The Arm, by Jeff Passan, is a well-researched and chronicled look at the abundance of UCL tears and Tommy John surgeries in recent years. Passan provides a wide array of possible causes with no clear, decisive thesis of why the injury is so widespread. Overuse? Year round showcases like Perfect Game? High Pitch counts? Bad mechanics? After 342 pages, he does not offer a concrete answer for the epidemic because, at this point, no one really knows.