The effect of age on muscle characteristics of the abductor hallucis in people with hallux valgus: a cross-sectional observational study

BACKGROUND The abductor hallucis muscle plays an important role in maintaining alignment of the first metatarsophalangeal joint. The aims of this study were (1) to determine differences in abductor hallucis muscle characteristics in people with hallux valgus between three age groups (20-44 years, 45-64 years, and 65+ years); and (2) to determine the association between age and abductor hallucis size and quality. METHODS Characteristics of the abductor hallucis muscle were measured in 96 feet with hallux valgus using musculoskeletal ultrasound. Muscle characteristics included width, thickness, cross-sectional area and echo-intensity. A one-way ANCOVA was conducted to compare the mean muscle characteristic values between the three age groups while adjusting for hallux valgus severity as a covariate. A Bonferroni post-hoc was used to adjust for multiple testing (p < 0.0167). Spearman's rho correlation coefficient was used to determine the association between age and the abductor hallucis muscle parameters. RESULTS There was a significant difference in dorso-plantar thickness (p = 0.003) and cross-sectional area (p = 0.008) between the three age groups. The Bonferroni post hoc analysis revealed a significant difference in mean thickness and mean cross-sectional area between the 20-44 age group (p = 0.003) and the 65+ age group (p = 0.006). No significant differences were noted between the three age groups for medio-lateral width (p > 0.05) or echo-intensity (p > 0.05). Increasing age was significantly associated with a reduction in dorso-plantar thickness (r = -0.27, p = 0.008) and cross-sectional area (r = -0.24, p = 0.019) but with small effect sizes. There was no significant correlation between age and medio-lateral width (r = -0.51, p = 0.142) or echo intensity (r =0.138, p =0.179). CONCLUSION Increasing age is associated with a greater reduction in size of the abductor hallucis muscle in people with hallux valgus. People over the age of 65 years old with hallux valgus display a significant reduction in abductor hallucis muscle size compared to those aged less than 45 years old. This is consistent with age-related changes to skeletal muscle.