to such an extent相当于so much
it litters up her desk to such an extent that every time she opens it, loose bits of paper fall out in every direction.= it litters up her desk so much that every time she opens it, loose bits of paper fall out in every direction.
that之后是结果状语从句
the air pollution is so serious to such an extent that some freeways have been put out of use
结构上没有问题,但是意思上好像略有点累赘（so 和to such an extent）,不过应该没事that后是结果状从 那请问to such an extent 是什么成分？谢谢！插入语