It’s putting the “bone” in — well, you know.

After falling on his rear end on the street, a 63-year-old man went to the emergency room for knee pain. Doctors, worried that he might have some additional broken bones, conducted a pelvic x-ray.

Instead, they discovered that his penis was turning to bone.

“An extensive, plaque-like calcification along the expected distribution of the penis was evident,” study authors — five of whom work at the Bronx’s Lincoln Medical and Mental Health Center — write in a report of the case, which will be published in Urology Case Reports next month.

The man with the member was not identified in the report, although it mentions that he walked with a cane and had a history of alcoholism.

He told doctors that he had experienced some pain in his pelvic region, but no other symptoms (such as swelling or discharge).

Then he left — against medical advice — before doctors could examine his rare condition further, or formally diagnose him.

“The diagnosis of penile ossification along the entire penile shaft was suspected,” the study says. But “no laboratory investigation, histological examination, or follow-up was done.”

Johnsons turning to bone “is exceedingly rare,” the researchers write: There are fewer than 40 published case reports of the painful-sounding phenomenon.

Even in those rare cases, the hardening typically only affects part of the shaft — but this unlucky patient showed signs of ossification on his entire member, the doctors say.

“Our patient probably presented in the acute phase of his disease due to the presence of a penile pain,” the report reads.

In most cases, penis ossification is linked to a condition called Peyronie’s disease, in which scar tissue develops inside the shaft. This can cause the organ to become inflamed and curve. It makes erections painful and can also lead to erectile dysfunction.

Besides Peyronie’s disease, syphilis, gonorrhea and trauma can also cause bone cells to form in a member.

Treatment depends on the severity of the condition, but can include painkillers, “stretching” or vacuum devices, shockwave therapy or surgery to remove the calcified tissues.