Veterinarian question about horse knees

I have started reading Daniel Deronda. I was amused young Rex calls his father 'Sir'.

I have a veterinarian question regarding horses' knees. Rex borrowed his father's nag to take part in a hunt. The horse was not very strong, fell over, threw Rex, and broke his knees. The horse was injured but managed to walk home. I read about a horse or pony breaking its knees in another book. It was either in Silas Marner by George Elliot or A Handful of Dust by Evelyn Waugh. I thought if a horse broke a leg, it had to be shot.