Theory, practice combine for custom orthoses

[Guest guest]

Biomechanics September 2006

http://biomech.com/showArticle.jhtml?articleID=193000715

Theory, practice combine for custom orthoses

By: Glaser, DPM, Don Bursch, PT, OCS, and Stuart Currie, DC

Bernard Lown, MD, inventor of the heart defibrillator, once

said, " The security provided by a long-held belief system, even when

poorly founded, is a strong impediment to progress. General

acceptance of a practice becomes the proof of its validity, though it

lacks all other merit. " 1 This is the exact situation we find

ourselves in today with regard to custom foot orthoses as a

biomechanical intervention. The vast majority of practitioners now

use a model and method for orthotic therapy that has not undergone

serious critical analysis or revision for almost 30 years. This is in

spite of very poor support in the professional literature for the

efficacy and value of this approach.

The application of podiatric biomechanics in clinical practice has

not been accompanied by an abundance of published articles describing

empirical studies.2-4 It is difficult to find a comprehensive

overview of this inherited, somewhat anecdotal methodology in order

to begin such a critical review. This would seem to reinforce the

appearance of general acceptance becoming proof of validity, for why

bother to explain what everyone already knows?

In order to discuss effective intervention strategy and methodology

for foot biomechanics, one must identify the goals of proper

biomechanical correction. The prevailing opinion has it that the goal

of orthotic design should be to bring the foot to operate close to

the neutral position of the subtalar joint and/or between the rear-

and forefoot during standing and gait.

Our collective clinical observation generally confirms that the

majority of foot and lower kinetic chain orthopedic disease and

deformities relate to excessive pronation and inadequate resupination

during the gait cycle. We propose that the following goals are most

pertinent:

That sufficient resupination of the foot occur after midstance to

stabilize or " lock " the tarsus in the sagittal plane and allow for

efficient propulsion;

That the forefoot contact the ground without imposed abnormal

compensatory motion proximally or in the transverse, sagittal, or

frontal planes;

That the first metatarsal be stably plantar-flexed against the ground

during forefoot loading;

That the first metatarsal accept 60% of forefoot loading force; and

That the first metatarsophalangeal (MTP) joint be free to dorsiflex

sufficiently for forward gait progression without compensations in

foot or lower extremity posture that would otherwise be necessary.

To accomplish the above goals, one must be able to impose adequate

mechanical control over the tarsus of the foot with primary emphasis

on the calcaneus and talus. The latter make up the subtalar joint,

which the work of Root et al,5 the accepted authority on foot

biomechanics, holds as the controlling joint for foot pronation and

supination, or foot " unlocking " and " locking. " The foot must unlock,

or pronate, for shock absorption and terrain adaptation; then it must

resupinate, or lock, for proper forefoot and first MTP function and

for efficient propulsion.

It is our experience, and we believe there is general consensus, that

the vast majority of biomechanical foot problems involve excessive

loss of the medial longitudinal arch (MLA) height and, therefore,

inadequate resupination during stance from full pronation through toe-

off. This is probably due to a number of common-sense factors, such

as the prolonged effect of body weight compressing the foot against

the hard, flat surfaces that are the norm in modern living. Over

time, foot ligaments tend to stretch out and this adds flexibility

and instability to the foot's structure. The dynamic or muscular

stabilizers of foot structure tend to either be underdeveloped due to

sedentary living or incapable of exerting sufficient control over a

progressively more flexible, flattened, and unstable foot posture.

The key clinical question, then, is what is the most reliable and

effective way to facilitate adequate resupination of the foot? The

current mainstream strategy, also derived from the work of Root et

al5-7 has been to affect frontal plane position of the calcaneus by

means of a sloped supportive surface known as a rearfoot post. This

theory presupposes that the calcaneus will spontaneously align its

vertical position according to the tilt of a weight-bearing surface,

such that, for example, a wedge-shaped heel support that is high on

the medial/low on the lateral side will cause an effective increase

in inversion of the calcaneus. Since calcaneal inversion is a

component of supination, it is further supposed that the entire

subtalar joint will supinate as well, thereby limiting pronation.

