Dwayne 'the Rock' Johnson is showing off his gnarly bruise from a ruptured bursa sac. What is that?

Dwayne Johnson speaks onstage.
Dwayne Johnson has big arms and a big bruise. (Jerod Harris/Getty Images for CinemaCon)

From muscles to action movies, Dwayne “the Rock” Johnson does everything big. Now he has a very large bruise — or “boo boo,” as he calls it — to match. Johnson shared a photo of his black-and-blue (and purple) injury on Instagram Thursday and explained that he had a “ruptured bursa sac that will eventually heal.” Johnson said that the injury started out as a “cantaloupe”-size lump at his elbow, and happened during filming for his forthcoming movie The Smashing Machine, a biopic about UFC champion Mark Kerr.

So what exactly is a bursa sac? And what’s a bursa sac rupture? Here’s what to know.

Bursa sacs are small, thin pouches, or sacs, of fluid found at many points on the body, including the elbows, hips and kneecaps. They act as cushions for bony, friction-prone joints, according to Mayo Clinic. When a bursa is injured or infected, the lining of the pouch can produce immune-cell-rich inflammation to help the area heal.

The most common cause of this swelling, known as bursitis, is trauma from a fall or repeated pressure from, for example, constantly leaning on your elbow, Dr. Asad Siddiqi, a sports medicine expert at Weill Cornell Medicine and New York-Presbyterian, tells Yahoo Life. Bursitis or bursa ruptures are common among athletes. “We see them on the outsides of hips in our football players, or over the kneecap,” says Siddiqi.

“Rupture of a bursa,” like Johnson’s, “is more rare than bursitis itself,” Siddiqi says. There are two main forms of bursa ruptures: septic, when the sac swells to fight an infection, and hemorrhagic, when the sac ruptures, often due to trauma, and fills with blood, he explains. Siddiqi believes that Johnson’s black-and-blue injury is a hemorrhagic rupture.

In most cases, a ruptured bursa sac will “heal on its own, and the body can form a new bursa in place of the old bursa, in response to additional compression, friction or pressure,” says Siddiqi.

It usually takes a matter of weeks to heal. Some inflammation at and around the location of the bursa sac is to be expected, says Siddiqi. “If it’s hemorrhagic,” like Johnson’s appears to be, “blood can be irritating to the area, so you try to mobilize it with anything from compression to gentle massage or myofascial release,” he explains, adding that warm compresses are good to keep the blood from clotting up too.

Ruptures usually don’t require any other treatment. When the bursa hasn’t ruptured but there is chronic swelling that doesn’t go away with time, anti-inflammatory medications and massage, it might need to be drained or, in extreme cases, surgically removed, explains Siddiqi.

Generally, no. “As bruising resolves ... there is not a prolonged recovery and it doesn’t have any long-term effects on function or length of motion,” Siddiqi says. However, he always checks bursa sac ruptures to be sure there are no cuts that could become infected, which can cause complications like pain and tendon weakness if left untreated.

Otherwise, anyone with a bursa sac rupture should rest while the injury heals and avoid whatever activity led to the injury. Will the Rock rest? “I’ve had much worse, and as always let pain — and tequila — be the guide 😈 Back to work,” he wrote.