(Disclaimer: any opinions are strictly my own, to do with as you will. Keep what you find helpful, toss the rest with yesterday's paper)
Rather like the movie 'SE7EN', this piece is violent and disturbing without being crass and vulgar. The violence is alluded to, and takes place behind the scenes. Perhaps it makes it that much more effective; leaving the brutality up to each reader's imagination, allowing us each to come up with our own 'right method'. As such, my only suggestion for this piece is that it may work even better if you opt to be more vague instead of 'scalpel' and opt for a generic tool/instrument/weapon/twisted metal -type descriptor instead.
The imagery you use to set the scene is well-developed and precise. Your descriptions are perfectly vivid. The reader finds their eyes fluttering; woozy. One can feel the course grain of the rope, the cramping muscles against the metal chair.
The final sentence is EXCELLENT. Kudos to you on such powerfully dark writing. Write On!