Pannkuk’s Rule: If the auxillary (MB2, MB3) canal has a shorter working length than the MB1 canal from the same reference point it is an independent root canal system with a separate portal of exit.
If this rule seems violated it means the MB1 is perforated or the file used for measurement is exiting an alternate lateral canal/resorption defect.
The clinical significance of establishing separate canal tracks is that you need to shape out the MB2 more to make sure you can fit a cone and properly treat it as a separate system. With a confluent MB1/MB2 you can often safely create a narrower tapered prep in the MB2 knowing you will get adequate deep shape and debridement of the MB1.
Many times you can validate confluence by placing a cone in one canal (usually the MB2) and then place a file in the MB1 to indent it, BUT you still never know if there will be a an apical branch off the confluence. Knowing that there is a shorter MB2 working length means a separate portal of exit gives you a bit more anatomical understanding should you get variable lengths on the electronic apex locator with different pre-curved file entries. Noting the fluid level decrease from the MB2 when you suction from the MB1 or vice versa, just means there is some common space at some point along the length of the root but is not of much value regarding your cleaning and shaping strategy.
Example Figure 1:
A: MB1 Working Length
B: MB2 Working Length
C: MB3 Working Length
A < B then there is a common POE (portal of exit)
A > C then there is a separate POE
This is true to near virtual certainty because mesiobuccal roots of maxillary molars tend to be sloped from the buccal to the palate. The path a file takes from an auxiliary orifice to join the MB1 canal can be presumed to be longer. The path a file takes from an auxiliary orifice which independently exits through a separate POE can be presumed to be shorter.
Figures 2 -6 show an actual case where the illusion of confluence from the cone fit radiographs shows that the Pannkuk Rule correctly indicated that the MB2 exited a separate POE to the lingual of the MB1. This working length knowledge helped develop an appropriate definitive shaping strategy to insure that the MB2 apical prep was better debrided before obturation.