Despite substantial success in research on gastroduodenal ulcer pathogenesis, the frequency of complicated and unfavorable course forms (of the disease) requiring operative intervention has not declined.
By "unfavorable course forms" I mean "forms with an unfavorable course." How can I fix this part? I suppose I could rewrite it as:
the frequency of complicated forms (of the disease) and forms with an unfavorable course requiring operative intervention
But then it sounds like only forms with an unfavorable course require operative intervention. The original in my native language literally translates to "of complicated and unfavorable-in-course forms."
I'm not sure if I should include "of the disease." It's in the original, but I'm afraid it might make the sentence even more confusing.
For example: Despite the substantial success of research into gastroduodenal ulcer pathogenesis, the occurrence of forms of the disease requiring operative intervention - complicated forms of it, and forms that run an unfavorable course - has not declined.
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For example:
Despite the substantial success of research into gastroduodenal ulcer pathogenesis, the occurrence of forms of the disease requiring operative intervention - complicated forms of it, and forms that run an unfavorable course - has not declined.