The principle objective of this study was a diagnostic tool for JH specific to the paediatric foot and ankle, with demonstrated test reliability and internal consistency, a defined cut-off point, and demonstrated validity.
The original 7 items returned Cronbach's α = 0.80, indicating good inter-item reliability. However, poor relationship with excessive subtalar movement component (0.23) directed us to exclude this item and achieve a reliability mean value of 0.59. Following this revision, the 6-item FAFI demonstrated good internal consistency with Cronbach's α = 0.82.
The BS cut-off score of 5/9 has previously shown sensitivity of 72% and specificity of 78%. Similarly, the LLAS cut-off point has previously shown sensitivity of 68% and specificity of 86%(14). The current study has determined that ≥4/6 was the optimal FAFI cut-off point, with high sensitivity (85%) and specificity (95%). The cut-off point was investigated for each gender, and both returned a cut-off point of 4, which is applicable for both genders. The present cut-off point has been calculated in a European ethnic group, and it is expected that there may be differences in other participants with different ethnicity, who are reportedly more hypermobile (3,4,15). The cut-off point from this study demonstrated positive predictive values of 95% and negative predictive values of 95%, thus measuring the efficacy of the diagnostic test.
Excellent intra-rater reliability (ICC= 0.96, p < 0.001) and excellent inter-rater reliability (ICC= 0.89, p < 0.001) were found. Previous reliability studies (16), have also provided psychometric data, and found inter-rater BS reliability of 0.73 (0.42-0.88) and LLAS inter-rater reliability of 0.78 (0.41-0.93), indicative of tools with excellent inter-rater reliability, for the identification of foot and ankle JH.
This research found similarities to other studies when hypermobility was compared with gender, age, and differences between left and right limbs. We found significant differences regarding gender, where girls were more flexible than boys at the foot and ankle level. These results agree with previous studies when compared with LLAS (17) and BS (7)(18)(19). When comparing left and right sides, no differences were found, in common with a previous stud (17). The current study found that joint hypermobility of the foot and ankle was inversely proportional to age (r= -0.23; p < 0.01), Similarly, other studies have found the same relationship in lower limbs. (17) and with GJH (20)(18)(19).
There was a proportionally positive correlation between the BS and FAFI (r=437; p < 0.01). A concordance of the data obtained in the study between BS and FAFI was found in 69%. For 48% of the results in which FAFI was positive, BS was unable to diagnose joint hypermobility. In 20% of cases, FAFI was negative in subjects with general joint hypermobility, indicating that there is overdiagnosis of JH when using the BS in studies that connect foot and ankle flexibility with BS findings. It is unlikely that the BS, with only knee range examined in lower limbs and predominantly upper limb signs, would always indicate foot and ankle JH. It is important to understand that for foot and ankle injuries, the BS should not be a primary test, as recent meta-analysis supports.(9), where generalized JH was used as a variable of JH for the relationship with lower limb injuries. Further investigations also suggest using JH tests located pertinent to the body area. (21,22). Malek et al. (21) have likewise considered integrating body region items in to a revised BS.
Regarding the LLAS, FAFI had a proportionally positive correlation (r=923; p < 0.01). Comparing it with BS if a higher correlation was found. There was a concordance of cases between FAFI and LLAS in 84%. In 27% of the cases in which FAFI was positive, LLAS obtained a negative value in hypermobility. As there were also 8% of cases in which FAFI was negative in subjects with lower limb joint hypermobility, therefore, LLAS could over diagnose cases of foot and ankle hypermobility. Clinicians should be careful when using the LLAS score as a tool that only assesses joint hypermobility of the foot and ankle, LLAS should be used when general joint hypermobility of the lower limbs is studied.
This supports the importance of incorporating this new reliable and valid tool for the diagnosis of foot and ankle joint hypermobility, which has a cut-off point for future foot and ankle research.
Further, clinicians need to appreciate joint hypermobility as both a local or systemic condition, and it’s overlap with hypotonia(23).
It has been suggested that children with hypotonia, present joint hyperlaxity as adults(24). Clinically, hypotonia is observed, indicated by the pull-to-sit test, the frog-sit posture, vertical suspension floppiness, and the scarf sign. No test has been validated for the diagnosis of hypotonia.
A limitation of this study is that only Caucasian participants were included, given that greater flexibility in Asian and African populations is reported (4,5). Therefore, the cut-off point is only applicable for this paediatric population.
Further investigations utilising FAFI should consider both wider age range, and more diverse ethnicities.
Our future research will compare FAFI with the validated Foot Posture Index, for the study of joint hypermobility of the foot and ankle and developing foot posture.