The dent in his torso is roughly an inch deep, five inches long, and an inch and a half in height. It is positioned where his lowest left rip is, it stops that area his torso being a smooth curve like the rest of his torso and turns it into the top of an ‘m’ shape.
The dent is hypersensitive - to pain and temperature - and hitting it can trigger full blow hallucinations of PTSD. The dent was caused by repeated punching from Colonel Sebastian Moran, the bone was never given a chance to heal properly before it was re-broken, over and over again until when it finally healed, without being set in place, it became the dent.
The dent is clearly visible through the fabric of his shirt even with the t-shirt he wears underneath it to try and make it less visible. Though the dent is visible should one pay attention to that side of Tyson’s torso, most people could go months without noticing it as a hollow in someone’s torso isn’t something people typically keep their eyes out for.
To the average passerby, it simply looks like an odd dip in the fold of his shirt. Sometimes clothes fold in weird ways. No big deal. Tyson’s habit of keeping his right hand over the dent in his torso, to shield it from accidentally getting struck, also shields it from view most of the time.
The dent in Tyson’s torso is an incredibly obvious weak spot that most people would never even notice. People see what they want to see, what they expect to see. In many ways, Tyson is more protective of the dent than he truly needs to be, but he cannot take the risk of leaving it exposed around people he doesn’t trust.
The reason the dent hasn’t been re-set and corrected is that the lower half of Tyson’s lower sternum was weakened by those attacks. To properly fix the dent in Tyson’s torso, it would require a lot of metal reinforcement and the possible complete replacement of a large portion - if not all - of Tyson’s rib cage.
To Tyson, it isn’t worth losing the flexibility and manoeuvrability for an operation that might not even solve the pain the dent in his torso causes.