Structural Deformities of Vertebrae: Bone Adaptation and Modeling in Experimental Scoliosis and Kyphosis

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1. Scoliotic and kyphotic curvatures were induced in eleven monkeys by means of resection of the sacrospinalis muscles and interspinous and flaval ligaments. Curve progression was followed roentgenographically and histologically.2. Wedge-shaped and irregularly quadrilateral vertebrae resulted, from adaptive modeling during growth. Their similarity to structurally deformed vertebrae of idiopathic scoliosis and juvenile kyphosis is demonstrated.3. Much of the scoliotic curve was accounted for by intervertebral-disc distortation.4. Failure of ligaments of the arch, as predicated by Ponseti, may have a role in the pathogenesis of the deformity.5. Adaptive deformity of growing bone depended on at least the following mechanisms:A. Asymmetrical chondrogenesis and osteogenesis at the metaphysis, the result of unequal compression or tension, or both. Growth suppression or epiphyseodesis followed sufficient compression. Acceleration of cartilage growth and consequently of osseous metaphyseal growth followed tension applied to this region. Together they caused vertebral wedging.B. Unequal apposition and resorption of compact bone were related to the state of tension in periosteum and contributed to modeling by accretion and erosion of bone respectively.C. Reorientation of spongiosa was accomplished in part by formation of new trabeculae at two sites: the endosteal surface of expanding compact bone and the active metaphysis.6. The most significant observation is that adaptational deformity of growing bone is the result of asymmetrical, quantitative alterations, not perversions of normal growth.7. Vertebral ring epiphyses contributed to the body rim and only slightly to its total height. They played no part in structural curves, and when compressed by curvature failed to develop, as the result of interference with their vascularization.8. The pathogenesis of idiopathic spinal deformity is believed to depend on separate conditions operating consecutively or together: predisposing, precipitating, perpetuating, and terminating. These factors are discussed within the context of the report.

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