What’s your strategy for occlusal adjustment? Specifically, where do you take away material from the functional surface of the restoration whether it’s a denture tooth or a full coverage restoration of some kind? Occlusion, of course, can be a rather broad topic. What are your thoughts on the BULL rule? I was taught this acronym as a way to think through an occlusal adjustment of an indirect restoration or removable prosthesis in dental school. I remembered my attempt to make sense of it when studying for the board examination.
The BULL rule: A guideline or an unbreakable rule?
What does it mean? If there is an occlusal discrepancy (premature contact), the idea is to maintain the functional cusp height while removing material to re-contour the non-functional cusps. According to the BULL rule, modification should be made to the buccal cusps of the upper posterior teeth and the lingual cusps of the lower posterior teeth. BULL stands for buccal upper lower lingual. For example, to illustrate the BULL rule, the palatal cusp of a maxillary first molar contacts prematurely in a centric position as well as in the working eccentric position. But the contact is correct in an eccentric or balancing position – where would you adjust? The answer, if you were taking a board exam, would be to deepen the central groove or marginal ridge of the opposing mandibular tooth to manage the centric interference and to reshape the mandibular lingual cusp to account for the working interference.
Is that all there is to it?
What about the discussion, techniques and effort focused on creating shallow cuspal inclines. One example would come in the form of a custom incisal guide table made specifically to set a boundary so as not to create inclines steeper than what the patient had worn into their dentition. Another example would be the shallow cuspal inclines available in posterior denture teeth that become even more shallow with time to account for the horizontal chewing patterns found in partially edentulous, or even completely edentulous, patients. Certainly, a true gnathological approach to occlusal adjustment would not subscribe to this type of generalized strategy for designing shallow inclines to simply flatten functional occlusal surfaces.
Is there a consequence to following the BULL rule?
When the goal is to deepen the opposing fossae while maintaining centric cusp height, what we are doing is increasing the angles of our cuspal inclines. In a classic article, Weinberg & Kruger measured directional force changes that were measured between the abutment and dental implant while altering cuspal inclines. Their results demonstrate that for every 10 degree change in contact inclination there is a 32 percent change in torsional stress at the abutment. For example, a 10 degree contact inclination results in 68 pounds of stress at the abutment; 20 degrees equals 100 pounds; 30 degrees equals 132 pounds. It would be reasonable to expect the magnitude of the force to be different when evaluating a complete denture primarily due to lower overall bite forces. It’s easy to make a comparison in that steeper cuspal inclines would likely destabilize the denture or, perhaps, contribute to premature wear of implant attachments for an overdenture.
So, back to the question …
Maxillary molar with a palatal cusp demonstrating premature contact in centric and working movements and correct in non-working movements. What if we break the BULL rule? Specifically, what if we adjust/shorten the maxillary palatal cusp – the centric holding cusp of the upper first molar? The result would require adjustment of the contra-lateral posterior teeth to re-establish a balancing or non-working contact. Each one of those adjustments results in shallower cuspal inclines. Is there a BULL rule penalty box?
- Weinberg LA & Kruger B. A comparison of implant/prosthesis loading with four clinical variables. International Journal of Prosthodontics 1995;8(5):421-33.