A prosthesis is a valuable artificial human organ, usually prescribed due to an amputation or congenital deficiency of a limb. Prostheses are as old as recorded amputations, making them very early examples of medical technology that humankind has valued with sufficient worth to create. Prostheses are referenced in Sanskrit literature, Greek mythology, Roman texts, and Viking sagas. They are illustrated on Incan pottery.1 Functional examples of limb prostheses have been discovered in ancient Egyptian tombs. This demonstrates how persons throughout history have valued and sought the replacement of the amputated limb as a universal and fundamental aspiration. There are many reasons a person with limb loss is motivated to use a prosthesis. These include restoration of physical functions such as ambulation, balance or grasp, for vocational needs, or restoration of body image and symmetry.2 A primary goal of the prosthetist is to ascertain the scope of these needs and the achievable goals of the individual with limb loss, and then utilize the growing selection of available prosthetic technologies and techniques to fulfill those needs and goals to the greatest extent possible. These basic motivations have been recorded in some form for millennia; it is the specialization of care and the technological options we have for treatment that have evolved.
Amputation is a very treatable, but otherwise a severely debilitating, chronic condition. Using a prosthesis is an effective treatment that offers the apparent and substantive benefits of standing and walking to the user. In recent years, increasing use and sophistication of technology has, in many cases, increased the associated costs, putting pressure on health care systems to understand the value of treatment provided. Beyond the demonstrated historical human desire for the prosthetic limb, what economic benefits might ensue from having the prosthesis? In this analysis, we consider what may be the most obvious benefit associated with the provision of a leg prosthesis, specifically the value of being able to ambulate in a bipedal manner. With this specific case in mind, the word “prosthesis” will be confined in the following text to mean an external prosthesis of the leg.
Too often, the provision of new prosthesis technology is a debate about only the cost of the prosthetic technology over that of a previous device, and not the value it provides as a medical treatment. This may be in part because the amputation is often a life-saving event and mortality has inherently been staved off in the short term. As a result, the prosthesis itself may not be seen through the same lens as a life-saving medical device. However, for the prosthesis user, his or her replacement limb can clearly be a profound life-restoring medical device. To assess the economic value of prostheses, like any medical device, it is important to select appropriate measures of effectiveness to gauge their value. No prosthesis, at no cost and offering no function, is only a cost minimization strategy and is anathema to the ultimate goal of rehabilitation medicine, namely, restoration. The value of a prosthesis is multifactorial, as are the health outcomes that may be calculated, such as quality of life, satisfaction, gait efficiency, comfort, and burden of disease. However, all of these can be related back to mobility as the core purpose of the applied prosthetic technology.
QUALITY OF MOBILITY AND QUALITY OF LIFE
Mobility and function, both of great importance to the person with limb loss, can be restored through the provision of a prosthesis and gait retraining.3 Consider the range of possibilities in treatment for an individual following amputation. At one extreme, no prosthesis is provided. One-legged hopping is possible, but is energy inefficient, unsafe, and likely injurious to the joints and ligaments in the long term. In such a case, the person may be provided with alternative mobility aids such as bilateral crutches or a wheelchair to get out of bed. However, both of these interventions have their own costs and lack the benefits of being bipedal. Crutches can provide three-legged mobility, but the use of crutches is disabling in itself, as both hands and arms are occupied with holding the crutches during weight bearing and balance tasks. A wheelchair is metabolically efficient but also leads to difficulty traversing barriers such as stairs, doorways, and curbs, and is also limiting and often results in a more sedentary life.
On the other end of the spectrum, provision of a prosthetic limb with sophisticated technology can quickly return a person’s functional gait capabilities. The Paralympian approaching 10-second times in the 100-meter race or the runner with amputation besting those with two healthy legs in a marathon are striking examples of this potential return of function. Whatever intervention is chosen, the user will be dealing with the ramifications for a time horizon that extends to the end of his or her life, impacting health, wealth, participation in society, and personal satisfaction.
