The force required to cause serious fractures to a young baby's legs would have been well in excess of normal care, an expert has told a jury.

"It is not force you would use when changing a nappy, nor when playing with a baby and using cycling motions," said Dr Joanna Fairhurst.

The consultant paediatric radiologist was giving evidence at the trial of parents Jody Simpson, 24, and Tony Smith, 46, who are accused of child cruelty.

Maidstone Crown Court has heard the baby boy was left with life-changing injuries after he was allegedly deliberately injured by one of the parents.

The child was just 41-days-old when he was found to have suffered eight fractures to limbs and then developed life threatening septicaemia.

"The ongoing effects of his injuries will be considerable and lifelong," said prosecutor Heather Stangoe.

"There is a high risk of disability."

Miss Stangoe said the parents delayed taking the boy to their doctor from their flat in Square Hill Road, Maidstone, after he became ill.

They later told police they were delayed because they were waiting for a plumber to mend a broken boiler.

Simpson phoned their GP on November 14, 2014 and reported the baby had cold-like symptoms and was crying.

She was advised to give him Calpol and take him into the surgery when he was due for his six week check-up.

But when he was taken to the surgery four days later he was gravely ill, having developed swelling and shock.

Experts expressed surprise that the mother had not called for an ambulance, said Miss Stangoe.

The baby weighed 7lb 7oz at birth and was "healthy and thriving".

When he was taken to the GP’s surgery at 41 days by Simpson he was gravely ill, said Miss Stangoe.

He looked grey, had froth at the side of his mouth and was grunting.

His eyes were closed and his lower limbs were hard and swollen. The doctor suspected septicaemia.

The child was taken to Pembury Hospital and then transferred to a specialist unit in London.

"He was in a parlous condition and required multi-organ support in intensive care,” said Miss Stangoe.

"On admission, he was drowsy and showed signs of respiratory distress."

X-rays revealed fractures to both thighbones, both lower legs, the right lower leg and ankle, and fractures to the base of the left thumb and two bones in the big toe.

"There was no evidence of any underlying bone disorder," said Miss Stangoe.

"A paediatrician and another doctor concluded that his injuries were not accidental."

The doctor's view was that the fractures led to the onset of septicaemia. The prognosis was poor.

"It was thought highly likely he would die imminently from multiple organ failure, secondary to his injuries and septicaemia," said Miss Stangoe.

The child needed a prolonged course of treatment.

"The dislocation of the right lower leg and ankle is the most significant of the fractures and would have required considerable force," said Miss Stangoe.

"For example, swinging the baby by the ankle.

"The severity of that injury would have been apparent to the carer present at the time.

"They would obviously have known. He would have shown distress whenever his leg was moved.

"This would have continued for several days after the fracture."

Dr Fairhurst, who inspected X-rays of the fractures, said babies bones were very pliable, bending a long way and springing back.

"It would have to be bent and deformed a long way before it breaks,” she told jurors.

"For injury to occur the foot would have to be twisted away from the ankle or around very suddenly.

"You could grip the ankle and foot and yank them away from the body and twist at the same time. That is one way this fracture would occur.

"It would require a lot of displacement of the foot. Another way this could happen is if the foot was trapped and the body was yanked in such a way as the foot was kept still and twisted.

"What you need is that twisting and distortion at the ankle to cause that fracture.”

Fractures to the big toe, she said, could be caused by a direct blow with something solid.

"More commonly, these fractures occur because the foot is tightly gripped and twisted," she continued.

"The ongoing effects of his injuries will be considerable and lifelong" - prosecutor Heather Stangoe

"You have to grip really hard and twist to cause bones to break."

Asked if it was something that could occur in every day handling, Dr Fairhurst replied: "Absolutely not. It is well in excess of anything a normal carer would use from day to day."

Dr Fairhurst said there would most likely have been swelling from the fractures. They would have been very painful injuries.

"I would expect the baby to remain distressed and crying for up to half an hour after the injuries occurred," she said.

"Every individual fracture would be painful.

"All these fractures occurred over the same time period. In theory they could have occurred at the same time.

"It is possible to apply twisting and pulling force around the knee that will cause fractures above the knee and the lower thigh bone and below the knee and the upper shin bone.

"Fractures around the right knee and left knee could have occurred from a single episode.

"Both injuries could have occurred at the same time, for example, if both legs are held and twisted at the same time."

While all injuries could occur in the space of minutes, she said, there would have to be at least four separate forces.

Simpson and Smith, now both of Sydney Road, Whitstable, deny causing or allowing serious physical harm to a child and cruelty to a person under 16.

The trial continues.