Inmates at Siu Lam Psychiatric Centre often got 'lost in the system' and received almost no rehabilitation, an expert testified yesterday. Dr K. Y. Mak told the High Court concerns over public safety and 'frequent changes of staff' conspired to trap inmates sent to Siu Lam for unspecified prison terms. Such sentences allow doctors to decide when an inmate should be released. Dr Mak, head of the Hong Kong University's psychiatric rehabilitation unit, was an expert witness at the sentencing of a pimple-ridden robber who tried to take revenge on a beauty salon that failed to clear his spots. Dr Mak recommended a short, specified period of detention for Cheng Chui-fai, 25, followed by intensive treatment and supervision. If Cheng were given an open-ended sentence, doctors would be reluctant to release him unless they were absolutely sure he would not re-offend, Dr Mak said. 'Also, Siu Lam is not a good place for rehabilitation, which is just as important as medication,' he added. A Correctional Services Department spokesman said outside the court the doctor's comments were unfounded. 'Inmates are given treatment,' he said. 'They are provided with the same services that any psychiatric patient would get outside.' Deputy Judge Colin Jackson sentenced Cheng to three months at Siu Lam, followed by outpatient care. In January, Cheng, using a fake gun, tried to rob the Elsa Beauty and Fitness Centre in Mongkok. He believed it had cheated him in 1987.