For months, a team of physicians and nurses had rehearsed for the delicate surgery. For hours they had prepped the two tiny bodies perilously joined at the head. And when it came time on that day in 1987 to put a knife to the large vein connecting them — the most fraught step in the groundbreaking operation to separate infant conjoined twins — Benjamin Solomon Carson, the brilliant young pediatric neurosurgeon who had overseen the babies’ case from the start, offered his scalpel to his boss.

It was a sign of deference and respect — and perhaps, a measure of caution. But Donlin Long, head of neurosurgery at Johns Hopkins Hospital, refused the gesture. Carson, he had already decided, should make the crucial cut.

“Part of me thought, maybe I should take the knife. If things go badly it would be terrible for the young doctor’s career,” Long recalled this month. “But I also know that if this was a success, if things go well, it would make his reputation, would make him famous, that people would grow up trying to emulate him.”

Donlin Long and Ben Carson speak at Johns Hopkins Hospital in Baltimore after the surgery that separated conjoined twins in September 1987. (Fred Kraft/Associated Press)

More than any other moment in a dazzling career, the separation of the Binder twins launched the stardom of Ben Carson. The then-35-year-old doctor walked out of the operating room that day and stepped into a spotlight that has never dimmed, from the post-surgery news conference covered worldwide, through his subsequent achievements in his medical career, to publishing deals and a lucrative career as a motivational speaker — all paving the way to his current moment as a leading candidate for the Republican presidential nomination.

But while Carson frequently deploys anecdotes from his compelling life story — a hardscrabble childhood in Detroit, his climb to the Ivy League, his journeys through spiritual faith and advanced medicine — he only occasionally cites and never dwells on the story of Benjamin and Patrick Binder.

Like many stories from the frontiers of medical science, it’s a hard one to fit into an inspirational narrative — a tale of risk and loss and brutally tough options. And although Carson and his team achieved something unprecedented, with long-term benefits for science, it did not result in a happy ending for the Binders.

“In a technological ‘star wars’ sort of way, the operation was a fantastic success,” Carson said in an Associated Press article from 1989. “But as far as having normal children, I don’t think it was all that successful.”

When reached for comment this week, Carson said: “The great reward of using your talents to save a child’s life can be a tremendous high, but you often find that there is nothing you can do to stop the pain of a patient’s journey through life. As a doctor, no matter how far we push medical science, we cannot conquer all pain.”

Updates on the children were limited after they returned to Germany following the surgery. The German magazine Bunte signed an exclusivity deal with the family until the boys turned 18. A search through its archives, in addition to new interviews conducted with members of the family and Carson’s medical colleagues, shows a more complete and complicated portrait of an event that shaped the lives of those involved.

“I will never get over this. . . . Why did I have them separated?” the boys’ mother, Theresia Binder, told the Freizeit Revue, a sister publication of Bunte, in November 1993. “I will feel guilty forever.”

In January 1987, Theresia Binder was eight months pregnant and suicidal.

“I wanted to kill them and myself as well,” she said, according to Carson’s best-selling book “Gifted Hands.” She had just learned that her babies were stuck together and felt as if “a sick, ugly monster” was writhing inside of her.

“I saw the babies [and] noticed only a huge head with two faces,” she told Bunte. “I thought: ‘My God, what will they look like, how will they live?’ ”

She debated swallowing pills. She considered opening up the window of a tall building and jumping out. Instead, on Feb. 2, 1987, she gave birth to her boys. They weighed a combined 8 pounds and 14 ounces. They shared a head, but Theresia’s fear was replaced with a new emotion.

“Not once did we ever not love them,” her husband, Josef, was quoted by Carson as saying in “Gifted Hands.” “They were our sons.”

Doctors informed the parents that if the sons remained joined, these pudgy blond babies would never be able to sit, crawl or turn over. Learning to walk was out of the question. But Johns Hopkins in Baltimore was world-renowned for taking on difficult cases. There they would meet a man who had grown up poor in Detroit, made it to Yale, gotten a medical degree at the University of Michigan, and at the age of 33 had become the youngest chief of pediatric neurosurgery in the country.

“After studying the available information, I tentatively agreed to do the surgery knowing it would be the riskiest and most demanding thing I had ever done,” Carson wrote. “But I also knew it would give the boys a chance — their only chance — to live normally.”

Conjoined twins occur in about one out of every 200,000 births. Almost 60 percent of them die before birth, and an additional 35 percent don’t survive beyond a day. Of all conjoined twins, only about 2 percent are craniophagus, or connected by the head.

