A Dutchman dubbed the "horror dentist" has been sent to a psychiatric prison unit after allegedly mutilating dozens of French patients.

Mark van Nierop, 50, claimed to be suffering from psychological problems including gender identity issues and suicidal impulses in an unsuccessful attempt to avoid extradition from the Netherlands to face trial, reports AFP.

A court approved his extradition to France, where last night investigating judges ordered him to be sent to a prison psychiatric unit in the Loiret department, south of Paris.

He is accused of harming scores of his 2,800 patients, with their injuries including broken jaws, and sepsis.

Authorities are also probing claims by van Nierop that he killed his second wife in 2006.

Van Nierop was detained in Canada in October and flown back to the Netherlands where he was arrested under an international arrest warrant.

Van Nierop allegedly pulled patients' healthy teeth for no good reason, and left patients in debt after fraudulent billing.

A criminal investigation was brought against him in 2013, after a group of 120 former patients formed a collective to file legal charges.

After he fled in December 2013, authorities tracked van Nierop down to the small town of Nackawic in the Canadian province of New Brunswick.

He was flown back to the Netherlands after an international warrant was issued for his arrest on charges of violence causing mutilation or permanent disability

A former patient described the trauma caused by van Nierop, after he replaced all her filings using dirty instruments, despite the fact that there were no problems with them, and she developed infections.

"Everything was infected. It hurt, it hurt," she told CBC.

"I am now for two years with another dentist. He is trying to remake, to rebuild, everything he has destroyed.

"I learned that other stories are even worse. People from 80 years old he had pulled off all their teeth and put in dentures.

"They have been to hospital and everything. People now, for their lives, they can't put anything in their mouth."

Van Nierop set up his practice in the remote Chateau-Chinon area in central France, where there were few medical practitioners.