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A Biomechanist's Perspective on Partial Foot Prostheses

Andrew H. Hansen, PhD

ABSTRACT

A systematic review of the biomechanics of walking after partial foot amputation was recently performed by Dillon and colleagues. Their review illustrated a high level of evidence from the literature that human gait is altered by partial foot amputation but low to insufficient evidence to demonstrate the effects of various types of prosthetic and orthotic devices on gait of persons with partial foot amputation. More work is needed to understand the biomechanical effects of various partial foot prosthetic devices on level and nonlevel walking surfaces. With this knowledge, medical personnel could more easily ascertain the safest and most functional device for a person with a partial foot amputation.

There appear to be parallel findings in rehabilitation research literature that may lend insight into the evaluation and design of partial foot prostheses. This article describes past efforts of various investigators in understanding the role of the ankle plantarflexors in walking, the function of various prosthetic feet for transtibial prosthesis users, the use of rockers to describe walking characteristics, and the use of rockers in walking toys and machines. The article then describes the roll-over shape concept, how it has been used to study able-bodied persons walking on level and ramp surfaces, and how it has been used to examine the effects of prosthetic feet on gait characteristics. Finally, this articles addresses the potential usefulness of the roll-over shape for determining relative biomimesis (i.e., the degree to which the personmachine system mimics the intact physiologic system) of various partial foot prosthetic systems. These measurements coupled with other outcome measures could be useful in the future design and prescription of these devices. (J Prosthet Orthot. 2007;Proceedings:P80–P84.)

'SILENCED' ANKLE PLANTARFLEXORS AND PROSTHETIC FOOT STUDIES

In the past, researchers have attempted to determine the specific role of the plantarflexors in walking by observing persons who have had isolated surgical excision of the gastrocnemius and soleus muscles or by "silencing" these muscles through nerve blocks.1–4 Measurements of the gait of these persons, without function of their posterior calf muscles, have shown an increased first peak of the vertical ground reaction force on the "sound" side (the side not affected by surgical excision or nerve blocks) as well as a shortened step length on that side.

Several studies were formed in the 1980s and 1990s to determine the effects of various prosthetic feet on gait.5–13 Most of these tests were repeated measures of persons using several prosthetic feet that were commercially available at that time. Perhaps the most consistent findings from these studies were that the use of many different kinds of prosthetic feet (but not all feet) correlated with an increased first peak of the vertical ground reaction force and a shortened step length on the sound side during walking.

Some partial foot studies have also shown an increased first peak of the vertical ground reaction force on the sound side when persons use particular partial foot prosthetic devices,14–16 (as reported in Dillon et al.17). However, the review by Dillon et al.17 indicates that there is insufficient evidence at this time regarding the effects of different types of partial foot devices on step length symmetry.

All of these studies incorporate a situation in which the ankle-foot system does not allow normal anterior movement of the center of pressure of the ground reaction force under the foot. "Silencing" the plantarflexors eliminates the counterbalancing mechanism that normally allows forward progression of the center of pressure. In fact, one of these studies noted decreased anterior excursions of the center of pressure.3 Prosthetic feet that led to increased sound limb loading were generally feet with shorter keel lengths or keels too compliant to allow anterior movement of the center of pressure. Persons did not have increased sound limb loading when walking with a foot having a longer and stiffer keel. Lastly, Dillon and Barker16 showed that none of the partial foot prosthesis designs in their study, with the exception of the clamshell prosthesis, allowed normal anterior movement of the center of pressure during walking. The study of anterior movement of the center of pressure and sound limb loading may have important implications for the health of the contralateral limb.11