Mobility is an important aspect of quality of life for the prosthesis user. In a study that looked at premorbid activity and the relationship with postamputation activity, researchers were able to control for the many potential confounding comorbidities in the population with limb loss. They found that satisfaction with mobility correlated strongly with satisfaction with life. Higher mobility function with a prosthesis was associated with overall improved quality of life. Even in a cohort with significant other health issues beyond amputation that limited physical activity, 50% who would achieve low to moderate mobility function after 12 months were satisfied with their level of mobility.3
AVOIDANCE OF DIRECTLY ATTRIBUTABLE NEGATIVE OUTCOMES
An argument can and has been made for negative outcome avoidance by not providing a prosthesis. This specious argument flows that a person with limb loss who does not have a prosthesis is not liable to be walking as much and is not going to have as many chances for falling. By reducing the likelihood of falls, additional costs from emergency department visits and hip replacement surgery are avoided. Let’s extend that argument. If you could keep the patient in bed, without having to transfer at all, that would be even better from a falls injury perspective. This is akin to saying the cheapest car to insure would be one where the insurance company requires that you not put wheels on it and keep it on blocks locked in a secure garage. Clearly, this car is of little day-to-day value for the owner. We should see the value of the prosthesis just as clearly. It is in the function that it affords the user, namely, mobility, that its intrinsic value is derived.
However, this example does provide us with important guidance pertaining to our understanding the value of improving prostheses and the technologies that they incorporate. Seen in this light, a prosthesis can itself be a factor in health care cost avoidance by providing mobility with safety by design. For instance, using a systematic review of the body of literature on prosthetics, the RAND Corporation issued a seminal research report in 2017 that concluded that technology can not only return mobility to a prosthesis user, but that advanced technology can be a cost-effective alternative to cheaper options.4 RAND’s analysis underscored the substantially increased risks of falls and osteoarthritis in the contralateral limb for patients with nonmicroprocessor knee (NMPK) technology, demonstrating that advanced computer-controlled microprocessor knees (MPKs) are safer for patients. Cost-effectiveness ratios were favorable for using the more advanced technology because of decreased serious health issues including falls and deaths in patients using the advanced technology. Over a 10-year time horizon, compared with NMPKs, MPKs increase quality-adjusted life years (QALYs) by 0.91 per person for additional costs of $10,604. MPKs have an incremental cost-effectiveness ratio of $11,606 per QALY, making them an extremely cost-effective technological enhancement for individuals with limb loss. They performed a sensitivity analysis that indicated, “MPKs actually cost less than NMPKs due to reduction in health care costs and indirect costs; even in the worst case, MPKs cost an additional $36,357 for every QALY gained, which is still well below the $50,000 threshold.”
BURDEN OF DISEASE WITH LIMB LOSS
There is a broad health economics focus on QALY, and with that comes a systematic bias toward life-saving interventions that directly change the rate and timing of death. Mortality is easily and reliably measured, and it can dramatically and positively impact the denominator of the QALY measure. Although these are important considerations, this bias devalues long-term interventions that may or may not help you live longer, but may significantly decrease the burden of disease (BoD) on the individual. BoD is more subtle and difficult to quantify, but for one living with chronic disability for years or decades, it becomes paramount. This realization has led to development of health econometrics that attempt to better incorporate the BoD concept within interpretation of health outcomes.5
One measure developed to augment the QALY is the disability-adjusted life year (DALY). The construct of the DALY was formalized in the 1990s to elicit the total BoD as subtracting from perfect health life years. One DALY is equivalent to one lost year of healthy life. In essence, it is the inverse of a QALY. The sum of these DALYs across the population, or the burden of disease, can be considered the difference between current health status and ideal health of a normal life span free of disease and disability. A disability weighting factor is used to correct the DALY measure for the severity of limitation that the specific disability creates while an individual is living with the condition. Until heterogeneous limb grafts or autologous regeneration become commonplace, amputation is a chronic condition and the primary medical treatment is a prosthesis. The burden of disease for a chronic condition has a significant negative economic impact for patients, families, and health care systems, especially since. According to the Global Burden of Disease study, there is a global trend toward more years lived with disability. The accepted disability weighting for a lower-limb amputation is 0.3.5 It can be interpreted as a 30% reduction in “healthy life years” for every year lived with limb loss—the burden of amputation.