Which means that few surgeons have had much practice separating them. So at Hopkins, a team of seven pediatric anesthesiologists, five neurosurgeons, two cardiac surgeons, five plastic surgeons and an array of nurses and technicians spent months practicing on dolls joined at the head by Velcro, according to a 1987 account of the surgery in Newsweek.

The Binder twins were lucky in that they had two brains. It meant that the surgery was at least feasible.

“From the time we started discussing it, we all tried to keep in mind that we wouldn’t proceed with surgery unless we believed we had a good chance of separating the boys without damaging the neurological function of either baby,” Carson wrote in “Gifted Hands.”

On Labor Day 1987, the ­7-month-old twins — who, according to Newsweek had been “giggling and kicking since entering Hopkins on September 2” — went in for surgery. For four hours, heart surgeons inserted “hair-thin” tubes into their veins and connected them to heart-lung machines that would keep them alive through surgery. Plastic surgeons sliced into their scalp, removing the bone tissue that connected them. The cardiologists then cut open their chests and removed small amounts of tissue from their heart to use later to construct new veins.

Doctors dropped the babies’ body temperature down to 68 degrees, stopping their hearts and allowing surgeons to operate without blood flow — the first time anyone had tried such a strategy for this type of surgery. A big clock on the wall counted down from an hour: Every minute without a heartbeat beyond the 60-minute mark threatened to cause irreparable damage to the boys.

“When the hour is up, just turn the pumps back on,” Carson told his team, according to his book. “If they bleed to death then they’ll have bled to death, but we’ll know we did the best we could.”

After Long gave him back his scalpel at the pivotal moment, Carson severed the primary thin blue vein that connected the twins. The doctors rapidly set about creating new veins from the heart tissue they had removed earlier. One twin was finished in 57 minutes, the other in 63.

“It got pretty intense in there,” said Bruce Reitz, director of cardiac surgery, according to a Washington Post article from the time. “We tried not to look at the clock.”

Separated for the first time in their lives, Benjamin and Patrick were placed in medically induced comas. The magnitude of this precarious success was lost on no one; a massive media scrum awaited the doctors as they emerged from the operation.

“The success in this operation is not just in separating the twins,” said Mark Rogers, the director of the department of anesthesiology, at the news conference. “Success is producing two normal children.”

Carson, second from left, performs brain surgery on Caroline Schear, 15, at Johns Hopkins Hospital in 2002. (Yoni Brook/The Washington Post)

Carson greets patient Katie Vanoli, 15, of Salinas, Calif., during his rounds at Johns Hopkins Hospital in 2002. (Yoni Brook/The Washington Post)

The Binders’ early recovery was closely charted in subsequent headlines: “Twins’ Doctors ‘Encouraged’ But Cautious, No Prediction Now For Health of Either,” “Siamese Twins Begin to Wake From Coma And Breathing Alone,” “Twins Face New Crisis,” “Breathing Problems for Twins,” “Fever Slowing Recovery of Separated Twin Boys,” “Separated Twins’ Status Delights Their Doctors; Last 10 Days’ Progress Called Remarkable,” “Separated Twins May Lead Normal Lives, Doctors Say,” “Twins Separated By Surgery Ready to Go Home to Germany.”

And they did go home to Germany, seven months later. And then, news of the Binders almost immediately evaporated. Even Carson lost track of them, he would say later in an interview, noting that he had written letters and never heard back.

Carson, though, remained very much in the news. In 1992 he published “Gifted Hands” — which became a made-for-TV movie starring Cuba Gooding Jr. — and a series of other books in an inspirational or self-help vein. Carson was appointed to lofty boards and commissions; the NAACP gave him its prestigious Spingarn Medal and the White House gave him a Presidential Medal of Freedom.

His fame — in particular for his work with conjoined twins — was such that when the filmmaking Farrelly brothers of “Dumb and Dumber” fame made a comedy, “Stuck on You,” about conjoined twins (played by Matt Damon and Greg Kinnear), they just had to have a Ben Carson cameo — and he signed on, once they agreed to host the movie premiere in Baltimore.

He became a wildly popular figure on the speakers’ circuit, his religious faith and social conservatism popular with black churches and red-state audiences. In 2013 he landed on stage at the National Prayer Breakfast, where his dark warnings of an America in decline were seen as a rebuke to President Obama, sitting just a few feet away — and Carson became the new darling of conservative radio hosts and activists. And now he is a presidential candidate, known for announcing at debates that among the field of rivals he’s “the only one to separate Siamese twins.”

Meanwhile, Carson’s first conjoined twins kept a lower profile back home in Germany.

Their mother would later tell reporters that they came home with the hope, instilled in them by the Johns Hopkins surgeons, that the boys might soon begin crawling, and then hit other milestones.

