There's slightly more info in Dutch on the Artis website[0]. It doesn't mention specifically how the bird got injured, but it does mention that the zoo still had the two broken-off pieces. They used these pieces (together with a CT scan) to create the new prostheses.

[0] https://www.artis.nl/nl/artis-park/verhalen/zuidelijke-hoorn...

The pictures don't really do it justice, it is a massive bird and very impressive up close. If you're ever near Artis do go and have a look (though with their prices I wouldn't blame you if you didn't...).