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Chapter Summary
• Context: Kyphosis is an integral component of the physiological sagittal curvature of the human spine, but becomes pathological when sagittal alignment alters in a manner incompatible with global balance, functional capacity, or neurological safety. Its clinical interpretation requires distinguishing normal physiological thoracic kyphosis from hyperkyphosis, and recognizing that curves with identical Cobb angles can exhibit vastly different morphometry, rigidity, segmental span, and biomechanical sequelae. Furthermore, compensatory adjustments in the cervical spine, lumbar lordosis, pelvis, and lower extremities can mask the true magnitude of underlying structural sagittal collapse. The chapter reviews a comprehensive etiological spectrum, including postural roundback, Scheuermann's disease, congenital vertebral malformations, iatrogenic post-laminectomy deformities, ankylosing spondylitis, metabolic disorders, neuromuscular diseases, and skeletal dysplasias. In every clinical scenario, diagnostic reasoning must synthesize the anterior column, posterior tension band, spinal canal patency, global sagittal balance, and disease natural history. Treatment selection depends not on an isolated angular threshold, but on the interplay among etiology, progression velocity, rigidity, symptoms, neural status, and spinopelvic harmony.
• Chapter Objective: To present the core concepts of sagittal alignment and elucidate the diverse etiologies producing pathological kyphotic deformities. The reader will master clinical and radiographic evaluation, recognize distinct pathognomonic features of major etiologies (postural, Scheuermann, congenital, iatrogenic, inflammatory), differentiate flexible from rigid structural curves, and understand the foundational principles governing observation, conservative orthoses, multi-level posterior column osteotomies (PCO/Ponte), three-column pedicle subtraction osteotomies (PSO), vertebral column resections (VCR), and targeted neural decompression.
• Kyphosis and global sagittal balanceThe normal human spine develops regional reciprocal curvatures postnatally that function in concert with the pelvis. Figures 1 and 2 illustrate this developmental biomechanics. The chapter emphasizes that sagittal evaluation cannot focus solely on the kyphotic apex: compensatory cervical hyperlordosis, lumbar hyperlordosis, and pelvic retroversion temporarily maintain global horizontal gaze and balance. Figure 3 differentiates anatomical from functional segmentation, while Figure 4 proves that two kyphotic curves with identical 70° Cobb angles can have completely different arc radii and clinical significance.
• Shared biomechanical failure mechanismsPathological kyphosis arises from structural failure of the anterior column under compression, insufficiency of the posterior tension band under tension, or a combined circumferential defect. As the gravity line shifts anteriorly, the flexor bending moment arm increases, mechanically accelerating deformity progression. The universal reconstructive principle is restoring anterior structural support and establishing posterior tension band stability while preserving spinal canal patency. Historically requiring combined anterior-posterior approaches, modern posterior-only osteotomies and resections allow single-stage posterior correction (Figures 6 to 8).
• Postural kyphosis and Scheuermann's diseasePostural kyphosis is fully flexible, lacks structural vertebral wedging, and responds reliably to physical therapy and posture re-education; surgery is never indicated. Scheuermann's disease is a structural deformity characterized by anterior vertebral body wedging (≥5° across ≥3 contiguous vertebrae), endplate irregularities, and Schmorl's nodes (Figures 10 to 13). Evaluation incorporates pain, skeletal maturity, curve rigidity, and progression. Most patients respond to conservative management (exercise, extension bracing). Progressive, rigid curves (>70-75°) with intractable pain may warrant posterior instrumented fusion with Ponte osteotomies (Figures 14 to 17).
• Congenital kyphosisCongenital kyphosis arises from embryonic vertebral formation failures, segmentation failures, or mixed defects (Figures 18 to 20). Natural history is aggressive, particularly in Type I formation failures, carrying high risks of progressive paraparesis. Early localized fusion in infants halts progression before extensive secondary structural curves develop. In delayed, rigid deformities, posterior vertebral column resection (PVCR) or three-column osteotomies are required (Figures 21 to 23).
• Acquired kyphosis and systemic conditionsIatrogenic kyphosis typically results from multi-level laminectomies without fusion (Figures 24 to 27). The chapter also details fixed sagittal deformities in ankylosing spondylitis ('chin-on-chest' deformity), achondroplasia, neurofibromatosis type 1, and skeletal dysplasias (Figures 28 to 35). In each disease, systemic biology dictates unique mechanical vulnerabilities and neurological risks.
• Clinical Application: In clinical practice, the initial step is determining whether the kyphosis is postural, physiological, or structural. Physical examination evaluates spontaneous posture, prone hyperextension active self-correction, pelvic tilt, hamstring tightness, and upper motor neuron signs. Standing full-spine radiographs must capture the entire sagittal profile from occiput to femoral heads. Prone bolster hyperextension radiographs differentiate flexible from rigid curves. CT and MRI evaluate bony bridges, endplate wedging, canal stenosis, and spinal cord draping over the kyphotic apex. Postural curves require only exercise and reassurance. In Scheuermann's disease, bracing (e.g., Milwaukee or modern bivalve TLSO) is effective during growth spurts (Risser 0-2), whereas surgery is reserved for severe progressive painful curves (>70-75°) in mature patients, fusing from the proximal measured vertebra down to the stable sagittal vertebra to avoid distal junctional kyphosis (DJK). In congenital kyphosis, observation is hazardous; early short-segment posterior fusion in young children prevents devastating neurological deficits and avoids extensive late osteotomies. In ankylosing spondylitis or severe iatrogenic flatback, planned three-column osteotomies (PSO) at L2 or L3 restore lumbar lordosis and horizontal gaze while protecting major anterior vascular structures.
