Paulina Porizkova is having hip replacement surgery at 58. Here's what to know about the procedure — and who needs it.

Paulina Porizkova is getting double hip replacement surgery. The model, 58, took to Instagram this week to tell fans that the procedure is "long overdue."

"It turns out I was born with congenital hip dysplasia — and because of that, the cartilage in my hips is worn out," she shared, adding that the "three doctors I've visited all recoiled when they saw my x rays." Doctors have told her that because she has no cartilage left, her hips are now "bone on bone."

"It's so bad, in fact, that doing one hip at the time is pointless," the former America's Next Top Model judge continued. "It would actually impede the healing and mobility. So. Had one last vacation with old hips — and now it's time for some new ones."

According to the American College of Rheumatology, over 450,000 hip replacements are performed in the United States each year, and this number continues to grow as the population ages. How does it work, and when might someone need one or both hips treated? Here's what to know.

Who might need hip replacement surgery?

Porizkova's hip issues were caused by developmental hip dysplasia. This is a condition in which the hip's ball and socket don't develop properly from birth, leading to a range of issues, from a shallow socket to a dislocated hip, Dr. Jessica Hooper, an orthopedic surgeon at Stanford Health Care, tells Yahoo Life. This condition increases the risk of arthritis at a younger age. The hip's cartilage wears away over time, resulting in bone-on-bone contact.

But hip replacement surgery isn't limited to those with hip dysplasia. In general, Hooper says, "it might be time to consider a consultation for hip replacement surgery if you have severe hip arthritis and consistent pain and/or stiffness that are affecting your quality of life on a daily basis." According to Verywell Health, you may also be a good candidate for hip replacement surgery if you have an injury or fracture of the hip, or hip osteonecrosis, a condition in which reduced blood flow leads to bone tissue death.

How does hip replacement surgery work?

As Verywell Health explains, a hip replacement can be either total (in which a portion of the pelvis and the head of the thigh bone are removed by an orthopedic surgeon and replaced with implants) or partial (in which only the damaged femoral head of the hip joint is replaced with a prosthesis). According to Hooper, the process also includes "smoothing out and resurfacing the inside" of the hip socket. To create a new surface, a metal socket is then inserted within the prepared bone, and a plastic liner is placed inside it. A metal implant is placed within the femur, and a ceramic ball is attached to it. When these parts are put together, the ball fits into the plastic liner inside the socket.

"All of the cartilage is gone, so the arthritis will not come back," Hooper says.

How common is double hip replacement surgery?

Porizkova's double hip replacement surgery, also known as bilateral hip replacement, is not as common as single hip replacement surgery. It is performed when both hips need intervention due to arthritis, injury or other conditions affecting joint function. According to Yale Medicine, there can be benefits to having both hips done at once, including only needing to go under anesthesia one time and only having to go through one single rehabilitation process.

Double hip replacement also doesn't require more downtime than single hip replacement. In fact, according to Johns Hopkins Medicine, recovery time may actually be shorter — however, it may be double the effort, as you must learn to strengthen both legs at the same time. Typically, patients undergo presurgical physical therapy, and continue their rehabilitation with the help of a physical therapist after the procedure.