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Outcome Measures 

Outcome Measures
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Outcome Measures
DOI:
10.1093/med/9780198849360.003.0016
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date: 29 November 2020

Summary

  1. 1. Core outcomes for patients with open fractures of the lower limb include:

    • quality-of-life

    • return to life roles

    • walking, gait and mobility

    • pain and discomfort.

  2. 2. Following a consensus process with patients, healthcare professionals and research methodologists the EuroQol-Five Dimensions-5L and the Lower Extremity Functional Scale are recommend to be used in future studies as a minimum.

  3. 3. UK funding bodies expect patient-reported outcome measures to be reported in studies.

What is an outcome measure and an outcome measurement instrument?

Outcome measures are a core component of clinical audit or research and need to encompass information relevant to patients and healthcare professionals. They may measure specific clinical events (e.g. absence or presence of infection) or they may capture a broader domain (e.g. quality of life) to demonstrate effects of an intervention on wider aspects of health (1).

An outcome measure refers to what is measured. It is also referred to as a construct, domain, or concept. In a clinical trial it refers to what is being measured on participants to examine the effect of the intervention (2). An outcome measurement instrument (OMI) refers to how the outcome is measured. It is a tool to measure the quality or quantity of an outcome (2) (e.g. the Oxford Hip Score). For example, when conducting a trial, the investigators may want to collect information on the quality of life of participants, and thus the outcome measure would be health-related quality-of-life, and this could be collected using an OMI such as the EuroQol-Five Dimensions (EQ-5D-5L), Short Form 36 (SF-36), or Sickness Impact Profile (SIP).

Selection of outcome measures that record the impact of an intervention on appropriate domains of health accurately is difficult. The World Health Organization’s International Classification of Functioning, Disability and Health model (ICF) identifies multiple interrelated domains of disability affecting a patient’s recovery. In the context of a severe lower limb injury these can be divided into impairments of body functions and structure, together with activity limitations and restrictions to participation that are influenced by the environment and personal factors (3). A common approach to assessing the multi-dimensional aspects of recovery in patients with open fractures of the lower limb is to use multiple outcome measures, aiming to capture different domains of health (4). Consequently, a battery of OMIs are often combined, including both general quality-of-life questionnaires and disease- or limb-specific instruments (5).

Increasingly, outcome measures should be patient-derived in an endeavour to focus on the relevant domains that enable better patient-centred care. The Core Outcomes in Clinical Effectiveness Trials (COMET) initiative promotes efforts to involve stakeholders, including patients, to reach a consensus on the most important outcomes to collect as a minimum in a specified disease entity. This is called a core outcome set (COS), which identifies what outcomes to measure. A Core Outcome Measurement Instrument Set (COMIS) identifies how to measure the COS, i.e. which OMIs to use. By establishing a consensus over what the most important outcomes are and how to measure them, consistency of outcome reporting across trials and clinical audit will be improved (1). In addition, future studies are more likely to measure appropriate outcomes, maximise the potential to contribute to systematic review and meta-analysis, and reduce selective outcome reporting in the future (1).

Different types of outcome measures

Outcome measures can be divided into different types:

  1. 1. objective clinical measures

  2. 2. patient-reported outcome measures (PROMs)

    1. a. general health and quality of life

    2. b. disease/region-specific

  3. 3. physical performance measures.

Examples of objective clinical measures

Assessment of severity of femoral and tibial shaft fractures:

  1. 1. fracture displacement

  2. 2. fracture comminution

  3. 3. soft tissue injury

  4. 4. energy of injury

  5. 5. mangled extremity scores.

Assessment of adverse events—bone and soft tissue healing complications. Commonly used outcome measures in this population have consisted of:

  1. 1. time to fracture union

  2. 2. malunion

  3. 3. infection

  4. 4. flap failure

  5. 5. secondary amputation.

Past studies have focused on objective clinical measures, but it is increasingly expected that current and future studies will be powered on outcome measures that are proven to be meaningful to patients, e.g. PROMs.

Examples of patient-reported outcome measures

PROMs are questionnaires that elicit responses to measure either patients’ perceptions of their general health or function in relation to specific diseases or conditions (6). Responses to questions are scored to provide a quantitative measure of an assumed underlying domain of health, known as the latent trait, which can be used as the estimated instantaneous status of that trait in that patient.

When investigating a specific condition, such as open lower limb fractures, disease- or region-specific PROMs provide a greater clinical focus than generic health measurement instruments as they are tailored to the symptoms and disability of the condition of interest (7). When selecting PROMs, it is essential to ensure they possess a degree of validity. Important components of validity to consider include (8):

  • Content validity, whether the instrument measures all important domains.

  • Construct validity, whether the instrument measures what it intends to measure.

  • Reliability, the extent to which measurements of individuals are similar when obtained in different conditions.

General health outcome measurement instruments

EuroQol-Five Dimensions questionnaire

EQ-5D-5L is a validated, generalisable, and standardised instrument for measuring generic health status (9). It comprises a five-level scale (no problems, slight problems, moderate problems, severe problems, and unable to perform task or extreme pain/anxiety) measuring across five domains related to daily activities: (i) mobility, (ii) self-care, (iii) usual activities, (iv) pain and discomfort, and (v) anxiety and depression (10). The respondent self-rates their health status by marking on a visual analogue scale (EQ-VAS; 0–100), labelled ‘best imaginable health state’ (100) and ‘worst imaginable health state’ (0 ). The EQ-5D-5L can be used to estimate quality-adjusted life-years (QALYs) as part of a health economic analysis and is used by the National Institute of Clinical Care and Excellence (NICE). It is responsive to change both when self-reported (9) and when proxy-reported for patients with cognitive impairment (11). EQ-5D-5L is one of the most commonly used generic questionnaires to measure health-related quality of life, is short and easy to use (taking about 5 minutes to complete), and is validated for use in a wide range of settings (12).

Medical Outcomes Study Short Form 12-Item

The Short Form-12 (SF-12) was developed as a shorter version of the Short Form-36. It uses the same eight subscales (physical functioning, the physical role, pain, general health, vitality, social function, emotional function, and mental health), which can be clustered into physical and mental component scores (13). It yields two summary scores comprising physical function and mental well-being. It was developed in an effort to reduce respondent burden and has proved to be useful in a variety of settings where a short generic health measure of patient-assessed outcome is required (14). The SF-12 was selected as an outcome measure in the UK Wound Management of Open Lower Limb Fractures (WOLLF) trial (HTA 10/57/20) and was used in combination with survival data to facilitate health economic evaluation (15).

Disabilities Rating Index

The Disabilities Rating Index (DRI) is a self-administered, 12-item questionnaire assessing the patient’s rating of their disability through assessment of activity and participation limitations (16). The DRI is quick to complete, taking <5 minutes. The 12 items are measured using a visual analogue scale (VAS 0–100) with low scores denoting little or no disability. The items are grouped into three distinct sections: basic activities of daily life, daily physical activities, and work-related or more vigorous activities (17). Questionnaire items were chosen to be applicable to disability secondary to pain, impairment of hip or knee function, and to impairment of gross body movements from pain, neurological, and muscle pathologies (16). The DRI was shown to be highly reliable with good validity, discriminating between different diagnostic categories. It met the need for responsiveness with good acceptability, practicality, and a high compliance rate (16). The DRI was selected as the primary outcome measure in the WOLLF study owing to its ability to assess ‘gross body movements’, making it a good choice for assessing patients presenting with a variety of fracture configurations (15).

Sickness Impact Profile

The SIP is a widely used 136-item patient-oriented general health questionnaire that covers domains of physical functioning, psychosocial health, sleeping, and work; it can be self- or interviewer-administered (18, 19). It assesses cognitive, social, emotional, and physical functioning in the performance of many activities of daily life and takes 20–30 minutes to complete (18). The SIP was developed in the 1970s with the intention of providing patient-oriented outcomes beyond the traditional measures of mortality and morbidity across a range of types of illness, severity, and demographic and cultural subgroups (18). The ability of SIP to measure function broadly and be sensitive to less severe outcomes following lower extremity fractures (19) was why it was selected as an outcome measure in the Lower Extremity Assessment Project (LEAP) (20). However, its length may limit practicality for use in routine practice and decrease compliance rates when used for research in the absence of sufficient patient incentive.

