facet is tilted medially or laterally. What the author should have done is set up his reference coordinate system and then expressed the normals to the planes of the facets in either spherical or cylindrical terms. It is noted that most podiatric texts express axes in cylindrical terms. So with no definitions in the quoted article, the data is useless.
To define the ease of movement on the orientation of one portion of the total joint surfaces that comprise it, and on no ligamentous restraints is totally to ignore mechanics. As Phillips and Lidtke pointed out, the subtalar joint can be clinically modeled to move around a single axis that is fixed to the talus. As the talus dorsiflexes in the ankle joint, the subtalar joint becomes more vertical, therefore vertical ground forces produce less torque around the subtalar joint axis and horizontal forces produce more torque. Therefore transverse plane leg rotations produce stronger subtalar joint torques, and these rotational forces are being generated by the movement of the swing leg. Also as the talus abducts, the subtalar joint axis moves laterally, producing longer lever arms for the vertical forces medial to the subtalar joint axis and shorter lever arms for the vertical forces under the lateral foot. So the ease of subtalar joint supination with the foot in a more supinated position can be fully explained without any need for the horizontal position of the anterior facet.
- The authors make the following claim: “A MASS Posture composite leaf spring applies an even distribution of force per unit of area by remaining in full contact with the foot throughout the gait cycle. The foot never has to drop down to hit the orthotic because it is already touching it, which minimizes impact and thus tissue stresses. It is the combination of full contact (redistribution of force per unit area) eliminating hot spots and the lack of repetitive impact that allow such a spring to apply a rather large corrective force while remaining comfortable to most patients. Once you have the correct geometry of the spring, it is time to adjust the spring constant.”
The authors have never demonstrated how their orthotic is constructed like a leaf spring, or why his orthotic is a leaf spring and the Root orthotic is not. Leaf springs are laminar with the thickest part of the spring in the middle, where the highest load area is. The MASS theory orthotic is a single lamina and is ground thinner in the middle.
All orthotics remain in full contact with the foot throughout the gait cycle as the foot will mold itself to fit the orthotic shape, which is also a basic tenet of Root orthotic therapy. A Root orthotic starts pushing against the bottom of the foot when the foot makes contact with it when the STJ is in neutral and the MTJ is pronated. The more the foot tries to deform from STJ neutral and MTJ pronated, the harder the orthotic pushes against the bottom of the foot. Blake and Kirby have both proposed modifications of the Root orthotic that initiates the orthotic producing a supinatory force against the heel before the subtalar joint tries to pronate beyond neutral [46-49].
How do the authors know there are no hot spots on the MASS orthotic? You have to have a pedobarograph to measure that, and there is no literature that has measured the force that the MASS orthotic puts against the bottom of the foot. This writer’s personal experience with the MASS orthotic is that it produces an extreme hot spot under the medial arch. If the orthotic is casted with the forefoot maximally plantarflexed against the rearfoot, then there will be a very high hot spot under the arch, unless the orthotic is flexible enough to lower under normal weightbearing to that point where tension develops in the plantar ligaments. Since there is no quantitative measures taken by the clinician before prescribing a MASS theory orthotic, neither goniometric nor pedobarographic nor kinematic measurements, only a qualitative judgement of the frontal plane mobility of the forefoot to the rearfoot, there is no way that the laboratory has any information about how much the forefoot plantarflexing mobility the patient has.
In conclusion, MASS theory has little to any support for its validity in the literature. There is only very limited literature on the use of the MASS orthotic. Currently there is only one source of MASS orthotics, and the authors of the reviewed article have a definite conflict of interest in the proposals offered. This writer will admit that there are definite problems in the classic “Root” approach that is commonly taught, and many authors since the original Root writings have definitely made valuable additions, clarifications and corrections to the Root approach. However, this writer, through study of the literature and clinical practice maintains that the literature better supports Root concepts, and therefore MASS theory cannot be accepted as a replacement for currently accepted practices and theories.
Robert D. Phillips, D.P.M.
Orlando Veterans Administration Medical Center
Disclaimer: the opinions in this paper are those of the writer alone, and do not represent the opinions of the U.S. Department of Veterans Affairs nor any other branch of the U.S. government.
- Root, M. L., Orien, W. P., & Weed, J. H. (1977).Normal and Abnormal Function of the Foot (Vol. 2). Los Angeles: Clinical Biomechanics Corporation
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