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INTRODUCTION
Whether owing to trauma, disease, or infection, amputation of the lower limb leads to a significant loss in physical function that results in limited mobility and a reduced quality of life for affected individuals [1]. The goal of rehabilitation following an amputation is to assist the patient in using and adjusting to a prosthesis, regaining the ability to walk, and returning to their previous level of physical activities of daily living (ADL). Rehabilitation specialists strive to enhance the quality of patients’ lives by maximising their ability to perform daily activities while wearing a prosthesis and helping them improve overall quality of life [2]. Measures of mobility in patients using lower limb prostheses commonly focus on physical activities [2]. For example, the 2-Minute Walk Test (2MWT) assesses walking distance over 2 minutes, while the Timed Up and Go Test (TUG) records the time required for patients to rise from a chair, walk 3 m, and walk back to the chair and sit down [3]. Despite their usefulness in evaluating an individual’s physical functions, these measures provide limited information as a tool for measuring real world ADL [4].
Patient-reported outcome measures (PROMs) assess patient mobility based on their actual experiences [5] and provide essential information that cannot be obtained in hospitals or laboratories; therefore, PROMs can be used in clinical settings and studies [6]. Several PROMs have been developed including the Locomotor Capability Index and Prosthesis Evaluation Questionnaire-Mobility Subscale; however, these questionnaires have poor psychometric reliability and validity [7,8]. The recently developed Prosthetic Limb Users Survey of Mobility (PLUS-M) is a brief item bank that measures mobility and exhibits good reliability [9,10]. This survey has been translated into more than 15 different languages and is widely used to evaluate mobility in individuals with lower limb amputations worldwide [11]. However, the PLUS-M has yet to be translated into Korean in a standardised manner given the Korean sociocultural context and studied for its psychometric properties for the official use of the PLUS-M in Korea [12].
Therefore, this study aimed to develop a Korean version of PLUS-M, obtain official certification, and analyse the psychometric properties of the PLUS-M/Short Form 12 (PLUS-M/SF-12). This study provided an instrument that can accurately assess the mobility of individuals with lower limb amputations.
METHODS
Study process
This prospective study analysed the psychometric properties of the Korean version of PLUS-M. It was approved by the Korea Workers’ Compensation & Welfare Service Incheon Hospital (KCIRB-2023-0003-002) on July 29, 2023 (Table 1).
Study instruments
PLUS-M
The PLUS-M contains 44 items and is available in the following two formats: a 12-item version and 7-item short form which can be completed in 3 to 5 minutes. The development of the PLUS-M followed strict psychometric methodology and there is a strong correlation between the total 44-item version and the short forms. Individuals with lower limb amputations should respond to each question while wearing their prostheses and those who use canes, crutches, or walkers should respond as if they were using their walking aids. Each item is scored using a 5-point Likert scale.
Korean translation
Permission from the developer and translation of PLUS-M into Korean
Permission to proceed with the translation study was obtained in consultation with the developer of PLUS-M (Brain J. Hafner, Department of Rehabilitation Medicine, University of Washington, Seattle, WA, USA). A rehabilitation specialist and researcher of physical therapy separately translated the instrument. For any inconsistency between the translations, data were compared to select the most suitable translation.
First and 2nd English-Korean translation compatibility verification
The translated PLUS-M was sent to two physical therapists working at Incheon Hospital and one prosthetist working at a Rehabilitation Engineering Center. Their task was to assess the appropriateness of term use and understanding of the instrument. Any unclear or unnatural items were sent to the rehabilitation specialist and researcher for correction.
Back translation and verification to the back translation
We requested a back translation from a native Korean speaker (whose mother tongue was English and was fluent in both English and Korean) from Editage (https://www.editage.co.kr). The PLUS-M translated into Korean and PLUS-M translated back to English were sent to the developer who verified the compatibility of the translated information. When necessary, any revision was reflected on the translated PLUS-M.
Third English-Korean translation compatibility verification
Two rehabilitation specialists and one professor from the Department of Prosthetics and Orthotics were asked to assess the suitability of the translated Korean version of the PLUS-M in comparison to the original. After incorporating the final revisions, the translated Korean version of the PLUS-M was approved by the developer. The entire translation process was conducted according to the guidelines from previous studies [13,14].
