This was done refers to verse 33 (they did not break his legs). Not one of his bones will be broken may be an allusion to the Passover lamb (Exo 12.46; Num 9.12) or to the “good man” whom the Lord watches over (Psa 34.20). In the context of John’s Gospel, it is best to take the reference as primarily to the Passover lamb. It would be misleading to introduce an agent for the passive expression Not one of his bones will be broken unless the agent can be made indefinite, for example, “they will not break one of his bones.” Sometimes a pseudopassive may be used, for example, “he will not experience the breaking of any of his bones.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .