2.1 Inclusion and exclusion criteria
Patients were recruited through convenience sampling from 2 private physical therapy clinics in Italy, over a 3-year period between 2019 and 2021. They have meet the following inclusion criteria: 1) aged over 18 years, 2) with clinically diagnosis of idiopathic FS by orthopaedic surgeons and physiotherapists, 3) a restriction of both active and passive ROM in multiple planes, especially in external rotation at variable degrees of shoulder abduction, and 4) a normal shoulder X-ray. Subject were excluded if they presented red flags, a secondary stiff shoulder, an history of trauma, serious disease, positive history of cognitive impairments or they were unable to understand Italian. All procedures involving human participants were in accordance with the 1964 Helsinki declaration and its later amendments. Personal information and informed consent were obtained from all of the patients to participate the study.
2.2 Outcome measures
Shoulder and Pain Disability Index: The Italian version of the SPADI (SPADI-I) was administered in this study. The Italian validation of SPADI (SPADI-I) has already been performed for the assessment of shoulder dysfunction in patients treated for neck cancer, in patients after shoulder surgery to solve an anterior instability and in patients with non-specific shoulder pain. Before the compilation process, the patient has to be instructed to place a mark on the NRS (Numeric Rating Scale): a 0-10 scale where the patient circles the number that describes pain or disability in a better way. The results of each subscale are summed and converted to a score out of 100: the closer the score is to a hundred, the greater pain and disability are. Patients may mark one only item as not applicable in each subscale and the item is omitted from the total score. If a patient marks more than two items as non-applicable, no score is calculated.
The minimal clinically important difference (MCID) is 8 points, instead, the Minimal Detectible Change (MDC) is 18 points.
Disability of Arm, Shoulder and Hand Questionnaire (DASH): The DASH is an outcome measure that assess the ability of an upper extremity to perform daily activities regardless of the site(s) and nature of musculoskeletal pathology. Thirty items regarding disability and symptoms (DASH-DS), and two optional modules compose the DASH questionnaire: the work one (DASH-W) and the sport/performing art one (DASH-SA), made up by 4 items each one. Items of DASH-DS tests the degree of difficulty when performing various physical, social and work-related daily activities together with the impact on sleep routine and the patient’s perception of him or herself in the light of upper extremity problems. The other two modules are optional and contains activity-specific items that rate the ability to work and to perform sports or play musical instruments. For each question, patients rates difficulty on a five-point Likert scale that ranges from ‘‘no difficulty or no symptoms’’ (scores 1 point) to ‘‘unable to perform activity or severe symptoms’’ (scores 5 points). The DASH-DS score ranges from 0 (no disability) to 100 (severest disability) and cannot be calculated if there are more than 3 missing items. The other two modules are scored separately but in a similar way. In this study has been used the longer form: translated, cross-cultural adapted and validated in Italian.
Numerical Pain Rating Scale (NPRS): NPRS is an eleven-point measure of pain in which patients rate their pain ranging from 0 (no pain) to 10 (worst imaginable pain). It has already shown good responsiveness in shoulder pain and in this study has been used to assess the patient’s pain perception in the last week.
36-Item Short Form Health Survey questionnaire (SF-36): SF-36 is an instrument that evaluates Health-Related Quality of Life. The SF-36 measures eight scales: physical functioning (PF), role physical (RP), bodily pain (BP), general health (GH), vitality (VT), social functioning (SF), role emotional (RE), and mental health (MH). It has been chosen in this study to assess patient’s general health because off his widely recognized properties in the evaluation of the shoulder disorders and has been used the Italian version.
SPADI was contextually administered to each subject with the Italian version of the DASH scale, the SF-36 and the NRPS. A private questionnaire was also administered to acquire clinic and demographic data. A sub-group of 29 participants have been re-tested after 7 days.
2.4 Statistical analysis
This study has followed the definitions and procedures proposed by the COSMIN initiative in the evaluation process of the SPADI’ s psychometric properties.
