A man's penis is slowly solidifying and turning to bone, leaving doctors scratching their heads.

The unidentified man is the subject of a new US medical study to be published next month.

In the video above: MMA fighter tears penis

The bizarre medical phenomenon started when the 63-year-old man fell on his bum and presented to an emergency room with knee pain.

Doctors, concerned he could have broken bones, conducted a pelvic x-ray and made the startling discovery.

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“The diagnosis of penile ossification along the entire penile shaft was suspected,” the study says.

However, no "laboratory investigation, histological examination, or follow-up was done".

Though the man was unidentified in the report, it says he had a history of alcoholism and walks with a cane.

Running with a limp (probably)

The man left the emergency room before doctors could examine the rare condition further.

Penile ossification is extremely rare and painful, with less than 40 case reports published worldwide.

Treatment depends on the severity and extent of the condition, but can include surgery to remove calcified tissue.

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