The hidden mechanism of vacuum bell therapy: Local fat hypertrophy drives cosmetic outcome in adolescents with pectus excavatum.

Purpose: Vacuum bell (VB) therapy is a non-invasive treatment option for pectus excavatum (PE), traditionally considered most effective in children under 12 due to greater skeletal remodeling potential. However, the extent and mechanisms of cosmetic improvement in older adolescents remain poorly understood. Methods: We retrospectively analyzed 19 male patients (median age 14.8 years) who underwent VB therapy for at least 1 year. Real-time MRI was performed before and after therapy to assess morphological changes. Key parameters included the Haller Index (HI), Correction Index (CI), and pectus excavation depth (PED). Additionally subcutaneous soft tissue thickness was measured at the deformity site and at the lateral thoracic wall. Results: After a median treatment duration of 1.8 years, HI and CI did not show significant reductions (HI: 5.4 to 5.3, p = 0.40; CI: constant 32 %, p = 0.96), while PED decreased significantly (16.7 mm to 112.7 mm, p = 0.009). Subcutaneous soft tissue at the deformity site increased markedly by a median of 6.5 mm, compared to a 0.5 mm increase at the lateral thoracic wall. Fat-selective T1-weighted Dixon MRI confirmed that the increased subcutaneous tissue consisted of adipose tissue. Conclusions: The improvement in visual appearance of PE morphology during VB therapy results primarily from localized adipose tissue hypertrophy, while skeletal remodeling is likely to contribute to a much lesser degree. From a cosmetic perspective, these findings might justify the continued use of VB therapy beyond the traditional age threshold.