Neck pain is one of the most common chronic pain problems, with a reported prevalence of 22%–30%. The types of disabilities resulting from neck pain have not been determined; however, it usually exerts a significant effect on daily life, resulting in extensive use of healthcare resources [1].

Pain in the neck (cervical pain) may be due to different reasons. The term “cervical pain” does not refer to a disease itself but rather to a symptom or the manifestation of a muscular pain disorder. Cervical pain usually involves multi-factorial clinical aspects because it may involve individual risk factors, either physical or emotional characteristics, or be related to ergonomics and work activities [2].

Many scales provided in English-language that measures neck pain and functional disability have been developed as follows: Neck Disability Index (NDI), Pain Disability Index, Copenhagen Neck Functional Disability Scale (CNFDS), Northwick Park Neck Pain Questionnaire (NPQ), Neck Pain and Disability Scale (NPDS), Neck Bournemouth Questionnaire, and Functional Rating Index [3].

To assess neck pain, many scales have been developed. The CNFDS is one such scale that comprises 15 items that assess the impact of neck pain and disability level. Three items assess the pain severity, including the patient’s acuity of the future impact of neck pain; eight items assess the disability during every-day activities; and four items focus on social interaction and recreation [4].

The CNFDS requires to be translated to make it easily understandable by the patients. In contrast to the NPDS, the CNFDS has no Visual Analog Scale that is perceived as difficult to apply by some patients. The amount of text is minimum compared with the 54 lines in the NPQ. Most importantly, the CNFDS uses qualitative items, and the clinical relevance of the changes in the item scores is readily perceived [5].

The scale index of clarity was 86.84%, as shown in Table 1. Expert proportion of clearance was calculated to be 81.58%, as presented in Table 2.

Item CVI of the final version was calculated; we found that the scale CVI (S-CVI) was 99.33%, as shown in Table 3. Expert proportion of relevance was calculated and is presented in Table 4. The study population comprised patients of both sexes (44 women and 30 men); further, 61 patients did and 13 did not perform the retest. Internal consistency was calculated for the final version; Cronbach’s alpha was 0.856 with lower bound 0.796 and upper bound 0.905 at 95% confidence interval (CI). Finally, Spearman’s correlation coefficients were calculated and are presented in Table 5.

The present study was designed to test the face validity, content validity, feasibility, internal consistency reliability, and test–retest reliability of the Arabic version of the CNFDS to measure the disability level in chronic neck pain patients. The Arabic version of the CNFDS exhibited good face validity, with scale index of clarity 86.84% and mean proportion of clearance (clear responses) 81.58%. The scale also demonstrated excellent content validity because the S-CVI was 99.33%, S-CVI/universal agreement was 93.33%, and the mean proportion of relevance (relevant responses) was 99.33%. The present results are in agreement with the reports of Polit and Beck [8] who stated that for a scale to be judged as having excellent content validity, it should be composed of items with item indices of content validity (I-CVI) that meet the following criteria (I-CVI of 1.00 with three to five experts and I-CVI ≥0.78 for 6–10 experts) and have S-CVI ≥0.90. The recommended standards may necessitate two rounds of expert review if the initial assessment suggests the need for substantial item improvements. This is consistent with the findings of Waltz et al. [9] who stated that S-CVI/average ≥0.90 is the minimum acceptable index and items that do not achieve the minimum acceptable indices are revised and reevaluated.

The Arabic version of the CNFDS demonstrated high feasibility as the scale items were filled in all the sheets by 99.15%, and ≤1.5 minute was required to complete the sheet in 64% of all sheets. Further, <5 minutes were required for almost all the sheets.

The present results agreed with the report by van Veen et al. [10], according to which the missing rate on the item level was considered acceptable if no single item had a missing rate >10%, and the completion time was considered acceptable if 95% of the sheets were completed in <15 minutes.

