Is Lighter Better? Thoughts on the Relationship between Device Weight and Function

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Sarah Mattes, MS

Most prosthetists agree that prosthetic devices should be as lightweight as possible while including the safest, most efficient, and most functional componentry possible. Contemporary ankle-foot components can adjust to changes in slope, making walking on ramps or other uneven surfaces easier and allowing a wider range of shoe heel heights to be worn. These components may contain hydraulic units, microprocessors, and force-generating actuators and are 100–250 percent heavier than lightweight, passive elastic-response feet. For example, Össur’s Vari-Flex® foot weighs 700g and the BiOM® foot weighs more than 2,000g.

What is the relationship between component function and weight, and at what point does “heavier” become “too heavy?” Can the added weight of an advanced component impact function detrimentally? The shift from lightweight, energy-returning ankle-foot prosthetic components to the significantly heavier hydraulic or powered versions requires further evaluation.

Why Does Prosthetic Gait Require More Energy?

One likely reason that prosthetists have embraced the idea that lighter is better is to minimize the muscular effort required during locomotion. This is based on the understanding that a major portion of metabolic demand during walking and running is associated with accelerating the limbs with each stride.1 Thus, as the weight of the prosthesis increases, it is assumed that more energy will be required to move it through space during swing phase. People with unilateral transtibial amputations generally require 20–25 percent more energy exertion and walk slower when using a prosthesis than able-bodied subjects.2 The slower walking speeds are associated with shorter contact times and longer swing on the prosthetic side compared to the intact side, creating asymmetries in gait patterns.3 When walking speed is controlled, Winter and Sienko hypothesized that the increase in energy cost comes from the increased muscular demand of the hamstrings to extend the hip during early stance to ensure knee stability during prosthetic gait.4 Individuals with unilateral transtibial amputations possess a substantial inertial asymmetry in their lower limbs. Lewallen and colleagues suggested that individuals with amputations intentionally walk slower and take shorter steps than able-bodied people in an attempt to achieve better temporal and spatial symmetry between the prosthetic and intact sides.5

Inertia Characteristics and Energy Cost Relationship Is Unclear

Most prosthetic limbs are considerably lighter weight than the anatomical limbs they have replaced. A typical transtibial prosthetic limb may weigh between 0.5 and 2kg, whereas the limb that it is replacing would likely be closer to 4kg for a 70kg man.6 Despite assumptions by practitioners regarding prosthesis weight, experimental research has not been able to gather sufficient evidence to draw firm conclusions about the effect of increased prosthetic mass on the metabolic energy cost to amputee gait. Gailey et al. found no correlation between energy cost and prosthetic mass for prosthetic masses between 2 and 2.7kg.2 Jans and Bach reported that energy costs were not changed when masses of less than 1.125kg were added to the prostheses of unilateral transtibial amputees.7

On the other hand, Mattes et al. and Smith and Martin examined energy cost and gait symmetry while matching the mass and moment of inertia of the prosthetic limb to the intact side by adding between 0.85kg to 2.6kg.8,9 These studies found that energy cost and gait asymmetry increased with increased mass. Smith and Martin added the load at several locations on the prosthetic limb and found that energy cost and symmetry were most negatively affected when the load was placed distally, at the ankle.9 This common result among studies measuring the impact of added mass supports Lehmann’s research that also found distally added mass negatively affected metabolic energy cost.10 Smith and Martin commented in their study findings that increasing the mass of a prosthesis distally should be avoided unless there are other benefits to be gained by the amputee. This conclusion is targeted at newer prosthetic ankle technology that seeks to incorporate motors at the prosthetic ankle.9

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Hydraulic Ankles Are Heavier…. But Maybe That’s Okay

Currently, only a few manufacturers offer microprocessor-controlled ankles: Össur’s PROPRIO FOOT® and Endolite’s élan foot are two examples. These feet weigh between 1 and 1.5kg, 50–100 percent more than an equivalent patient-appropriate, passive energy-storing foot. Although the motors in these ankles control the action of the hydraulic units, which increase and decrease the rate of compression based on the user’s real-time needs, these devices do not include actuators to actively plantarflex the ankle at terminal stance.

Experimental research and anecdotal reports demonstrate a positive reception to some of the features of these feet. Portnoy and colleagues concluded that using a hydraulic ankle decreased internal socket stress, which could protect the distal end of the residual limb from pressure-related injury.11 De Asha et al. found that use of a microprocessor-controlled hydraulic foot increased self-selected walking speed and provided a smoother weight transfer to the prosthetic limb.12 Several other studies have discussed the benefits of walking up and down ramps and stairs with hydraulically controlled dorsiflexion and plantarflexion.13

Studies have also documented benefits to the intact side, which include less impact force across the knees, hips, and spine.14 It has also been documented that active toe dorsiflexion during swing increases ground clearance, making the foot safer for users who present fall risks.15 Experimental data regarding metabolic energy cost of ambulation with these devices is not readily available in the published literature. Endolite reported in its own research, by Moser et al., that “compared to conventional energy-return prostheses, the findings obtained from level, uphill, downhill, upstairs, and downstairs extended walking trials showed that the new hydraulically assistive foot offered the amputees the ability to walk with up to 8.5 percent less effort.”16 This research has not yet been published in a peer-reviewed scientific journal.

