Most Prosthetists believe that when it comes to prosthetics, lighter is better. This stems from the thinking that energy spent during walking is concentrated on the limbs. Thus, the heavier the prosthesis, the more energy is used while walking or running. This is supported by various studies.
For instance, a research published in the Prosthetics Orthotics International Journal shows that lower-leg amputees’ heart rates and metabolic costs are 16% higher, and the pace, 11% slower, compared to those with intact limbs. Metabolic cost is the amount of energy, normally measured in calories, used in performing a physical task. The amputees’ slower pace was linked to shorter time their prosthetic feet spent on the ground and longer swing phase as they walked.
Amputees tend to walk more slowly and take shorter steps to achieve evenly timed and spaced steps between the prosthetic foot and the sound side foot. When an amputee controls his walking pace while using a prosthesis, another study, “Biomechanics of below-knee amputee gait” by Winter and Sienko, claims that the increase in energy used is concentrated on the hamstrings as the hip extends to stabilize the knee before the prosthetic leg swings during walking.
It is no surprise then, that manufacturers produce conventional prosthetics that weigh less than the original limb.
However, the studies that support this thinking are limited, and offer no irrefutable proof that lighter prosthetics are better for you.
In particular, despite assumptions on the negative effects of heavier prosthesis, there is still no sufficient evidence that conclusively shows the adverse effect of prosthesis weight on the energy consumed by an amputee while walking.
In a study conducted by Gailey, Wenger, Raya, Kirk, Erbs, Sypropoulos and Nash published in Prosthetics and Orthotics International, they found no effect caused by an additional 2.7-kg weight in the prosthesis to energy consumed while walking. In another study presented during the 8th World Congress of the International Society for Prosthetics and Orthotics in Melbourne, the researchers observed that an additional weight of less than 1.125 kgs on the prosthesis had no effect on the energy consumed during walking.
On the other hand, some studies say otherwise - that the increase in weight causes the prosthetic user to use up more energy, and even result in an abnormal gait. Particularly Matte, Martin & Royer’s study published in Archives of Physical Medicine and Rehabilitation, Smith and Martin’s research published in the Journal of Applied Biomechanics in 2000, and the study made by Lehman et. al., also published in the Journal of Applied Biomechanics in 1998.
The second study also says that amputees use up more energy as the added weight is placed further down the prosthetic leg - specifically in the ankle, where the weight is concentrated in powered prosthetic feet. Based on these results, the researchers recommend that amputees avoid using heavier prosthetic limbs, particularly those with additional weight in the ankles.
Heavier might be better
There are currently only a few manufacturers that offer microprocessor controlled prosthetic components, and they weigh between 50 to 100% heavier than the more conventional passive energy-storing feet. Two models, Ossur’s PROPIO FOOT and Endolite’s Elan Foot are between 1 to 1.5 kgs heavier.
Currently, most microprocessor feet do not contain “actuators”, a mechanism to make the ankle plantarflex. Plantarflexing is a movement of the ankle that points the foot away from the leg, such as when you tiptoe. This movement is necessary in pushing the leg into the swing phase of walking. Despite this limitation, these models are already capable of increasing and decreasing the rate of compression depending on what’s needed at any phase in a prosthetic user's stance. This and the rest of the features available in microprocessor feet have garnered a number of positive feedback and research outcomes.
One of them is the alleviated stress on internal socket, which protects users from pressure-related injury. This was noted in a study titled “Outdoor dynamic subject-specific evaluation of internal stresses in the residual limb: Hydraulic energy-stored prosthetic foot compared to conventional energy-stored prosthetic feet.”
Another observed benefit is that they allow users more flexibility in controlling their walking speeds, and provide smoother weight transfer to the prosthetic limb. This was evident in the study, “Attenuation of centre-of-pressure trajectory fluctuations under the prosthetic foot when using an articulating hydraulic ankle attachment compared to fixed attachment”, published in Clinical Biomechanics.