The Arabic version of the CNFDS exhibited good internal consistency and good test–retest reliability, with a Cronbach’s alpha of 0.856 (0.796, 0.905) and all Spearman’s rank correlation coefficients between test and retest results being statistically significant (item 1=0.872, item 2=0.813, item 3=0.837, item 4=0.910, item 5=0.889, item 6=0.891, item 7=0.946, item 8=0.817, item 9=0.851, item 10=0.748, item11=0.700, item 12=0.924, item 13=0.863, item 14=0.871, item 15=0.939, and total score=0.958). Therefore, according to Darren and Mallery [11], an α value between 0.7 and 0.9 indicates good internal consistency. Further, Spearman’s rank correlation coefficient between 0.9 and 1 (as in items 4, 7, and 15) indicates excellent test–retest reliability, and Spearman’s rank correlation coefficient between 0.7 and 0.9 (as in items 1, 2, 3, 5, 6, 8, 9, 10, 11, 12, 13, and 14) indicates acceptable test–retest reliability.

These results agree with a study that tested the outcome measures of functionality, social interaction, and pain in patients with cervical spondylotic myelopathy. A validation study for the Iranian version of the CNFDS and a form including the individual characteristics of the patients were applied to 93 subjects with cervical spondylotic myelopathy. The Cronbach’s alpha coefficient was 0.84, and intraclass correlation coefficient (ICC) was 0.95, certifying that the Iranian version of the CNFDS had acceptable reliability. Moreover, the correlation of each item with its hypothesized domain on the CNFDS was acceptable, suggesting that the items had a good relationship with their own domains. These results also indicate that the instrument was responsive to change (p<0.0001). Therefore, the Iranian version of the CNFDS was considered to have acceptable psychometric characteristics, based on the promising results of the ICCs for the scale scores and the good stability of the Iranian version of the CNFDS over time. Therefore, the CNFDS appears to provide internal consistency and reproducible results for patients with neck pain [12].

These results were similar to those reported by Yapali et al. [13] who conducted a study to adapt the CNFDS to the Turkish language and evaluate the psychometric properties of the Turkish version of the CNFDS in patients with chronic neck pain. The scale was translated from the source language to the target language, synthesized, back translated, revised, and pre-tested. Overall, 101 patients participated in the study. Test–retest reliability was tested using ICCs. The internal consistency was excellent, with an ICC value of 0.86 at 1 week, confirming the reliability of the Turkish version of CNFDS for patients with neck pain [13].

The present results are strengthened by the report by Misterska et al. [5] who conducted a study to assess the validity and the reliability of the NDI and CNFDS in patients with neck pain due to degenerative and discopathic disorders. The psychometric properties of the Neck Disability Index Polish versions (NDI-PL) and Copenhagen Disability Index Polish Version (CDI-PL), 60 patients were treated due to degenerative and discopathic disorders in the cervical spine filled out the completed patient-reported questionnaires (NDI-PL and CDS-PL). The Cronbach’s alpha values were excellent for the NDI-PL in the test and retest (0.84 and 0.85, respectively) and for the CDS-PL (0.90 in the test and retest). ICCs were excellent for the CDS-PL and NDI-PL at 0.93 (95% CI, 0.89 to 0.95) and 0.87 (95% CI, 0.80 to 0.92), respectively. This study found that the NDL-PL and CDS-PL scales were valid and reliable for patients with degenerative changes in the cervical spine.

The validity and reliability of the translated tools were assessed in two or three studies. The first study was designed to translate the tool to the targeted language; thereafter, the translated version was tested for face and content validity and reliability on a monolingual population. The second study was designed to test the full psychometrics of the translated tool in bilingual participants. The third study tested all the psychometric properties of the translated tool on a monolingual population, which indicated that there is no need for the second study to be made [6,7]. To our knowledge, the present study is the first trial to test the validity and reliability of the Arabic version of the CNFDS.

The final version is considered the base for future research that will establish all the psychometric properties of the Arabic version of the CNFDS.

The present results and the corresponding discussion show that the Arabic version of the CNFDS possesses adequate face and content validity, feasibility, internal consistency, and test–retest reliability to enable the measurement of the disability level in chronic neck pain patients.