If Michigan's print is made of 2 to 3 mm thickness evenly on maxillary arch. How to, uh, deal with even distribution of forces if the mandibular occlusal surfaces are uneven or suprarected in some of the areas? How will occlusal splint serve its purpose?
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Michigan occlusal splint fabrication occlusal adjustment
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occlusal splint bruxism temporomandibular disorder occlusal equilibration
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Michigan occlusal splint uneven mandibular occlusion force distribution clinical adjustment technique
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Here is a thorough answer to your question:
Michigan Splint on a Maxillary Arch: Handling Uneven or Supra-Erupted Mandibular Teeth
The Core Problem
A Michigan (stabilization) splint is fabricated to 2-3 mm of hard acrylic on the maxillary arch, creating a flat occlusal platform. Its fundamental purpose depends on even, simultaneous, bilateral posterior contacts in centric relation - this is what distributes masticatory and parafunctional forces uniformly, decompresses the TMJ, and relaxes the elevator muscles. When mandibular teeth are uneven (supra-erupted, tilted, drifted, or have a steep curve of Spee), they will contact the splint surface at different vertical heights, creating premature contacts on the high spots and leaving gaps at others - which defeats the entire therapeutic mechanism.
How to Manage This Clinically
1. Occlusal Adjustment of the Splint Surface (Primary Approach)
The acrylic of the Michigan splint is the variable - not the teeth. The splint surface is selectively ground to:
- Eliminate high spots created by supra-erupted mandibular cusps
- Create flat "stops" (shallow fossae) for each opposing cusp so every tooth lands simultaneously
- Allow freedom of movement within splint centric without premature contacts
Per Ramfjord & Ash's classic description of Michigan splint protocol (Bauer Smiles PDF):
"A principal objective of the initial adjustment is to provide even bilateral and posterior contacts in freedom of splint centric, including centric relation if possible."
The adjustment process is iterative - articulating paper (ideally Shimstock foil or T-Scan) is used to identify where the mandibular cusps are hitting first, and those areas on the splint are reduced until simultaneous, even contact is achieved across all teeth.
2. The Role of Thickness (Why 2-3 mm Matters Here)
The 2-3 mm thickness is not arbitrary - it gives the clinician enough acrylic material to selectively grind high spots without perforating the splint. If a supra-erupted mandibular tooth is creating a premature contact, that area of the splint can be reduced by 0.5-1 mm to bring it into simultaneous contact with neighboring teeth. Without adequate thickness, you run out of material to adjust.
In severe cases of supra-eruption, the supra-erupted tooth may require reduction in the splint area (creating a deeper stop/indentation), while the adjacent teeth need slightly thicker contact zones. This is achieved during the adjustment phase, not during fabrication.
3. Managing Supraerected Teeth - Specific Strategies
| Situation | Solution |
|---|---|
| Single supra-erupted mandibular molar | Reduce the splint surface at that cusp contact; ensure the adjacent contacts remain even |
| Steep/exaggerated curve of Spee | The flat splint surface will contact anteriorly first - grind anterior contact areas and build up/preserve acrylic over the deeper posterior areas to eventualize contact |
| Multiple uneven teeth | Systematic articulation paper marking - adjust in order from heaviest (earliest) to lightest contacts |
| Grossly supra-erupted tooth (>2 mm discrepancy) | Consider selective occlusal adjustment of the supra-erupted tooth itself before splint fabrication, OR design the splint with slightly increased thickness in that region |
4. Pre-Fabrication Planning for Severely Uneven Arches
If the mandibular arch is grossly irregular before splint fabrication, the better approach is:
- Mount accurate study models on a semi-adjustable articulator in centric relation
- Identify the extent of discrepancy before designing the splint
- If a single supra-erupted tooth is the main offender, consider limited occlusal adjustment of that tooth prior to splint delivery (with patient consent), rather than compensating entirely on the splint
5. Follow-Up Adjustments Are Mandatory
The Michigan protocol specifically requires adjustment at every recall visit because:
- As muscle pain/spasm resolves, the mandible repositions and new prematurities appear
- Supra-erupted or tilted teeth may create new contacts as the patient's posture and muscle tone changes
- The splint surface must be repeatedly re-equilibrated to maintain even force distribution
Failure to do this is one of the most common reasons Michigan splints fail therapeutically.
6. Digital Occlusal Analysis (T-Scan)
Conventional articulating paper has a well-documented limitation - it shows the location of contacts but not their force magnitude. A T-Scan study (ADTT journal) found that conventionally adjusted splints had 24% vs 75% force split left-to-right - markedly imbalanced despite appearing adjusted by paper marks. T-Scan guided adjustment brought this close to 50:50. In complex cases with supra-erupted teeth, T-Scan guidance significantly improves the clinical outcome.
How the Splint Still Serves Its Purpose Despite Uneven Arch
The Michigan splint serves its purpose through the adjustment process, not at the moment of delivery. The flat acrylic platform compensates for the mandibular irregularities by:
- Acting as a "corrective equalizer" - the soft acrylic (during final cure or adjustment) can be selectively shaped so that every mandibular tooth lands on a level surface at the same vertical height, regardless of their natural irregularity
- Removing cusp inclines as an occlusal factor - the flat surface means cusps contact without lateral vectors, reducing torquing forces even on uneven arches
- Providing bilateral simultaneous contact - once adjusted, even a severely uneven mandibular arch can close against the flat splint and have all teeth contact at the same moment, distributing force simultaneously across all posterior supporting areas
- Protecting from premature contact-related muscle hyperactivity - by eliminating the tooth-to-tooth prematurities that would otherwise trigger proprioceptive muscle guarding
The key clinical principle: the splint is an adjustable interface. The irregularity of the mandibular arch is corrected at the splint surface, not in the teeth. This is why hard acrylic (not soft liners) is recommended - it can be precisely adjusted, and soft liners make equilibration unreliable.
Summary: Uneven or supra-erupted mandibular teeth are managed by selective grinding of the splint's acrylic surface during the adjustment phase, using articulating paper (or T-Scan for precision) to progressively achieve simultaneous bilateral posterior contact. Adequate 2-3 mm thickness ensures there is enough material for this adjustment. Grossly uneven arches may require pre-splint tooth preparation or slightly increased splint thickness in specific regions. Ongoing recall adjustments are not optional - they are the mechanism by which the splint continues to serve its therapeutic purpose as the patient's occlusion responds to treatment.
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