How was the skull of Jesus pierced when none of his bones were broken?
You ask a worthwhile question, given that Jesus’ head was undoubtedly wounded by his crown of thorns (John 19:2-5, Matt. 27:27-31), and yet John 19:36 conveys that none of Jesus’ bones were broken, because he is the New Covenant fulfillment of the Passover lamb (see Exodus 12).
However, one’s head can be wounded significantly without one’s skull being broken. Consider a concussion, even a severe one that knocks a person out. There is not necessarily an associated fracture of the skull. In addition, the skin of the scalp is the thickest on the human body, meaning it can be pierced and experience great trauma without the skull/bone beneath it being fractured. The scalp has five layers, with the skin being the thickest.
In this light, Christ experienced great head trauma in his Passion, and it’s possible his skull may have been bruised, but definitely not broken as Scripture makes clear.