This theory, however, does not stand up to a rudimentary mechanical

analysis. For one thing, the inferior surface of the calcaneus is

relatively rounded. The chances that a spheroid object will align

itself perpendicular to a sloped plane are similar to the chances

that a ball will remain aligned to the side of a bowl or the head of

the femur not revolve within the acetabulum. There is simply not

enough effective torque to hold the calcaneus aligned to the wedge

and allow it to resist the momentum of body weight and ground

reaction forces. Then there is the additional problem of the

considerable soft tissue between the calcaneus and the wedge,

including the subcalcaneal bursa and a large fat pad. So whatever

miniscule torque the wedge may be able to exert on the calcaneus

would surely be more than nullified by the potential displacement of

overlying soft tissue.

That posting may not translate into measurable differences in the

proximal kinetic chain is supported by McPoil and Cornwall, whose

work noted no difference in internal tibial rotation with a posted

plastic orthosis compared to a nonposted orthosis.8 Add to all of

this the very short instant of time when only the heel makes contact

with the ground in the gait cycle, and it becomes even more unlikely

that a wedge could exert enough force in enough time to significantly

affect supination and limit pronation. Even conceding any effect of

rearfoot posting, limiting pronation-and not assisting resupination-

of the foot is the only possible effect it could have since the heel

is in the air during mid- to late stance phase when the critical goal

of resupination must be met.

From a mechanical standpoint, the only effective way to apply enough

leverage to the subtalar joint is through full and direct support to

the MLA. We know that ideally muscle activity does not contribute

significantly to the height of the MLA. That foot arches do not sag

in the absence of muscle action has been shown in lower leg specimen

studies in which the supporting muscles were removed and a loading

force was applied to the tibia.9 By definition there is a direct

relationship between arch height and degree of subtalar pronation.

Therefore, if you control arch height, you will effectively control

pronation.

This was understood intuitively in the past, when attempts were made

to fully support the arch. The problem was that materials such as

steel plate, solid wood, and laminated leather, being entirely

unyielding, completely blocked pronation, so were inherently

uncomfortable and unusable. Pronation is an important foot function

and should be not eliminated, but controlled. Such early,

unsuccessful experiments with full arch support tended to discredit

the general strategy. Now, the full arch support strategy has been

revived using modern thermoplastic support materials. This allows for

creation of custom supports with the proper blend of rigidity for

control and flexibility for limited pronatory function and comfort.

A misleading question is often posed in orthotic evaluation: should

the device be rigid or flexible? The question is misstated. The real

question is: If we place a curved piece of plastic on a flat surface,

how much vertical force should the plastic exert on the human body?

The answer is found in Newton's third law: For every action, there is

an equal and opposite reaction. In other words, before the upward

vertical force can be established, the downward force must be

determined. The downward force is influenced primarily by a person's

weight, foot flexibility, and activity level. Before one can

determine the amount of force the orthoses should apply to the body,

one must first ascertain the downward forces the body is applying to

the plantar feet through gravity and momentum. This assumes the feet

are in the properly corrected position, which will be defined later.

Unlike the attempts to impose subtalar neutral, this full arch

support strategy acknowledges and allows for the possibility that the

subtalar joint does not operate absent external forces. The whole is

more than the sum of its parts. This idea was originally outlined by

Huson10 when he proposed that dynamic foot pronation and supination

result from all foot segments (tarsal, tarsometatarsal, and

metatarsophalangeal) working together rather than separately. Work in

support of this idea by Cornwall and McPoil11 further elucidated that

a kinematic coupling exists within the subtalar and

talocalcaneonavicular components of the tarsal mechanism, and that

movement of the calcaneus, navicular, and first metatarsal occur in a

similar and coordinated pattern during gait. This underscores the

arguably simple fact that control of the whole arch and midfoot is

critical to control of the foot. In addition, they found that the

magnitude of navicular movement was actually greater than that of the

calcaneus during dynamic gait. It should follow that, through full

and direct contact with the entire plantar surface of the foot

(including the navicular), it is possible to control the movement of

all components of the MLA.

