Every year, Major League Baseball spends more than $1.5 billion on pitchers. Their import is exceeded only by their fragility. One tiny band of tissue in the elbow, the ulnar collateral ligament, is snapping at unprecedented rates, leaving current big league players vulnerable and the coming generation of baseball-playing children dreading the three scariest words in the sport: Tommy John surgery. Jeff Passan traveled the world for three years to explore in-depth the past, present and future of the arm, and how its evolution left baseball struggling to wrangle its Tommy John surgery epidemic.