A closer look at bite registration, a crucial part of denture fabrication for fit and wear, and how to capture the position you need when treating an edentulous patient.
Dr. Shirin Khoynezhad, director of second-year pre-clinical dentistry at the University of Alabama, is a prosthodontist that lectures on denture materials and computer-aided design and computer-aided manufacturing (CAD/CAM) dentures. She explains that bite registration records how the teeth articulate or come together. Next, the record transfers to the articulators or casts for treatment planning.
The significance of bite registration in dentures
The bite registration is crucial for all types of oral rehabilitation, per the Foundation for Oral Rehabilitation (FOR.org). Without this critical measurement and documentation, dentists and dental lab technicians cannot create accurate occlusal relationships for the patient’s prosthesis.
Centric relation (CR) is an important factor in bite registration. However, there are many definitions of what it means. Per an article from 2017 in the Journal of Prosthetic Dentistry, CR is a spatial relationship that is clinically determined by the relationship between the mandible and the maxilla that is a physiological position both repeatable and independent of tooth contact.1
CR is also a starting point for vertical, lateral or protrusive movements and is restricted to a “pure rotary movement about a transverse horizontal axis.”2 It is a repeatable reference position for both mounting casts or, at a determined vertical dimension, the development of functional treatment occlusion.3 That said, the article also suggested that the CR might vary a bit depending upon the recording method.4
“Without a correct centric relation (CR), the denture you build will disturb the normal function of the masticatory system,” Dr. Khoynezhad says. In other words, whenever the patient bites down they'll have premature or interruptive contact between the dentures.
“The dentures should come together all at once because they sit over soft tissue,” she says. “In a regular denture, everything sits on soft tissue which is movable, so everything should come together at once. Otherwise the denture will have instability inside the mouth.”
This “rocking” of the denture causes the prosthesis to become unstable and can interfere with function in the short term. In the long term, Dr. Khoynezhad says, it can cause joint problems that range from headaches to joint pain, clicking or popping in the joints. In some cases, the denture patient can develop temporomandibular disease (TMD).
Dr. Khoynezhad compares this to balancing tires on a car. If you don’t have a proper balance, you will have more pressure on one tire. This causes premature wear and vibration problems that shorten the life of the tire. The same kind of wear can occur in the mandibular joint of a patient with an unbalanced denture.
“You will have initiated disease in that joint,” Dr. Khoynezhad says. “The CR bite registration is very important in denture patients.”
Advice for capturing accurate measurements in bite registration
For.org offers the following treatment guidelines regarding complete denture maxillomandibular relations:5
Continue to page two to read more...
When making a restoration, clinicians analyze both static and dynamic occlusion. The static occulsion is how the upper and lower teeth come together when the jaw is closed and is recorded when the teeth have the most contacts. The dynamic occlusion is how the teeth connect when the jaw is moving in all directions and depends on both the teeth and the temporomandibular joint (TMJ) shapes.
The correct bite registration for the static occlusion requires the following, per Infodentis:
Capturing the bite registration
Once these parameters are met, the bite registration is recorded. Sometimes the interocclusal relationship is captured in bite wax strips or plates. However, one criticism of bite waxes is they can be less stable than other materials.
Many times, the bite is recorded in impression material for a conventional bite registration. Once the material sets, the bite record is sent to the dental lab or scanned into a CAD/CAM system.
Per Ivoclar Vivodent, the ability to capture these images of the impression material digitally can save time and increase accuracy for dentists. These scannable bite registration materials are vinylpolysiloxanes. Ivoclar Vivodent has a Virtual CADbite registration material with a vinylpolysiloxane base. The fillers in the material were designed to help prevent the light scattering that can occur when scanned by a laser scanner.
In some rare cases, a clinician can record a digital bite registration for use in conjunction with a CAD/CAM system. The system uses a scanner or camera to capture the antagonist bite registration.
What about when the patient doesn’t have any teeth?
This explains what to do when recording a bite registration when the patient has teeth. However, the edentulous patient doesn’t have any teeth. They cannot establish the vertical dimension needed for the prosthesis.
The answer for an edentulous patient is that the bite record comes from what remains, the patient’s muscles and TMJ.
Dr. Khoynezhad explains that a proper bite registration needs to capture the bite while the mandibular jaw is in a repeatable position, meaning it can hit the mark over and over again with few misses. When you record the bite in a position that cannot be repeated easily and then fabricate the prosthesis based on that bite registration, the denture will not fit.
The problem with using the muscles to capture the bite, Dr. Khoynezhad says, is they are not reliable. Moreover, they can contract in different positions. If there is a difference in the contraction of the muscles of the jaw, it can change the position of the lower jaw.
So, without teeth and because the muscles are unreliable for a repeatable position, clinicians are left with the joint position to record the bite. Dr. Khoynezhad says seating the condyles in a repeatable position is crucial in edentulous cases to capture the most accurate bite registration.
“We need to take the centric relation for bite registration, so you set the condyle in the most superior anterior position. That's where the disc of the joint is the closest bone-to-bone contact. It’s repeatable because there is no cushion of the joint between the condyle and the joint,” Dr. Khoynezhad explains.
Different techniques exist for capturing centric relation. The new digital world of dentures is exciting for clinicians and patients alike. For patients with teeth, you can capture the bite both conventionally and digitally. However, for edentulous patients, Dr. Khoynezhad recommends still capturing the bite conventionally and then scanning it into the computer to assemble the information and design the prosthesis digitally for fabrication.
“Because we are dealing with the soft tissue and joint, we have to do it manually on the patient to be able to find the most comfortable position,” she says. “You can guide the patient to the CR position and then scan the arch to be able to fabricate some tools to get the correct registration. Capturing the bite registrations purely digital is not happening for the edentulous patient.”
Dr. Khoynezhad recommends two main things to ensure excellent fit and stability for the denture. First, capture the muscles by border molding to get an accurate impression of the ridge and mobile tissue around it so the denture will stay in place. The second is getting the CR bite record.
“It's a crucial part of fabricating dentures,” she says. “There are many ways to do it, but clinicians have to take time and double check to make sure they got the CR bite before they proceed with the fabrication of dentures. That's the most important thing.”
1. Wiens, Jonathan P. et al. “Defining centric relation”Journal of Prosthetic Dentistry , Volume 120 , Issue 1 , 114 – 122. From Web. 9 March 2019. < https://www.thejpd.org/article/S0022-3913(17)30708-4/pdf>.
5. “Complete Denture - maxillo-mandibular relations.” www.for.org. Web. 9 February 2019. < https://www.for.org/en/treat/treatment-guidelines/edentulous/treatment-procedures/removable-prosthetics/complete-dentures/complete-denture-maxillo-mandibular-relations>.