By Stu Currie, DC; Don Bursch, PT, OCS; Ed Glaser, DPM

Professionals in the field of physical medicine rely on the study of human form and posture to determine deviations from normal and apply appropriate corrections to the underlying structure. It is commonly accepted that departures from correct form, posture, or structure, through either acute trauma or insidious degradation, affect function. Muscles exert their pull and force more efficiently across joints, and the human body is better able to counteract the effects of gravity and the ergonomics of our sedentary lifestyles, when ideal postural balances are maintained. This concept is illustrated by studies of the cervical spine, which have demonstrated that changes in joint position and moment arms affect the moment generating capacities of muscles,¹ and that posture has an effect of motion coupling.² There is a conservation of energy and an efficiency of function essential to the biomechanical workings of the human body.

When these concepts are considered within the scope of current popular foot orthosis intervention strategies, the question arises as to why they have not been applied to the foot to any significant degree. Although practitioners may consider these concepts in their clinical evaluation, it seems that these ideas have lost traction when it comes to foot orthosis design and correctional modeling. In many common models, emphasis is less on correcting foot position or posture that may have deteriorated and more about shifting tissue stresses and forces. Although shifting tissue stresses may provide pain relief, it may be too narrowly focused to provide the most complete preventive solution to the problem.

The concept of designing a foot orthosis around the tenet of subtalar neutral has been called into question. Investigations have demonstrated the lack of correlation between rearfoot motion during gait and a valid measure of subtalar joint neutral position during weight bearing.^3,4 This underscores the fact that, when considering orthotic intervention to affect dynamic function, the subtalar joint neutral position cannot be relied on to predict the corrected position. In addition, the ability and degree to which a custom orthosis can even control rearfoot motion is debatable. Davis et al showed that there are few differences between a custom and a semicustom device in the ability to control the rearfoot,⁵ and that foot orthotic devices do not produce significant change in rearfoot-tibial coupling.⁶

The pioneering work of Merton Root⁷ gave us measurements that helped define a “normal” foot based on the concept of subtalar neutral. These static measurements have been used as part of the biomechanical assessment of a patient and form the basis for restoring normal function with the use of a foot orthosis.⁸ However, investigations into the assumptions behind these measurements have shown that goniometric measurement of the forefoot to rearfoot relationship is unreliable regardless of clinical experience.⁹ In addition, one study revealed that when comparing groups of casters (inexperienced, experienced, and “expert”) the neutral-position measurements of the frontal plane forefoot to rearfoot relationship in a single foot ranged from 10° everted to 6.5° inverted across the groups.¹⁰ When considering that rearfoot posts typically range from 4° to 10°, this variability casts doubt on the practicality of rearfoot control with a posted orthosis. Foot type analyses that involve primarily frontal plane static measures may have less to offer than more dynamic and robust analyses. Clinical measures of static foot structure that have included subtalar range of motion and calcaneal eversion and inversion have been shown to have poor interrater reliability.¹¹ Moreover, these rearfoot measurements are poor predictors of dynamic rearfoot motion.¹²

To summarize, if these static measurements are unreliable and unrelated to the function of the patient’s foot in motion, then skepticism on the part of the clinician regarding these types of foot posture measurements is warranted. However, abandoning the concept of foot posture altogether because the rearfoot measurements don’t correlate does not help with the great incongruity that exists: We generally accept the concept of a proper architecture to the rest of the human body, so why should this not apply to the foot?

In contrast to the clinical murkiness of the measurements discussed above, we do know that there are statistical differences in biomechanical function between the planus and rectus foot.¹³ It is thought that changes in foot structure affect dynamic function,¹⁴ and foot morphology has been implicated in a variety of lower extremity overuse injuries.^15,16 A pronated foot posture is thought to be a factor in various pathologic conditions of the foot; for example, excessive foot pronation has been cited as a cause of limited dorsiflexion at the first metatarsophalangeal joint during gait.^17,18 Munteanu et al also postulated that people with pronated feet are more likely to exhibit limitation of dorsiflexion at the first MPJ during gait and found that orthoses focusing on the forefoot to rearfoot relationship (Blake-style inverted) did not significantly change the first MPJ range of motion.¹⁹ Could this be due to the focus of this type of intervention on the forefoot to rearfoot relationship, rather than on restoring proper orientation or posture to the entire foot? Since the days of Merton Root, the correction of perceived imbalances in the forefoot to rearfoot relationship has been the goal of orthotic intervention. It is clear the relationship of these measurements to the improvement of the human gait cycle is questionable.

