Disuse bone atrophy was induced in 10 adult dogs by means of brachial plexus section and/or elbow disarticulation. After 9 to 12 weeks of disuse intact humeri were examined by X-ray, and their physical properties determined. The whole humeri were isolated, defatted, dried to constant weight, and their mineral and collagen content determined.Eight out of 10 experimental (non-ueed) humeri demonstrated evidence of decreased radiodensity. The non-used limb demonstrated parallel loss in dry, fat-free weight (--23.2 % ), in collagen (--25.3 %), and in mineral (--26.1% ), as compared to the normal limb. The data indicated that the major portion of the lost bone tissue in disuse osteoporosis is replaced by water, fat, and other unidentified organic materials rather than fibrous tissue, and that collagen is lost in equal proportion to mineral. The proportionately greater loss of collagen and mineral than of dry, fat-free weight appears to be due to an increase of non-collagenous, non-lipid organic material, presumably protein.Atrophic osseuse d'immobilisation 6fair induite par section de la plexus brachial et/ou Immobilisations-Knochenatrophie wurde in 10 erwachsenen Hunden herbeigefiihrt dutch Brachialplexusschnitt und/oder Ellenbogenexartikulation. Nach 9--12 Wochen der Immobilisation wurden die intakten Humeri unter R6ntgenstrahlen untersucht und ihre physischen Eigenschaften bestimmt. Der intakte Humerus wurde isoliert, entfettet, zu konstantem Gewicht getrocknet und der Knochenaschengehalt und Kollagengehalt bestimmt.Acht der 10 experimentellen (immobilisierten) Humeri demonstrierten Beweis yon verringerter R5ntgendichte. Die immobilisierte Extremit~it zeigte ~ihnlichen Verlust in trockenem, fettfreiem Gewicht (--23,2%) in Kollagen (--25,3%) und Knochenasche (--26,1%) im Vergleich zur normalen Extremit~t. Die Daten deuten an, dal3 der Hauptanteil des Verlustes
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