23/01/2026
🦴 Thoracic Scoliosis Biomechanics – Convex vs Concave Side Explained
Thoracic scoliosis is a three-dimensional spinal deformity involving lateral curvature, vertebral rotation, and rib cage distortion. Biomechanically, the spine does not simply bend sideways; it twists and adapts under gravitational and muscular forces, leading to asymmetric loading of the vertebrae, ribs, muscles, and soft tissues.
🔄 Convex Side Biomechanics
On the convex side of the curve, the ribs are pushed outward and separated, causing the associated muscles to become lengthened and mechanically disadvantaged. Muscles such as the erector spinae, intercostals, and stabilizing scapular muscles are placed in a stretched position. Although they appear longer, these muscles often become weak and inefficient, reducing their ability to stabilize the spine against gravity.
The rib cage on the convex side rotates posteriorly, contributing to the characteristic rib hump seen during forward bending. This altered rib position changes thoracic mobility and affects breathing mechanics by limiting effective expansion of the lungs.
🔒 Concave Side Biomechanics
On the concave side, the ribs are drawn closer together, and muscles become shortened and overactive. Intercostals, paraspinal muscles, and accessory respiratory muscles on this side remain in a chronically contracted state. This increases compressive forces on the vertebral bodies and facet joints, often leading to stiffness, discomfort, and reduced spinal mobility.
The rib cage on the concave side rotates anteriorly, reducing thoracic volume and restricting lung expansion. Over time, this asymmetry can impair respiratory efficiency, especially in more pronounced curves.
⚙️ Vertebral & Disc Loading
Biomechanically, uneven muscle forces and gravity cause asymmetrical loading of the intervertebral discs. The concave side experiences greater compression, while the convex side undergoes tensile stress. This imbalance influences vertebral growth and disc health, potentially allowing the curve to progress if left uncorrected, especially during growth phases.
🚶 Posture & Kinetic Chain Effects
Thoracic scoliosis does not affect the spine in isolation. Pelvic alignment, shoulder positioning, and head posture all adapt to maintain balance. These compensations alter the mechanics of the cervical spine, lumbar spine, hips, and even gait, increasing energy expenditure and muscular fatigue during daily activities.
💨 Breathing & Rib Cage Mechanics
The rib cage plays a major role in respiration. In scoliosis, asymmetric rib motion leads to uneven lung expansion, with reduced ventilation on the concave side. This places increased demand on accessory breathing muscles and can contribute to early fatigue and reduced exercise tolerance.
🩺 Clinical & Rehabilitation Perspective
From a biomechanical standpoint, treatment should aim to decompress the concave side and activate the lengthened muscles on the convex side. Corrective exercises focus on improving thoracic mobility, restoring muscle balance, enhancing postural awareness, and optimizing breathing mechanics rather than simply strengthening indiscriminately.
📌 Key Biomechanical Insight
Scoliosis is a dynamic, three-dimensional problem driven by asymmetric forces. Understanding the difference between convex lengthened weakness and concave shortened stiffness is essential for effective rehabilitation and long-term spinal health.
