Written by J. Patrick Ford, DDS Thursday, 09 January 2014 10:28
The architecture of poverty has touched us all in both our personal lives and in our professional careers.
“Doc, my dog ate my top denture. I gotta have a new plate by Monday morning. I won’t go to work with no teeth. I’ve got 3 dollars.”
The stress axis/DNA denture protocols are an outgrowth of the development of a value denture service. The stress axis/DNA clinical and laboratory protocols provide not only a personalized denture for both the modest and reduced income demographic but also, when applied, a profitable denture service for the premium and elite clinical and laboratory denture programs.
Recent studies indicate that, in the United States, there will be more persons wearing full dentures in 2020 than are presently wear full dentures.1 Nondentist providers in the state of Oregon may now provide a denture service directly to the public.
The need to develop a traditional, conservative, profitable, personalized denture service within a general dental practice/dental laboratory protocol is evidenced within these data. The stress axis/DNA denture is an effort to address this need utilizing and complementing the existing infrastructure of this traditional dental college/dental laboratory removable prosthetics training.
This is a 2-part introduction to the stress axis/DNA protocol denture. This first article includes the historical and scientific background, the transfer of the DNA data and the stress axis to an edentulous model, and clinical examples of the stress axis/DNA denture procedures. The second article will present case examples and discuss case problem solving which is simplified by the use of stress axis/DNA denture protocols.
The Stress Axis of the Bimler Analysis
Factor 6 of the Bimler cephalometric analysis, the green arrow in Figure 1, is termed the stress axis. The cephalometric occlusal stress axis is the radius of, and therefore the 90° perpendicular of, the curve of Spee. It is drawn from the Mentale point on the mandible (Figure 1: the green arrow); through the functional anterior-posterior (A-P) plane of occlusion (the curve of Spee); to an end point within the upper left quadrant of the cephalograph, termed the centromasticale. The curve of Spee is the dotted, curved line in Figure 1, and cephalometrically may be extended through the retromolar pad (RMP) (the blue arrow) to the capitulare (the center of the condyle).
|Figure 1. The stress axis.|
If the full RMPs are impressed, the DNA expression on the RMPs and of the mandibular plane of occlusion through the RMPs (as evidenced in Figure 1 by distally extending the curve of Spee in the cephalograph tracing) may be located on a fully or partially edentulous model. The occlusal plane of the mandibular denture will then be set to the correct, DNA-determined, mandibular plane of occlusion allowing the wax setup of the mandibular denture teeth to be set to each individual’s DNA-regulated (original and natural) plane of occlusion referenced at the RMPs.
The Bimler stress axis analysis is a maxillary bone to mandibular bone relationship, and therefore is an applicable measurement for the edentulous patient. For stability, the removable dental prostheses must be placed in alignment with the mandibular bone-to-maxillary bone stress axis.
Golden Mean Gauge/DNA Measurements
The width of the DNA molecule is 20 Ångströms, and the length of one complete double helical turn of the molecule is 34 Ångströms. This ratio (1.618…) is termed phi. The phi ratio gauge (PRG) opens and closes to this DNA length-to-width ratio.
|Figure 2. Direct phi ratio gauge (PRG)—3 x 5 or 5 x 3.|
|Figures 3a and 3b. Overlapping PRG centimeter ratios (3 to 8 and 5 to 8).|
The ratio may be measured directly, as in the crown-to-root length ratio (Figure 2) or overlapped, as the length-to-width ratio exampled in the human finger joints (Figure 3).
The overlapping relationship seen in the bone-joint-bone relationship is the DNA proportional template used to determine the mandibular plane of the occlusion-to-freeway (speaking) space and the maxillary plane of occlusion-to-freeway (speaking) space configuration on the RMP (also a bone-joint-bone relationship).
At the posterior border of the dental component of the speaking (freeway) space, the direct DNA ratio at the base of the RMP is the measurement used to determine the average mandibular-to-maxillary incisal edge ratio visible during normal speech at the anterior edge of the dental component of the oral speaking space (the freeway space). This A-P speaking space relationship is explained in more detail in the esthetic guideline segment.
The Articulator Template
The vertical component of the waxing template, transposed on the Bimler analysis, is the perpendicular of the stress axis/curve of Spee interface (Figure 4). Placement of the distal edges of waxing template on the mandibular plane of occlusion at the RMPs (blue arrow in Figure 1) transfers 2 of the 4 measurements of the stress axis data necessary to reproduce the mandibular plane of occlusion. The third and fourth measurements needed are the midline millimeter depths of the mandibular and maxillary vestibules to the heights of the upper and lower lips. The anterior measurements are subsequently adjusted to the DNA guidelines existing on the RMP.
