We really do not know for sure. Some suggest there were no nails in Christ's feet. Here is what the Catholic Encylopedia says:
The question has long been debated whether Christ was crucified with three or with four nails.
The treatment of the Crucifixion in art during the earlier Middle Ages strongly supports the tradition of four nails, and the language of certain historical writers (none, however, earlier than Gregory of Tours, "De glor. mart.", vi; for the supposed sermon of St. Cyprian, "De passione", is a medieval fabrication), favours the same view.
On the other hand, in the thirteenth century, Western art began to represent the feet of the Crucified as placed one over the other and pierced with a single nail. This accords with the language of Nonnus and Socrates and with the poem "Christus patiens" attributed to St. Gregory Nazianzus, which speaks of three nails. More recent archaeological criticism has pointed out not only that the two earliest representations of the Crucifixion (the Palatine graffito does not here come into account), viz., the carved door of the Santa Sabina in Rome, and the ivory panel of the British Museum, show no signs of nails in the feet, but that St. Ambrose ("De obitu Theodosii" in P. L., XVI, 1402) and other early writers distinctly imply that there were only two nails.