June 2020

A Multifactorial Perspective on Lower-Extremity Amputee Rehabilitation

Limb loss alters pain, touch, and proprioception in countless ways that are not yet fully understood. Understanding how to more realistically evaluate the efficacy of both proactive and reactive interventions meant to address these alterations will improve outcomes and quality of life for those living with lower-extremity amputation.

Figure 1. Assessing sensitivity to ground slope. On each trial, the participant steps from a flat platform to a ramp whose slope is manipulated by the experimenter. The participant then reports whether she perceived the ramp to be sloped or not. Over a series of trials, this paradigm can be used to determine the ramp slope that each participant can reliably identify as sloped.

By Austin Davids, MSOP, CPO, and Jeffrey Yau, PhD 

People who suffer lower-extremity amputations contend with a host of sensorimotor complications as they attempt to regain function and mobility. Protective sensation may be irrevocably lost depending on the etiology of the amputation. Most amputees experience pain and phantom sensations. Prosthetic comfort may be complicated in some patients due to issues with their residual limb; prosthesis users must learn to ambulate with reduced and unfamiliar patterns of sensory input. Many complications related to limb loss can be addressed by both proactive and reactive interventions, including surgery, pharmacological pain management, therapeutic strategies, and prosthetic design choices. Emerging technologies, such as advanced neuroprostheses, are also poised to reshape the outcome landscape for this population. Here, we briefly summarize what is currently known of how pain, touch, and proprioception are altered by limb loss and the interventions available to address these complications. We conclude by offering a multifactorial perspective that integrates these components and highlights the critical need to additionally emphasize active sensing and haptics when considering outcomes in lower-extremity limb loss.

Residual and Phantom Limb Pain

No matter the cause, an individual who undergoes lower-extremity amputation will experience significant pain that requires management. Amputation-related pain can be divided into two general categories: residual limb pain and phantom limb pain. Residual limb pain refers to pain that is localized at, or proximal to, the amputation level of the affected limb. This includes pre-surgical pain, acute post-surgical pain, and chronic pain from complications associated with adhered scar tissue, neuromas, and heterotopic ossification, among other conditions. Phantom pain is pain that is localized distal to the amputation level of the affected limb. Phantom pain is well-documented in lower-extremity amputees and the neurobiological origin of these painful experiences—which involve signaling in the peripheral and central nervous system—remain the focus of a number of research efforts. For most individuals, even without treatment, the severity and frequency of phantom pain decreases with time; however, such pain can persist in some individuals with devastating effects on quality of life.

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Numerous strategies have been implemented to manage amputation-related pain. Proactive methods, such as the administration of multimodal analgesia regimens that simultaneously target multiple pain pathways, are implemented perioperatively. Such proactive methods can reduce the duration and severity of amputation-related pain, which implies that blunting pain signaling before and during surgery serves as management for both short-term and long-term pain outcomes. Additionally, the specific approaches used during surgery can impact long-term pain experiences; surgical techniques such as targeted muscle reinnervation and targeted sensory reinnervation can reduce the likelihood of neuroma formation and phantom pain complications.

Reactive methods are used both acutely and chronically following amputation surgery, with some being more appropriate than others, depending on the cause and severity of the pain. The case has been made for using mirror therapy, transcutaneous electrical nerve stimulation, and percutaneous electrical nerve stimulation to treat amputation-related pain and substantial research efforts are currently aimed at demonstrating the efficacy of these and similar interventions. Non-primary targeted muscle innervation, performed as a revision surgery, can be effective for reducing neuroma-related pain. Importantly, prosthetic design choices can substantially influence pain outcomes. Indeed, pain can be minimized with a well-fitting and comfortable prosthesis, designed with judicious use of gel liners, localized low-durometer reliefs, and volume adjustment systems (eg, RevoFit). Additive manufacturing and other advanced materials, such as silicones and flexible resins, give prosthetists additional tools with which they can design comfortable interfaces for their patients.

As long as the cause of the pain can be correctly identified, in most cases there will be options to manage that pain and maximize the patient’s functionality; proactive and reactive methods offer a panoply of pain management approaches.

Effects of Limb Loss on Touch

Limb loss can be associated with dramatic changes in how touch is experienced on the residual limb. Unfortunately, how limb loss affects touch can be inconsistent as there are reports of increases or decreases in tactile sensitivity and complications can arise from either. With increased sensitivity, it may be difficult to achieve acceptable socket comfort, and this can limit the individual’s prosthesis use and functionality. With reduced sensitivity, the individual may not reliably report discomfort from their prosthesis. This increases the risk of residual limb ulceration, which has potentially serious consequences. Additionally, impaired touch on the affected lower-extremity and on the intact limb can contribute to poor balance in quiet standing and frequent falls. Changes in touch, which include non-painful phantom limb experiences, may be associated with altered neural activity throughout the afferent nerves, the spinal cord, subcortical centers, or the brain. In some instances, changes in touch may not be due to limb loss, per se, but they may instead be a consequence of other conditions, such as diabetic neuropathy, that impact the health and function of sensory nerves.

