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Edentulous Soft Tissue Anatomy
The convergence of conventional and digital workflows
By Robert Kreyer, CDT
There are both clinical and technical needs to achieve excellent impressions and accurate records that are the cornerstones of success with conventional and digital denture workflows. Complete-denture prosthetics is in the process of eliminating workflow variables to improve consistency and quality. One of these variables is the stone cast/model, which will not accurately replicate the impression if gypsum/stone is not mixed using the proper water-to-powder ratios. The digital workflow completely eliminates the need for a conventional stone master cast and reduces such physical variables.
How laboratory technicians and dentists communicate and the vocabulary used is critical to optimal collaboration on denture cases, whether using conventional or digital workflows. Understanding basic soft tissue anatomy is fundamental to success with complete prosthetics. The objective of this article is to show how to effectively communicate technical needs for capturing excellent impressions for either conventional or digital denture workflows.
Current Removable Prosthetic Trends
Complete-denture prosthetics is experiencing a digital revolution, as techniques and materials evolve for these new subtractive (milling) and additive (printing) technologies. The digitization of removable prosthodontics is changing the way dental technicians work, think, and visualize treatment for prosthetics planning. This new digital world of removable diagnostic case will no doubt change the curriculum for complete and partial dentures. New dental school graduates comfortable in a digital world with smartphones and immediate access to information will drive the adoption of digital technology for removable prosthetics. This will require a change in how clinicians and technicians communicate prosthetic information and collaborate in the design and manufacturing processes. Digital technologies are amazing tools that—when properly combined with conventional techniques—become an extension of the clinical and technical artistic skills.
Currently, removable prosthodontics is experiencing a convergence between conventional and digital denture workflows. There will be a growing demand to understand when to use conventional, digital, or integrated conventional/digital workflows depending on the prosthetic variables involved. Conventional denture workflows and processes will still be used for the next 20 years or so, depending on the future demand by clinicians graduating from dental schools. Non-conventional complete-denture prosthetic workflows must be developed based on the variables involved. There will be conventional denture workflows, integrated conventional/digital, and digital options. Having these three workflow options provides clinicians and technicians with the best technology available to design and manufacture complete-denture prosthetics for a compromised edentulous patient population.
Regardless of advances in technology, the basic principles of creating complete-denture prosthetics will not change. Literature published 100 years ago by Dr. Alfred Gysi is as relevant today as it was in 1918. It will be these principles of prosthetics combined with this digital revolution that will drive an evolution in clinical and technical prosthodontic education. In the last 2 years, many papers on digital dentures have been published in prosthodontic publications such as the Journal of Prosthetic Dentistry and Journal of Prosthodontics. A study by Dr. Brian Goodacre1 found that the CAD/CAM fabrication process was the most accurate and reproducible denture fabrication technique when compared with pack-and-press, pour, and injection denture base processing techniques. Dr. Bryan McLaughin, et al, in the study, “Comparison of Fit of Dentures Fabricated by Traditional Techniques Versus CAD/CAM Technology,”2 found that, overall, injection molding and CAD/CAM fabrication methods produced equally well-fitting dentures, with both having a better fit than compression molding. Shallow palates appear to be more affected by shrinkage than medium or deep palates. Shallow, ovoid arch forms appear to benefit from the use of injection molding compared to CAD/CAM or compression molding.
In the only published clinical study on digital dentures—“An update on computer-engineered complete dentures: A systematic review on clinical outcomes”—Drs. Mathew T. Kattadiyil and Abdulaziz AlHelal3 found that a positive trend was seen in the outcomes with computer-engineered complete dentures (CECDs), although patient selection might have also contributed to favorable outcomes. Significantly reduced clinical time, improved retention, and digital archiving were the main advantages associated with CECDs. This published prosthodontic paper indicates that there are advantages to using digital workflows and conventional injection workflows depending on the prosthetic variables involved. An integrated conventional and digital workflow will be used widely with more complex prosthetic applications such as implants and for patients with very high esthetic expectations.
Edentulous Soft Tissue Anatomy
The maxillary and mandibular edentulous soft tissue anatomy within the denture space of the oral environment is shown in Figure 1. Anatomical landmarks such as the retromolar pads, external oblique, mentalis muscle, frenum attachments, mylohyoid ridge, tuberosities, hamular notches, incisive papilla, labial sulcus, and buccal vestibule are critical and must be captured in the positive (and easily read in the negative) aspect of any edentulous impression.
Figure 2 shows a fully edentulous maxillary and mandibular impression that has proper vertical and horizontal peripheral border extensions. The impressions are excellent, but it's also critical to see “beyond” the impression. Have the anatomical landmarks been captured and do they effectively communicate the soft tissue anatomy? These questions are very important to communication and collaboration with converging conventional and digital technologies. It's absolutely critical for technicians to understand basic soft tissue anatomy; it's the foundation for success with complete-denture prosthetics.
