A 20-year-old man who paid for dental work with some of the money from four Oahu armed robberies was sentenced Thursday to 16 years in federal prison.

Marcus Kalani Watson previously pleaded guilty to committing the robberies during a five-week span last year.

He pointed a pistol at the chest of a Reynolds Recycling center worker and threatened to shoot him after the worker threw cash on the ground, according to a criminal complaint filed in court. He later posted on Facebook a photo of himself holding a large amount of cash, the documents said.

An indictment accused him of also holding up tellers at the Wahiawa branch of Central Pacific Bank, and the Pearlridge and Salt Lake branches of American Savings Bank.

An informant who passed a lie-detector test told the FBI that Watson spent some of the stolen money on an old Lexus and a root canal. The manager of the Honolulu dental office where Watson was treated confirmed to investigators that Watson paid $2,810 in cash for a root canal and crown.

During a phone call with the informant, Watson recounted the recycling center robbery and described his anger toward the man throwing money on the ground.

Lawyers representing the banks spoke in court Thursday about the lingering trauma of employees involved in the robberies. Lana Rapoza, of Central Pacific Bank, said employees fear coming to work early or staying late, experience paranoid thoughts, suffer panic attacks, take antidepressants and feel hatred toward Watson.

Fortunately no one was killed during the robberies, said Assistant U.S. Attorney Tom Brady. "But for the grace of God, we’d be dealing with a murder case right now," he said.

Watson apologized in court.

"I take full responsibility for my wrongdoings," he said, adding he’s grateful no one was physically injured.

Watson had an "underprivileged, at-risk" childhood, noted U.S. District Judge Derrick Watson, who isn’t related to the defendant. But the judge said many others with challenging backgrounds don’t commit crimes.

The judge didn’t go into detail about the young man’s childhood but said it involved bouncing back-and-forth between the mainland and Hawaii under the care of various family members.

Two co-defendants were previously sentenced. A.J. Williander, described as a getaway driver for one of the robberies who later cooperated with authorities, received a sentence of about four years. Rogussia Eddie Allen Danielson, who was involved in two robberies, was sentenced to about 15 years.

The three of them will have to pay back the $39,000 that was stolen.