How To Cross-Mount a Case and Why It's Important To Do So

Why is cross-mounting so important? It is used in the laboratory to verify, replicate, or correct the horizontal plane. Cross-mounting is an important process used to verify the clinical scenario and make the final restorations with predictability.

In a more complex case, it is a very helpful way to minimize errors in the esthetics and the functional outcome. The smallest case for which cross-mounting is effective is when restoring the maxillary central incisors, and continues to be a crucial step in the fabrication process through cases as extensive as a full-mouth rehabilitation. It is used to establish the following on the maxillary anterior teeth:

• the horizontal plane,
• the midline angulation,
• the incisal length,
• the anterior/posterior position,
• their inclination, and
• the envelop of function.

The most important thing you can do to develop an acceptable esthetic outcome is establish harmony between horizon, the interpupillary line and the smile, particularly a line between the commissures and the maxillary and mandibular occlusal planes. 

• If a facebow transfer is taken of a patient's approved provisional restorations and subsequently mounted on the articulator, the clinical scenario is now able to be visualized in the laboratory.
• If there is a slight cant to the incisal plane of the maxillary provisional cast or a midline cant, this should be confirmed by the photographs taken of the patient's smile relative to horizon.   
• If the facebow of the tooth preparations is made and the cast is not cross-mounted, it is not possible to confirm the facebow is correct relative to horizon. If photographs are taken of the facebow transfer relative to horizon, this could be used to confirm the mounting. 
• If the cast of the provisionals is correctly mounted with the facebow transfer, then the mandibular cast can be mounted and the occlusion properly established.

This should help you understand why it is important to provide the correct sequencing of bite registrations for the lab. 

If you are interested in understanding the process the lab uses to cross-mount a case or are cross-mounting your own cases and want to avoid potentially introducing error into the procedure, I suggest you complete the steps in the following order. Doing so increases the predictability that the esthetics and function of the case will meet the expectations of you and your patient.   

1. Take a face bow of the patient-approved provisional.
2. Mount the patient-approved provisional case.
3. Mount the opposing mandibular cast (natural dentition or a cast of the provisionals).
4. Once the mandibular cast is mounted, then the cast of the maxillary preparations is mounted with a bite registration to the mandibular teeth or provisionals, again at the vertical dimension of occlusion.
5. The cast of the mandibular prepared teeth is now mounted to the cast of the maxillary preparations.
6. When taking the bite registration in an extensive case, the maxillary and mandibular anterior provisionals are left cemented to establish the vertical dimension of occlusion and establish the anterior stop. This bite is now used to mount the cast of the maxillary prepared teeth.

This completes the steps involved in the cross-mounting of casts. Remember, this process helps the lab verify the information sent from the doctor’s office and increases the likelihood that the cementation process is smooth and uneventful. If the mounted models are not aligned the same as the clinical photographs, the laboratory may photograph the case and send it to the dentist to discuss the discrepancy. 

(Click this link to read more dentistry articles by Dr. Bob Winter.)

Bob Winter, D.D.S., Spear Faculty and Contributing Author