The injuries are not in dispute.

The Canberra prison guard had his nasal bone shattered, and the base of his skull and a neck vertebrae fractured.

The guard required immediate corrective surgery on his injuries, spent eight days in hospital, has nerve damage causing numbness and a lack of sensation around his face, and has been diagnosed with post-traumatic stress disorder and had not returned to work.

But an ACT Supreme Court jury must decide if a prisoner had acted self-defence, if the injuries constituted grievous bodily harm, and if the prisoner had acted recklessly.

The 27-year-old prisoner has been on trial this week accused of recklessly causing grievous bodily harm, causing actual bodily harm, assault and threatening to kill.

He has pleaded not guilty, claiming self-defence.

The court has heard an alert had been raised from one of the Alexander Machonochie Centre's remand cells on January 15 last year.

When the guards responded, it is alleged the accused told them the intercom had been knocked by accident.

However, when the accused became distracted, his cellmate whispered a plea to "get me out of here".

The officers ordered the accused to put his hands through the hatch so he could be handcuffed.

One cuff had been locked to his wrist before he pulled back from the door and allegedly started punching the older detainee.

The man then allegedly used the handcuffs to choke the detainee.

It is alleged that when the guards entered the cell to stop the attack, the accused man punched one guard before pulling him down towards the back of the cell, causing his head to strike a wooden bench.

In his closing address, prosecutor Anthony Williamson told the jury they should have no difficulty in finding the accused guilty, urging them not to overthink the case and use their common-sense in their deliberations.

“It’s a simple and straight forward matter,” he said.

Mr Williamson also addressed the evidence of the alleged prisoner victim of the assault, who claimed he could not remember the day's events.

Mr Williamson said this evidence did not prevent jurors from reaching a guilty verdict.

The prosecutor said it was an unwritten rule of the underworld that criminals don’t cooperate with the authorities, or they risked reprisals.

However, defence barrister John Masters urged jurors to look at the evidence in detail, saying the whole approach of the investigation had been not to overthink it.

“Because the devil is in the detail,” he said.

He argued the investigation had only worked to one hypothesis, and not probed the entire incident fully.

Mr Masters also said that it had been for the Crown to prove it’s case, and reminded jurors that they should not hold the fact the accused had been prison against him.

Mr Masters told jurors to remember that the way a person would conduct themselves in prison, where freedom is limited, is different to way one would conduct themselves in society.

Chief Justice Helen Murrell sent jurors home for the day upon the completion of the closings.

The judge said she would to give her final directions to jurors on Friday morning before sending them to deliberate on a verdict.