Teenaged baseball players who throw more than 100 pitches per week are at heightened risk of an injury that could permanently mar normal shoulder development, says a new study.

The injury, dubbed “acromial apophysiolysis” by the authors, is characterized by incomplete fusion of the bones that form the top portion of the shoulder joint, the acromion, and local swelling and fluid (edema).

“Over the years,” said lead author Dr. Johannes Roedl, the study team had noticed young baseball players “who came in at the end of the season with shoulder pain, but with MRI imaging on which we really didn't see anything besides the abnormality, that edema at the acromion.”

Roedl, a radiologist in the musculoskeletal division at Thomas Jefferson University Hospital in Philadelphia, said that neither the radiologists nor orthopedic surgeons knew if these abnormalities were clinically relevant, but they knew something was going on, so they decided to take a closer look at all the cases they had seen.

“We looked at all the clinical data that we had from the orthopedic surgery department,” Roedl said. “In terms of how did the patients present, where was the pain, and of course, their history in terms of sports - what kind of sports did those patients play?”

He said they were surprised at how many patients appeared to have the condition.

“I mean it's still a relatively rare condition – only about 2.5 percent of patients at the age range between 15 and 25 who come in with shoulder pain have that acromial apophysiolysis,” he said.

Roedl said it is most likely an overuse injury from too much pitching because most of the patients who had it were “really avid pitchers - the majority pitched more than one hundred pitches per week.”

He and his colleagues reviewed medical records for more than 2,000 patients, both male and female, between the ages of 15 and 25 who had MRIs for shoulder pain between 1998 and 2012. Most of the patients were pitchers.

A total of 61 patients had pain at the top of the shoulder and incomplete fusion of the acromion but no other radiological findings. The study team compared them to a control group of 61 similar patients who had other identifiable causes for their shoulder pain.

The researchers found that 40 percent of the patients with acromial apophysiolysis threw more than 100 pitches per week compared to 8 percent of the control group, they report in the journal Radiology.

One patient underwent surgery and all of the patients rested their pitching arms for three months and took nonsteroidal anti-inflammatory (NSAID) medications.

The research team was also able to review follow-up images, either MRIs or X-rays, for 52 participants after they were 25 years old, the age when bone development would be complete. The average age at follow-up was 27 and a half, and the average interval since the original images was eight years.

Of the 29 patients with apophysiolysis as teens, 25 showed incomplete bone fusion at the acromion at follow-up, compared to only one out of the 23 controls.

Torn rotator cuff muscles were also more common - and worse - in the adult patients who had apophysiolysis than in the control group.

“You can imagine if the bone in the shoulder doesn't fuse it’s kind of unstable,” Roedl said. “This is not a medical term, but it's essentially ‘floating’ and doesn't really have a fixed point.”

Roedl said that when a bone floats around it can press on tendons and those tendons can rupture or tear.

“And that's what we saw in those patients - that they more often had tendon tears of the rotator cuff . . . so it actually had a long-term effect on those patients,” he said.

“Overall, I think it’s a great radiology study. The authors describe the presentation of a new condition in a fairly large sample of patients that had previously not been done in that capacity,” Kyle Aune told Reuters Health in an email.

Aune is a clinical researcher with the American Sports Medicine Institute in Birmingham, Alabama. He wasn’t involved in the study.

But, Aune isn’t yet convinced the condition is the sole cause of rotator cuff tears.

“While possible, it is likely that these rotator cuff injuries would be due to a combination of factors including general overuse and impingement from biomechanical flaws within a pitcher’s throwing mechanics,” he said.

Aune also wasn’t sure that female softball pitchers should have been grouped with male baseball pitchers since the throwing motions are quite different.

“The majority of the girls in the study played softball – it’s a different pitching style, but they still have an over the head movement, and that's probably what causes the problem,” Roedl said.

Roedl added that in the future, they want to look closely at other sports to see if it’s really just confined to pitching or if it’s found in other athletes who use overhead motions, such as tennis, lacrosse or swimming.

As for prevention, he said that not overdoing the same pitching motion over and over is the key.

“It's important to pitch less than 100 pitches per week when you're young,” Roedl said. “Take the off-season off, take a break of two or three months,” he said.