The structural validity was assessed by a Two-Factors Confirmatory Factor Analysis (CFA) using the following indicators :
- The Comparative Fit Index (CFI) and the Tucker-Lewis Index (TLI) ≥0.95
- The Root Mean Square Error of Approximation (RMSEA) ≤0.06
- The Standardized Root Mean Square Residual (SRMR) ≤0.08
Reliability was assessed as internal consistency, test-retest reliability and measurement error. These analyses were undertaken on both the two subscales, separately: Pain and Disability subscale. Internal consistency was valuated according to these criteria:
- Cronbach’s Alpha (α), where values are recommended between 0.70 and 0.95.
- α if an item was deleted, where the alpha was calculated after removing each item in turn; values below the total Cronbach’s alpha are expected.
- item to total correlation, which are the nonparametric correlations (based on Spearman’s ϱ) between each item and its rest score (i.e. the total score minus the item score); values≥0.40 were considered satisfactory.
To examine the test-retest reliability, the intraclass correlation coefficient (ICC2,1) with 95% confidence intervals (CI) was calculated. The minimal ICC value that is requisite for a reliable measure on groups is >0.75, but value >0.90 are considered essential to reach excellent reliability in a clinical measurement at the individual level. The Standard Error of Measurement (SEM) and Minimum Detectable change (MDC) were used to test the measurement error. SEM was calculated with the following formula: SD*√(1–ICC), where SD is the baseline standard deviation of the measurements, and the ICC value is the one of intra-rater reliability. MDC was calculated by multiplying the SEM by 1.96, i.e. the z-score associated with the 95% confidence level and the square root of 2.
The administrated scales have been used for a comparative analysis to study the construct validity. To analyse the pain construct, the authors compared SPADI with NPRS, while, to analyse shoulder disability, compared SPADI with DASH questionnaire, despite it refers to a disability of the arm, shoulder and hand and has been shown not be specific for shoulder disorders.
Construct validity has been tested though hypothesis testing by the means of the Spearman’s rank correlation (rs) coefficients between the two subscales of the SPADI and the other three scales (DASH, NPRS and SF-36). Based on the obtained rs coefficients, an Hypothesis Testing of the following assumption was made (Table 4):
- the correlation between the Pain subscale and the NPRS scores is > 0.60 because they measure the same construct, and this value of correlation has be found in another SPADI validation study;
- the correlation between the Pain subscale and the DASH total scores is > 0.60 as found in other SPADI validation studies[48, 49];
- the correlation between the Pain subscale and the physical functioning (PF) subscale of the SF-36 scores is > 0.30, because shoulder pain globally compromises physical health status;
- the correlation between the Pain subscale and the bodily pain (BP) subscale of the SF-36 scores is > 0.30 and < 0.60, based on the weak values found in the literature, together with the consciousness that they measure similar constructs;
- the correlation between the Disability subscale and the DASH total scores is > 0.60, as reported in other SPADI validation studies[23, 48, 49];
- the correlation between the Disability subscale and the NPRS scores is between >0.30 and < 0.60, because pain showed a strong correlation with disability in affine patients[18, 47];
- the correlation between the Disability subscale and the role physical (RP) subscale of the SF-36 scores is > 0.30[47, 51];
- the correlation between the Disability subscale and the social functioning (SF) subscale of the SF-36 scores is > 0.30[47, 51].
The cut-offs of rs > 0.60, 0.30< rs < 0.60, and rs < 0.30 represent strong, moderate, and weak correlations, respectively. Construct validity was considered satisfactory, moderate, or low if ≥75%, >50% but <75%, and <50% of expected hypotheses were met, respectively.
R-package lavan was used to run the CFA, while SPSS package, version 21 for Windows (SPSS Inc., Chicago, IL; 2004) was used for all other statistical analyses. The p-value was set for α<0.05 for all analyses.