In addition to answering how one can control the foot, one must

answer this: in what position should we capture the foot to best

model the orthosis? Most podiatrists still use some variation of

subtalar neutral as proposed by Root et al.12 There is much

disagreement about the nature and role of neutral position in the

professional literature. Root and colleagues derived their reference

position from the relaxed calcaneal stance position of two subjects

in a study by et al published in 1964.13 Among other things,

the possible conclusions from this study were severely limited by its

extremely small sample size. In addition to the limitations of this

original study, its definition of " resting calcaneal stance position "

was misinterpreted by Root and colleagues to mean subtalar neutral.

There was no basis in the original article for this leap.

For Root, the concept of neutral position apparently means two things:

That the foot is neither pronated nor supinated, but displays

talonavicular joint congruity. This is a vaguely defined point in the

range of motion between extremes of supination and pronation. The

thinking was that the foot operates best around a single position and

that excessive deviation from that position will cause certain

deformities. In other words, the foot should avoid extremes in range.

This meaning of neutral position, according to the studies of Root

and colleagues, is the position the foot will be in when palpation of

the talonavicular joint finds maximal joint congruity.

A balanced relationship between the forefoot and the rearfoot in the

frontal plane. This is a position in which any evident forefoot varus

or valgus angulation relative to a supposed ideal rearfoot position

(one-third the total available range from inversion to eversion;

i.e., a " vertical " calcaneus) is eliminated. Once this position is

achieved during the casting process, the foot is " locked " by

dorsiflexing the lateral column (fourth and fifth metatarsal heads)

of the foot. This, according to the theory adopted by Root from

Elftman14 stabilizes the midtarsal joint by placing it into its fully

pronated position. This position theoretically causes the axes of the

midtarsal joint to be askew, which is deemed the locking mechanism of

the tarsus.

The first sense, of correcting the range of the subtalar joint to

neutral, is counter to the goal of controlling pronation: why start

to control the foot halfway through its pronation range? By the time

the foot makes contact with the orthosis, it has already pronated

enough to unlock. What will help the resupination effort? This so-

called corrected position of the foot is already significantly

pronated. Root and colleagues failed to identify a lack of adequate

resupination as the primary biomechanical challenge.

The second sense, of correcting frontal plane relationship to

neutral, is irrelevant to the goals of biomechanical correction

outlined above. A theoretical foot position in the open chain,

without a frame of reference, does not achieve the goals of closed

chain function. The pronation and supination mechanism has been shown

to change between the open and closed chain positions.15,16

Recent works that have used 2D and 3D studies in an attempt to

determine what typical rearfoot motion is have cast serious doubt on

the use of subtalar joint neutral as a basis for determining typical

rearfoot motion or the direction of treatment with orthotic

intervention.17,18 Elftman's theory of midfoot locking also does not

hold up to mechanical analysis. In order for this theory to begin to

make sense, there would have to be discrete and static axes of motion

for both the calcaneocuboid and the talonavicular joints that can set

them either parallel to each other or askew. One has only to consider

the nature of the talonavicular joint to know that this cannot be the

case. It is a ball and socket-type joint, according to Sarrafian,19

the " acetabulum pedis " (acetabulum of the foot). This type of joint

has nearly infinite axes of motion, such that any axis of motion of

the calcaneocuboid joint will find a parallel match in the

talonavicular. By this argument, the foot would always be unlocked.

Kinematic analysis also calls into question the assumption that

mobility is increased by making divergent running axes parallel.