ROCKERS-WALKING TOYS, MACHINES, AND HUMANS

Rockers have been used by many investigators to describe walking. Perry18 described the functions of the normal foot and ankle as creating three rockers to facilitate forward progression during walking: the heel rocker, ankle rocker, and forefoot rocker. Other investigators have used rockers as a critical component in their study of walking toys and machines.19–22 Morawski and Wojcieszak19 studied the use of rockers in walking toys and suggested that rockers could be useful for the design of prostheses and orthoses. McGeer20 created mathematical and physical models of mechanisms that could walk down gentle slopes using only passive dynamic properties (i.e., without the use of external power). A key component of McGeer's model was the circular rocker used to replace the function of the foot and ankle. Using a simple model and calculation, McGeer suggested that the "equivalent radius" that is created by the lower limb system for human walking would be roughly 0.3 times the length of the leg. Collins et al.21 later developed more lifelike walking machines that incorporated rockers in place of the feet and ankles and that were able to walk on level ground with small amounts of input energy. Wisse and van Frankenhuyzen22 demonstrated that increasing the radius of the rocker on a passive dynamic walking machine increased the amount of disturbance it could tolerate without falling down. Their study suggests that rockers provide an inherent sense of stability during walking that is increased with rocker radius. Adamczyk et al.23 examined the effects of wearing rocker boots on the metabolic energy expenditure rate of ablebodied ambulators. Subjects walked at 1.3 m/s on a treadmill while wearing rigid-ankle walking boots connected to wooden rockers. The metabolic energy expenditure rate was estimated from respiratory gas exchange data measured during treadmill walking trials and was examined as a function of rocker radius. Adamczyk et al.23 reported that the subjects walked with a minimum metabolic rate when the rocker radius was approximately 0.3 times the leg length, matching the "equivalent radius" suggested by McGeer.20 The results of these studies suggest that using the correct rocker shape may provide efficiency and inherent stability during walking.

Rockers provide a logical avenue for anterior progression of the center of pressure under the foot during walking. In the earlier examples of "silenced" plantarflexors, short- or soft-keeled prosthetic feet, and certain partial foot prostheses, we can think of these systems as providing insufficient rockers for the forward progression of the body over the foot. A method of measuring these effective rockers on humans (both able-bodied and disabled) has recently been developed. Examining the effective rockers of lower limb systems may help to improve the function of partial foot prostheses.

ROLL-OVER SHAPES FOR ABLE-BODIED PERSONS WALKING

The roll-over shape has recently been defined by our group as the effective rocker shape that a lower-limb system conforms to during walking, specifically between initial contact and opposite initial contact.24 There are three types of rollover shapes: the foot, ankle-foot, and knee-ankle-foot rollover shapes. These shapes are found by transforming the center of pressure of the ground reaction force from a laboratory-based coordinate system to a body-based coordinate system. For example, the ankle-foot roll-over shape is created by transforming the center of pressure into a shankbased coordinate system. The ankle-foot roll-over shape illustrates the effective rocker (or cam) created by the combined effects of the ankle and foot during walking. Foot and knee-ankle-foot roll-over shapes are found by transforming the center of pressure into foot- and leg-based coordinate systems, respectively. Through a variety of experiments, we have found that able-bodied persons adapt to various changes in level ground walking to maintain essentially unchanged ankle-foot and knee-ankle-foot roll-over shapes.24–26 Best fit arcs to the knee-ankle-foot roll-over shapes have a median radius of approximately 0.3 times the leg length, regardless of walking speed, shoe heel height, or amount of added weight being carried. This value, as indicated earlier, is the same as suggested by McGeer20 and Adamczyk et al.23 During ramp walking, the ankle-foot and knee-ankle-foot roll-over shapes change "alignment" on the limb, presumably to help accommodate the surface.27

These measurements of physiologic lower limb systems suggest goals of the person-device system for walking on both level and ramped surfaces. In particular, if the ankle of a person with partial foot amputation is strong and left unrestricted by a partial foot prosthesis, it may allow the person to more easily adapt to different walking surface inclinations. On the other hand, these systems may not be able to adequately distribute pressures and have sufficient coupling with the residual foot to allow movement of the center of pressure beyond the distal end.16 For level walking, a clamshell prosthesis, as described by Dillon and Barker,16 seems to allow anterior progression of the center of pressure under the foot in a more normal fashion. Best-fit radii of partial foot prosthesis roll-over shapes (being used by the person) could indicate whether forefoot materials in clamshell prostheses have appropriate stiffness properties or whether they should be stiffer or more compliant.

