A prominent sportsman has been charged with causing the injuries and the prosecution told the jury at the Dunedin District Court it probably occurred during a moment of frustration when the child’s mother was at the gym on July 16, 2023.
The man, who currently has name suppression, has pleaded not guilty to injuring with reckless disregard, as well as an alternative assault charge.
Dr Juliet Soper, who was a consultant paediatrician at Starship Hospital at the time, told the court on Friday t would take a "high magnitude of forces" to cause rib fractures to a baby.
But the defendant and the infant’s mother told police repeatedly they had not caused the damage, nor could they come up with any viable theory as to how the injuries may have been caused accidentally.
There was a suggestion the mother may have rolled on the baby while co-sleeping but she said she could not recall a situation where that occurred.
Dr Soper said the injuries would not have been sustained while the child was on his back on a bed.
"The magnitude of the force required to cause rib fractures in children is large," the witness said.
"Rib fractures are not caused by common infant-care activities."
As other Crown medical specialists have said throughout the trial, Dr Soper did not believe there was evidence of any bone-fragility disorder that meant the child was any more susceptible to fractures.
"In the absence of an obvious accidental mechanism, child maltreatment needs to be considered as the cause," she said.
The infant was diagnosed with severe vitamin-D deficiency when he was admitted to hospital but the doctor concluded that alone did not increase the risk of bone breaks.
Prolonged lack of vitamin-D can lead to rickets in the most serious cases, but Dr Soper and other specialists opined there was no evidence of the disorder in the baby.
She cited one study which showed 94 children admitted to hospital with rickets, presenting with 216 fractures between them.
None of them featured rib breaks, she said.
Dr Soper also referenced another medical report which looked into rib fractures in babies.
Of the few cases where it had occurred, one was the result of an unrestrained child in a crash, another when an older sibling fell on them and a third where a father holding his baby fell down a flight of stairs.
Dr Soper highlighted the possible explanations raised by the other medical expert witnesses: vitamin-D deficiency; bone disease or rickets; an inherited genetic disorder; young age; trauma at birth; straining due to constipation or vomiting; bed sharing; massage and craniosacral therapy.
She cast doubt on all of them and said the "most likely" cause of the injuries remained inflicted force.
Cross-examination of Dr Soper - the final Crown witness - will begin on Monday, after which the defence will open its case.