Huson has noted that parallelism between the talonavicular and

calcaneocuboid joints would require coordinated rotation about both

axes (similar to the doors of two cupboards opening at the same time

and same rate), which is very unlikely due to the anatomical

connections between these joints.20

The argument for neutral position as the model for orthoses is

further compromised by the standard practice of capturing the foot

hanging off the edge of a treatment table in the open chain. From an

engineering perspective this invites an intolerable amount of

variation and is clearly an unreliable methodology. The idea that

this traditional method of static casting is not representative of

the dynamic arch is supported by Hamill et al21 and Pierrynowski et

al.18 This technique ignores context. It attempts to capture an

enhanced functional position of the foot in the open chain when the

foot must function on the ground in the closed chain. We know from

previous studies that traditional static measurements are not good

predictors of dynamic limb function.22,23 In addition, there are

significant differences in the measurements of closed- and open-chain

calcaneal eversion.24 Adding to measurement error is the fact that as

much as one-third of the apparent total arc of motion of the hindfoot

actually comes from the tibiotalar joint.25 Finally, reinforcing this

issue is the wide interexaminer variability in neutral-position

casting of the foot that has been recently documented by Chuter et

al.26

It must also be noted that even when a practitioner follows all the

above conventional guidelines for the corrected foot position, it is

common lab practice to " cast correct " the plaster received from the

practitioner (also called " arch fill " ). This is probably because the

labs have no faith that slipper casting yields a foot model that is

reliable and accurate. So to facilitate comfort they reduce the size

of the arch dramatically. There is, however, no standardized

technique for determining the correct amount of plaster arch fill. It

is a common industry belief that arch-filled orthoses may be less

challenging to the foot and may obviate fears that the patient will

desire to return the product. These orthoses lack the full contact

necessary for biomechanical control. Perhaps this is why so many

major studies have found custom orthoses to be no more effective than

prefabricated ones or other modalities in the treatment of common

conditions such as plantar fasciitis.4,27

The neutral-position method fails to adequately control pronation

through midstance, therefore it will fail to lock the first ray as

the heel leaves the ground. When the subtalar joint is in neutral

position, the first ray will be vulnerable to

dorsiflexion/abduction/eversion about the first metatarsocuneiform

joint due to ground reaction forces. An orthosis should be able to

lower the head of the first ray against ground reaction forces to

enable normal forefoot loading, with 60% of that load borne by the

first metatarsal head. This is supported by the Dananberg model,

which suggests that a smooth transition of weight from the heel to

the forefoot depends on a full range of pivotal motion for the

metatarsophalangeal joint.28 " Lowering the head of the first " refers

to the relative change in first ray position that occurs when the MLA

is raised. In addition to ensuring normal forefoot loading, a

stabilized first ray is required for normal range and function of the

first MTP.

Advanced electromagnetic motion analysis by Cornwall and McPoil

reveals that during the stance phase of gait, the calcaneus and

navicular undergo an eversion moment in the frontal plane.11 This is

not surprising and conforms with observation of the gait cycle. What

is noteworthy, however, is that the first metatarsal undergoes a much

quicker eversion and remains maximally everted throughout midstance

until heel-off. This underscores the importance of lowering the head

of the first in a controlled manner, which these authors suggest

requires a device designed to control the tarsometatarsal

articulations as well as the subtalar joint.

So why, in the face of a significant accumulation of evidence in

refutation of the original concept of subtalar neutral, do so many

practitioners still adhere to the old tenets? In our opinion, the

primary reason is that a viable alternative has not been presented

for true peer evaluation.

We therefore present the MASS position as an alternative model for

correction. MASS stands for maximum arch subtalar stabilization, a

phrase used to describe a position achieved by a sequential, gait-

referenced impression of the foot (patient seated) in foam supported

by the floor (floor as frame of reference). Gait-referenced casting

of the foot in the MASS position involves a sequence of steps that

attempts to pass weight through the foot in as close to an ideal gait

pattern as the particular anatomy of each foot can tolerate. The

sequence of foot impressions in foam is:

heel strike with the foot held in optimal inversion;

lateral foot impression;

release of the plantar fascia by flexing the toes;

metatarsal head impression from lateral to medial; and

a thrust in line with the subtalar joint axis (posterior, inferior,

lateral) to seat the heel.

