Melvyn Megitt, 56, who ran four surgeries in the Manchester area until 1997, is accused of serious professional misconduct.
He prescribed treatment that was too extensive for his patients' needs, an orthodontist told the professional conduct committee of the General Dental Council.
The hearing was told that in one case a teenage girl had been treated for nine years for protruding front teeth when she might only have needed less than a year's treatment.
Amina Begum, who was 16 when Mr Megitt began treating her in 1987, had what was described as "a very mild" condition. However, she endured numerous tooth extractions. The hearing heard that the overjet (the protruding front teeth) "had been worsened from 5mm to 12mm and six teeth had been extracted".
Ms Begum said that despite starting treatment in 1987 she was still wearing a brace at night.
Dr David Lawton, a practising orthodontist since 1962 with more than 20 years' experience as a lecturer at the London Hospital and chairman of the British Orthodontist Society, told the hearing the measures Mr Megitt had used in nine particular cases had not produced satisfactory results.
He said Mr Megitt had followed the philosophy and practice of the eminent clinician Harry Orton, but he added: "He did not understand fully what Mr Orton, who was a superb clinician, was doing." Dr Lawton said the result on several occasions had been an "increased" protrusion of front teeth which was disadvantageous to the patient's health.
After detailed assessments of 11 cases, Dr Lawton concluded: "Mr Megitt's clinical standards are low and unacceptable in a speciality like orthodontics.
"I find it totally unacceptable. It is some of the worst treatment I have ever come across and I have unfortunately seen quite a range of treatment.
"His diagnosis was terrible. His records were terrible. He was just not up to standard. The treatment was substandard all the way through."
Mr Megitt, who was not at the hearing in London, is accused of serious professional misconduct for seeing too many patients and failing adequately to examine them or get permission for treatment. The bulk of the charges refer to 11 NHS patients he saw between 1987 and 1997 when he ran surgeries in Altrincham, Oldham, Sale and Salford.
He is also accused of failing to provide a high enough standard of care after it was noted the number of braces he fixed was unusually high.
Dr Lawton said Mr Megitt handled and estimated 1,200 to 1,500 cases per year, which meant he had a caseload of 2,500 - "an excessive number" - compared with the average seen by most dentists, which was between 500 and 700.
Earlier, Timothy Preston, representing the council, said that between 1994 and 1995 Mr Megitt, from Sale, Cheshire, was the highest earner among general dental practitioners in the country, making pounds 600,000 - the average was pounds 80,000.
The hearing continues.