Developmental anomalies of the scapula-the "omo"st forgotten bone

Congenital malformations of the scapula, ranging from complete absence, to abnormal shape and position (Sprengel anomaly) are encountered, not infrequently, in genetic practice. Despite this, little is known of the embryologic origin of the scapula and the relationship of the embryology to the observed birth defects. Standard embryology texts, when discussing the subject at all, generally consider the scapula as part of the upper limb. The pattern of associated birth defects suggests that this is at least an oversimplification and may be inaccurate. Sprengel anomaly is the most frequently encountered malformation of the scapula. It can be seen in isolation, but is often seen in association with other defects that include; scoliosis, hemivertebrae, segmentation abnormalities of vertebrae and ribs (including Klippel-Feil sequence), spina bifida, clavicular abnormalities, renal abnormalities and hypoplasia of the muscles of the neck and shoulder. The ipsilateral limb is usually normal. An unappreciated association between Sprengel anomaly and diastematomyelia of the lumbar spine also raises questions about the embryologic origin of the scapula. 25-50% of Sprengel anomaly patients have an associated omovertebral band or bone that arises from the posterior process of a vertebral body and attaches to the superior angle/medial portion of the scapula. This is felt to be of scapular origin, but the report of at least one patient with an omovertebral bone, not associated with a Sprengel anomaly questions this assumption. Scapuloiliac dysostosis (Kosenow syndrome), a rare skeletal dysplasia, is associated with marked hypoplasia of the scapulae, clavicles and pelvis. Associated anomalies include eye anomalies, rib anomalies and spina bifida. The limbs are normal. Knockout of the Emx2 gene in mice yields a similar skeletal phenotype. Mutations in EMX2 in humans are associated with schizencephaly, not skeletal anomalies. Data on gene expression in the scapula will be reviewed. Based on this information, the author proposes that the scapula arises from 2 or more distinct embryologic anlage under different genetic control than the upper limb.