Check the TNG series for [the episode](http://en.memory-alpha.org/wiki/Genesis_(episode)) where a synthetic T-cell becomes a viral infection that causes the crew to "de-evolve". Worf's proto-self is not a creature to be toyed with, and the cranial ridge is much more pronounced. As Teknophilia said, there's an extra skeletal shell around their skull.

Klingons are built to survive, as was mentioned in [another TNG episode](http://en.memory-alpha.org/wiki/Ethics_(episode)) (the one where Worf becomes paralyzed from the waist down). The visiting doctor who is working on an experimental paralysis surgery is reviewing Klingon anatomy and remarks offhand that whoever designed them certainly believed in spare parts.