NEW YORK - "Embedding" is a severe form of self-injury among teenagers that involves inserting objects into the skin or muscle and appears to be linked to thoughts of suicide and major psychiatric disorders, according to a US study.

The behavior, while rare, is on the spectrum of self-harming behaviors as a much more severe form, added the study, which appeared in "Pediatrics." It noted that all the patients involved had bipolar disorder.

"There's clearly a more severe intent to hurt themselves than cutting," said William Shiels, a radiologist at Nationwide Children's Hospital in Columbus, Ohio, and one of the authors of the study.

"Inserting a 16 cm paperclip -- not just to do that on one arm, but both arms -- the intent that's required to cause that much self harm is significant."

Self-injury, which often takes the form of cutting or burning, is a fairly common behavior, with estimates ranging between 4 percent and 30 percent of youth who have hurt themselves in some way.

The pain involved in self-harm is thought to provide a sense of psychological relief, and is generally not considered part of a suicide attempt.

Shiels and his colleagues had noticed that several patients at his hospital required objects to be removed from their bodies -- objects that were intentionally put there.

To see if there were similar cases, they looked through 600 medical records of children who had material removed from their tissue, and found 21 instances of intentional embedding among 11 patients between the years 2005 and 2008.

All of the patients were teenagers between 14 and 18 years old, most of them girls, and had come to the hospital because they had admitted embedding an object or because they ended up with an infection at the site.

Staples, pencil lead, and paper clips were the most common objects, often inserted into the arm.

All the patients had bipolar disorder and most suffered additionally from post-traumatic stress disorder. Others also had depression, borderline personality disorder, or attention deficit-hyperactivity disorder.

The study also found out that 90 percent of those who embedded had suicidal thoughts.

Ten of the patients lived in a group psychiatric home, though not the same one.

Shiels said that the multiplicity of disorders among the teens was one of the differences between embedding and other forms of self-injury.

"With self-embedding behavior we see more severe behavioral health abnormalities," he added.

Shiels could not estimate how common self-embedding is and said he'd like to set up a national registry to get a better idea of how many people do it.

Nancy Heath, a professor at McGill University who was not involved in the study, said that self-injury among children is usually tied to a sense of relief or feeling better.

"Self-embedding behavior does not appear to have that. It's much more about self-harm," she said.

"I think it is definitely a phenomenon that is out there."