Disease- or region-specific outcome measures

A recent systematic review of outcome measurement tools used for leg, ankle, and foot conditions identified 12 distinct OMIs (21). A selection of relevant region-specific OMIs are described in the chapter. The two most validated OMIs identified were the Foot and Ankle Ability Measure (FAAM) and the Lower Extremity Functional Scale (LEFS), represented in two and six studies, respectively (21). Outcome measures described are scored with respect to reliability, content validity, and construct validity in Table 16.1.

Table 16.1 Evidence for frequently used general lower limb and ankle patient-reported outcome measures.

Based on Martin and Irrgang (2007) (25).

Measure

Focus

Reliability

Content validity

Construct validity

MCID

AAOS-FA

Regional

+

+

+

FAAM

Regional

+

+

+

+

FAOS

Regional

+

+

+

LEFS

Regional

+

+

+

+

+ denotes presence of validation studies.

MCID, minimal clinically important difference; AAOS-FA, American Academy of Orthopaedic Surgeons lower limb outcomes assessment instruments Foot and Ankle module; FAAM, Foot and Ankle Ability Measure; FAOS, Foot and Ankle Outcome Score; LEFS, Lower Extremity Functional Scale.

Foot and Ankle Ability Measure

The FAAM was designed as a region-specific OMI to evaluate changes in the self-reported physical function for patients with musculoskeletal disorders of leg, ankle, and foot (22). It contains two subscales, a 21-item assessment for activities of daily living (ADL), and an 8-item sports subscale compiled with the intention of providing information across a spectrum of ability. The items are scored using a Likert scale, where higher scores represent better function (22). The FAAM was validated on 1027 patients receiving treatment for leg, ankle, and foot musculoskeletal disorders referred for physiotherapy (22).

Lower Extremity Functional Scale

The LEFS is a broad region-specific OMI suitable for patients with musculoskeletal disorders of the hip, knee, leg, ankle, or foot (23). The LEFS contains 20 items specifically assessing International Classification of Functioning, Disability, and Health model domains of activity and participation. It too uses Likert scales, where a higher score represents better ability (23). The LEFS was originally validated on 107 subjects presenting predominately with knee disorders (n = 71) but also included other lower limb impairments, including in the leg (n = 8) and thigh (n = 1) (23).

American Academy of Orthopaedic Surgeons lower limb outcomes assessment instruments Foot and Ankle module

The American Academy of Orthopaedic Surgeons (AAOS) developed OMIs designed for patients of all ages with musculoskeletal conditions affecting all body regions (24); for example, the AAOS lower limb outcomes Foot and Ankle module (AAOS-FA) is their region-specific OMI for patients with a foot- and ankle-related diagnosis (25). It consists of a lower limb core scale, a global foot and ankle scale, and a shoe comfort scale. The lower limb core scale contains nine items that assess symptoms and functional status (25).

Foot and Ankle Outcome Score

The Foot and Ankle Outcome Score (FAOS) is a region-specific OMI designed to evaluate symptoms and functional limitations in patients with foot and ankle disorders (26) and is adapted from the Knee Injury and Osteoarthritis Outcome Score (27). The FAOS consists of five subscales: pain (nine items), ADL (17 items), sports and recreational activities (five items), foot- and ankle-related quality of life (four items), and other symptoms (seven items). Each subscale is scored separately using a Likert scale where a higher score represents a better function (25, 26).

Physical performance measures

Physical performance measures (PPMs) are defined as clinician-observed measures of physical function that assess a task (28). PPMs of mobility can quantify the ability of an individual with an open lower limb fracture to perform certain tasks that require movement. PPMs focus on the assessment of the ICF domain classed as ‘activities’ and relate to ‘the ability to move around’ (29) and ‘the ability to perform daily activities’ (30) rather than direct tests of body structure, function, or impairment (28). PPMs are often assessed directly by an observer while the activity is being performed by timing, counting, or distance measurement (28). One such method of assessment is the Enneking Score.

