A third doctor, testifying for the defence, tried to convince jurors that a nine-month-old baby was injured prior to a fall that sent him to hospital last February, as the murder trial of a British au pair continued.
Dr Lawrence Thibault, co-author of a 1987 paper which defined shaken baby syndrome, told the nine women and six men on the jury panel that if, as the prosecution maintains, the baby was violently shaken and then slammed against a hard surface the injuries would be much more extensive.
Prosecutors have charged au pair Ms Louise Woodward (19), of Chester, with first degree murder in the death of Matthew Eappen.
They maintain that, frustrated and angry with the baby's parents who objected to her late nights out, on February 4th she violently shook and then slammed the baby's head against a hard surface causing the massive brain damage which led to his death five days later at Boston's Children's Hospital.
Lawyers for Ms Woodward, who has been held without bail since her arrest eight months ago, have argued the injuries occurred earlier and the baby may have suffered from a genetic disposition making him susceptible to fractures.
In a complicated lecture involving velocity, torque and high impact injuries, Dr Thibault said if Matthew had been injured in the way prosecutors maintain, ligaments in his spine would have been torn, parts of his spinal column would have been fractured and he would have lacerations on his head. None of these symptoms was present when Matthew entered the hospital.
Under questioning by defence lawyer Mr Barry Scheck, who gained fame as part of O.J. Simpson's "dream team", Dr Thibault said: "Matthew did not suffer a violent impact to the head and certainly not on February 4th."
If convicted of the first degree murder charge, Ms Woodward faces a sentence of life in prison without parole.
Earlier Dr Alisa Gean, chief of neuroradiology at San Francisco General Hospital, testified that the baby did not die from having his head violently slammed or shaken.
She said she based her diagnosis on pre-operative CT scans of the child. They showed, Dr Gean said, that the baby suffered a skull fracture and blood clot three to four weeks before his death.
Dr Gean said she first came to that conclusion after looking at the scans for 15 minutes. Subsequent medical evidence served only to confirm her theory that the early injury oozed blood into the brain until finally there was "a catastrophic bleed" which led to the infant's death.