title: [Biomechanical principles in diarthroses and synarthroses. II: The humerus articulation as a ball-and-socket joint].
abstract: The shoulder joint does not represent a simple ball-and-socket joint with three degrees of freedom unlike widely believed. Series of X-ray pictures show that in abduction the momentary rotational axis does not meet the center of the humerus. Morphological measurements yield the result that the radius of curvature of the cavitas is slightly greater (2 mm) than the humeral one. This physiological incongruity of the articulating surfaces is functionally necessary. Thus, the shoulder joint represents a force-locked dimeric ball-and-socket joint. In abduction e.g. the humerus simultaneously rotates around the humeral center and around the cavitas center in opposite direction. The resultant momentary rotational axis is therefore shifted towards the scapula as in vivo measurements confirm. Fundamental kinematical and statical properties of the joint are derived which explain substantial experimental finding in literature.
authors.affiliation: IV. Physikalisches Institut, Univ. Göttingen.
authors.forename: H
authors.initials: H
authors.lastname: Nägerl
authors.forename: D
authors.initials: D
authors.lastname: Kubein-Meesenburg
authors.forename: H
authors.initials: H
authors.lastname: Cotta
authors.forename: J
authors.initials: J
authors.lastname: Fanghänel
authors.forename: S
authors.initials: S
authors.lastname: Kirsch
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D006811:Humerus / Q000033:anatomy & histology; D009119:Muscle Contraction; D011859:Radiography; D016059:Range of Motion, Articular; D012785:Shoulder Joint / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000502:physiology*
vernacular_title: Biomechanische Prinzipien in Diarthrosen und Synarthrosen. Teil II: Die Articulatio Humeri als Dimeres Kugelgelenk.