At first, it seemed like an ordinary search-and-rescue run for Ladder Company 6 in Chinatown. The World Trade Center was burning, and Bill Butler, then an eight-year veteran of fighting fires in New York, was on the 28th floor of the North Tower. But at 9:59 a.m. on Sept. 11, 2001, the adjacent tower collapsed, and he was told to evacuate.

Butler and five others from his unit reached the 20th floor, where they encountered a civilian struggling with an exhausted Josephine Harris, a 59-year-old bookkeeper. Butler and his crew slowed their pace to take over escorting her.

They reached the fifth floor, with Butler and a partner carrying Harris, when the North Tower collapsed. Miraculously, Harris, Butler and 13 other firefighters, including those from another unit, survived — their stairwell was spared in the collapse — although they were trapped for five hours, inhaling dirt, soot and toxic chemicals before they managed to escape.

The pain began almost immediately.

“I was spitting up drywall and other parts of the building. I coughed like crazy, came down with bronchitis several times a year, plus, I got back and hip pain,” Butler, now 53, tells The Post. He endured constant aching in his left hip, dulling it with 12 or so aspirin per day and treating it with physical therapy and chiropractic care.

By 2009, the hip pain got bad enough that Butler struggled to do his job, barely able to climb six-floor walk-ups and respond to fires in Fordham, where he was reassigned in 2004. He downplayed his agony for fear of being forced to retire prematurely, but by November 2015, the suffering — now in both hips — became too unbearable to ignore. He stopped actively fighting fires and was reassigned to do desk work.

“My right hip went from no pain to absolutely killing me,” he says. “There were sleepless nights. I couldn’t get comfortable in bed and ached all day. Guys working with me noticed that I had slowed down quite a bit.”

He finally went to a pain-management physician, who referred him to Dr. Nakul Karkare, an orthopedic surgeon and joint-replacement specialist in New York.

“Bill had severe arthritis,” says Karkare. “WTC rescue workers are at increased risk of seronegative [rheumatoid] arthritis because of an autoimmune disorder that chemicals and debris [that were in the air after the attack] can contribute to. There is a good chance his arthritis was caused by that.”

Karkare, however, works with a cutting-edge robotic arm called Mako, which assists in hip-replacement surgeries and makes them more effective and long-lasting. Most insurance companies cover Mako-aided operations just as they would surgery done by hand, rendering the robotic procedures no more expensive than the old-fashioned ones — and they increasingly are being recognized as the gold standard for bone work.

Mako allows surgeons to customize replacements for each patient and achieve new levels of precision when shaving diseased bone and inserting the new hip, making it less likely to pop out, which often can leave patients worse off than they were before surgery.

“It is much more accurate than a human can be,” says Karkare.

The high-tech promise did not initially assuage Butler’s fears of robosurgery.

“When you hear about a robot doing an operation on your bones, it does give you pause,” says Butler. “In reality, though, the doctor is working the robot.”

Plus, the software has been upgraded significantly since the robot was first used for total hip procedures in 2010, according to Bill Huffnagle, president of the reconstructive division at Stryker Orthopaedics, which owns the Mako technology.

Karkare, who was been using Mako since 2013, is also enthusiastic about it being configured to help with total knee replacements, beginning in April, and believes it may represent the future of orthopedic surgery. He has completed about 100 hip-replacement operations with Mako’s help and believes that in the next five years, the bot will be able to assist with shoulder, spine and ankle procedures.

Following the standard Mako approach, a few days before his hip-replacement surgery, Butler went for a CT scan, which was sent to Stryker’s headquarters in Michigan, where a computer program helped doctors determine the correct implant size, position and alignment. It also aided in mapping the precise bone areas surgeons needed to remove.

“All the planning gets done ahead of time, based on results of the CT scan,” says Karkare. Before Mako, doctors would cut into the patient and make refinements on the fly. Now, they know what they are getting into before initiating the first incision.

“When doing it without the Mako, we have an X-ray and ballpark figures as to what we will do,” adds Karkare, “but we do not know for sure until we actually see the hip in terms of the size of the bone that will be replaced.

“Info was fed into Mako,” says Karkare. “It figured out the exact orientation of the bone milling and insertion of the replacement bone.”

Butler was opened up, and Mako went to work.

“The robotic arm has a reamer [essentially, a fast-spinning grater], which reams the bone with absolute accuracy,” says Karkare, who controlled the arm as it shaved away the damaged bone. The arm, with Karkare controlling it, then inserted the replacement piece of hip, made from titanium and compressed plastic, at exactly the right location and angle.

When doing it without the Mako, we have an X-ray and ballpark figures as to what we will do, but we do not know for sure until we actually see the hip.

One day after the five-hour operation, Butler was up and walking in the hallway.

“The only pain I experienced, I would attribute to the sutures,” he says.

After five days of hospitalization, Butler went for five days of rehab at Helen Hayes Hospital in West Haverstraw, NY, even though Karkare said it wasn’t necessary.

“He’s young, and he could have just gone home to heal,” say Karkare. “That’s the norm. Patients are usually back to normal in four-to-six weeks with or without rehab” — which is no different from the recovery time for normal hip-replacement surgery.

Nine days after surgery, Butler was going up and down steps with a cane. A couple days later, he walked almost two miles to a friend’s house.

“Today,” he says, “I am 100 percent healed. Dr. Karkare advised me against running marathons, and the city won’t let me fight fires anymore. I’m now a captain with the marine division of the FDNY, where I oversee inventory. I miss the old days of actually being out there, but I’m happy to not be hurting.

“For the first time in so many years I am pain-free.”

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