Get ready for the latest installment of what is becoming an annual or even biannual tradition: the worlds of Grey's Anatomy and its spinoff Private Practice will cross over again.

Private Practice leading lady Kate Walsh is heading back to Grey’s Anatomy in January in a crossover involving Addison, Mark, and a patient played by guest star Leven Rambin.

The situation as we know it is as follows:

On Grey’s Anatomy half of the crossover, Dr. Mark Sloan (McSteamy, a.k.a. Eric Dane) pleads with Addison to come to Seattle and help perform surgery on Rambin's character.

One would assume that the girl is pregnant, given that Addison is the preeminent neonatal surgeon in, like, the whole country. But that has not been confirmed at this point.

Then, the story continues on Private Practice when, according to creator Shonda Rhimes, “complications arise and Mark takes [her] back down to L.A. for more surgery.”

As for Mark's connection to the new patient, whether sparks fly between Addison and Mark, and how Mark and Lexie are affected, that all remains to be seen. For now.

Are you excited for the latest Grey's Anatomy-Private Practice crossover?

We'll reserve judgment on this for now, but do you think they're overdoing it with these crossovers. By our quick calculations, this will be the fourth since April 2008:

• Addison returned to Seattle near the end of Season 5 of Grey's Anatomy. We forget the exact reason right now, which probably tells you all you need to know.
• Last season, Addison summoned Derek to treat her ailing brother, Archer (Grant Show). This story line continued for several episodes and on both programs.
• Just this fall, Bailey (Chandra Wilson) took a transplant patient to Los Angeles, where she resides, after she was unable to get the help needed in Seattle.

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.