Drop foot impact drives practitioner innovation

[Guest guest]

April 2007 Biomechanics Magazine

Drop foot impact drives practitioner innovation

By: Ian Engelman, CPO

http://www.biomech.com/showArticle.jhtml?articleID=198900243

Deconstructing elements of popular culture through a medical lens

has given us some interesting insights. Like the Mona 's

pregnancy or the Mad Hatter's mercury poisoning, it may be that

someone " tripping over his own feet " is not just a cliche, but the

description of an actual medical problem. Some authors have

speculated that the biblical story of wrestling with the angel

may be the first recorded case of foot drop.1

In patients with drop foot, the dorsiflexor muscles of the foot and

ankle aren't able to lift the foot clear of the ground during swing

phase of gait or control plantar flexion during heel strike. The

cause is often neurologic, though trauma to the relevant muscles may

also be a causative factor.1,2 Though the terms-drop foot and foot

drop-are interchangeable, the condition is the same.

Practitioners in the biomechanics mainstream who treat the condition

should seek a solution that takes both swing phase and stance phase

into consideration. The gait deviations and clearance issues of the

patient with drop foot are relatively easy to detect and, from the

clinician's standpoint, the many swing phase solutions that handle

the clearance issue generally work well. However, drop foot also has

a stance phase element that is nearly invisible to the clinical eye,

but is significant to the patient. Solutions that ignore this aspect

may create knee instability, poor gait efficiency and/or uneven step

lengths.

In other words, the dorsiflexors are also responsible for managing

impact by lowering the foot from heel strike to foot flat. This

seems to have been overlooked in brace design since most braces do

not take it into consideration, which can lead to stance phase

issues for drop-foot patients who wear a brace. Clinical experience

reveals that in a stiff brace design with too much plantar flexion

resistance at heel strike, the floor reaction force will cause a

knee flexion moment and subsequent knee instability. Too little

plantar flexion resistance, however, results in repetitive trauma to

the foot due to the lack of shock absorption (normally achieved by

eccentric contraction of the dorsiflexors).

Describing drop foot as an anterior compartment deficit may

facilitate a better understanding of the condition as both a swing

phase and stance phase problem.

To fully appreciate the impact to the patient, one has to understand

the importance of the line of ground reaction force as it relates to

the knee and ankle. Two equivalent-force ground reaction vectors

will create greater or lesser knee flexion moments depending on how

closely they pass through that joint. From physics, we remember that

the moment of force (M) (in newton-meters) equals force (F) (in

newtons) times distance (d) (in meters) from the joint (M = F x d).

The distance depends on the location of the ground reaction force.

With a stiff ankle foot orthosis (AFO) design that doesn't allow the

foot to plantar flex (Figure 1B) the ground reaction point is pushed

posteriorly and its distance from the knee joint is increased.

Allowing the foot to come to foot flat earlier in stance phase

(Figure 1A) shifts the floor reaction anteriorly and the resultant

force vector is closer to the knee joint. If the difference in

distance between the situation in Figure 1A (d1) and the situation

in Figure 1B (d2) is a factor of three, then the patient with the

ability to plantar flex (Figure 1A) has to work only a third as hard

to stabilize the knee (M1 [F x d1] < M2 [F x d2]).

So why don't AFO designs allow the foot to come down to foot flat

immediately and shift the ground reaction point forward as quickly

as possible? Because with such a device, there would be little or no

deceleration at the ankle. In our clinical experience, patients who

use such devices, like elastic straps, for instance, find that they

get excessive foot slap and some develop calcaneal bone bruises.

Hence, they often compensate with a shorter step length.

Shock absorption

In normal gait, knee flexion and ankle plantar flexion are the

mechanisms that absorb shock.4 Normally, the eccentric contraction

of the dorsiflexor muscles would help, along with the knee flexion,

to absorb the impact of body weight. But with no deceleration at the

ankle, the drop foot limb has only knee flexion left for shock

absorption. Creating the proper deceleration in an AFO can restore

this shock absorber at the ankle joint.

Adjustable plantar flexion resistance in some AFO designs allows the

practitioner to adjust timing and quantity of shock absorption.

Variables for the practitioner to keep in mind when determining the

correct amount of shock absorption are: patient's weight, patient's

step length, quality of the shoe heel, and patient's activity level.

When setting the resistance level, the practitioner should aim to

achieve two goals: to keep the force vector close to the knee joint

axis and to maintain some deceleration for shock absorption.

Establishing this setting is best accomplished with an articulated

AFO.

In our clinical experience, articulating designs are often more

effective than static designs because they allow enough dorsiflexion

range so as not to inhibit sit-to-stand, allow normal functional

benefit and range of plantar flexion, promote and normalize

neurological patterning, and allow for a shorter and more

cosmetically appealing solution.

Designing a brace for the shoe is as important as designing a brace

for the foot. The interaction of the brace and the shoe plays a

substantial role in stability and fit. Several design features can

be used to optimize space inside the shoe (see tables, pages 53 and

54).

Stance phase dynamics can also change substantially with footwear.

Especially early in stance when hard heels without bevels can make

maintaining stability even more challenging. Like a hard shoe heel,

a significant determinant of the ground reaction force location is

the heel shape of the brace (Figure 3).

