What I mean is, which episodes started off where a guest star suddenly has some soft of affliction...and then the opening credits role? The principle players would be completely absent until after the credits.
I can think of two episodes:
"Silken Threads and Silver Hooks"
"Just a Little Courage" - The opening of this one is so much like House MD it's scary.
Can't give you a real number but on page 152 in his book Playing Doctor Television, Storytelling, & Medical Power Joseph Turow explains why it was done:
Other approaches began to fall into place as the series took shape. The creative impulses of the producers and writers came together with the standard demands of TV storytelling in such a way as to generate a flow of routines that became rather predictable after a while. David Victor had it down pat. "The ideal show," he said, "would have to do with a good strong human theme. And you get involved with somebody in the opening minutes of the show...so that there's an emotional tie to somebody- in the early moments. So then you care what happens to them through this travail."
That meant the introduction, before the title. In those days, such prologues always included the main character. But Victor and O'Connell argued that sometimes it was less important to see Marcus Welby or Steven Kiley in the prologue than it was to get audience sympathy with the person who, anthology-like, would be the focus of the episode. ABC programming executives allowed them to do it.