ROLL-OVER SHAPES OF PROSTHETIC FEET AND EFFECTIVE FOOT LENGTHS

The ankle-foot roll-over shape has been used by our group to describe the function of prosthetic feet because they typically replace both the ankle and foot.28–30 Our work on roll-over shapes of prosthetic feet coupled with results and discussion from other investigators led us to the hypothesis that a shortened anterior lever arm of a prosthetic foot would lead to a drop-off effect. This drop-off could then lead to an increase in first peak of the vertical ground reaction force and a shortened step length on the sound limb. The effective foot length ratio (EFLR) was created as a method to quantitatively assess the variety of anterior keel lengths and properties provided by a series of prosthetic feet commercially available in the 1990s.31 The EFLR is simply the effective foot length, which we consider as the distance from the heel of the foot to the most anterior part of the roll-over shape, divided by the total foot length. Assuming a heel-to-toe gait, this distance indicates the amount of forward rolling on the ankle-foot system during the step (i.e., from initial contact to opposite initial contact). Values of EFLR for prosthetic feet ranged from 0.6 to 0.8, and our estimation of the EFLR for the physiologic ankle-foot system was 0.83.31 Based on these data, we created experimental prosthetic feet that could be altered quickly and easily to represent the span of effective foot lengths of commercially available feet. In our study using these experimental prosthetic feet,32 we found that shortening the effective foot length led to a significant increase in the first peak of the vertical ground reaction force and, for many of the subjects, a reduction in the sound limb step length. These results support the theory that an insufficient forefoot lever arm on one side alone can lead to increased loading to the other limb, which may result in negative long-term effects.

POTENTIAL USE OF ROLL-OVER SHAPE IN EVALUATION AND DESIGN OF PARTIAL FOOT PROSTHESES

The concept of effective rocker shapes and the measurement of roll-over shape seem appropriate to future research studies of partial foot prostheses. The roll-over shape can be used as an outcome measure if one works under the general theory that these devices should mimic the intact physiologic system that they intend to replace or augment. The roll-over shapes can be modeled as circular arcs, of which several parameters can be calculated (e.g., radius, arc length, anterior position of the arc's nadir). Perhaps the most useful measure could be the EFLRs that are achieved with various devices and how these values relate to other gait parameters, including sound limb step length and sound limb loading (In the case of partial foot prostheses, the effective foot length should be divided by 0.15 times the height, the estimated total length of the foot based on anthropometric data.33). For example, Figure 1 shows two possible roll-over shapes that could arise from the use of two different partial foot prostheses and the physiologic ankle-foot roll-over shape. Clearly, one of these roll-over shapes is closer to the physiologic goal and may provide more efficient and robust walking. From a mechanical standpoint, it seems that a partial foot prosthesis should 1) off-load sensitive areas of the residual foot, distributing pressures to more tolerant areas; 2) provide adequate coupling between the residual foot and the device; and 3) provide sufficient rigidity of the prosthesis portion to allow the center of pressure to move beyond the distal end of the residual foot.16 Future work using roll-over shape analyses may assist in determining the appropriate stiffness values for partial foot prostheses and may provide a general outcome measure of the success of these mechanical goals.

The roll-over shape is a potential tool for furthering our understanding of the function of partial foot prostheses; it suggests goals for these devices on both level and ramp walking. However, it gives only a global view of the system output and does not describe issues internal to the overall systems. For example, roll-over shape does not describe pressure distributions between the residual foot and the prosthesis, which are of vital importance to both safety and function of the person-device system in walking. There are many other important factors involving the right prescription of partial foot prostheses of which clinicians are well aware, including cosmesis. It is hoped future breakthroughs in understanding and design of partial foot prostheses will be made by multidisciplinary teams of doctors, prosthetists, orthotists, therapists, and engineers, which will make partial foot amputation a more attractive and viable option.

ACKNOWLEDGEMENTS

The author thanks Dr. Stefania Fatone and Mrs. Pinata Sessoms for their helpful suggestions during the preparation of this article.

Correspondence to: Andrew H. Hansen, PhD, Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, 345 E. Superior Street, Room 1441, Chicago, IL 60611; e-mail:


ANDREW H. HANSEN, PhD, is a research health scientist at Jesse Brown VA Medical Center and a research assistant professor in the Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago, Illinois.

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