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Anatomy and Biomechanics
The shoulder forms as a single cartilaginous anlage at 5 weeks gestation. Between 6 and 7 weeks of gestation, a cleft forms in the anlage creating the glenohumeral joint. The upper humerus ossifies from three separate centers:
- The head, appearing at 6 months,
- The lesser tuberosity, appearing at 5 years,
- The greater tuberosity, appearing in the 2nd or 3rd year of age.
The tuberosities unite at age 5 and fuse with the head between 7 and 14 years of age. The head subsequently fuses with the shaft by age 19. The scapula develops from seven or more ossification centers.
The articular surface always overlies physeal cartilage. The growth plate of the proximal humerus is concave, with its apex posteromedial where the periosteum is also strongest. The medial most portion of the proximal humeral epiphysis is intra-articular. The teres minor, infraspinatus and supraspinatus insert posteriorly and superiorly; the subscapularis inserts into the region of the lesser tuberosity; and the pectoralis major inserts into the metaphysis of the proximal humerus.
The shoulder joint has little intrinsic stability, allowing for a wide range of motion. The ligamentous anatomy is similar to that seen in adults. In children, the physis is more likely to fail than the capsule of ligaments, making physeal fracture more likely than a dislocation of the shoulder (Slide 1 and Slide 2).
The proximal humeral physis accounts for 80% of the longitudinal growth of the physis. Before age 2, less than 75% of growth occurs, increasing to 85% at age 8, and 90% at age 11.
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