Available braces

Sometimes carbon graphite devices that provide some ground reaction

energy return are used for drop foot. This is curious since most

cases of drop foot do not include corresponding plantar flexion

weakness. Like an energy-storing prosthetic foot, the benefit of

these carbon graphite AFOs is that the spring is compressed by body

weight as the tibia travels over the foot. Then, late in stance

phase, the energy is released by the material the brace is composed

of and returned for use in toe-off. This energy return may be

desirable for the flail foot, but the typical drop foot case does

not need this feature.

A preflexion angle in an articulated AFO is desirable for a " drop

and slap " foot (Figure 4). With a dorsiflexion assist/plantar-

flexion resist joint and a preflexion angle, instantaneous

deceleration is possible. Here, resistance to plantar flexion is

effective immediately at heel strike. Articulated AFO designs with

proper ankle joints provide this preflexion angle. When donned, the

brace is under tension and produces a condition of preload. This

instant resistance is due to the preexisting tension in the system.

Typical off-the-shelf posterior leaf spring (PLS) AFOs do not have

an adequate amount of preflexion. Without the preflexion, the shock

absorption quality will not become effective until 20 degrees to 30

degrees of plantar flexion. This is too late because the foot is

already at foot flat before the plantar flexion resist joint

activates.

A custom version of the PLS AFO may be more common. It is difficult

for the practitioner or manufacturer to create a PLS device that has

the equivalent preflexion angle, preload, and range of an

articulated AFO. The reason we don't see PLS AFOs in 40 degrees of

preflexion is that the plastic used to fabricate them does not have

the resiliency to withstand the 70 degrees of deformation required

without losing integrity. Usually custom AFOs are too stiff in the

plantar-flexion range and create instability. Some adjustments can

be made by tailoring the trim lines, but the range and dynamics

cannot match those of an articulated AFO.

Step length will also be affected by a poor AFO design. In a stiff

design, which is unstable at the knee, the patient will take a short

step to reduce the instability. In a design that is too weak, a

short step length will also be manifested, due to patients' desire

to reduce the shock of impact.

Features and benefits

Without dorsiflexion assistance during swing phase, individuals with

drop foot use various compensatory gait movements to make up for the

dragging foot or feet (if bilateral), such as hip hiking, lateral

lean, circumduction, vaulting, and/or excessive knee and hip flexion.

Without a tailored shock absorption setting at stance phase,

individuals with drop foot use various gait deviations to compensate

for the improper floor reaction vector location and direction,

including short step, use of a cane, overuse of the quadriceps, and

crouch gait.

A complete bracing solution for anterior compartment deficit (drop

foot) will allow for ease of walking and gait efficiencies. AFO

designs need to address both swing and stance phase dynamics. An

adjustable articulated design is best suited for achieving these

goals. Two opposing needs must be considered for stance phase:

providing shock absorption (for deceleration deficit) and creating

knee stability (for gait efficiency). Choosing the correct ankle

joint will allow the practitioner to choose settings that will

achieve the perfect balance between these two considerations.

Nonbrace treatments

A few innovative solutions for drop foot have come on the market

recently that do not fall into the AFO category. Functional

electronic stimulation (FES), or neuromuscular stimulation of the

dorsiflexors through electrodes, has been effective in jump-starting

swing phase in some cases. The companies that offer this technology

generally recommend that the patient maintain a bracing alternative.

Another option, elastic strapping, is also heavily marketed. These

work well for clearance, but fall short on effective shock

absorption. One creative solution is a plastic dorsal splint.

Patients can walk barefoot with this design, but the friction on the

dorsum (especially in late stance phase) can be limiting.

Lastly, surgical options include transfer of the posterior tibialis

tendon and arthrodesis. Results have been varied and success is

sometimes loosely defined, since even successful cases still require

bracing.

Conclusion

The ultimate solution for drop foot will act in every way like the

dorsiflexors in normal human locomotion. It would fit well into all

types of shoes, weigh nothing, and be invisible. While that may be

unrealistic, there are signs that some of these objectives can be

met. Joaquin A. Blaya and Hugh Herr, PhD, at MIT created a variable-

impedance device capable of delivering varying amounts of resistance

as needed throughout the gait cycle.2 For instance, the resistance

would be reduced to zero at toe-off to avoid impeding the plantar

flexors, but a torsional spring-damper control during swing would

work to keep the toes from dropping. For the rest of us who wish to

provide the best current solution, there are articulated designs

that allow the user to dial in the preferred amount of plantar-

flexion resistance. For the nonarticulated design, care must be

taken to find the best trim line. Whatever solution is chosen, the

practitioner should consider the patient's stance phase needs.

Ian Engelman, MS, CPO, an engineer who holds many patents, owns

Insightful Products, an R & D company based in Scarborough, ME.

References

Pritchett JW, Porembski MA. Foot drop. eMedicine,

www.emedicine.com/orthoped/topic389.htm, accessed 3/15/2007.

Eidelson SG. Drop foot (foot drop) and steppage gait (footdrop

gait). SpineUniverse,

www.spineuniverse.com/displayarticle.php/article2620.html, accessed

3/15/07.

Blaya JA, Herr H. Adaptive control of a variable-impedance ankle-

foot orthosis to assist drop-foot gait. IEEE Trans Neural Syst

Rehabil Eng 2004;12(1):24-31.

biomech.media.mit.edu/publications/Active_Ankle_Foot_Orthosis.pdf,

accessed 3/15/07.

Buczek FL, Cavanagh PR. Stance phase knee and ankle kinematics and

kinetics during level and downhill running. Med Sci Sports Exerc

1990;22(5):669-677.