The snapping scapula and subscapular exostoses.

complains of an audible, and often palpable, grating and snapping sound on shoulder girdle movements. Scapular movements are uncomfortable, and the noise can often be localised by the examiner to a specific part of the scapulo-thoracic “joint”. The condition was first described by Boinet in 1867 when he presented a young man of nineteen with discomfort on scapular movements and “une cr#{233}pitation rude qui rappelle assez bien celle de l’arthrite s#{232}che” . Demarquay, at the same meeting, described the necropsy finding of an exostosis arising from the deep surface of the scapula. Mauclaire in 1905 attempted to evaluate the significance of the various noises to be heard: a gentle friction sound was considered physiological but louder sounds were always pathological. In the early part of this century further accounts appeared in the European literature and one account appeared in America (McWilliams 1914). In 1933 Milch and Burman reviewed the literature and described as one cause of the condition a forward curve of the superior angle of the scapula, which gave symptoms by its articulation with the thoracic wall. Later, Milch (1950) ascribed the snapping to “some anomalous condition existing between the thoracic wall and the undersurface of the scapula”. Under normal circumstances the scapula glides over the thoracic wall, cushioned from the undulating surface of the ribs by the serratus anterior and subscapularis muscles. Its superior and inferior angles, and its medial border, are relatively poorly cushioned, and it is here that small projections or angles may cause friction effects. An exostosis in this situation would produce friction while relatively small, whereas exostoses arising elsewhere on the deep surface would be well protected by subscapularis and serratus anterior. In the latter case the exostosis would have to penetrate or produce pressure atrophy ofthe muscles before symptoms occurred. Milch (1950) described four cases of snapping scapula, one of which, in a girl of thirteen years, was caused by a subscapular exostosis articulating with the thoracic wall. Scapular