Analysis of psychometric properties (verification of psychological reliability and validity)
Study participants
Notices were posted on the bulletin boards at the Rehabilitation Center of Incheon Hospital. Brochures were also made available at the reception desk of the orthotic room at a Rehabilitation Engineering Center. Additionally, advertisements were placed on the bulletin board of the Korea Amputee Association Website to recruit research participants. The translated version PLUS-M was sent to two physical therapists working at Incheon Hospital and one prosthetist working at a Rehabilitation Engineering Center for them to assess the appropriateness of term use and understanding of the instrument.
The inclusion criteria were as follows: patients who had undergone unilateral lower limb amputation at least 1 year prior, used a prosthesis indoors, could respond to questionnaires, and voluntarily decided to participate in this study and signed informed consent before the study began.
The exclusion criteria were as follows: patients who had bilateral lower limb amputations, had cognitive impairment, wore a prosthesis for aesthetic purposes, or had limited indoor ADL; as well as individuals who fulfilled the above criteria were selected as study subjects.
Variables measured
Data collected from every participant included demographic and general variables (age, sex, level, cause and time since amputation and type of prosthetic equipment), findings for functional performance tests (TUG and 2MWT), questionnaires assessing mobility in individuals with lower limb amputations (Activities-specific Balance Confidence [ABC] scale, PLUS-M/SF-12), and one-on-one interviews. Scores obtained from 12 items of the PLUS-M/SF-12 questionnaire were converted to T-scores based on the scoring criteria proposed by the developer (Supplementary Material 1) [15].
(1) ABC scale
The ABC scale measures confidence in performing various activities and consists of 16 items regarding specific activities and requires the patient to rate his/her “confidence for performing each activity without falling or losing balance.” Participants are asked to answer their levels of confidence in performing certain activities, with a score from 0% to 100%. The total score is reached by adding all the scores together [16-18].
(2) TUG
The TUG evaluates walking ability, functional mobility, balance, and the capability to perform tasks related to walking. The test begins with the participant seated in a chair with armrests, measuring the time taken for the participant to stand up from the chair, walk a distance of 3 m, turn, walk back to the chair, and sit down upon the rater’s starting signal. The participant is then asked to repeat the course 3 times. The mean TUG score in this study was calculated using all scores. This test demonstrates high reliability with consistent results when performed multiple times by the same rater (intra-rater reliability of 0.99) and similar results when performed by different raters (inter-rater reliability of 0.98) [19].
(3) 2MWT
The 2MWT assesses walking distance over a predetermined time to evaluate functional ambulation ability and physical strength in clinical settings. The American Thoracic Society recommends a 6-Minute Walk Test (6MWT) for evaluating ambulation ability and physical strength. However, the 6MWT may not be a useful under certain circumstances owing to time constraints. Therefore, the 2MWT is commonly used as an alternative test. 2MWT has a strong correlation with the distance walked in the 6MWT and excellent test-retest reliability with an intraclass correlation coefficient (ICC) of 0.97 [20,21].
Outcome measures of efficiency
Construct validity
To assess construct validity, the correlation between the questionnaire assessing mobility in individuals with lower limb amputations, functional performance tests (TUG, 2MWT), and PLUS-M/SF-12 was analysed.
Reliability test (test-retest reliability)
We calculated the ICC between 2 completions at a 2-week interval. For any participant who was unable to make the second visit due to personal circumstances, the survey was administered via e-mail and Naver Office Form.
Face validity and usefulness
At the end of each visit, the investigator conducted an individual interview with participants on the content of the PLUS-M/SF-12 and its form.
Floor and ceiling effect
Among the participants scoring the lowest or highest T-scores, 15% or more indicated a significant floor or ceiling effect [22].
Data analysis
Statistical analyses were performed using SAS ver. 9.4 (SAS Institute Inc.). The participants’ general characteristics were analysed using descriptive statistics and all correlation calculations. Partial correlation coefficients were classified into the following five categories: r≥0.91, very high; 0.90–0.71, high; 0.70–0.51, moderate; 0.50–0.31, low; and <0.31, negligible [23]. The ICC was classified as <0.40, poor; 0.40–0.59, fair; 0.60–0.74, good; and 0.75–1.00, excellent [24]. p<0.05 was considered statistically significant.
