The external Haller index as a new, reliable, and accurate evaluation of pectus excavatum associating clinical assessment, 3-D surface scanning measurement, and MRI

INTRODUCTION: Pectus excavatum (PE) is the most common chest wall deformity. The traditional Haller index (HI), as measured using computed tomography (CT), guides surgical decision-making. Radiation-free magnetic resonance imaging (MRI) and less expensive alternatives, such as clinical thorax calipers or 3-D imaging, can measure the external Haller index (EHI). MATERIALS AND METHODS: PE patients and controls with normal thoracic anatomy (other diagnoses) were enrolled prospectively (2015-2021). All participants underwent cardiac MRI as part of routine clinical practice. PE patients also underwent a clinical assessment, pulmonary tests, and a dedicated 3D surface scanning measurement. Inter-rater reliability was assessed for every subject's HI, measured using MRI, and for each PE patient's EHI, measured using the intraclass correlation coefficient (ICC). The EHI's and the HI's diagnostic accuracy for discriminating PE patients from controls was evaluated using the area under the curve (AUC), and best cut-offs were identified using the Youden method. RESULTS: We enrolled 51 PE patients (14.1 ± 2.0 years) and 30 controls (14.4 ± 1.8 years) with similar demographic characteristics. Inter-rater reliability was excellent for both MRI-HI (ICC = 0.97) and EHI (ICC = 0.96). Both indices demonstrated excellent diagnostic accuracy (MRI-HI AUC = 0.99, cut-off = 3.29; MRI-EHI AUC = 0.95, cut-off = 1.68). The EHI derived from MRI strongly correlated with values from clinical caliper and 3D surface scanning (p < 0.05). CONCLUSION: The traditional HI cut-off of 3.25 can be effectively replaced by an EHI cut-off of 1.68, obtained with a clinical caliper or 3D imaging. Its excellent reliability makes it ideal for use in pediatric orthopedic and surgical clinics, bridging the gap between complex imaging and actionable clinical decision-making.