Upon creating a master impression, if the necessary anatomy or biology of the denture space within this oral environment is not easily read in the negative impression, then the impression must be taken again until it is correct and all necessary soft tissue anatomy is visible. Understanding how to read the negative aspect of an edentulous impression is critical to creating good master casts or digital models for complete-denture prosthetics. Inspect the impression to see if any defects such as bubbles, pulls, voids, or show-through of tray exist in the impression material. Look for proper peripheral border extensions and anatomical landmarks such as retromolar pads in mandibular impressions and tuberosity extensions with hamular notches (pterygomandibular raphe) in a maxillary impression. In a mandibular impression you should see in the lingual aspect of peripheral borders an S-curve, which shows proper border molding. This S-curve of the mandibular lingual border is a result of the extrinsic movements of the tongue during function.
Scans of the mandibular and maxillary impression in STL (standard tessellation language) file format are shown in Figure 3 and Figure 4. When an STL file is created from an impression of edentulous soft tissue, it's important to analyze the file to ensure a good scan and that all the anatomical landmarks were captured in the scanning process. The ability to read an impression in the negative aspect, visualizing all the soft tissue anatomy, is critical in the digital model building process. This check and double-check quality control procedure must exist in any digital workflow process to avoid internal rescans and reworks. Look at the scans, starting with the mandibular from the most posterior aspect on one, inspect the retromolar pad, pear-shaped pad, peripheral denture borders, residual ridge, mylohyoid area, frenum attachments (buccal, labial, and lingual), buccal vestibule or shelf area, and labial extension or muco-buccal fold area. On the maxillary, inspect the extensions into the hamulus (hamular notch), tuberosity, fovea palatine, posterior hard palate, soft palate, anterior palatal rugae, midline or midpalatine raphe, residual ridge, incisive papilla, frenum attachments (labial and buccal), and denture border extensions (vertical and horizontal). All these soft tissue anatomical landmarks must be evaluated in the impression and the digital scan file.
In Figure 5 and Figure 6, the maxillary and mandibular soft tissue anatomy is highlighted and labeled. These illustrations help remind technicians and clinicians of the anatomical landmarks that must be captured in an edentulous soft-tissue impression and an edentulous scan. Understanding the biology and function of the oral anatomy will definitely improve clinical and technical communication. The basic principles of communication for conventional or digital workflows start with knowledge of the edentulous oral environment. Having a common “language” for this knowledge is requisite for collaboration.
The soft tissue anatomical landmarks that affect the peripheral border design include:
• Hamular notches
• Fovea palatina
• Labial and buccal frenums
• Retromolar pads
• Sublingual crescent
• Lingual frenum
• External oblique/buccal shelf
• Buccal frenum
• Mentalis muscle
• Labial frenum
Dr. Alfred Gysi's illustration from over 100 years ago (Figure 7) shows the forces of the edentulous oral environment's finite denture space. For every person, each oral environment is different with its own set of variables. With digital technology, using scan data to create STL design files, we can now analyze the forces or variables involved in complete denture treatment.
Understanding palatal contours (Figure 8) will aid in determining the classification of throat form that affects the posterior palatal extension of a maxillary complete denture. The posterior palatal extension or border is very critical for proper placement of the posterior palatal seal or post-dam area. The House Classification of Throat Form guides us in determining the posterior palatal extension from a Class I to Class III. A House Class I palatal throat form is very broad and shallow, with the most posterior extension. A House Class III throat form is very deep or vaulted and has the shortest posterior extension as described by McLaughlin, et al. These throat forms are very important to understand in the conventional and digital process. Sometimes it will seem like an impression scan did not capture the entire palate when it's a Class III palatal throat form.
As stated and found in the conclusions of McLaughlin, et al, shallow palates appear to be more affected by shrinkage than medium or deep palates. Shallow ovoid arch forms appear to benefit from the use of injection molding compared to CAD/CAM and compression molding.
Always look at the actual impression and compare this with the digital model, verifying the posterior soft tissue anatomy has been captured properly. Soon, conventional impressions will be replaced by digital impression scans. Prosthetic dentistry must ready itself for this evolving impression process.
As more fixed-prosthetic dental laboratories expand their product lines to include removable prosthetics, the ability to effectively communicate and analyze the impressions or scans they receive will affect their productivity and success. Being able to speak the language of removable prosthetics using proper terminology will improve internal and external workflow processes. As we look at impressions and scans, this soft tissue anatomy must be top of mind if we are working to a higher standard of quality control.
The current interest in removable prosthetics is truly amazing and invigorating for those who have been in the dental technology profession for many years. The conventional, integrated, and digital options now available to utilize alternative workflows will truly provide value-added dentures from diagnostics to delivery.
About the Author
Robert Kreyer, CDT
Kreyer Prosthetic Consulting
San Ramon, California
Disclosure: The author has no disclosures to report.
1. Goodacre B. Comparison of denture base adaptation between CAD-CAM and conventional fabrication techniques. J Prosthet Dent. 2016;116:249-256.
2. McLaughlin JB, Ramos V Jr, Dickinson DP. Comparison of Fit of Dentures Fabricated by Traditional Techniques Versus CAD/CAM Technology. Journal of Prosthodontics. 2017; Nov. doi: 10.1111/jopr.12604.
3. Kattadiyil MT, AlHelal A. An update on computer-engineered complete dentures: A systematic review on clinical outcomes. J Prosthet Dent. 2017;117:478-485.