The sportsman, who has interim name suppression, is on trial at the Dunedin District Court after the Crown alleged he caused more than a dozen rib fractures, and a broken clavicle, to the baby.
He has pleaded not guilty to injuring with reckless disregard (and an alternative assault charge) and told the jury this week he would never hurt the child.
When the defendant gave evidence on Monday, the Crown suggested he had harmed the unsettled child on July 16, 2023, during a momentary "brain explosion" when the baby’s mother was at the gym — something the man vehemently rejected.
Yesterday, the defence case continued with expert evidence from Dr Douglas Benson, an orthopaedic surgeon and expert in metabolic bone conditions.
His evidence got off to a slow start with the California-based witness struggling to hear the lawyers and understand their New Zealand accents.
Live transcription was required for the cross-examination.
After reviewing the baby’s medical records, Dr Benson concluded the fractures were caused at birth, but were only minor "cracks" at the time.
These did not heal because of the baby’s vitamin-D deficiency.
Handling of the baby and issues such as coughing, vomiting or straining could have caused these minor fractures to become "displaced", Dr Benson said.
"I am certain that his rib and clavicle fractures were birth related.
"When deficiencies are present ... healing is much delayed and the remodelling process can be delayed for months until sufficient minerals are available for normal bone repair and growth."
The lack of bruising on the child when he presented to hospital also pointed to a cause other than abuse.
"Usually with traumatic injuries there are associated bruises.
"Birth injuries infrequently have any associated bruising in my experience.
"The fractures occurred due to bone failure rather than excessive force."
Under cross-examination though, the witness accepted not all rib fractures would show external signs.
Crown experts who testified vitamin-D deficiency alone would not cause bone fragility or lead to fractures were "fish swimming against the tide", Dr Benson said.
He questioned why the baby in this case was given "massive doses" of the vitamin after his deficiency was discovered if it was not a significant issue.
The doctor said there were recent claims that gave new insights into bone fragility, but revealed he had discovered that information through the use of artificial intelligence.
He assessed the baby’s X-rays as showing other evidence of weak bones and said a low level of a pancreatic enzyme also meant the child was unlikely to have suffered trauma.
"It’s as if the pancreas is a very sensitive indicator of other trauma."
Crown prosecutor Robin Bates said the witness had disagreed with many experts with more experience than him in their fields.
"You have gone well outside your lane in numerous areas of your evidence, genetics, endocrinology, radiology, obstetrics, you name it."
Dr Benson responded his evidence was based on "objective facts" and he "would put those against any credentials or claims".
The trial, which is being presided over by Judge David Robinson, continues today.