Previously, the midfoot was modeled as one rigid segment based on the assumption that negligible motion took place within the midfoot.²⁰ We now know that movements including walking and slow running involve considerable motion between midfoot bones.^21-24 It is logical to consider this midfoot motion in any attempt to control the foot with an intervention such as a foot orthosis. Perhaps there is evidence that returning the focus to the midfoot, medial longitudinal arch, and plantar vault could yield measurements that relate more directly to the dynamic gait cycle and result in improved clinical outcomes.

Postural assessment tools

The Foot Posture Index (FPI) was designed to be a valid, simple, and clinically useful tool. Its most recent version²⁵ is based on six static foot morphology criteria (talar head palpation, curves above and below the medial malleoli, inversion/eversion of the calcaneus, bulge in the region of the talonavicular joint, congruence in the medial longitudinal arch, and adduction/abduction of the forefoot on the rearfoot). Although there is some improvement in intrarater reliability and validity with this tool, there continues to exist significant doubt as to its interrater reliability²⁶ and applicability to dynamic function.²⁷

The foot line test (FLT), which is a measure of the medial prominence of the navicular bone in a mediolateral direction, was developed to investigate the relationship between foot morphology and injury development. The measurement of medial/lateral movement of the medial prominence of the navicular is primarily a frontal plane measurement, so although the FLT has been shown to be a reliable measure,²⁸ future work is needed to determine if this measure correlates with dynamic function.

The longitudinal arch angle (LAA) is defined as the angle formed by two vectors: one passing through the midpoint of the medial malleolus to the navicular tuberosity and the other passing through the midpoint of the medial aspect of the first metatarsal head to the navicular tuberosity (Figure 1). What is interesting about the work done with these angles is that it is starting to reveal measurements that show good interrater reliability,²⁹ are prognostic of dynamic foot posture during walking and running, and may have some value in predicting the risk of lower extremity injuries.^30,31 This is good news for the clinician who looks to the published research to confirm and validate his or her clinical measures.

Importance of the arch

It seems plausible that the closer the measurements get to a measure of the medial longitudinal arch and plantar vault of the foot, the more applicable they are to the dynamic gait cycle. This correlates with the knowledge that significantly more sagittal plane motion is required during each step of the gait cycle than in the transverse and frontal planes.³² Put quite simply, there is an accepted clinical relevance to the arch of the foot and its perceived position—either low or high—and it may very well be that the closer a measurement gets to describing this position, the more dynamically relevant it is.

Therefore, a distinction is needed between those foot type classifications that focus on frontal plane measurements and foot posture that is evaluated with respect to the medial longitudinal arch, or a 3D posture. Looking at some of the more recent measurements that have focused more on the medial longitudinal arch reveals progress toward defining more reliable, clinically relevant measures.

To date, much of the study of foot posture has included discussions of frontal plane orientations such as calcaneal inversion. The aforementioned evidence suggests that more clinically relevant measures may be found in the midfoot and medial longitudinal arch. Continuing along this logic path, it is possible that accounting for the entire plantar vault may give us a better understanding of which changes in foot posture are relevant to our clinical decisions. In an ideal scenario, a measurement would be accurate, reliable, and backed by clinical outcomes. It would describe the entire foot and, perhaps most important, predict the dynamic function of the foot.

