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The Rootian Revolution in the late 1970's brought a much needed focus on foot biomechanics, especially the need to improve or manage it in the individual patient. While Root et al did an admirable job analyzing normal and abnormal foot function7, justifiably establishing their preeminence in the field, they did a curious thing when it came to intervention strategy. Without the rigorous guidance of research, they proposed the now mainstream orthotic methodology of casting in
"neutral" and posting under the rearfoot8. Such is the power of notoriety and perceived expertise to dictate common practice.
If we try, though, to find the relevance of “neutral” to healthy stance-phase gait, we immediately run into significant problems1-6,9,10. No one seems to be able to connect the neutral model of correction to what should be happening in gait. This is certainly true when we look at the functional problems of the flexible over-pronator. In this article, we outline the following
six functional goals on which the casting and design of truly effective orthotics should be based.
| 1. Sufficient supination at heel strike.
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Pronation will proceed more rapidly and deeply if the foot arrives pre-pronated on the ground. This is because the subtalar joint requires a tightly-nested, i.e. supinated, posture to resist rapid unraveling towards pronation.
| 2. The medial forefoot contacts the ground without imposed abnormal compensatory motion proximally or in the transverse, sagittal or frontal planes.
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This is what can frequently happen when the medial longitudinal arch structure of the foot loses integrity with excessive pronation. Whether it be a forefoot tilting towards varus or valgus or a rearfoot fixed in varus, loss of arch integrity allows for numerous abnormal compensations of foot posture that often become the etiology for many common foot and lower kinetic chain diagnoses.
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3.
Sufficient re-supination of the foot occurs after midstance |
...to stabilize or
"lock" the tarsus in the sagittal plane to allow for efficient propulsion. Payne et al have demonstrated that the amount of final pronation at midstance directly affects the difficulty of re-supination11
(See diagram Below). This must happen to ensure goal #4.
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4. The first metatarsal is stably plantarflexed against the ground during forefoot loading. |
Otherwise, this critical element becomes
"unlocked", free to drift dorsally and medially, causing failure of goal #5.
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5. The first metatarsal accepts 60% of forefoot loading force.
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Without this, the lesser metatarsals must accept abnormal loads to compensate.
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6. The first metatarsophalangeal (MTP) joint is free to dorsiflex sufficiently |
...to avoid compensations in foot or lower extremity posture that would otherwise be necessary to allow sufficient dorsiflexion or forward gait progression. This depends on achieving goals 3 – 5. Failure here leads to the very commonly observed degenerative changes in this joint.
There are three main forces during midstance that are pertinent to achieving re-supination for push-off: 1) The ground reactive force in the pronated foot has a greater lever arm to cause further pronation. 2) The re-supination torque of the windlass effect has a greater lever arm to facilitate supination if it starts from a supinated position. 3) Leg external rotation, the re-supination force of which is facilitated by starting from a less pronated position. A simple way to think about this is that the supination resistance is less when the foot is less pronated and increases dramatically when it is in full pronation11. The determination of how much supinatory force is required is central to the decision making process in the prescription of custom foot orthoses.
In light of these goals, where does neutral position correction have an impact? Neutral methodology calls for at least 50% pronation of the subtalar joint and maximum pronation of the midtarsal joints. Clearly, we need to facilitate significantly more supination than this if the foot is to achieve the above goals. Neutral methodology assumes that a rearfoot post is able to influence calcaneal inversion in the instant of heel strike. That this is a mechanical improbability we will explore in future articles. One would also have to accept the idea that influencing some degree of calcaneal inversion by itself would significantly limit pronation or the ability to re-supinate. The latter is unlikely since, as the heel lifts off the ground, how do we facilitate re-supination when the post makes no forcible contact with the heel? These are the issues that beg for a new model of correction. We will begin to explore this new model in future articles.
REFERENCES
1.
Sobel E, Levitz SJ. Reappraisal of the negative impression cast and subtalar joint neutral position. J Am Podiatr Med Assoc 1997;87(1):32-33.
2. McPoil T, Cornwall MW. Relationship between neutral position and pattern of rearfoot motion during walking. Foot Ankle Int 1994;15(3):141-145.
3.
Elveru RA, Rothstein JM, Lamb RL, Riddle DL. Methods for taking subtalar joint measurements. A clinical report. Phys Ther 1998;68(5):678-682.
4.
Pierrynowski MR, Smith SB.
Effect of patient position on the consistency of placing the rearfoot at
subtalar neutral. J Am Podiatr Med Assoc 1997;87(9):399-406.
5. Pierrynowski MR, Smith SB, Mlynarczyk JH. Proficiency of foot care specialists to place the rearfoot at subtalar neutral. J Am Podiatr Med Assoc 1996;86(5):217-223.
6.
Pierrynowski MR, Smith SB. Rear foot inversion/eversion during gait relative to the subtalar joint neutral position. Foot Ankle Int 1996;17(7):406-412.
7. Root ML, Orien WP, Weed JH. Clinical biomechanics: normal and abnormal functions of the foot, vol. II Los Angeles: Clinical Biomechanics, 1977
8.
Lee W. An historical appraisal and discussion of the
Root model �as a clinical system of approach in the present context of theoretical uncertainty. Clin Podiatr Med Surg 2001;18(4):555- 684.
9.�
LaPointe SJ, Peebles C, Nakra A, Hillstrom H.
�The reliability of clinical and caliper-based calcaneal bisection measurements.� J Am Podiatr Med Assoc 2001;91(3)121-6.
10.�
Mathieson I.� Restructuring Root.� An Argument for Objectivity.� Clin Podiatr Med Surg 2001;18(4);691-702
11.
PAYNE P, MUNTEANU S, MILLER K:
Position of the Subtalar Joint Axis and Resistance of the Rearfoot to Supination. JAPMA 93(2): 131, 2003.
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CLICK to VIEW
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This lecture begins with a review of
the major studies that have shown no
additional benefit from custom
orthotics when compared with prefab
orthoses or other treatment
modalities. The biomechanics of the
subtalar joint and the key
biomechanical events during stance
phase gait are then reviewed as the
rationale for defining a new
paradigm for correcting foot
function. This paradigm is applied
to numerous common diagnoses that
afflict the foot and the lower
kinetic chain. The tendency of many
of our current mainstream treatment
strategies to address symptoms
rather than the biomechanical causes
is discussed. A comprehensive set of
biomechanical goals for stance phase
foot function is presented in order
to evaluate the relative efficacy of
the traditional and new MASS
paradigms. The relevancy of Neutral
Position is challenged.
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