Much of the torture devices typically associated with the Middle Ages, like the iron maiden or the "pear of anguish," were later anachronisms meant to show the barbarity of the medieval period. In much the same way as historians and philosophers would use pejorative terms like "gothic" to describe certain aspects of medieval society, they also invented or exaggerated devices to symbolize the perceived backwardness of that time. Granted, there was still a lot of crime and torture in the Middle Ages, although the use of some of the fiendish torture devices that have since become synonymous with the Middle Ages are not grounded in the medieval sources. I am not a specialist on medieval crime and punishment, but these sources should get you started.

Esther Cohen. The Modulated Scream: Pain in Late Medieval Culture. Chicago: University of Chicago Press, 2009.

Chris Bishop. “The ‘Pear of Anguish’: Truth, Torture and Dark Medievalism,” International Journal of Cultural Studies, Vol.17:6 (2014).

Anthony Bale. Feeling persecuted : Christians, Jews and images of violence in the Middle Ages. London : Reaktion 2010.

Amy Remensnyder. “Torture and truth: Torquemada’s ghost.” Why the Middle Ages Matter: Medieval light on modern injustice, eds. Celia Chazelle et al (London, 2012).

Followup question. Where there so called medieval torture devices that were never really used as a medieval torture device?

Also, how widespread was their usage?