Sydney forensic pathologist Professor Johan Duflou told the Victorian Supreme Court he was no fan of the show.

"I think it's fair to say that with a program like CSI specifically is the bane of the forensic expert witness's life," Dr Duflou told the jury in the trial of Paul Ta Vuong, who has pleaded not guilty to murdering his girlfriend Kate Leithhead's 12-month-old son Silas.

"Unfortunately I seem to spend most of my time in the witness box saying, 'I'm not sure', 'I don't really know', 'possibly it is', 'possibly it's not', while in CSI you hear absolute statements of fact and unfortunately that's not really the way it works in the real life of forensic pathology - well, certainly not as perceived by me.

"A lot of forensic pathology examining bodies involves a lot of interpretation and, frankly, a lot of uncertainty."

Dr Duflou was called to give evidence by Mr Ta Vuong's defence team after Melbourne forensic pathologist David Ranson, who conducted the autopsy on Silas, told the jury he did not believe the boy's head injuries were caused by a simple fall.

Mr Ta Vuong claimed he tripped and likely fell on top of Silas when he was taking the boy out of the bathroom at Ms Leithhead's Richmond apartment on May 15 last year.

Ms Leithhead had left Silas with Mr Ta Vuong to buy heroin from her local dealer.

Mr Ta Vuong later told police Silas cried for a short time after the fall but he was alive and breathing when he put him in his cot. He did not tell Ms Leithhead about the fall when she returned with the drugs.

Silas died from blunt force trauma to the head.

Crown prosecutor Andrew Grant has told the jury Mr Ta Vuong allegedly boasted to two prisoners how he had dropped Silas on his head five times because he was angy at having to wait too long for Ms Leithhead to get heroin from the drug dealer.

A prisoner, known as Witness A, gave evidence that Mr Ta Vuong confessed to the murder when on remand after his arrest.

Witness A claimed Mr Ta Vuong told him he had been frustrated that Ms Leithhead was taking too long to get the drugs and suspected she was "f...king the dealer or she was f...king around with the dealer" so he decided to "play rough" with Silas.

Defence barrister Saul Holt, SC, told the jury the prisoners were lying.

Dr Ranson, when asked if Silas' head injuries could have been caused by falling to the floor from chest height, replied: "I don't think I would see this extensive nature of the fracturing. I believe the fracture pattern is too great for an explanation to be a simple uncomplicated low- level fall."

But Dr Duflou said that in the past medical experts had believed head injuries to babies were almost invariably an indication of abuse but recent studies had shown "complex skull fractures ... can occur in relatively simple falls and ... you really end up with major difficulty in terms of trying to understand what happened."

In his report for the defence after viewing all the relevant documents, Dr Duflou said: "In this case a history is provided that the accused fell on top of the infant with both the infant and the accused possibly falling on to a tiled floor. Such an impact, especially if the accused was not able to arrest the fall with his hands or other parts of his body, could have resulted in the infant's head striking the tiled surface with some force followed by the body weight of the accused further impacting with the head and other parts of the body of the child".

The trial, before Justice Karin Emerton, continues.