In the proposed biomechanical model, this technique achieves the

maximum closed-chain supination that is easily obtainable at

midstance in any given foot while maintaining a flush position of the

rearfoot and forefoot relative to the floor (no net varus or valgus

angulation of the forefoot). This position insures adequate

supination of the foot at heel strike. Pronation is delayed and

controlled by starting from a maximally supinated position. The shell

material should have memory and be calibrated to flex enough when

loaded to absorb shock and adapt to the terrain (Newton's third law).

When these conditions are met, the orthosis acts as a return spring

for resupination. Since the corrected position we have captured is

identical to that needed for resupination, the device accomplishes

this goal by simply returning to its original shape. Mechanical

efficiency is achieved by full contact of the orthosis with the

plantar foot (zero arch fill). Hodgson et al found the

orthosis " appeared to be more effective in achieving the goals of

custom-molded orthotic intervention " when orthoses using the MASS

position and a gait-referenced cast impression were compared with

those made using standard Rootian principles.29

By facilitating optimal supination in late midstance, the first ray

is held stable against ground reaction forces during forefoot

loading. Abnormal loading of the lesser metatarsals is avoided when

the first metatarsal absorbs its full complement of force. The

sesamoids beneath the first MTP can dissipate shearing forces on the

plantar skin as they are designed to do. Excessive medial/lateral

splay of the forefoot is also controlled when adequate supination of

the foot is maintained. The pathomechanical etiology of hallux valgus

is avoided by preventing dorsiflexion and abduction of the first

metatarsal.

First MTP range of motion is maintained when its head is effectively

lowered and stabilized against the ground. If, on the contrary, the

foot is not resupinated enough to stabilize the head of the first

metatarsal and the foot elongates with overpronation, the plantar

fascia's windlass mechanism will tighten excessively and limit first

MTP dorsiflexion. With this limitation, the foot must compensate to

allow forward progression.30 These compensations can contribute to

the etiology of dorsal bunions, first MTP osteoarthritis,

metatarsal/cuneiform exostoses, chronic pinch callus, and

neurotrophic ulcers.

Conclusion

Orthotic theory and methodology continue to be based mainly on

neutral position concepts originated by Root and colleagues in 1977.

There is general disagreement about these principles and how or

whether they relate to the goals of biomechanical management of the

foot. The goals themselves have not been consistently or thoroughly

defined. We have presented them as we see their pertinence to the key

clinical problem of inadequate resupination at the end of stance

phase. We propose a model based on a more coherent theory and

methodology with which to address these goals. There is a

considerable need for research to help confirm these ideas.

Ed Glaser, DPM, is the owner and founder of the firm in Lyles, TN,

and Don Bursch, PT, OCS, is president. Stuart Currie, DC, is research

director of the firm and maintains a private practice in Denver, CO.

References

1. Quotes for the Openminded Scientist; against excessive skepticism.

www.amasci.com/weird/skepquot.html, accessed 8/3/06.

2. Payne CB. The past, present, and future of podiatric biomechanics.

J Am Podiatr Med Assoc 1998;88(2):53-63.

3. Ball KA, Afheldt MJ. Evolution of foot orthotics-part 2: research

shapes long standing theory. J Manipulative Physiol Ther 2002;25

(2):125-134.

4. Landorf K, Keenan AM. Efficacy of foot orthoses. What does the

literature tell us? J Am Podiatr Med Assoc 2000;90(3):149-158.

5. Root ML, Orien WP, Weed JH. Normal and abnormal function of the

foot. Los Angeles: Clinical Biomechanics, 1977.

6. Root ML, Orien WP, Weed JH. Biomechanical evaluation of the foot,

vol 1. Los Angeles: Clinical Biomechanics, 1971.

7. Root ML, Orien WP, Weed JH. Neutral position casting techniques.

Los Angeles: Clinical Biomechanics, 1971.

