The injury, termed acromial apophysiolysis by the researchers, is characterized by incomplete fusion and tenderness at the acromion. The acromion, which forms the bone at the top or roof of the shoulder, typically develops from four individual bones into one bone during the teenage years.
“We kept seeing this injury over and over again in young athletes who come to the hospital at the end of the baseball season with shoulder pain and edema at the acromion on MRI, but no other imaging findings,” said Johannes B. Roedl, M.D., a radiologist in the musculoskeletal division at Thomas Jefferson University Hospital in Philadelphia.
To investigate the unexplained pain, Dr. Roedl and a team of researchers conducted a retrospective study of 2,372 consecutive patients between the ages of 15 and 25 who underwent magnetic resonance imaging (MRI) for shoulder pain between 1998 and 2012. The majority of the patients, which included both males and females, were baseball pitchers.
“Among high school athletes, pitching is the most common reason for shoulder pain,” Dr. Roedl said.
Sixty-one of the patients, (2.6 percent) had pain at the top of the shoulder and an incomplete fusion of the acromion but no other findings. The patients were then age and sex-matched to patients who did not have the condition to form a control group.
Pitching history was available for 106 of the 122 patients included in the study. Through statistical analysis, the researchers found that throwing more than 100 pitches per week was a substantial risk factor for developing acromial apophysiolysis. Among the patients with this overuse injury, 40 percent threw more than 100 pitches per week, compared to 8 percent in the control group.