RESULTS
PLUS-M Korean version
In this study, the PLUS-M was translated into Korean. Table 2 shows the original English PLUS-M/SF-12 and the translated Korean version used for assessing psychometric properties (Supplementary Material 2). Before the developer’s final approval, three patients with lower limb amputation inspected the translation in advance to ensure that the translated information was understandable.
Psychometric validation of PLUS-M/SF-12
General characteristics of participants
Out of 40 initial patients who agreed to participate in the study, 8 dropped out due to personal reasons. The final analysis included 32 male participants with unilateral lower limb amputations resulting from trauma. Their average age, height, and weight was 52.22 years, 170.7 cm, and 73.7 kg. On average, the time since amputation was 7.25 years. Of the participants, 18 (56.2%) had transfemoral amputations and 14 (43.8%) had transtibial amputations. Additionally, 10 participants (31.2%) had right-sided amputations and 22 (68.8%) had left-sided amputations. Among all participants, nine (28.1%) used single-point/four-point canes or crutches, whereas 11 (33.4%) were employed (Table 3).
Participants were graded from K-Level 0 to 4 based on the amputee’s ability to perform activities and ambulation levels. All lower limb prosthetic products were rated according to the recommended activity levels. A user with Level 0 could not have the ability or potential to amblate, Level 1 could walk in the household, but could not perform outdoor activities. Level 2 could walk in limited outdoor walk, Level 3 could walk in free outdoor walk and Level 4 could walk in high functional ability outdoor walk. The level of activity increases with grade number [25]. Patients who required a prosthetic foot could select a prosthetic type based on their level of activity. The class of prosthetic foot increases with the amputee’s level of activity [26]. Of the participants who used lower limb prosthetic equipment, 4 (12.5%) used a prosthesis suitable for K-Level 2, while 28 (87.5%) used a prosthesis suitable for K-Level 3, 4. Overall, 12 participants (66.6%) used microprocessor-controlled knees, which are designed for individuals with lower limb amputations and require a high level of activity (Table 4).
Construct validity
Data from 32 participants were analysed. PLUS-M/SF-12 Korean version had a high correlation with the ABC scores (r=0.88, p<0.05). PLUS-M/SF-12 Korean version exhibited a moderate correlation with 2MWT (r=0.59, p<0.05), but a weak correlation with TUG (r=0.40, p<0.05) (Fig. 1).
Test-retest reliability
The ICC calculated for the 32 participants who completed the Korean version of PLUS-M/SF-12 twice was 0.91 (95% confidence interval [CI], 0.82–0.95), which was excellent.
Internal consistency
The internal consistency of PLUS-M/SF-12 Korean version was analysed. Cronbach’s a was 0.95 at the first test and 0.95 at the second test, which demonstrated significantly excellent internal consistency.
Face validity and usefulness
The participants understood all items well and evaluated that the PLUS-M/SF-12 Korean version appropriately included all items necessary for assessing lower limb amputees’ mobility. However, some comments were as follows: the evaluation format did not consider variables such as skin conditions on the residual limb and physical strength; previously unexperienced questionnaire items; insufficient definition about certain situations (e.g., without lights and slippery); a lack of items that evaluate the use of public transportation.
DISCUSSION
This study translated the PLUS-M into Korean to account for cultural differences. The tool was demonstrated to have good reliability and validity.
Assessments aid in making clinical decisions and are essential for selecting appropriate therapeutic interventions and increasing therapeutic effects [27]. Those using self-reported instruments are highly suitable for measuring health outcomes since they reflect an individual’s experience, perception, and opinion. These instruments have great value in a clinical setting in terms of patient screening, health outcome monitoring, facilitation of communication between patient and clinician, provision of information on therapeutic decision, and evaluation of therapeutic effect [28]. Furthermore, self-reported data are used to provide important information on health care policy and service payment [29]. This study translated the 44 items of PLUS-M into Korean and analysed the psychometric properties of the PLUS-M/SF-12 instrument through official procedures.
