The first sternocostal joint is uniquely a synchondrosis (not a synovial joint) — it fuses the first rib cartilage directly to the manubrium with no synovial cavity. This makes it immobile and resistant to trauma in a different way from the synovial sternocostal joints 2-7. The manubriosternal joint (angle of Louis) between the manubrium and body of the sternum is a secondary cartilaginous joint (symphysis) at the level of the second rib.
The first sternocostal synchondrosis calcifies with age, becoming radiographically apparent as the 'Tic Tac Toe' calcification pattern of the upper costal cartilages. The angle of Louis (manubriosternal joint) is the landmark for counting ribs — the second rib articulates at this level.
First rib fracture through the synchondrosis from repetitive overhead activity in throwing athletes producing neck-shoulder pain managed with rest from throwing.
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