So what is the ideal foot posture if it is unrelated to subtalar neutral? Several theories have emerged over the past decade or two that provide possible alternatives to the subtalar neutral theory originally posited by Root. These include (but are not limited to) the tissue stress theory, the preferred movement pathway theory, the sagittal plane facilitation theory, the subtalar joint axis location/rotational equilibrium theory, and the MASS position theory.^33-37 A detailed analysis of each theory is beyond the scope of this article; the remainder of this discussion will involve the MASS (maximum arch subtalar supination) position theory, which focuses its orthotic intervention on direct support to the medial longitudinal arch and plantar vault, as it relates to a defined and corrected foot posture.

MASS position theory

The MASS position orthotic is designed to capture the foot in a corrected position without reference to subtalar neutral position. The position is defined as the maximal amount of closed chain supination that is achievable for any particular foot at midstance, with the heel, first, and fifth metatarsals in contact with the ground. The goals of this postural correction include:

1) adequate supination at heel strike

2) full contact of the forefoot on the ground at midstance

3) positioning of the majority of forefoot load on the first metatarsal joint at heel lift

4) no limitation of first MPJ dorsiflexion.

This posture is captured using a gait-referenced sequence with semi-weight-bearing loading. The casting method is the same for all feet, though each foot yields a unique 3D shape due to unique anatomical variations and flexibility differences. It is based on the concept that if there is a corrected foot posture that is to be achieved, it needs to be emulated and induced directly by the intervention, in this case a foot orthosis. This direct intervention takes the form of a full-contact, weight-calibrated orthosis (Figure 2).

It has been shown that orthoses with higher medial arch profiles provide advantages, including significantly reduced tension on the plantar aponeurosis compared to their lower-arched subtalar neutral functional orthosis counterparts, and it is further theorized that control of the medial longitudinal arch is facilitated by support to the apical bony structures of the arch.³⁸ Hence it seems plausible that a correction strategy such as MASS position, aimed at directly supporting the arch in a supinated posture (with no cast correction), should provide some of these benefits. Although this has yet to be directly explored, there is some evidence of improved clinical outcomes with orthotics based in the MASS position.

These investigations have shown that foot orthoses based on this corrected position (or posture) provide relief of lower extremity musculoskeletal pain³⁹ and improved economy of gait.⁴⁰ In addition, a reliability study showed that MASS position casting had significantly better intracaster reliability than did neutral plaster casting, significantly less forefoot to rearfoot variability in the standard error of measurement, and similar interrater reliability.⁴¹

The full contact quality itself is not one that comes with inherent contraindications; in the attempt to provide a corrective force with a foot orthotic device, contact with the foot is a requirement. Certainly there are situations where rigidity or flexibility become issues (such as with recent surgical fixations). However, the casting technique will not allow an overcorrection past the anatomical limits, and in these cases the orthotic may facilitate the dispersal of force per unit area.

MASS position theory requires the flexibility of the orthotic shell to be unique to the requirements of each foot, allowing more or less flexibility on an individual level. Patient-specific thicknesses of high-molecular-weight plastic are determined based on known clinical variables to deliver the required correctional force, while at the same time being tolerable. These variables can include patient body weight, foot flexibility, arch morphology characteristics, and activity levels. Two variables that have been correlated with supination resistance of the rearfoot are body weight and medial deviation of the subtalar joint axis.⁴² In other words, a heavier person with a more medially deviated subtalar joint axis requires a greater amount of force from an orthotic to balance the pronatory forces. The converse would be true of a lighter person. An orthotic device must account for all of these variables to apply an effective corrective force to the bony architecture of the foot.

Foot posture measurements taken in the MASS position may prove particularly applicable to clinical decision making with regard to orthosis manufacture, as they can be compared volumetrically to a pronated foot posture or relaxed calcaneal stance position. Knowing that there is an optimal foot posture that may be different for each individual, but based on the same reference points, logic follows that there is a spectrum of function (or dysfunction) and a zone of optimal control. Future writings and investigations will focus on the zones of postural control and their application to treatment with foot orthoses.

Stu Currie, DC, directs in-house research and development and coordinates outside university investigations for Sole Supports in Lyles, TN. Don Bursch, PT, OCS, is the president of the company and creates the company’s visual educational training material. Ed Glaser, DPM, is the owner and founder of the company and originator of the MASS theory.

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