Material
Patients were recruited during all visits made with a specialist rehabilitation physician or orthopedic surgeon at the Specialist Hospital in Rudna Mała, Poland from January to May in 2018. All patients were diagnosed by a specialist based on: the results of an interview, physical examination, and imaging studies (depending on the needs: USG, MRI, CT, X-ray).
People with a variety of lower limb conditions were included, provided they met the following inclusion criteria: age >18 years, native speaker of Polish, informed consent to participate in the study was provided, diseases/injuries located in any part of the lower limb - osteoarthritis, arthroplasty due to osteoarthritis, conditions after injury (of muscles, nerves, ligaments, menisci, bones, joints, as well as contusion/hematoma), patellar chondromalacia, joint deformations, and symptoms, provided the duration was >4 weeks.
Exclusion criteria were coexisting neurological disease, failure to provide written consent, and an inability to read Polish.
Design
This was a two-stage, cross-sectional study with repeated measures on two variables during re-test examination.
Stage 1 involved translation and cross-cultural adaptation of the LLFI. This was completed in accordance with the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) guidelines and is approved by regulatory agencies such as the Food and Drug Administration and the European Medicines Agency [21].
It consisted of nine steps, each of which was documented with a written report (Figure 1).
Figure 1: Flow chart of the translation and cultural adaptation process of the Lower Limb Functional Index from English to Polish
Step 1. Two forward translations were performed by two independent translators whose native language is Polish. This process allows Polish language equivalents to be introduced in place of terms that are otherwise difficult to translate.
Step 2. Reconciliation meeting between the two forward translators and the authors of the Polish adaptation (experts in health/physiotherapy, expert with experience in instrument development and translation) - to create a common version of the two forward translations.
The team analyzed the individual items, the sets of answers to questions, the instructions for questionnaire completion and score calculation.
Step 3. Back translation by an independent English native speaker, fluent in Polish, who was not familiar with the original version or different language versions of the LLFI to create a back translation version.
Step 4. Back translation review. The English back translation version was compared to the original English version by the author of the original LLFI, the “back” translator, and authors of the Polish adaptation.
Step 5. Review by clinicians working in the relevant medical field who were bilingual. The received version of the questionnaire was assessed by a three-person expert panel: an orthopedic surgeon, one physician - a specialist in rehabilitation medicine and a physiotherapist/expert with experience in instrument development and translation. They qualitatively assessed compliance with a 5-0 scale (5=full compliance to 0=non-compliance) of each question in the source version with the relevant question in the Polish version, i.e. whether both questions measured the same symptom/problem.
Step 6. Cognitive debriefing. In this stage the resulting version was pilot-tested on a group of five symptomatic patients (two women, three men) with lower extremity problems that were present > three months (ACL injury, knee osteoarthritis, knee joint arthroplasty, hip osteoarthritis, and patellar chondromalacia) to assess face and content validity through the accuracy of questions and clarity of the wording. The group assessed whether a given position of the scale was fully understood or raised doubts using a three-point (2,1,0) scale where: 2=completely understood, 1=partially understood and 0=completely incomprehensible. In the event that the question was incomprehensible to the respondent, they were asked to indicate the reason for the lack of understanding.
Step 7. Review of cognitive debriefing results and finalization to create a final Polish version of the LLFI (the LLFI-PL).
Step 8. Proof reading. In this stage, a Polish language teacher checked the LLFI-PL for any minor errors (spelling, grammatical or other), which could have occurred during the translation process.
Step 9. Final report. The final report provided a description of all translations and cultural adaptation decisions and was sent to the author of the original version of the LLFI.
Stage 2 involved a prospective evaluation of the essential psychometric properties of the LLFI-PL. The subjects were evaluated twice with an initial baseline examination that consisted of completing the Polish versions of all questionnaires by the respondents: the LLFI-PL, the WOMAC, the EuroQol 5-Dimensions, five-level version questionnaire (EQ-5D-5L), and the Pain Numerical Rating Scale (NRS). During the second examination (re-test), patients completed the LLFI-PL and the Pain NRS. This second visit took place three to seven days (average=six days) after the first examination, during a period of non-treatment, which is considered adequate and reasonable [4,22]. Patients completed the Pain NRS (to eliminate unstable participants from reliability analysis for whom the pain difference between baseline and retest was more than +/-1 point on the 11-point NRS). There is a low probability of changes in symptoms in this period, and the recollection of original responses is reduced [22].
Research tools
The LLFI assesses the impact of any lower limb problem on everyday activities. It is a 25-item regional PROM with a three-point response option of ‘Yes’ (points=1), ‘Partly’ (points=1/2) and ‘No’ (points=0) with a raw score range of 0-25 points. The final score is calculated by simple addition of the responses from the 25 items. The sum is multiplied by four and then subtracted from 100 to generate a 0-100% score (100%=no disability). Up to two missing responses are permitted[17].
The WOMAC index v. 3.1 was used to subjectively assess the functional status of patients. It includes 24 questions on a five-point (0-4) scale that determines symptom intensity in three domains: pain (five items), stiffness (two items) and function (17 items). The final score is obtained by summing all items (0-96) where the higher the score, the worse the functional status suspected. The data could be standardized to a range of values from 0-100 on a percentage scale, where 0 represents the worst health status and 100 the best health status. The WOMAC has been adapted into multiple languages including Polish [14,23,24].
