The American Journal of Sports Medicine. This one, which began in 1999 before pitch counts were the norm in youth baseball, used innings pitched as the primary risk factor to be measured. The study also looked at pitchers who played the position of catcher and those who threw curveballs before the age of 13.
The study found that youth pitchers throwing more than 100 innings per calendar year were 3.5 times more likely to experience a serious arm injury. It found a small increase in injuries for those pitchers also playing catcher. But it was unable to find a correlation between curveballs and injuries.
The details of this study, as well as a good summary on youth pitching and previous research on the subject, are in the AJSM publication. This is not exactly new information - we know quite a lot more about overuse injuries than we did in 1999. But it is further confirmation and another reminder of how careful we need to be about the number of pitches our boys throw, how fatigued they get when pitching, and how much rest they get between outings. Taking these precautions, which are mostly mandated by Little League rules, and teaching proper mechanics will go a long way toward keeping our pitchers safe.