“‘I took it seriously,’’ she told Freizeit Revue in 1989. “But maybe they just wanted to encourage me.’’

It soon became apparent, though, that the boys were hopelessly delayed, according to a 1993 interview with the Revue. Benjamin would moan occasionally, but Patrick remained completely silent; he had had a setback in the Baltimore hospital when he choked on a piece of food, going without oxygen for a short time. Years later, neither boy could get around on his own or feed himself.

The doctors say they always knew this outcome was a possibility, that the swelling from the surgery and the time without blood flow left the children very much at risk. They say they hoped that the twins would mature into normal lives, but that it was always just a hope.

“We certainly made the risks clear,” Long said. “And we made it clear that it had never been done before.”

When Freizeit Revue caught up with Theresia in November 1993, she said her children’s brain damage had destroyed her marriage.

“Josef has never been able to cope with this blow of fate,” she said in the little-seen interview written in German and translated into English. She said Josef, who has since died, according to Theresia’s brother, became an alcoholic, lost his job, cheated on her and spent all their money, leaving her and the children to live “from hand to mouth.”

“He never touched them,” she said. “He was appalled.” Unable to care for the twins on her own, she brought them to a home for disabled children, where they became wards of the state.

“The first thing I think of every morning is ‘today I’m going to get them,’ ” she said in 1993. “But then I can’t.” She said she had lost her faith in God. She remarried and had another child, and in 1993 was pregnant with another.

“When we go for a walk with the children in their wheelchair, people look at me, as if I’m a monster,” she said. “I need this child. To heal. I still have to prove to myself that I’m not a monster.”

Katharina Korn was a neighbor of Theresia Binder in Ravensburg, a town in southern Germany famous for its picturesque medieval buildings, until Theresia moved away about eight years ago. Korn recalled seeing the toll the twins’ conditions took on Theresia.

“She was feeling bad, because her children were severely disabled, much worse than before the operation,” Korn told The Post. “They promised her much more than what the actual outcome was.”

Patrick Binder died sometime in the past decade, Theresia’s brother Peter Parlagi said. Benjamin is 28 now and still cannot speak, but according to Parlagi is doing “relatively well.” He’s grown up; his hair is now dark. His younger half-brother, Florian Vosseler, still checks in on him two or three times a year, often with their mother, who declined to be interviewed for this story. They’ll take walks, feed him cake and help him drink coffee, which he prefers with extra milk.

“It’s difficult to communicate with him,” Vosseler said. “But you can tell by the way he looks at you that he’s happy to see you.”

Carson looks on as Dr. Rafael Tamargo speaks after a surgery separating conjoined twins Lea and Tabea Block in September 2004. (AP Photo/Chris Gardner)

At the office for his plastic surgery practice in Fairfax, Va., Craig Dufresne has hung a framed copy of a 1987 New York Times article trumpeting the ­22-hour surgery that he participated in with his longtime friend Carson. As he sees it, the doctors never explicitly promised the Binders anything, though he can understand how they got their hopes up.

“We said that if all goes well, they would be completely normal,” said Dufresne, who went on the initial trip to examine the twins in Germany. “And we really thought there was a good chance it would turn out with a happy ending.”

Dufresne said the plan was for the twins to return to Hopkins a year later, but that once they left for Germany it was impossible to keep track of the family. When there was no news, he figured there must have been bad news. He and others on the team would have to take the surgery for what it was: a big step forward for medical science, even without the ideal result.

“My job as a doctor is to make sick people well, and when I fail to do that, regardless of exactly why, I still failed,” Long, the former head of neurosurgery, said in a phone interview about the Binder surgery. “So in that way, the simple answer is no, I do not think it was a success.”

The surgery did prove that it was technically possible for twins conjoined at the head to be separated and for both to survive. It created a blueprint for other doctors to follow in the future. Carson himself would go on to participate in four other similar surgeries — one of which, a 1997 surgery of Zambian twins, left both patients not only alive but neurologically normal.

“You can’t go into something like this with a pessimistic attitude,” Carson told Johns Hopkins Magazine after a 2004 separation of twins joined at the head. “A lot of younger guys are learning a lot, pushing on the next border, which is how progress is made. We all act as stepping stones.”

Carson would go on to perform hundreds of other difficult and impressive surgeries, including operating on babies inside the womb and removing large chunks of the brains of children plagued by repetitive seizures.

But it was the Binder twins who launched Carson onto the national stage he now dominates, barely three years after retiring from surgery.

“If I hadn’t given him the knife, he probably wouldn’t be there today,” Long said. “He still would have received a great deal of credit, but it would have been divided. It became the thing that got him everybody’s notice.”

David Farenthold contributed to this report.