Abstract
BACKGROUND: Fatty infiltration, or fatty atrophy, of the abductor digiti minimi (ADM) muscle of the foot is proposed to be associated with entrapment of the first branch of the lateral plantar nerve (i.e., Baxter's neuropathy) as part of plantar heel pain (PHP). However, this association has not been rigorously investigated. The aim of this study was to determine if there is an association between PHP and fatty infiltration of ADM. METHODS: This cross-sectional observational study compared 50 participants with PHP to a control group of 25 participants without PHP who were matched at recruitment for age (± 5 years), sex and body mass index (BMI) (± 10%). Fatty infiltration of ADM was assessed on all participants using magnetic resonance imaging (MRI). Prior to assessment, four grading scales of fatty infiltration were investigated for reliability by two independent assessors using Kappa or weighted Kappa and the most reliable scale was chosen for the primary data analysis. Following this, participant characteristics were compared between the PHP and the no-PHP groups using Chi-squared, Mann-Whitney U or independent samples t-tests to ensure there were no significant differences between the two groups in characteristics that could have confounded the findings. The association between PHP and fatty infiltration was analysed using the Chi-square test (χ(2)). RESULTS: In the PHP and no-PHP groups, respectively, the mean age was 49.1 and 48.9 years, women comprised 58% and 56%, and the mean BMI was 30.6 and 30.2 kg/m(2). A four-point grading scale was found to be the most reliable scale (0.87 and 0.92 for inter- and intra-rater reliability, respectively). There were no significant differences (p > 0.05) for important participant characteristics (e.g., age, sex and BMI). There was some fatty infiltration (grades 1-3) in 78% and 96% of participants in the PHP and no-PHP groups respectively. No significant association was found (χ(2) = 6.176, df = 3, p = 0.103) between PHP and fatty infiltration according to the four-point grading scale. CONCLUSIONS: After accounting for age, sex and BMI, there was no association between PHP and fatty infiltration of ADM. These findings suggest that adults with PHP do not have a greater predisposition for or amount of fatty infiltration of ADM compared to adults without PHP. Therefore, fatty infiltration of ADM is likely to be an incidental finding on MRI rather than a diagnostic sign of PHP, where Baxter's neuropathy may be present. Accordingly, clinicians should not focus on fatty infiltration of ADM on MRI to diagnose Baxter's neuropathy, or view it as a proxy marker, particularly when surgery for PHP is being considered.