AN 81-year-old dentist who, because of his poor eyesight, allowed his son to examine patients was struck off the register for nine months yesterday. A Dental Council inquiry found Dr Yeung Chung-mou guilty of professional misconduct after hearing of a police investigation into his Sai Ying Pun practice last October. The inquiry heard that police Constable Tam Kam-chiu posed as a patient in an investigation prompted by patients' complaints and was examined by Dr Yeung's 46-year-old son, Yeung Ling-wai, who told him one of his molars needed treatment. During the examination other police officers burst into the room and Dr Yeung's son, who had no training in clinical dentistry, was arrested. In a police statement, Yeung Ling-wai said he helped his father in his 18-year-old clinic and sometimes checked patients' teeth, although he had only been doing so for a few years. The statement went on: ''My father has poor vision and so I would look at patients' teeth and tell my father of any problems, although it would be up to him to offer treatment.'' At the moment, elderly dentists do not need a health certificate to practise, but under proposals to amend the Dentists Registration Ordinance dentists over a certain age will have to obtain a health certificate. Yeung Ling-wai told the inquiry his father had taught him how to examine teeth using dental instruments and that he had a certificate in dental mechanics. He admitted he had examined Constable Tam's teeth although he said he told him it was only a preliminary examination and that he was not the dentist. Yeung Ling-wai added: ''My father was feeling sick on that day and he knew I was carrying out the preliminary examination, while I would have referred the patient to my father for any treatment.'' But Constable Tam said he had not been told that the examination was preliminary or that treatment would be carried out by someone else. Dr Yeung's son pleaded guilty to practising dentistry as an unregistered person when he appeared in court last October and was fined $4,500, although no charges were brought against Dr Yeung. The legal officer said: ''He clearly held himself up as a dentist who was prepared to give treatment when he said he was his father when he answered the door. ''His father's knowledge of this cannot be disputed and clearly there was some sort of pre-existing understanding that Yeung Ling-wai was authorised to carry out whatever examination or treatment was necessary.'' Dr Yeung's defence counsel argued that the delegation of duties from father to son was proper and that he had only been allowed to carry out a preliminary examination. He added: ''The police decided there was no evidence of Dr Yeung aiding and abetting his son to practise dentistry as an unregistered person which is effectively what he is being accused of here.'' Dr Yeung has one month to appeal against the Dental Council's decision. Following the inquiry, Dental Council chairman Dr Jeffrey Tsang Yick-sang said he did not think there was a problem with the general fitness level of older practising dentists.