The Powered Ankle Is Really Heavy!

A powered prosthetic ankle-foot that was originally developed by Hugh Herr, PhD, and his Biomechatronics research group at the Massachusetts Institute of Technology (MIT) Media Lab, Cambridge, is capable of mimicking some dynamics of the anatomical ankle; it is now commercially available under the name BiOM. This foot uses a traditional, J-shaped carbon foot at the base and a unidirectional screw-type actuator that replaces the action of the gastrocsoleus complex. This foot was designed to match the inertial characteristics of the missing anatomy; it weighs 2kg.

As discussed, a prosthetic mass of this magnitude has been shown to negatively affect energy costs.8 However, Herr and Grabowski demonstrated that the metabolic energy cost to users of this foot was decreased by 8–12 percent and that energy demand was proportionally decreased as walking speed increased.17 They also noted that preferred walking speed was the same as that for able-bodied subjects, and temporal and spatial symmetry were improved. This indicates that the power-generating ankle provides more metabolic benefit than the energy cost associated with the increased mass.

The developers of the powered ankle are hoping to see the energy cost of ambulation among those with amputations actually decrease to a point lower than the requirements of able-bodied individuals.17 They suggest that since the battery and motor are doing some of the work of locomotion, then there should be less metabolic demand, so less energy should be required for walking. They point to the lack of a rigid connection between the residual limb and the socket as a place where mechanical energy may be lost in lighter-weight, passive feet.

Suspension Systems May Alter Perception of Weight

Perception of weight is subjective and may be related to suspension methods. Few of the studies that have investigated mass on any type of foot have reported information about the suspension systems that were used. Board et al. found gait symmetry increased when subjects were using active vacuum suspension compared to passive suction suspension.18 They proposed that the increased volume of the residual limb maintains a better fit inside the socket under higher vacuum, improving proprioception and allowing the user to transfer forces to the prosthesis more efficiently. This is consistent with the thoughts of the MIT group, whose subjects were using passive suction suspension.

Users Don’t Always Prefer Lighter

User preference for more or less weight seems unpredictable. A comprehensive study by the U.S. Department of Veterans Affairs on amputee satisfaction with prosthetic devices indicates that weight was generally referred to as good or bad, but “the common point being made was that getting the weight correct matters a lot.”19 Other research corroborates this conclusion. Hale reported subjective preference in an experimental mass condition where the prosthetic limb weighed 75 percent of the estimated mass of the intact side.20 Prosthetic users don’t always prefer “as light as possible,” and it’s not uncommon for an individual to request more mass. The categories of “heavy” or “light” seem to be less of an issue than “not right.”

Sacrifice Function for Weight? Maybe Not….

Certainly not all patients are candidates for powered ankles or active vacuum suspension systems because of the shape of their residual limbs, build heights, gadget intolerances, or financial considerations. Combining some of these options with more traditional tools may provide greater functionality without the expected metabolic or perception consequences. “There is quite a lot of scientific evidence now which shows that added functionality appears to outweigh issues of weight as the correct design can offer improvements in almost every area of biomechanical performance,” says David Moser, PhD, senior mechatronics design engineer at Chas A Blatchford & Sons, headquartered in Basingstoke, England.

It is clear that weight is an important characteristic to consider during the evaluation of prosthetic components, among many other factors. Prosthetists should consider multiple factors, including vocational and recreational activities, health of the residual limb, cosmesis, and gadget tolerance, and not automatically discount a component due solely to its mass characteristics. There is little evidence to support considering device weight disproportionately over other design factors, especially in the case of higher-end components.

Sarah Mattes, member of the Academy Gait Society, is a prosthetics resident at Hanger Clinic, Orange, California. She finished her P&O education at California State University, Dominguez Hills (CSUDH), Long Beach, in May 2012, and her master’s degree in biomechanics at Arizona State University, Phoenix, in 1997.

The author would like to thank the following individuals for their help and direction with this article: John T. Brinkmann, MA, CPO, LPO, FAAOP, lead prosthetic instructor with Northwestern University Prosthetics-Orthotics Center, Chicago, Illinois; Mark Muller, MS, CPO, FAAOP, senior instructor with the CSUDH P&O program; Brian Ruhe, PhD, research, gait, and prosthetics instructor with the CSUDH P&O program; and Scott Hornbeak, MBA, CPO, FAAOP, director and clinical instructor with the CSUDH P&O program.

Academy Society Spotlight is a presentation of clinical content by the Societies of the American Academy of Orthotists and Prosthetists in partnership with The O&P EDGE.


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