Introduction

Background

An estimated 1.9 million people in the United States are living with limb loss, a number expected to double by 2050 mostly due to the rising prevalence of diabetes.1, 2 However, fewer than half of amputees ever receive a prescription for a prosthetic device.3, 4 Prescription rates are even lower among older amputees, African Americans, and Americans living in the southern United States4 The management of lower limb amputees with respect to lower limb prostheses (LLPs) is a complicated problem. LLP candidates are a heterogeneous group with distinct needs dependent upon age, etiology of limb loss, level of amputation, comorbidities and health status, postoperative stage, and rehabilitation status. Many LLP options exist, comprising numerous permutations of components, the anatomy they replace, their sophistication, and other attributes, including those pertaining to cosmesis and comfort. In addition, patients may require multiple LLPs (initial, preparatory, definitive, or replacement prostheses, or those for specific types of activities). Compared to the general population, LLP patients exhibit lower overall physical and emotional health (e.g., increased risk for cardiovascular disease,8 anxiety, and depression9) and higher mortality (estimated 5-year mortality rates for amputees range between 5010 and 74 percent11; estimated 1-year mortality is 36% for amputees >65 years old12). However, in a study of Medicare beneficiaries, amputees who received an LLP were significantly more likely to remain in the home and less likely to have an emergency room admission.13

The most common cause of major lower limb loss among adults is dysvascular disease, primarily due to diabetes and peripheral artery disease, accounting for about 81 percent of lower limb amputees.2 Trauma accounts for about 17 percent of major lower limb amputation. Cancer is a relatively uncommon cause of lower limb amputation in adults (2%). However, individuals who lost their limb from etiologies other than dysvascular disease are disproportionately represented in the total limb loss community.2 This is likely due to the relatively high mortality rate among those with dysvascular conditions. About two-thirds or all amputees are men; although among older adults (≥65 years), 46 percent are women. Dysvascular disease is a more common amputation etiology among older than younger adults. Amputation etiology has an important impact on patient survival and functional ability. Among Medicare recipients, about the same percentage of lower limb amputees have transfemoral as transtibial amputations.14

The current standard approach for matching patients to prostheses relies heavily on performance-based assessments, self-assessments, and, in some instances, wearable monitoring technologies that record patient activity;5 although prosthetists and treating clinicians often rely on clinical judgment to match patients to prostheses. Insurance coverage policies often dictate which prostheses and components are selected for a given patient. Numerous instruments exist to assess the patient functional status, but no consensus “gold standard” assessment schema exists. Similarly, numerous instruments (or techniques) are used to assess current amputee function or status and tools have been developed to predict future outcomes, including successful use of LLPs. Constructs of reliability (e.g., test-retest, interrater, internal consistency) or validity (e.g., face, content, construct, criterion) of existing outcome measurement tools (OMTs), assessment techniques, and prediction tools have been evaluated in the amputee population for the most frequently used instruments.15 However, it is unclear to what degree studies with functional and patient-centered outcomes use validated instruments and outcomes. It is also unclear whether the population of amputees included in validation (etc.) studies is generalizable to the population of participants in studies of LLP components and, in turn, whether these study populations are applicable to the more general population of users of LLPs.

LLPs replace the functionality of a missing limb, ideally, to as great a degree as possible. Medicare covers custom fabricated LLPs in accordance with Local Coverage Determination (LCD): Lower Limb Prostheses (L33787).16 As for all items to be covered by Medicare, it must: (1) be eligible for a defined Medicare benefit category, (2) be reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member, and (3) meet all other applicable Medicare statutory and regulatory requirements. An LLP is covered when the beneficiary: (1) will reach or maintain a defined functional state within a reasonable period of time; and (2) is motivated to ambulate. Potential functional ability is based on the reasonable expectations of the prosthetist and treating physician, considering factors including, but not limited to, the beneficiary’s past medical history, the beneficiary’s current overall health condition including the status of the residual limb, and the nature of other medical problems. Some prosthesis components are limited to beneficiaries with a functional ability at or above a certain level.

As indicated by Medicare coverage guidance,16 clinical assessments of beneficiary rehabilitation potential must be based on the classification levels listed in . The Medicare Functional Classification Level (MFCL or K level) system broadly defines five classification levels that can be attained with an LLP and range from 0 (no ability or potential to ambulate or transfer; LLP will not enhance quality of life or mobility) to 4 (ability or potential to exceed basic ambulation skills). The classification level assigned is used to determine the potential value of certain componentry, and thus to match the most appropriate LLP to fit the beneficiary’s clinical needs.

Table 1

Lower limb extremity prosthesis Medicare Functional Classification Levels (K levels).

In practice it is difficult for clinicians to determine the most appropriate component for a given patient (whether of higher or lower level or sophistication). Determination of a patient’s potential functional abilities requires an assessment of current condition and ability and potential to ambulate. In practice, therefore, OMTs must both assess and predict function to help guide prosthetists, treating physicians, and beneficiaries. However, it is unclear to what extent measures of current function and status are able to predict future function.

A major methodological challenge in addressing selection of OMTs for routine use pertains to the assessment of predictive validity. Predictive tests should be valued with respect to their ability to predict future important outcomes. However, outcomes are determined by the whole patient management strategy which involves the baseline assessment, the LLP that a patient is given based on this assessment, patient health and changes in patient health, and any additional care (e.g., physical therapy, rehabilitation) that the patient receives. Thus, it is inherently challenging to assess the value of a baseline OMT assessment by itself, particularly if the choice of LLP is influenced by the initial OMT assessment.

Variability and subjectivity in assigning or predicting the K level of prospective LLP recipients may inadvertently lead to inefficient or inappropriate LLP matching.17 This can occur if a person receives an LLP allowed for lower K levels when an LLP allowed only for higher K levels would enable better function, or if a person receives an LLP approved for higher K levels, which might be unnecessarily complex for an individual who would have equivalent or better function with a simpler component.

Options for configuring LLPs are abundant, as LLP are highly customized devices, comprising combinations of components that replace missing anatomy and function. Components of a given type can differ in terms of functional sophistication (e.g., articulated componentry may be passive, with undamped movement, have mechanical or hydraulic dampening, or have electronic control), materials used, weight, aesthetics, comfort, and other factors. A major question is how to match patients with LLPs (both by K levels as well as by other characteristics) to optimize functional and other patient-centered outcomes. Because there are many different patients and many possible LLPs, there are numerous possible matchings. However, it is unclear which patient-level characteristics or LLP-level attributes predict a good matching, or how to weigh patient functional potential against their current functional level in the matching process.

The major contextual challenges in providing data to inform matching of LLP components to patients pertain to the large heterogeneity in patient characteristics and attributes of the LLPs; the lack of data on patient characteristics and LLP attributes that are important to best match a patient to a specific LLP; disagreements about what constitutes an optimal matching of patients with LLPs; and poor clinical outcomes and wasted resources associated with suboptimal LLP matching. Specifically, patients who are in need of LLPs are heterogeneous in terms of etiology of limb loss, amputation type (level of amputation, uni- or bilateral), age, comorbidities, frailty, general health status factors, expected life span, mental health status (e.g., depression, posttraumatic stress syndrome), family and social support, and many other factors, including whether they have fragile skin or allergies towards socket liners or other materials. These factors may affect their actual and perceived current and maximum attainable functional ability, and the likelihood that they will receive and use an LLP.8,16