The man, who has interim name suppression, is on trial in the Dunedin District Court after he pleaded not guilty to wounding with reckless disregard and an alternative charge of assault.
The Crown case is that the defendant applied a squeezing or crushing force to the child’s torso during a moment of frustration while the infant’s mother was at the gym in July 2023.
But the defence has offered other explanations, including the baby’s severe vitamin D deficiency causing him to fracture easily, the injuries being caused at birth or the fractures being caused through unsafe co-sleeping.
Paediatric radiologist Dr Julie Mack was the second defence expert to give evidence, speaking to the jury by video link from the United States.
Her assessment of the evidence was pitted against her counterpart, the Crown’s radiological expert Starship Hospital’s Dr Russell Metcalfe.
Dr Mack said she generally agreed with his findings regarding the locations and extent of the child’s fractures but disputed his conclusion that the circumstances indicated the injuries were “almost certainly” the result of abusive trauma.
She told the court another major disagreement was around the limitations of x-rays.
“A significant amount of bone mineralisation must be lost before they appear abnormal on the imaging.
"Loss of bone cannot be detected on imaging until at least 23% of bone strength is lost,” Dr Mack said.
Shortly after the baby was admitted to hospital, doctors discovered a severe vitamin D deficiency and prescribed a large dose to the child to compensate.
While prosecution witnesses previously spoke about the importance of the vitamin to bone health, they believed that alone would not cause weakness.
Dr Mack disputed that conclusion and suggested the vitamin D level may have been dismissed as a factor in the bone breaks because child abuse was assumed to be the cause from early in the case.
She stressed there was “little or no” recognition by medical staff that the abnormality may have affected the infant’s bones.
“Fractures can occur from abnormal force on normal bone . . . or from normal force on abnormal bone. In this case, the evidence supports the latter,” Dr Mack said.
In a statement read to the jury, the radiologist also spoke about the problem of disproven allegations of child abuse not being reflected in medical data.
“There’s no feedback loop where the physicians can learn from their mistakes,” Dr Mack said.
The witness pointed out the lack of bruising on the baby and said while rib fractures could occur with no external signs “it doesn't support significant trauma when you have no bruising”.
She could not exclude the possibility of the injuries being caused during the baby’s birth, but being “microscopic” at the time, and becoming visible later.
“This is a bone fragility case, I don't think we have good evidence that these were fractures caused by major force,” she said.
Dr Mack also said the ribs being fractured on both sides did not necessarily indicate that force was applied to both sides of the infant because of the connections in the rib cage.
The fractures being in a row was also common when force was applied to the rib cage.
“You can get rib fractures like this in a row from non-severe forces,” Dr Mack said.
“It just tells me the rib cage was bent and it failed in the places where the fractures are.”
Dr Mack will continue giving evidence this afternoon.