8. McPoil TG, Cornwall MW. The effect of foot orthoses on transverse

tibial rotation during walking. J Am Podiatr Med Assoc 2000;90(1):2-

11.

9. RL. The human foot. An experimental study of its mechanics,

and the role of its muscles and ligaments in the support of the arch.

Am J Anat 1941;68(1).

10 Huson A. Functional anatomy of the foot. In: Jahss MH, ed.

Disorders of the foot and ankle: medical and surgical management.

Philadelphia: WB Saunders, 1991:409.

11. Cornwall MW, McPoil TG. Three-dimensional movement of the foot

during the stance phase of walking. J Am Podiatr Med Assoc 1999;89

(2):56-66.

12. Landorf KB, Keenan AM, Rushworth RL. Foot orthosis prescription

habits of Australian and New Zealand podiatric physicians. J Am

Podiatr Med Assoc 2001;91(4):174-83.

13. DG, Desai SM, WH. Action of the subtalar and

ankle-joint complex during the stance phase of walking. J Bone Joint

Surg 1964;46-A:361-382.

14. Elftman H. The transverse tarsal joint and its control. Clin

Orthop 1960;16:41-46.

15. Rockar PA. The subtalar joint: anatomy and joint motion. J Orthop

Sports Phys Ther 1995;21(6):361-372.

16. Sell KE, Verity TM, Worrell TW, et al. Two measurement techniques

for assessing subtalar joint position: a reliability study. J Orthop

Sports Phys Ther 1994;19(3):162-167.

17. McPoil TG, Cornwall MW. Relationship between neutral subtalar

joint position and pattern of rearfoot motion during walking. Foot

Ankle 1994;15(3):141-145.

18. Pierrynowski MR, SB. Rear foot inversion/eversion during

gait relative to the subtalar joint neutral position. Foot Ankle

1996;17(7):406-412.

19. Sarrafian SK. Biomechanics of the subtalar joint complex. Clinic

Orthop Relat Res 1993;(290):17-26.

20. Huson A. Biomechanics of the tarsal mechanism. A key to the

function of the normal human foot. J Am Podiatr Med Assoc 2000;90

(1):12-17.

21. Hamill J, Bates BT, Knutzen KM, Kirkpatrick GM. Relationship

between selected static and dynamic lower extremity measures. Clin

Biomech 1989;4(4):27-25.

22. McPoil TG, Cornwall MW. The relationship between static lower

extremity measurements and rearfoot motion during walking. J Orthop

Sports Phys Ther 1996;24(5):309-314.

23. Knutzen DM, Price A. Lower extremity static and dynamic

relationships with rearfoot motion in gait. J Am Podiatr Med Assoc

1994;84(4):171-180.

24. Lattanza L, Gray GW, Kantner RM. Closed versus open kinematic

chain measurements of subtalar joint eversion. J Orthop Sports Phys

Ther 1988;9(9):310-334.

25. KF, Bojescul JA, RS, et al. Measurement of isolated

subtalar range of motion: a cadaver study. Foot Ankle Int 2001;2

(5):426-432.

26. Chuter V, Payne C, K. Variability of neutral-position

casting of the foot. J Am Podiatr Med Assoc 2003;93(1):1-5.

27. Landorf KB, Keenan AM, Herbert RD. Effectiveness of different

types of foot orthoses for the treatment of plantar fasciitis. J Am

Podiatr Med Assoc 2004;94(6):542-549.

28. Dananberg HJ. Sagittal plane biomechanics. J Am Podiatr Med Assoc

2000;90(1):47-50.

29. Hodgson B, Tis L, Cobb Set al. The effect of 2 different custom-

molded corrective orthotics on plantar pressure. J Sport Rehabil

2006;5(1):33-44.

30. Hall C, Nester CJ. Sagittal plane compensations for artificially

induced limitation of the first metatarsophalangeal joint: a

preliminary study. J Am Podiatr Med Assoc 2004;94(3):269-274.