The EQ-5D-5L consists of two parts. The initial part’s questions are grouped into five life-domains: movement, self-service, everyday activities, pain/discomfort, and emotional state (anxiety/despondency). Each question is assessed on five levels, from 1=minimum (no problems/no pain) to 5=maximum (impossible to perform/max pain). The second part consists of a 0-100 point visual analogue scale (VAS) where 0=minimum (worst self-rated health status) to 100=maximum (best self-rated health status). A basic subdivision can be made according to the structure of the EQ-5D-5L: presenting results from the EQ-index value (1-5, the higher the score, the worse the condition) and presenting results of the EQ-VAS as a measure of overall self-rated health status (0-100, the higher the score, the better health status). The questionnaire has been adapted to a Polish version [25,26].
The 11-point Pain NRS was used, where 0=minimum (no pain) and 10=maximum (the highest imaginable intensity). The recall period was ‘last week’, and during the re-assessment it was specified that it concerned the period between test and re-test [27].
Statistical analysis
All statistical analyses were conducted using SPSS Statistics software version 24. The level of statistical significance was assumed at p<0.05. Normal distribution of the results of this study was verified using the Shapiro-Wilk test.
Sample size
Internal consistency
The internal consistency was determined using Cronbach`s alpha (α) coefficient, where α should be between 0.70 and <0.95 [22,30]. Data from the first examination, at baseline, were included in the analysis (n=125).
Reliability
The intra class correlation (ICC2.1, CI=95%) was used to assess test-retest reliability in patients whose completed the LLFI-PL twice and for whom the difference on the Pain NRS between baseline and retest was +/- 1 point (n=94). In addition, the Pearson's correlation coefficient (PCC) was also estimated between two LLFI-PL measurements. Fisher’s F test was used to assess statistical significance of the PCC. Reliability was good when the ICC>0.70 and the PCC was r³0.70 [31].
Measurement Error
To assess error the Standard Error of Measurement (SEM) and Minimal Detectable Change at the 90% level (MDC90) the sample was all participants who completed the LLFI-PL at baseline and reassessment and for whom the difference on the Pain NRS between baseline and retest was +/-1 point (n=94).
The SEM was calculated using the formula: SEM=SD√(1-R), where SD is the standard deviation of measurements repeated from the test and retest and R - the reliability parameter (ICC2.1) [32].
The MDC is the minimum change in a patient's score that ensures the change is not the result of measurement error. The MDC90 was calculated using the formula: MDC=SEMx1.645x√2, where 1.645 reflects the 90% Confidence Interval (CI) of no change, and √2 indicates two measurements assessing change [33].
Construct Validity
In order to evaluate the LLFI-PL construct validity the PCC was calculated between the LLFI-PL, the WOMAC (total, pain, stiffness, function), the EQ-5D-5L index value, the EQ-5D-5L-VAS, and the Pain NRS (n=125). Consequently, the LLFI-PL should correlate highly with the WOMAC, which is also used to assess the function of the joints of the lower extremities. It should correlate moderately with the EQ-5D-5L index value because the generic questionnaire was designed to measure both the functional state, pain, and the emotional state. The LLFI-PL should correlate moderately with the EQ-5D-5L VAS, because this part of the questionnaire assesses the overall sense of health. The LLFI-PL should also correlate moderately with the Pain NRS, as it assesses only the severity of pain.
Therefore, the a-priori hypotheses were proposed as follows:
1a-d. The LLFI-PL should correlate highly with the WOMAC total; and with each domain (pain, function, and stiffness);
- The LLFI-PL should correlate moderately with the EQ-5D-5L index value;
- The LLFI-PL should correlate moderately with the EQ-5D-5L VAS.
- The LLFI-PL should correlate moderately with the Pain NRS.
If fewer than (25 %) of the hypotheses were rejected, construct validity of the LLFI-PL was considered high, and for moderate validity 25–50 % and for low validity, more than 50% should be rejected [22,34]. The indications for PCC r strength for validity were <0.30=low, 0.30-0.70=moderate and ³0.70=high [31].
Factor Structure
Exploratory factor analysis (EFA) was used with maximum likelihood extraction (MLE) and Varimax rotation. The Kaiser-Meyer-Olkin measure of sampling adequacy (KMO) was set at 0.80-1.0 to indicate adequate sampling and the Barlett Test of Sphericity significance level was p<0.001 indicating that the EFA could be used for the data analysis. A single-factor structure was indicated if extraction requirements satisfied all three a-priori criteria: 1) ‘Screeplot’ inflection at the second point, 2) Eigenvalue >1.0 and 3) Variance >10% for a population >100 [35]. The item loading was also calculated for the one-factor solution using the maximum likelihood extraction (MLE) method.
Practical Characteristics
Readability was performed qualitatively as part of the face and content validity. The completion and scoring times were calculated from the average of three separate measures of n=15 participants performed by n=15 therapists.