WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 179, "Victor's Scheme," now streaming on Crunchyroll, as well as for manga chapters not yet adapted to the anime.
Naruto concluded a shocking note as Naruto and Sasuke, thanks to their Rasengan and Chidori respectively, lost their hands in the final battle. As both lay on the ground, missing their limbs, somehow they had time to joke about their past as they finally buried the hatchet. It was a metaphor for the sacrifice it took to defeat Kaguya and reset the shinobi world, understanding they needed to put petty differences aside.
Now, in Boruto Episode 179, Naruto's missing hand is finally addressed and it comes via an underrated and most ominous upgrade.
Sasuke never replaced his missing arm as he got older. Instead, he began using his katana more and perfecting his other techniques. It was a reminder of how much he needed to atone for attacking Konoha. As for Naruto, his arm was recreated using Hashirama's cells but bandaged up to avoid looking like the evil White Zetsu.
However, Katasuke, head of Konoha's science division, meets Naruto in Episode 179 of Boruto and Shikamaru, presenting the perfected prosthetic arm. He considers it his greatest invention as it's attuned to Naruto's chakra, not an easy feat considering Naruto has Kurama, the Nine-Tails Fox, inside him. This means Naruto can harness all his power as if it were his real hand -- which leaves Katasuke glowing as he has to make up for being brainwashed into leaking Konoha's secrets during Boruto's exams by a mysterious enemy.
Naruto's excited that he can spar with Boruto and feel like the good old days again. To him, it's not just a prototype, it's a memento of his past. In the manga, this new arm allowed him to fight as normal. However, this also foreshadows death and destruction. In the manga, Katasuke would give Kawaki another prototype of this, with Naruto lending the kid chakra to stay healthy and for the arm to stay attached. It's ironic because the anime now has Kawaki being found by Konoha.
The Boruto manga storyline involves Jigen using Naruto's chakra signature and connecting it to Kawaki's arm. This arm would help the villain track and locate Kawaki so he could teleport the kid in and use him as a vessel to be reborn in. So while it seems all is well, this hand is going to place Kawaki in a very dark spot.
Additionally, it's also what drives a wedge between Naruto and Boruto as Boruto gets jealous seeing his dad bond with Kawaki in a way they never did. Thus, as Katasuke celebrates his milestone, he has no clue his gift is going to upend Naruto's world by tearing his family apart and inviting in his deadliest enemy.
