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Pistorius was born without fibulas in his lower legs, leading to amputation at the age of 11 months.
Testifying for the defence about his mental state, forensic psychiatrist Merryl Vorster said that because the surgery was at such a young age, it would have been impossible to reason with the baby, making the pyschological scars much deeper.
"He was too young to understand why," Vorster told the court. "His mother could not have comforted him because he was pre-language phase. It would been perceived as traumatic assault."
Pistorius is on trial for murder after shooting and killing his girlfriend, 29-year-old law graduate and model, Reeva Steenkamp on Valentine's Day last year.
The athlete says the shooting was a tragic mistake, and that he shot at a closed toilet door in his luxury Pretoria home thinking an intruder was hiding behind it.
The states alleges he fired at Steenkamp in a fit of rage after the couple had a row in the early hours of the morning.
If convicted of murder, Pistorius faces life in prison.
Vorster also said his parents' divorce when he was a child would have added to the 27-year-old's general feelings of anxiety and insecurity.
After his parents split, Pistorius was brought up by his mother, who died of cancer when he was aged 15. Pistorius has frequently spoken of his love for his mother and the difficulty her death caused him.
However, in her assessment of the athlete, Vorster said Pistorius' mother grew increasingly anxious and drank excessively, making it impossible for her to address the fears and concerns of her children.
Terrified of intruders breaking into the house, she slept with a pistol under her pillow, Vorster told the court.
Pistorius competed against able-bodied spinters on carbon-fibre prosthetics, becoming one of the most recognised names in athletics. Besides a clutch of Paralympic medals, he reached the semi-finals of the 400m at the London 2012 Olympics.