Enneking Score

The Enneking Score is a physician-administered OMI that includes functional assessment measures as well as donor site morbidity. It was originally designed to assess pain, function, walking distance, use of aids, gait, and emotional acceptance following musculoskeletal tumour excision and reconstruction (31). It has also been used to assess limb function following reconstruction after severe open tibial fractures (Grade IIIB) (32) and severe open ankle fractures (33). The scoring system contains components for assessment of the lower and upper limb for which the clinician assigns a numerical value (0–5) for each of the six assessment categories based on their clinical judgement. A numerical score and percentage rating is produced, with the final score expressed as a percentage of the patient’s uninjured limb, allowing for control of confounding variables (e.g. co-morbidities) (31, 33).

Quantitative physical performance measures

Perhaps the most fundamental goal of functional recovery from an open lower limb fracture is pain-free walking. This can be assessed easily and directly in a variety of ways including a simple 10-meter timed walk test, self-paced walk test, or 6-minute walk test (28). There are also PPMs of other mobility tasks such as the stair climbing, chair stand, and timed up and go tests (28); however, these activities can be viewed as extensions of walking (34). Typically, simple walking measures focus on gait speed, which has been demonstrated to be a strong indicator of pathology (35, 36). This can be performed on patients with cognitive impairments where PROMs may not be feasible. Despite the practicalities of performing simple timed walk tests in the clinic, these are limited by an inability to discriminate between normal and pathological gait in situations where patients adopt a compensatory gait pattern in order to maintain velocity (37). This inability can be overcome by comprehensive gait analysis using modern measurement apparatus, thus enabling the capture of detailed aspects of movement on walking (34). Following lower limb pathology, highlighted by studies of ankle fractures, the main disturbance of walking gait is asymmetry (38, 39). See Table 16.2 for methods of assessing gait asymmetry.

Table 16.2 Methods for assessing gait asymmetry.

Method

Mechanism

Motion Analysis

Measurement of the magnitude and timing of joint movements using body markers to allow multiple cameras to capture 3D motion in a fixed gait analysis laboratory (e.g. VICON®, Oxford, UK)

Force Plates

Force plates are used to quantify ground reaction forces during weight bearing to provide an indication of the functional demands on the foot. Plantar pressure movement is closely related and can identify regions of high pressure between the ground and foot (34)

Electromyography (EMG)

EMG captures coordination and timing of muscle activity

Energy Expenditure

Energy expenditure can be calculated to examine the efficiency of gait mechanics

Temporo-spatial Footfall Analysis

Combines gait muscle kinetics (effect of muscle forces on joints) and arthrokinematics (movement of joints) to provide a robust measure of gait quality (34, 40). This is commonly measured using a walkway containing electronic pressure sensors to capture the spacing and timing of footfalls (e.g. GAITRite®, CIR Symptoms, Havertown, PA) (34)

Many of the methods for assessing gait symmetry described in Table 16.2 are currently not practical for use in routine clinic follow-up of patients with open lower limb fractures as they require specialist training and facilities. However, measuring gait symmetry in the clinic is an increasingly realistic option by use of temporo-spatial footfall analysis systems, and wearable inertial motion sensors, as these systems are portable and increasingly user friendly.

Outcome measures for resource use

There has been little work to date investigating the economic burden to healthcare providers and patients following open lower limb fracture. The LEAP study was a large multicentre prospective cohort study conducted in the United States reporting outcomes from 601 participants who underwent reconstruction or amputation following severe, lower limb trauma (41). The LEAP study made the first detailed economic evaluation of the associated healthcare treatment cost of sustaining an open lower limb fracture by comparing the lifetime cost of reconstruction vs amputation (42). The main drivers of resource use after injury were identified as direct healthcare costs and projected lifetime costs. Direct healthcare costs included the initial hospitalisation, readmissions, inpatient rehabilitation, outpatient clinic, physical and occupational therapy attendances, and prosthetic devices and related services. Projected lifetime costs included projected healthcare costs and projected prosthetic costs (42).

