An expert advisor engaged by the Hospital Authority said an incident in which a surgical light at United Christian Hospital fell last week happened because all six screws holding the light together had broken, probably due to metal fatigue.

At a press conference on Friday, engineer Eric Lim said he found marks indicating metal fatigue on the surface of at least four of the six screws. He said signs of erosion from at least six month ago were also visible on the screws.

The authority said the vendor had checked the lighting unit in December, with records showing it had tightened the screws.

A subsequent check by the HA in all public hospitals revealed that 23 out of 195 surgical lights from the same brand, Getinge, were potentially unsafe. Some 270 lights of other brands, meanwhile, were found to be safe.

Dr Michael Wong, director of quality and safety with the authority, said they are asking the vendor to submit an investigation report into the incident, and to replace the screws as soon as possible.

He said the HA is very dissatisfied with the vendor's performance in maintaining the lights, and reserves the right to take legal action.

"[The] Hospital Authority has always been monitoring the performance of different maintenance vendors," he said. "We regret that from our findings, this vendor seems to be substandard."

Dr Wong added the HA would not carry out additional checks on surgical lights since they were low-risk equipment. He refused to say if there was any oversight by the authority.

"We will increase our vigilance and try to make sure future vendors will follow their procedures and we will try to have surprise checks... to ensure they do their job properly."

The surgical light hit an anaesthetic assistant on the shoulder on Saturday. No patients were present when it fell.