The Michigan splint was developed in the 1950s at the University of Michigan. It is a hard, transparent acrylic splint that covers all the maxillary teeth and has flat, occlusal centric stops for the mandibular teeth so that, on jaw closure, there are even and simultaneous occlusal contacts of the mandibular teeth on the splint surface. This centric relation splint can be fabricated in an adjustable articulator and is fitted with a canine guidance that has a sufficiently steep inclination to prevent both working and nonworking contacts as well as anterior guidance contacts.
The canine guidance of approximately 1 to 2 mm permits only minimal lateral or protrusive movements under full occlusal contact of the lateral splint surface before disocclusion occurs. These minimal movements are referred to as freedom in centric occlusion, and they have to be individually determined and worked into the canine guidance. The degree of freedom of movement permitted by the canine guidance must be adapted to different occlusal positions:
- Centric occlusion
- Habitual occlusion
- Occlusal position when swallowing
- Occlusal position when sleeping
Incisor guidance such that the splint disoccludes must not be present. The Michigan splint in its basic form is hence a stabilizing centric relation splint with a defined occlusal elevation of 2 to 3 mm, canine guidance, and freedom of movement in the centric position (freedom in centric occlusion). It is used to treat malocclusions, to manage diseases of the muscles of mastication and pathologic TMJ changes (myoarthropathies), and to control bruxism. Figures 11-14 to 11-18 illustrate the actions and functions of the Michigan splint.
Because changes of mandibular position influence the neuromuscular system, especially the length of the muscles, muscle activity is markedly reduced by the use of a Michigan splint. A positive effect of the Michigan splint can be accurately demonstrated; ie, the splint therapy works even though the mechanisms of action cannot be clearly verified. There is hence something of a placebo effect. The beneficial effect is based on the following conscious perceptions:
- It compensates for occlusal anomalies.
- It allows unloaded joint positioning.
- It counteracts bruxism and clenching.
- It prevents pathologic occlusal positions.
To fabricate the Michigan splint, the models are mounted in the articulator in the centric occlusal position with a facebow record. Joint values should be individually set, and the occlusal height and inclination of the incisal guide plate should be adjusted as follows:
- The vertical opening is arranged so that an inter-occlusal distance of 1 to 2 mm is present in terminal occlusion and in protrusive movements.
- The inclination of the incisal guidance is set up to allow border movements within the canine guidance.
- The height of the canine guidance should be designed so that all parts of the dentition dis-occlude in the edge-to-edge situation of the canines.
The splint in the maxilla is then carved in wax:
- The palatal border makes contact with the area of the palatal ruga, and the vestibular margin extends slightly above the tooth equators so that the splint can later be firmly clasped. After fabrication, it must be firmly seated without additional retentive components.
- Occlusally, a flat bite plane is created until only the buccal occluding cusps touch the splint. The bite plane follows the sagittal and transverse occlusal curves.
- The sloping canine guide plane is built up, and the movement paths of laterotrusion and protrusion are established on freedom in centric occlusion so that continuous contact of the canine guidance can ensue and until disocclusion of the posterior teeth occurs.
The splint is then fabricated in transparent thermoplastic polymer, the maxillary model is rearticulated, and the splint is reground in the articulator. The final grinding is done by the dentist. The Michigan splint must have an absolutely stable seating; it cannot rock. A splint that is not seated in a stable position must be remade.