More recently the WOLLF trial conducted a prospective economic evaluation alongside a UK multicentre randomised controlled trial to estimate the cost-effectiveness of negative pressure wound therapy (NPWT) compared with standard dressing in patients following open lower limb fracture (15). A cost-utility analysis was undertaken and expressed in terms of incremental cost attributable to the NPWT per QALY gained as recommended by NICE. The EQ-5D was used to measure health-related quality of life from which utility was valued using the York A1 model derived from a UK population using a time-trade off technique (43). Using this model, responses to each of the five domains in the EQ-5D can be valued on a health utility-scale from –0.59 to 1, where negative values represent health states considered worse than death, 0 being equivalent to being dead, and 1 being in a state of perfect health. The SF-12 was also collected within WOLLF and responses were converted into a form from which QALYs could be calculated.

When measuring and valuing health effects, health effects should be expressed in QALYs. The EQ-5D-3L is the preferred measure of health-related quality of life in adults for NICE (44).

Outcome measurement instruments developed for open lower limb fractures

While generic outcome measures are proven to be valid across multiple disease states and different populations (45, 46, 47), their use in measuring outcomes during recovery from injury is disputed (48). It has been recognised that many disease- or region-specific outcome measures used currently in trauma failed to involve patients at the item-generation stage of development (47). It is notable that the tools used focus on the physical aspects of recovery, overlooking the psychosocial impact of an open fracture, such as dealing with scarring and changes in appearance.

This deficit is addressed through the development of a novel patient-centred recovery score for lower limb trauma by a research group based in Swansea, UK. A qualitative approach has been used with the aim to assess how patients perceived their recovery following open tibial fractures (47). Topics of pain and mobility were shown to be central to descriptions of recovery by all patients interviewed. Domains of health additionally identified as important in the patient experience of recovery were sleep, flexibility, temperature (in relation to its effect on symptoms), fear, appearance, employment and finance, recovery, goal setting, adaption, and impact on others. This work has resulted in the creation of the Wales Lower Limb Recovery Scale (WaLLTR), which has undergone validation on patients with open lower limb fractures (49). The WaLLTR scale consists of a Likert and non-Likert section. The Likert section has 10 items covering 8 domains:

  1. 1. Self-efficacy

  2. 2. Pain

  3. 3. Body image

  4. 4. Activity participation

  5. 5. Fear

  6. 6. Social impact

  7. 7. Financial considerations

  8. 8. Perception of recovery.

Core Outcomes for Open Lower Limb Fracture Study (CO-OLLF)

In March 2019 following COMET guidance, a COS was developed for patients recovering from open lower limb fractures to identify what outcomes to measure. A consensus was reached for four core outcomes:

  1. 1. Quality of life

  2. 2. Return to life roles (e.g. employment, military duty, caring)

  3. 3. Walking, gait, and mobility

  4. 4. Pain or discomfort.

Further work identified how these outcomes should be measured in an additional consensus meeting in February 2020 attended by patients, clinicians, and expert methodologists. The OMIs that reached consensus to be included in the COMIS to measure core outcomes were the EQ-5D-5L and the LEFS to measure ‘quality of life’ and ‘walking, gait, and mobility’, respectively. Consensus was not achieved to identify an OMI for the core outcomes ‘return to life roles’ and ‘pain or discomfort’. The WaLLTR Scale was considered the best OMI to measure ‘return to life roles’ but failed to reach a consensus for inclusion in the COMIS. It was recommended that the WaLLTR scale be revised to specifically address measuring ‘return to life roles’. OMIs discussed for pain were considered inadequate due to being too non-specific for patients with open fractures, and in particular not addressing the chronic pain element that many patients describe during recovery. Although no OMI was selected for ‘return to life roles’ or ‘pain and discomfort’ the EQ-5D-5L does have face validity to measure all four core outcomes.

Conclusion

There is significant variation in outcome measures used in studies to date investigating outcome following treatment of open fractures of the lower limb. The CO-OLLF study addresses this by establishing a COMIS recommending the EQ-5D-5L and the LEFS to be collected as a minimum in all future studies on patients recovering from open lower limb fractures. This will reduce data heterogeneity in future studies and improve research quality.

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