The DNA Template on the Anterior-Posterior Curve of Spee
The double helical slope of the DNA molecule measures 17°. The 8-inch waxing template (Myotronics-Noromed) has a 17° slope; therefore, rather than arbitrarily selecting the occlusal curve, the 8-inch waxing template is the recommended waxing template for the A-P curve of Spee and the lateral curve of Wilson. Clinical modifications to this may be necessary (trauma, etc), but with the stress axis/DNA denture protocol, the DNA slope is the initial point of reference for the A-P and lateral occlusal curves. The 17° slope of the DNA molecule provides a stable academic and clinical platform of reference to evaluate and compare traditional and/or future models used to determine an individual’s occlusal curves.
|Figure 4. Bimler with template.|
The DNA expression on the RMPs and the A-P curve of Spee permits the guided transfer of the Bimler cephalometric data to an edentulous, or partially edentulous, model. In concert with traditional intraoral measurements, this DNA-regulated measurement will very closely, if not exactly, duplicate the original mandibular plane of occlusion. An individual lateral cephalometric x-ray of each patient is unnecessary.
CLINICAL PROCEDURES FOR FULL MAXILLARY AND MANDIBULAR DENTURES
DNA Overlap Measurement of the Retromolar Pad
The overlapping measurements of the RMP, as pictured in Figures 5a and 5b, is the DNA-PRG template used to bilaterally locate the freeway space between the maxillary and mandibular planes of occlusion, which is marked in red in Figure 5c. The posterior boundary of the A-P curve of Spee is the caudal mark on both RMPs (Figure 5a). The maxillary plane of occlusion is the coronal border of the freeway space (Figure 5b). The freeway space, marked in red in Figure 5c, separates the maxillary and mandibular occlusal planes.
The direct measurement (Figure 5a) of the caudal portion of the overlapping DNA relationship is the millimeter measurement used to determine the average mandibular incisal edge visible during normal speech at the anterior edge of the oral speaking space (the freeway space).
|Figure 5a. Retromolar pad (RMP) marking mandibular plane of occlusion.||Figure 5b. Retromolar pad marking maxillary plane of occlusion.|
|Figure 5c. Retromolar pad freeway space marked in red.||Figure 6. Baseplate trimmed to RMPs.|
The baseplate is trimmed to accommodate the 8-inch waxing template (Figure 6). The freeway space is marked in red (Figure 5c). The heated base of the template is placed into the notches (Figures 7a to 7c) and then the wax bite rim adjusted to both the anterior and posterior guidelines, the anterior guideline being the high lip-line measured from the midline depth of the mandibular vestibule.
Stress Axis Location in the Edentulous Model
The stress axis can be reconstructed on the edentulous (or partially edentulous) mandible by locating and utilizing 4 anatomic reference points.
These 4 reference points are: (1) the right, DNA-regulated, mandibular plane of occlusion on the right RMP; (2) the left, DNA-regulated, mandibular plane of occlusion on the left RMP; (3) the millimeter measurement of the mandibular anterior midline (the vestibule-to-lower lip height) at rest (Figures 8 to 10); and (4) the millimeter measurement of the maxillary anterior midline (the vestibule-to-upper lip height) at rest.
|Figure 7a. Right RMP notched.||Figure 7b. Left RMP notched.|
|Figure 7c. Template to notch on right RMP.|
The vestibule-to-lip heights will be refined at the wax try-in and set to the DNA-regulated, A-P, freeway space (the speaking space) utilizing the DNA template on the RMPs (Figure 5a), the height of the RMP to the mandibular plane of occlusion.
The full mandibular wax-up is oriented to the DNA template on the RMPs, and the mandibular denture then set to the functional stress axis of the mandibular arch, which is the vertical component of the waxing template seen in Figure 4. The maxillary bite rim, after the clinical modifications for centric and esthetic considerations, is mounted on the articulator to the previously mounted, Bimler stress axis orientated, mandibular wax try-in.
Mounting the Mandibular Cast
Before mounting the mandibular model on the articulator, the waxing template is bilaterally set to the notched, DNA-guided mandibular plane of occlusion at the RMPs (Figure 7c).
After placing the posterior portion of the heated metal template into the notches in the RMPs of the mandibular model, the template is rotated to the predetermined mandibular incisal height marked on the mandibular wax bite rim (Figures 8 to 10).
The notches of the RMPs provide the stable, posterior fulcra from which the heated template may be pivotally rotated, melting the wax bite rim to the clinically predetermined mandibular lip height (Figure 10).
With the vertical component of the waxing template connected to the upper arm of the articulator (Hanau) utilizing an adaptor (IMF Machine and Fabricating), the case can be mounted to the individual stress axis of each patient (Figure 11).
To allow the inclusion of protrusive and lateral bite records and monitor wax shrinkage (Figures 12 and 13), the utilization of a conventional, semi-adjustable articulator (Hanau) (Figures 13 and 14) with an anterior adjustable pin is recommended.
Waxing the Mandibular Denture
The early detection of any waxing discrepancies (Figures 12 and 13) is an advantage of using a mounted stress axis waxing template. Unless detected and corrected, these discrepancies are carried forward to final processing and finishing.
|Figure 8. Wax rim set to prescribed (millimeter) height.||Figure 9. Lip-line mark with template.|
|Figure 10. Heated template set to right lower RMP and lip-line.||Figure 11. Stress axis/DNA protocol bite rim mounting.|
|Figure 12. Early detection of wax shrinkage.||Figure 13. Wax shrinkage after cooling.|
When the full wax try-in is allowed to cool, the shrinkage is apparent and easily corrected (Figure 13). This step ensures that the original DNA-guided plane of occlusion (Figures 8 and 10) is maintained during the subsequent clinical and laboratory procedures.