According to a study published in the Journal of Rehabilitation Research and Development titled “Comparison between microprocessor-controlled ankle/foot and conventional prosthetic feet during stair negotiation in people with unilateral transtibial amputation”, related benefits were also observed when users walk up and down ramps and stairs using these types of feet, particularly higher symmetry between the sound side and amputated limbs, which makes users less prone to falling.
Aside from benefits of using microprocessor-powered prosthetics, researchers also observed advantages for the sound side leg, such as less stress and exertion on the knees, hips and spine, as observed in “Walking speed related joint kinetic alterations in trans-tibial amputees: Impact of hydraulic 'ankle' damping”, a study published in Journal of NeuroEngineering and Rehabilitation.
As plantarflexing involves tiptoeing, dorsiflexing is the opposite movement - when the feet point up towards the shin. As mentioned earlier, microprocessor-powered feet cannot plantarflex, but are capable of active dorsiflexion during swing phase of walking. According to a study published in Prosthetics and Orthotics International, this capability allows greater ground clearance for the user and makes the prosthesis safer for use among amputees with fall risks, such as those recovering from trauma, or those with chronic illnesses.
However, research supporting the use of these prosthetic components and their effect on energy cost is not yet available. Endolite, a prosthetic brand with a microprocessor-powered foot, reported a result of a self-funded study that shows its hydraulically assistive foot provides 8.5% less effort for amputees’ ability to walk, but the study is still pending publication by a peer-reviewed journal.
The more human-like foot you have, the heavier
There is currently one brand of prosthetic feet that has an actuator and thus can mimic the movement of the calf and an intact ankle. This is a brand called BiOM, created by Hugh Herr, PhD, which has a traditional j-shaped foot made of carbon at the base, and weighs a whopping 4.4lbs.
As we discussed earlier, the assumption is that because this model is heavier, it causes the amputee to use up more energy while walking. However, a study done by Herr and Grabowski published in Proceedings of the Royal Society B: Biological Sciences in 2012 showed that the energy spent by users of this particular foot were 8-12% lower, and that the need for energy commensurately lowered, the faster the user walked. The researchers also observed that the speed with which the users walked were identical to those with intact limbs. What’s more, the gait became more even! This is evidence that the actuator-powered ankle releases more energy than what is spent by its increased weight.
The goal of Herr and his team is for the energy consumed by amputees during walking and other walking movements to be lower than those with intact limbs. The idea is for the prosthesis’ battery and motor to replace some of the energy normally provided by the prosthetic user.
Prosthetic weight should not be the only consideration
Interestingly, most studies related to the weight of prosthetics and their effect on energy did not specify what method of suspension the prosthetics used. This is important because in a study made by Board, Street and Caspers published in Prosthetics and Orthotics International in 2001, the users’ gait became more even when they were using prosthetics with active vacuum suspension, rather than passive suction suspension. The researchers claim that the amputated limb fits better in a socket with higher vacuum, and gives the user a more secure feel, enabling him to shift more power onto the prosthesis.
Still another point to consider is that users don’t necessarily prefer what’s lighter. In measuring the satisfaction users have on their prosthesis during a study made by the US Department of Veteran Affairs, it seems that the issue lies not in whether the prosthesis is heavier or lighter. Rather, it’s mostly about what’s the right weight. Researchers noted that the appreciation of the prosthesis’ weight tends to be subjective. When asked for preference, users did not always choose “as light as possible”. In fact, requests for additional weight was common.
It's becoming evident that weight should not be the sole consideration for choosing a prosthetic component. There are a host of other factors that we need to consider, such as the prosthetic user’s level of activity, the residual limb shape and size, the user’s value for aesthetics, cost considerations, and the level of tolerance for certain mechanisms and technologies, among others. Certainly, there is little evidence that shows prosthetic weight and mass as the only important factors. Numerous studies support the use of powered ankles, showing us that heavier might not be bad at all. In fact, it might even be irrevelant.
Powered ankles are not for everyone. Choosing what prosthetic components are right for you is a concerted effort among your medical team, your Prosthetist, and you. Depending on your need, it is worthwhile to check options that combine some of these technologies with the more traditional components, to balance the benefits with the disadvantages, so you can end up with the prosthesis that is most appropriate and best suited for you.