Conversely, the dramatic reduction of the burden of amputation associated with a prosthesis can have a large positive impact on economic value. As an example, if one uses the standard criterion of $50,000 to $150,000 of value per QALY gained as good value for the money spent on a health care intervention,6 a completely perfect prosthesis, completely removing the burden of disease of living with an amputation, would be worth $15,000 to $45,000 per year for the life of a person with limb loss. Although no prosthesis in existence succeeds at perfectly removing the burden of disease, it is the aspiration of researchers, engineers, and businesspersons who are continuously reinvesting in the development of improved prosthesis technologies.
A descriptive epidemiological study calculated DALY for all 1,183 cases of amputation surgeries pursuant to complications from diabetes mellitus that occurred in public hospitals in a state of Brazil over a 5-year period. The research included amputations due to all types of diabetes mellitus, unilateral or bilateral lower-limb amputations in both sexes, for all levels of amputation, and in all age brackets. The findings showed that some 80% of the amputations were at the transtibial level. More than 8,475 DALYs were recorded and importantly, it was found that disability accounted for 93%, and mortality for 7.5% of the DALY calculated in this study (Figure 1).7 This finding highlights how a discussion of the prosthesis intervention would differ greatly whether or not an analysis adequately considers the burden of disease due to amputation.
If the calculated economic value of the prosthesis is driven by the condition of the individual living with amputation rather than his or her eventual mortality, the primary economic goal would naturally be to decrease the burden of disease by providing the most capable prosthesis within the accepted value for money as this would have the greatest economic impact. From a health economics perspective, the goal of prosthetic rehabilitation should not be to provide the mobility aid that provides minimal function at lowest cost, but rather a prosthesis that most reduces the burden of disease (by enabling as near normal ambulation as possible) within an acceptable value for money in line with other health technologies.
VALUING HEALTH BENEFIT OF PHYSICAL ACTIVITY WITH PROSTHESIS
Health care costs due to inactivity are substantial. Annual costs to society of physical inactivity in the United States has been reported to be as much as $6,049 (2010 US dollars) per inactive person.8 However, physical activity has beneficial effects on 23 diseases or health conditions.9 There is no reason to suspect that this is any less true for a person who has had an amputation. The preeminent clinical goal in treatment with a prosthesis is to provide for maximum useful mobility to the person with limb loss. One positive health effect from mobility with a prosthesis is the potential reduction in what would otherwise be a virtually assured sedentary lifestyle.
It has been noted that a sedentary individual who takes up exercise twice a week will likely have bigger health gains when compared with an individual that increased exercise behavior from twice to four times a week.10 Given the obvious increased difficulty for persons with limb loss to be physically active without a prosthesis, looking at the impact of sedentary lifestyles on future health is an important way to analyze the impact of having a maximally functional (technologically advanced?) prosthesis to use.
Mobility encourages and enables physical activity with concomitant health benefits. The World Health Organization (WHO) estimates that physical inactivity causes a total of 1.9 million deaths and the loss of 19 million DALYs annually. Being inactive is a top 10 risk factor of premature death in developed nations. Further, inactivity is estimated to be a direct contributing cause to 10% to 22% of all cases of metabolically related diseases such as colorectal cancers, diabetes mellitus, and ischemic heart disease.1 According to a Swedish assessment of the world’s most developed economies, physical inactivity is estimated to cause 12% of all mortality, 8% of all lost years as a result of premature death, 2% of lost years as a result of morbidity, and 5% of the overall burden of disease.11
Clearly, on a population basis, promoting physical activity can be considered as a positive public health intervention and providing two legs to walk on instead of one or none must certainly fall into this category. There is arguably more impact for the subpopulation of persons with limb loss than there is for the general population since a large percentage of amputations are due to metabolic disease such as diabetes mellitus and cardiovascular disease.