Currently, there are relatively limited options for treating aberrant tactile experiences related to limb loss. Hypersensitivity on the residual limb can be reduced, in part, through desensitization exercises. Careful and deliberate prosthetic interface design can also help minimize discomfort. In cases where the sense of touch has been reduced or lost, targeted sensory reinnervation surgery can lead to near-normal skin sensitivity on the residual limb, with said sensations referred to the patient’s missing extremity. Crucially, whether and how prosthetic interface design and cutaneous sensing impact a prosthesis user’s overall functionality is unknown at this time. This remains a critical knowledge gap and research efforts need to establish a consensus on how best to use the intact and available skin regions after limb loss.

Importantly, prosthetic design choices can substantially influence pain outcomes. Indeed, pain can be minimized with a well-fitting and comfortable prosthesis.

Effects of Limb Loss on Proprioception

Proprioception is the ability of a person to sense the position and posture of his or her limbs in space. This sense is critical for moving and balancing. Much proprioceptive signaling comes from specialized receptors in the muscles and tendons. The loss of these due to amputation understandably results in an impaired ability to perceive the location or posture of one’s limb. Interestingly, proprioceptive deficits may be more evident when individuals respond to passive movements of their limbs as compared to when they move their limbs to produce specific postures. This implies that individuals are able to exploit multiple cues, including signals related to motor control, to perceive their limb state. Because less is known regarding the processing of proprioception compared to pain and touch, even in able-bodied individuals, strategies for treating amputation-related proprioceptive deficits are still being developed and evaluated. Surgical procedures such as targeted muscle/sensory reinnervation and osseointegration have the potential to proactively improve proprioception in lower- extremity amputees. Myoneural interfaces that leverage agonist-antagonist muscle relationships have also been shown to be effective in improving proprioception. Interface design and fit may also contribute substantially to proprioception. Importantly, physical therapists can train lower-extremity prosthesis users on how to interpret proprioceptive feedback to maximize their safety and functionality.

Adopting a Multifactorial Perspective That Emphasizes Functionality

Figure 2. Slope perception with sound and prosthetic limbs. (Left) Participants with 2 sound limbs had similar slope perception thresholds with their dominant and non-dominant legs. Higher thresholds indicate reduced sensitivity to differences in ramp slope. (Right) Participants with limb loss were able to perceive ramp slope using their prosthetic limb, albeit with higher thresholds compared to their sound limbs.

Lower-extremity limb loss has devastating effects on an individual’s functionality and quality of life. In the proceeding sections, we have offered a brief summary of how limb loss impacts the experience of pain, touch, and proprioception. We have also reviewed proactive and reactive strategies for mitigating the deleterious effects related to amputation in these domains. While a reductionist perspective, which focuses on pain, touch, and proprioception individually, can be helpful and clarifying, we propose the adoption of a perspective that incorporates multiple factors in evaluating the experiences and rehabilitation of lower-extremity amputees. Moreover, we also suggest that research efforts address how individuals with limb loss experience their environment in addition to how they perceive their body. Our preliminary work on the perception of ground slope offers an illustrative example. Volunteers were asked to perform a simple behavioral task requiring them to step on a ramped platform, the slope of which could be adjusted from flat to steep (Figure 1). Each time the participant walked over the platform, they indicated whether they felt that the ramp was sloped downhill. By repeating this process over many different slopes, we were able to estimate how sensitive individuals were to ground slope differences and how this sensitivity differed between legs (Figure 2). Our preliminary results indicated unilateral transtibial prosthesis users had impaired slope sensitivity on their prosthetic side compared to their sound side. In contrast, control participants had similar sensitivities on their dominant and non-dominant legs. Importantly, beyond simply confirming the presence of impaired ground slope perception, our method allows one to quantify the degree of functional impairment in a more behaviorally relevant context. Conceivably, this more precise assessment may be helpful in evaluating the efficacy of the proactive and reactive interventions described above.

In conclusion, while it has been useful to evaluate the impact of lower-limb loss on how amputees experience their body, we recommend that these efforts be complemented by efforts to gauge how limb loss impacts more naturalistic behaviors. How the effects of limb loss on pain, touch, and proprioception relate to active behaviors, such as walking and ground slope perception, is an open question. Answering these questions will hopefully inform the development and refinement of rehabilitation strategies for improving the function and quality of life for lower-extremity amputees.

Austin Davids, MSOP, CPO, is a certified prosthetist-orthotist with Hanger, Inc., in Washington, DC. Jeffrey Yau, PhD, is an Assistant Professor of Neuroscience at Baylor College of Medicine in Houston, TX.

This work is based on a poster by the same authors (“A Novel Method to Quantify the Haptic Perception of Slope”) that was presented at the 45th Annual Meeting & Scientific Symposium of the American Academy of Orthotists and Prosthetists held in Orlando, Florida, in early March 2019.

ARTICLES OF INTEREST

Pain

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Targeted Muscle Reinnervation/Targeted Sensory Reinnervation

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Osseointegration

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Measuring Proprioception

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AMI

Clites TR, Carty MJ, Ullauri JB, et al. Proprioception for a neurally controlled lower-extremity prosthesis. Sci Transl Med. 2018;10(443):eaap8373.

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