Liam Neal, 18, repeatedly struck the victim in the head and body with the smashed bottle during an unprovoked attack in Falsgrave Park, Scarborough.
The victim’s injuries were so severe he needed 50 stitches to the shocking wounds, York Crown Court heard.
Another boy who was in the park with the victim was punched and kicked in the head by two other youths when he tried to intervene.
This boy, who cannot be named for legal reasons, was caught by the broken bottle during the melee.
He suffered a cut to his hand so deep that the bone was showing through, said prosecutor Mike Greenhalgh.
The incident on May 25 last year had arisen out of nothing more than the girlfriend of one of the victims walking past Neal and the two other youths to meet her boyfriend in the park.
Neal was staring at the girl as she walked past and then motioned as if to hit her over the head with the bottle, added Mr Greenhalgh.
When she reached her boyfriend and the other boy, they asked her if she was okay.
Neal “took issue” with this and started shouting, “What’s your problem? Come down here.”
“Neal broke the bottle he was holding,” said Mr Greenhalgh.
He held the bottle by its neck with the jagged edge jutting outwards, then pointed it towards the girl, “making stabbing motions towards her”.
Her boyfriend got between Neal and his girlfriend to protect her, but then Neal “stabbed out” at him, catching the sleeve of his jacket.
Neal then “slashed out” at the other boy who threw a punch back but missed, falling over as he did so, whereupon Neal “set about him with the bottle, slashing him in the head and body several times while (the victim) was on the floor”.
The other boy tried to get the bottle off Neal, at which point the two other youths joined in and one, or both, of them started kicking and punching the lad in the head “repeatedly”.
Neal then “slashed out with the bottle” at the boy, causing a cut to his hand so deep “that the bone was visible” and which also had to be stitched.
At this point another group of youths appeared and the victims ran away.
Police were informed and Neal and the two other youths were arrested.
Neal, of Aberdeen Walk, Scarborough, denied having a weapon at the scene but ultimately admitted wounding the first victim with intent to cause grievous bodily harm and unlawfully wounding the second boy.
The two other youths who were with him that day both pleaded guilty to affray.
Neal appeared for sentence via video link on Friday after being remanded in custody.
His two co-defendants’ cases were remitted to the youth courts.
They had been given court bail in the meantime on condition that they did not contact witnesses.
One of the victims said he thought he was going to pass out when he saw the other boy’s bloodied face following the “stabbing” with the bottle.
“He said that if it hadn’t been for (the other victim’s intervention), he thinks he could have died,” added Mr Greenhalgh.
He said Neal had previous convictions and was subject to a youth referral order at the time for offences including assault occasioning actual bodily harm and damaging property.
The two other defendants had convictions for criminal damage, being drunk and disorderly and shoplifting respectively.
Sean Smith, for Neal, said the teenager had lived a troubled life and had been taking drugs since he was 13 years’ old.
However, Mr Recorder Preston said that Neal had caused “horrific, life-changing” injuries with a “dangerous weapon”.
“I’ve no doubt (the victim) will be scarred for a long time, if not for life, and the emotional scarring can only be imagined,” he added.
He told Neal: “Your drug intake and your drink intake resulted in this explosion of violence.”
Neal was given an eight-year detention order in a young-offenders’ institution but will be released halfway through on licence.
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