There are two issues here: grammatical and rhetorical/literary.
 He saw Case and smiled, h͟i͟s͟ ͟t͟e͟e͟t͟h͟ ͟a͟ ͟w͟e͟b͟w͟o͟r͟k͟ ͟o͟f͟ ͟E͟a͟s͟t͟ ͟E͟u͟r͟o͟p͟e͟a͟n͟ ͟s͟t͟e͟e͟l͟ ͟a͟n͟d͟ ͟b͟r͟o͟w͟n͟ ͟d͟e͟c͟a͟y͟.
the underlined part is a supplement in the form of a verbless clause (CGEL, pp 1359-1360).
As was pointed out in the comments, the meaning is the same as if we had
 He saw Case and smiled. His teeth w͟e͟r͟e͟ a webwork of East European steel and brown decay.
More on the vebless clauses below.
Rhetorically, the difference between  and  on the one hand, and
 He saw Case and smiled, his teeth l͟i͟k͟e͟ a webwork of East European steel and brown decay.
on the other, is that  uses a simile, whereas  and  use a metaphor.
More on verbless clauses as supplements, from CGEL, pp 1359-1360. Note that supplements are 'elements which occupy a position in linear
sequence without being integrated into the syntactic structure of the sentence' (p. 1350).
(f) Verbless clause
 i The tourists, m͟o͟s͟t͟ ͟o͟f͟ ͟t͟h͟e͟m͟ ͟f͟o͟r͟e͟i͟g͟n͟e͟r͟s͟, had been hoarded onto a cattle truck.
ii The defendants sat in the dock, t͟h͟e͟i͟r͟ ͟h͟e͟a͟d͟s͟ ͟i͟n͟ ͟t͟h͟e͟i͟r͟ ͟h͟a͟n͟d͟s͟.
iii The only household chore men excelled at was - d͟r͟u͟m͟r͟o͟l͟l͟ ͟p͟l͟e͟a͟s͟e͟ - taking out the
In [i] the supplement is comparable in function to a relative clause: compare who were most of them foreigners (or most of whom were foreigners). If the supplement consisted of foreigners on its own, it would be an ascriptive NP (noun phrase), like those in  (e.g. Her father, a͟ ͟d͟i͟e͟-͟h͟a͟r͟d͟ ͟c͟o͟n͟s͟e͟r͟v͟a͟t͟i͟v͟e͟, refused to even consider the proposal.); most of them, however, does not function as a modifier in NP structure, so most of them foreigners must be analysed as a reduced clause - one which could not stand alone as a sentence. The supplement in [28ii] could likewise not stand alone, but differs in its internal structure in that their heads is subject. An equivalent integrated construction would have a modifierwith the form of with + verbless clause: with their heads in their hands. The supplement in [28iii], by contrast, could stand alone as a sentence. It is simply a fragmentary main clause (with the illocutionary force of a directive) used as an interpolation.