The cooling-corrected mandibular teeth are then correctly set to the metal waxing template (Figure 14). The mandibular wax-up and the maxillary bite rim (set to the patient’s maxillary millimeter depth at the midline of the upper lip) are forwarded to the dental office.
The maxillary bite rim (Figure 15) is then configured at the dental office for the midline tracing, the Cupid’s bow (high smile-line) tracing, and the leveling of the bite rim to the lateral canthus of each eye (eye sockets and smile patterns may not be bilaterally symmetrical). A face-bow (ear-bow) transfer is not used.
If mould and shade selections, maxillary and mandibular vertical heights, and full RMPs are impressed, a full maxillary wax setup may be included with the mandibular setup at the wax try-in appointment. Full anterior esthetic adjustments are then feasible, reducing the number of appointments needed.
Several options are available to the chairside clinician at the wax try-in/bite record appointment. They are:
- The stress axis/DNA mandibular wax try-in with a full maxillary bite rim, or
- The stress axis /DNA mandibular wax try-in with a 6-tooth set up in maxillary anterior portion of the bite rim, or
- In special clinical circumstances, such as time constraints or patient request, a full maxillary and mandibular wax try-in, or
- The stress axis/DNA mandibular wax up may be integrated at any point in a traditional waxing protocol.
The millimeter distance between the lower border of the RMP (Figure 5a) to the line which marks the mandibular plane of occlusion on the RMP is a guideline for the average amount of mandibular incisal edge which needs to be visible above the mandibular lip-line during normal speaking movements (Figure 16).
Ideally, the maxillary and mandibular incisal speaking edge lengths, visible during normal speech patterns, are the PRG proportion. In the natural dentition, this is often not the case. If the distance in Figure 5a is 3 mm, then the average visible length of the lower anterior teeth is set at 3 mm (Figure 16). A direct measurement of the PRG on a millimeter rule indicates that the average visible speaking length of the maxillary anterior teeth will be 5 mm (Figure 16). This incisal edge speaking length may be observed during the course of an average conversation.
|Figure 14. Wax try-in with 8-inch waxing template.||Figure 15. Cupid’s bow.|
|Figure 16. Phi ratio gauge showing mesiodistal anterior speaking space.||Figure 17. Finished buccal cusp occlusion.|
|Figure 18. Close-up of intercuspal occlusion.||Figure 19. Full upper and lower completed dentures.|
|Figure 20. Is stress axis important?|
This guideline is the DNA-regulated distance to the mandibular plane of occlusion (the posterior speaking space) on the RMP (Figure 5a). This millimeter measurement is transferred to the average incisal edge PRG measurements. To be in concert with the DNA imperative located on the RMP, this millimeter measurement is used as the average speaking space height of the mandibular incisal edges. This direct measurement of the DNA proportion is similar to the measurement of the crown-to-root ratio measurement in Figure 2.
Admittedly, natural dentition will have variations of this “ideal”; therefore, the PRG proportion is offered as a guideline. Patient and clinician preferences would, of course, take precedence, but transferring the RMP DNA template to the visible incisal edges will help facilitate the normal speech patterns of any spoken language.
Wax Try-In to Completion
Each buccal cusp or incisal edge in the mandibular setup will be in contact with the 8-inch template (Figure 17). A cusp/fossa occlusion, rather than a lingualized occlusion, is used in the stress axis/DNA denture protocol.
Following clinician and patient approval of the wax try-in (Figure 18), the denture is returned to the laboratory for normal processing and finishing (Figure 19).
The exfoliated mandibular right second molar (Figure 20) is an example of the result of the misalignment of the stress axis. Placing any nonstress axis orientated dental appliance (fixed or removable) on a well-constructed implant compromises the long-term success of the prosthesis.
Author’s Note: The patient referenced in the introductory remarks received her maxillary denture by Monday morning. In addition to the $3 fee, this clinician received an editorial comment or 2.
The author would like to acknowledge the following for the editorial, case history, laboratory, and equipment/supplies: Intrawest Machine and Fabricating Inc, Grand Junction, Colo; American Tooth Industries, Oxnard, Calif; Lincoln Dental, Modesto, Calif; Pierce Dental Laboratory, Grand Junction, Colo; Dani Dental, Tempe, Ariz; Master Craft Dental Lab Corp, Loveland, Colo; R. Wurtzebach, DDS, Denver, Colo; C. Belting, DDS, Norwood, Colo; J. Murray, DDS, Glenwood Springs, Colo; M. Gadeken, DDS, Grand Junction, Colo; R. Ford, BS, Grand Junction, Colo; J. Drazek, DDS, MS, Grand Junction, Colo; and Richard Hurd, DDS, Grand Junction, Colo.
- Douglas CW, Shih A, Ostry L. Will there be a need for complete dentures in the United States in 2020? J Prosthet Dent. 2002;87:5-8.
Disclosure: Dr. Ford reports no disclosures.
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