In a meta-analysis study of 91 interventions promoting physical activity without regard to the target population, researchers established a relationship of metabolic equivalent (MET) hours gained to the economic costs of means of promoting physical activity. Physical activity benefits were considered significant if they provided at least the US guideline–recommended levels of an additional 1.5 MET-hours per day for adults (equivalent to being able to do light housecleaning for 30 minutes).12 Although not specifically referencing prosthesis users, one could reasonably posit that the reason for providing a prosthesis as an intervention after amputation is to enable greater physical activity.13 Further, the type and amount of additional activity from use of a prosthesis is consistent with the MET per hour gains desired (Table 1).
Formal cost-effectiveness ratios were then calculated as cost per MET-hour gained per day basis per individual reached. This helped to establish a benchmark economic cost of $1 per MET-hour added as a threshold for interventions to be considered cost-effective.12
To make an estimate for the physical activity value of use of a given prosthesis, one can take the average number of hours per day of ambulation with a prosthesis, times the number of useful days of use for that prosthesis, times the METs achievable with the type of activity that the user might undertake. In the following example (Table 2), empirical data are used that measured how many hours per day an individual with amputation might take complete steps on his or her prosthesis.14 The useful life of the prosthetic technology used is considered to be 3 years since that is the standard by which common components are validated for safety and functionality by regulatory bodies.15 The minimum METs for ambulation is drawn from the third edition of a compendium of METs for different activities in adults.16
Using the previously cited estimate of $1 intervention cost per MET-hour gained, as a threshold to consider an intervention cost-effective, an additive economic value of $13,797 for a transtibial prosthesis is estimated through increased physical activity that helps avoid other negative health consequences and increased care costs.
Amputation is a chronic health condition characterized by lifelong adaptation, meaning that incremental reductions to the disability index are multiplied by years and decades of life. Amputation is also a very treatable condition, and effective treatment offers readily apparent benefits. Coupled with direct health impacts from being more physically active and participatory in community, provision of prostheses can provide large positive economic impacts. Provision of an artificial leg can quickly return much of the individual’s functional capability for a long term. The primary purpose of the lower-limb prosthesis is to provide mobility. Logic follows then that the key impact (returning mobility) should be central to analyses of the value of provision of prosthetic care. The prosthesis is not just a physical commodity purchased to accommodate the disabled person, like a hospital bed. The prosthesis itself is medical treatment for creating mobility, and its value should be measured by the health impact attributable to the intervention.
There are many additional positive impacts on a person’s health that may result from provision of a prosthesis. For example, comfort is a multifaceted requisite for the prosthesis. Precise fit and material selection of the socket, together with stability, dynamic function, and reliability of technology, can all play a part in comfort for the user. This analysis has not taken into account such things as the psychological impact of restored body image on the prosthesis user or the value of physical restoration to the psyche. Cosmesis (natural or unnatural in appearance) may be highly desired if it is a subtle facet of overall function for the person for whom the prosthesis is an outward representation of him or her. Some persons with amputation will even prefer to sacrifice physical function in their prosthesis for improved cosmesis. Loss of a limb is psychologically traumatic, so part of the value of a prosthesis is helping the person after amputation to transition from the immediacy and shock of loss to the forward-looking prospects of prosthetic rehabilitation. These are additional areas of tangible value that merit further study and consideration when assessing the economic value of prostheses.
This article has focused on health economics related to the impact of decreasing the burden of disease as well as other health benefits from enabling persons with limb loss to be physically more active. The intent has not been to provide a definitive answer to a very complex question (What is the value of being mobile?), but rather to bring awareness to and address the inadequacy of evaluating prosthetic technology on cost alone, separated from value.
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