Rare concomitant thoracic deformities: navigating the interplay between Scheuermann kyphosis and pectus excavatum. Illustrative case.

BACKGROUND: Scheuermann kyphosis and pectus excavatum rarely present concomitantly. The sternal-rib complex functions as a "fourth column" of thoracic spine support; its disruption can destabilize the spine and exacerbate kyphosis progression. Traditional surgical indications include curves exceeding 70°-75°, but compensatory mechanism assessment may provide additional indicators. OBSERVATIONS: A 16-year-old male presented with thoracic kyphosis (77°) and pectus excavatum (Haller index 2.8). Staged treatment included the Nuss procedure followed by observation. Despite physical therapy, kyphosis prog ressed to 90° over 28 months. Equal standing versus supine extension lumbar lordosis measurements (78°) indicated exhausted compensatory mechanisms. The patient underwent T2-L2 posterior spinal fusion with Smith-Petersen osteotomies while the Nuss bar remained in place, with bar removal 8 months later. LESSONS: This case demonstrates the biomechanical interdependence between thoracic spine and anterior chest wall as a fourth column and introduces compensatory mechanism assessment as a novel surgical indication. The successful retention of the Nuss bar during spinal correction represents a technical innovation minimizing surgical morbidity. This paradigm shift from curve magnitude-based to functional capacity evaluation may revolutionize timing decisions for spinal deformity correction. https://thejns.org/doi/10.3171/CASE25496.