Expert Insights On Forefoot And Rearfoot Posting

These expert panelists discuss rearfoot posting for custom orthoses, forefoot posting for varus deviations and which measurements are most effective during a biomechanical exam.

Q:

What is your preferred method for determining the amount of rearfoot correction/posting you order in a custom foot orthosis prescription?
 

A:

Jeffrey Cusack, DPM, acknowledges that he “came into the profession fully indoctrinated in the Root-ian approach to mechanics, and although much of his theorems have been challenged over the years, as a basis to begin a reasonable approach to treatment, clearly Dr. Root’s work was ahead of its time.”

Having said that, Dr. Cusack admits that over the past few years, he has become increasingly more intrigued with the “tissue stress” approach to the biomechanical management of the dysfunctional patient. With this approach, after identifying the specific structure under duress, he notes the podiatrist must then determine what anatomical and functional position to place that structure into in order to reduce some of the excessive tensile forces causing the structure to remain in a plastic state (position of abnormal stretch).

Dr. Cusack says the tissue stress approach, while freeing the podiatrist from the minutia of attempting to measure positional relationships of rearfoot to leg and forefoot to rearfoot, does require an absolute comprehension of the specific structures under duress and their functions in gait (both primary and, if applicable, secondary).

One one hand, Dr. Cusack says podiatrists are freed up from dragging out the protractor. On the other hand, he says the specificity of the design of tissue stress is more reflective of the structure under duress if one applies a tissue stress approach. Citing Kirby’s work, Dr. Cusack says the orthosis becomes a functional extension of the foot and its design must reflect those needs.¹

Dr. Cusack says Kirby’s example of a dysfunctional posterior tibial tendon serves to illustrate how one might apply tissue stress theory to design an appropriate orthosis.² If the posterior tibial tendon is being abnormally stretched due to an abnormally pronating foot, he notes placing the foot into a more inverted attitude will decrease tension and healing can begin. This would require the construction of an orthosis with either a rigid or semi-rigid shell and adequate rearfoot posting to achieve enough of an inversion moment to relax tension on the complex. This, in turn, relaxes tension on the “stressed” structures. Dr. Cusack says an enhanced heel seat coupled with a Kirby skive may serve the patient well. If necessary, he suggests considering a Blake inverted approach.

Recognizing that the posterior tibial tension is also a secondary ankle plantarflexor, Dr. Cusack says the addition of modest heel lifts further serves to decrease tensile forces along the compromised tendon. He acknowledges that if a below ankle approach fails to provide the desired clinical response, an above ankle approach providing increased leverage may be necessary.

Q:

Do you prefer using frontal plane subtalar and tibial varum measurements from your biomechanical exam, or do you use a more empirical approach based on the severity of compensation in the patient?  

A:

For Barbara Pelc, DPM, a combination of the two approaches is most effective. As she notes, the range of subtalar joint motion and neutral position can be a starting point, but one needs to consider superstructural influences leading to additional compensatory influences, such as hip, knee and ankle function. Dr. Pelc notes sometimes physicians will need to employ a higher post or additional heel lift to address one of the most common of those superstructural influences, sagittal plane equinus.

Noting he has shifted away from specific mathematical measurements, believing the data show little validity and reliability of these measurements, Robert Eckles, DPM, has long been convinced that whatever posting one applies to the orthosis, the actual force applied to the medial heel is really a function of how the device interacts with the footwear in question. In other words, he says the effect of the posting is always going to be based on the response of the footwear. Soft or worn shoe interiors (and outsoles) will totally mitigate the effect of posting, according to Dr. Eckles.

“This doesn’t mean I don’t post,” says Dr. Eckles. While he believes the addition stabilizes the orthosis under the right conditions, he does not rely on posting as the primary functional component of the orthosis. Dr. Eckles gives patients an informed responsibility for their footwear choices. His standard rearfoot post is the customary 4 degrees unless his patient has a high subtalar joint or proximal limb varus. For those patients, he will go higher but rarely posts over 6 degrees as patients will tend to slide off the device in those cases.

Q:

What are your thoughts on forefoot posting for varus forefoot deviations? Do you post only for structural varus or do you consider that some acquired supinatus would benefit from post control?

A:

Dr. Eckles notes forefoot posting is the classic suggestion when there is structural varus. However, as physicians do see situations in which the degree and nature of the hindfoot valgus position have caused longstanding supinatus deformity, he suggests it is at least initially reasonable to add a forefoot post. Time and relaxation of the medial column may make this unnecessary. Dr. Eckles says reducing or eliminating this forefoot post over time would be a benefit for shoe fit as well as first metatarsophalangeal joint motion. He would evaluate patients in six to 12 months or as required to see if a new device is necessary. When a new device is required, Dr. Eckles emphasizes that one should take a new cast.

When the clinician confirms a small forefoot deformity, either varus or valgus, and the patient is going to wear the device in dress or casual shoes, Dr. Eckles posts proximal to the metatarsal heads, if extrinsic, or tries to keep the posting intrinsic. For larger deformities of more than 5 to 6 degrees, and for deformities in athletic patients for whom control is important in late stance (such as a fast-paced runner), he tends to post extrinsically to the sulcus.

Dr. Pelc stresses that one needs to identify forefoot varus deviations clearly as structural varus or acquired supinatus. After documenting a structural varus, she favors extrinsic posting on an orthotic with a rigid shell. Most frequently, Dr. Pelc says a post to the sulcus, which supports the metatarsal heads later in the stance phase, is very effective. The choice of shoe gear often limits that kind of bulky post so she suggests an alternative tip post in low volume shoes. That tip post, she cautions, will end proximal to the metatarsal heads and will not function quite the same way.

If there is an acquired supinatus, Dr. Pelc suggests modifying the casting method. As she notes, one should dorsiflex the hallux to reposition the first ray into plantarflexion, eliminating the existing supinatus and locking the oblique axis of the midtarsal joint. She says this position will create a stable and natural contour to the longitudinal arch.

If one has cast the patient with the supinatus deformity still present, Dr. Pelc says a cast modification for intrinsic forefoot posting will come the closest to the hands-on technique of manipulating the foot during casting.

Dr. Cusack is an Assistant Professor in the Department of Orthopedics and Pediatrics at the New York College of Podiatric Medicine. He is affiliated with the Foot Center of New York in New York City.

Dr. Eckles is the Dean of Clinical Studies and Associate Professor, Department of Orthopedics and Pediatrics at the New York College of Podiatric Medicine. He is affiliated with the Foot Center of New York in New York City.

Dr. Pelc is in private practice in Glen Head, NY. She is a consultant to Langer Biomechanics.

Dr. Volpe is a Professor in the Department of Orthopedics and Pediatrics at the New York College of Podiatric Medicine in New York City. He is in private practice in New York City and Farmingdale, N.Y.

References
1. Kirby KA. Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc, Payson, AZ, 1997, pp. 267–268.
2. Kirby KA. Prescribing orthoses: has tissue stress theory supplanted Root theory? Podiatry Today. 2015; 28(4):36–44.