Back in 2011, Trey Parker came up with a hilarious satirical spin on Tom Six's first Human Centipede film, in the South Park episode 'HUMANCENTiPAD'. That 22-minute animation was packed full of spiky observational humour about the way in which people who blindly agree to things are often cruelly exploited. The final part of Six’s grotty vision takes on a similarly bawdy tone, turning its pincers on US correctional facilities and their questionable management techniques.

Just as South Park reserves the right to be equally offensive to all, so does Six, but he doesn’t know how to handle his material, other than to raise the shock stakes and the length of the surgically conjoined monstrosity to 500 people (up from the original film's three).

Six starts off by playing with the dumb blonde routine, casting ex-porn-star Bree Olson as dim-witted secretary Daisy, who is in the employ of the greedy and sadistic prison warden Bill Boss, played by a scenery chewing Dieter Laser – star of the first instalment. The second film's lead, Laurence R Harvey, is also recast as the warden’s accountant Dwight, who comes up with the cost-cutting idea of stringing all the prisoners together.

Unlike, say, Judy Holliday in Born Yesterday, Daisy learns nothing and is simply used as a plaything by the head of the operation. The level of degradation she's subjected to is saddening rather than shocking. Is Six suggesting that America views women in the same way? Probably. But his decision to not give Olson anything to do other than parade around in skimpy attire and get shafted, before ending up as a punch-line to his unfunny jokes, feels more sexist than scathing.

As Six smugly reminds the audience, the Human Centipede franchise is now part of the zeitgeist whether you like it or not. It comes to a welcome close with this juvenile and, apparently, intentionally crappy final outing that has nothing insightful to add to an already overstretched idea.

Selected release from Fri 10 Jul.