BULL Rule or Guideline? - Spear Education

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?

References

Weinberg LA & Kruger B. A comparison of implant/prosthesis loading with four clinical variables. International Journal of Prosthodontics 1995;8(5):421-33.

Douglas G. Benting, DDS, MS, FACP, Spear Visiting Faculty and Contributing Author. [ www.drbenting.com ]

Comments

Robert Stewart
February 13th, 2015

I am assuming you are referring to a denture-type arrangement of fully balanced articulation. BULL is a guideline of course...however the 'rule' does allow for the following actions: If the maxillary palatal cusp is in line with the other palatal cusps of the adjacent maxillary prosthetic teeth (i.e. not below the compensating curve or dipping into worn occlusal surfaces of the lower molar), then slight reshaping of the inner incline of the lower tooth's buccal cusp is warranted for any working interferences. This may be done without changing the centric contact that cusp maintains. The lower fossa may also be adjusted FiRST if the contact is premature. Maintaining the centric contact is done with EITHER the palatal cusp of the upper tooth into the lower fossa, or with the buccal cusp of the lower teeth to the upper fossa...pick one. If we are talking about a natural dentition, then eccentric contacts on molars will not be the norm, with the exception with unilateral group function schemes. The more typical scheme is the anterior disclusion concept, where the anterior teeth and posterior teeth are assigned different roles. The anterior teeth perform a deflective role so that posterior teeth disclude out of centric. The close cousin (often considered synonymous) is the mutually protected articulation philosophy where the anterior teeth (and possibly first premolars) "protect" the other posterior teeth from potentially harmful eccentric contacts (and huge bending moment stresses on them), and the posterior teeth "protect" the anterior teeth from centric contact stresses. Therefore, the generally accepted contacts on natural molars is centric only.

Doug Benting
February 17th, 2015

Thank you for your response Robert. The idea of a topic on occlusion is to bring up a little debate in anticipation for the “Point Counter-Point” discussion coming up at the Faculty Club Summit this fall will certanly touch on a few of these topics. Of course, I have to ask a follow up question related to anterior disclusion or mutually protected occlusion. The way that I look at “canine guidance” is as a functional prosthetic convenience position – it’s easier to adjust eccentric contacts if the canines are handling the contacts in lateral movement positions. How often do we see canine guidance as a natural presentation? One paper finds that for 17 to 25 year olds cuspid protected occlusion in lateral movement was present bilaterally in 57% of the subjects, unilaterally in 16% no guidance provided by the canines 26% of the time as in more of a bilateral balanced denture type occlusion. Other numbers quoted show how often each occurs in nature observing Bilateral Canine Guidance in 2.3% of the samples, Unilateral Canine Guidance in 10%, Bilateral balancing interference in 40%, Unilateral balancing interference in 60% of the samples. I assume that unilateral or group function situations occur more often as the teeth wear with age. While I completely understand that if we have the opportunity to complete a full oral rehabilitation on every patient with abrasive wear, we might choose canine guidance as a means to protect the posterior teeth from contacting in eccentric movements. I believe it is a more common scenario that presents where we are working with either a single or multiple full coverage indirect restorations but not a situation that includes all remaining natural teeth. There must be a thought on the BULL rule that at least crosses the mind as a guideline that would likely be similar in fact to what is found for complete dentures. Certainly there are scenarios that arise where a steep canine guidance would not be indicated where one example would traditionally involve a dental implant supported canine. The question comes up … what about the mandibular movement potential as demonstrated by wear on the natural posterior teeth? Are we able to fix that simply by adding canine guidance and created a mutually protected situation where the molars only contact in centric without creating tension in the positioning muscles of the mandible? What about the functional situation chewing a piece of food such as meat in the posterior segment? While it is certainly possible to see how steeper cuspal inclines could potentially improve the overall chewing efficiency, there will be a significant amount of lateral force in the palatal inclines of the buccal cusps while the meat is being pulverized before the canines can perform their guidance function. I can only imagine there comes a point where in order to maintain steep cuspal inclines in the posterior segment that the canines would have to be contoured in such a way to provide steep vertical overlap with a very shallow horizontal overlap in order to function in a manner that would protect the posterior teeth. There is an esthetic risk of un-naturally long canines and the corresponding reverse smile line as well as the potential to “lock” a patient in where the mandibular movement is restricted. I’m always curious for more thoughts and ideas!