This study evaluated internal consistency to determine whether the items within each measurement category measure the same characteristics in survey-based studies. The “first PLUS-M/SF-12” and “second PLUS-M/SF-12” validations indicated excellent internal consistency among the questions, suggesting that the questionnaire is highly reliable. Test-retest reliability was assessed using the ICC to examine the stability of the questionnaire across repeated measurements. The analysis revealed a correlation coefficient of 0.91 (95% CI, 0.82–0.95), indicating excellent reliability. The overall results indicate that this questionnaire produces consistent outcomes, and are similar to those reported by Karatzios et al. [11], who evaluated the psychometric properties of the French version of the PLUS-M/SF-12. This implies that the Korean version of PLUS-M/SF-12 has excellent stability, consistency, and predictability. Accordingly, this study demonstrated that the Korean version of PLUS-M/SF-12 has greatly excellent reliability and efficacy as a psychometric instrument, which implies that it has great potential to be used in research and clinical settings.
This study tested validity using different instruments, such as ABC, 2MWT, and TUG, to assess various characteristics related to mobility in individuals with lower limb amputation [30-32]. The PLUS-M/SF-12 scoring guidelines indicate that the percentage (0.2%–98.4%) represents percentiles converted to T-scores. These T-scores show the proportion of individuals who scored lower than the patients in the sample population. In the present study, the average T-score for participants’ PLUS-M/SF-12 was 50 points, suggesting that approximately 51.9% of the sample population has lower mobility compared to the study participants. Consequently, this indicates that the participants’ ambulation level is higher than the average level (Supplementary Material 1). There was a strong correlation between the PLUS-M/SF-12 and ABC scores. Similar results were found by Hafner et al. [10], who analysed the correlation between the original version of PLUS-M/SF-12 and other instruments (Korean version r=0.88 vs. original r=0.81). This suggests that balance is a component of physical function and is closely linked to the ability to walk. The PLUS-M/SF-12 shows a moderate correlation with 2MWT but a low correlation with the TUG. With regards to 2MWT, participants performed the test in a different environment from their usual living conditions. The low correlation with TUG may be due to a high ceiling effect because the average time was 15.6 seconds (n=32), with nine participants (28.1%) having an average time of less than 10 seconds.
The higher the score of PLUS-M/SF-12, the longer the distance of 2MWT, the higher the score of ABC, and the shorter the time of TUG, the better the mobility. The hypothesis regarding the gradual correlation between the PLUS-M/SF-12 scores and comparable instruments has been validated. The lower the TUG score or the higher the 2MWT and ABC scores, the stronger the correlation. Finally, based on feedback obtained from individual interviews, it is advisable to consider adding variables related to physical condition, specifying the situation in questions, and including items related to the use of public transportation in the assessment questions.
Study limitations
Sample composition: All the study participants had undergone amputation as a result of a traumatic incident. Notably, because traumatic amputations account for less than 10% of the total amputee population as per epidemiological data, the study sample may not fully reflect the characteristics of the broader amputee population. This lack of representation can potentially impact the study results. Furthermore, the study’s sample is exclusively male, deviating from the demographic norm.
Sample size: Although the calculated number of participants was 50, only 32 (64.0%) were included in the actual analysis. Accordingly, future research should recruit more participants and expand the sample size to increase the generalisation of the study results.
Criteria for use of PLUS-M: The PLUS-M is recommended for skilled individuals who have at least 1 year of experience using prosthetic equipment. However, the rehabilitation goals of inpatients may not cover all the activities described in the PLUS-M/SF-12. It is important to consider this when interpreting the study results.
Cultural and empirical differences: Some of the questions may be difficult to understand due to cultural and lifestyle differences between Eastern and Western countries. The participants could assume a similar situation or make a judgement based on their own experiences.
Conclusion
It is important to curate a reliable instrument that can assess the health of individuals with lower limb amputations, as the prevalence of this condition is increasing globally. The PLUS-M is a reliable and valid self-reported questionnaire survey. It is an effective tool for monitoring the mobility of individuals with lower limb amputations in clinical settings, as well as for improving communication between patients and rehabilitation specialists, evaluating therapeutic interventions, and enhancing the patients’ condition. This study translated the PLUS-M into Korean to account for cultural differences and demonstrated the reliability and validity of the translated instrument through psychometric verification procedures. Developing new language versions of the instrument in the future will open opportunities for collaboration and advancement of new research in rehabilitation and prosthetic assistive devices for individuals with lower limb amputations.
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