Mandibular Denture Anatomy

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Getting from treatment plan to completion takes careful consideration and planning.

Anatomy for Complete and Partial Dentures Lips Vermilion Border Denture provides lip support Affects vermilion border width Lips Philtrum Depression below nose Lips Nasolabial Angle Angle between columella of nose & philtrum of lip Normally, approximately 90° as viewed in profile Lips Tissue of the Upper Lip Loose tissue of the upper lip can be gathered between your thumb and index finger. Complete dentures are worn by patients who are missing all of the teeth in a single arch—i.e., the maxillary (upper) or mandibular (lower) arch—or, more commonly, in both arches.

No longer is a complete denture the standard of care in restoring the edentulous mandible.1 The new standard of care is an overdenture that uses implants to help retain and support it. This article is aimed at dentists who are beginning to incorporate implant placement and restoration into their practices.

Maxillary denture anatomy

Bar Overdenture Considerations

A bar overdenture is an excellent prosthetic option that many patients choose over less expensive alternatives for a number of reasons, including:

  • having more retention than a conventional denture.
  • providing better support than a tissue-supported prosthesis.
  • functioning better because it is more stable and moves less.
  • being more comfortable to the patient.

Because of the amount and the pattern of resorption of the edentulous mandible, the only area available for implant placement is often the intraforaminal area.2 For the purposes of this article, the author will not discuss short implants or grafting procedures.

Mandibular

Some basic conditions must be present for the patient to be a candidate for a bar overdenture, and a good protocol for examination and treatment planning will determine whether or not these conditions are present.3

First, there must be adequate volume of bone in the intraforaminal area.4 This means having adequate bone in three dimensions—arch length, width, and height.3 (Note that approaches for dealing with inadequate bone volume are beyond the scope of this discussion.) Arch length is not a problem for placing four implants between the mental foramina; adequate bone height should allow a 12-mm or 13-mm implant to be placed without damage to any structures, and width should be approximately 6 mm to allow for placement of the standard 3.7-mm to 4-mm wide implants with 2 mm of bone facial and lingual to them.

There must be an interarch space of about 15 mm, which is not uncommon in the edentulous mandible, but in case there is not enough space, an alveoloplasty would create the space necessary.

Patient Selection

To determine if the patient meets the requirements described, at the initial examination the clinician should take preoperative photographs and impressions for study models and mount them using a facebow record and bite registration in centric relation. In addition, a radiographic survey consisting of a panoramic X-ray and selected periapical films are exposed as needed. As radiographs show only two dimensions, it is necessary to supplement that information with clinical observations or use a cone beam volumetric tomography scan to obtain very accurate cross-sectional measurements.5-7 Once the required information is obtained and a treatment plan is prepared, they are presented as part of informed consent to the patient along with information on all alternative treatment—eg, a fixed prosthesis, a fixed removable (hybrid), an implant-supported prosthesis (bar overdenture), a tissue-supported prosthesis such as an O-ring, a locator- or mini-implant-retained overdenture, or a conventional complete denture.

Case Presentation

A 50-year-old woman with an unremarkable health history and who wore a maxillary complete denture against a terminal mandibular dentition, chose to have her remaining teeth extracted and replaced with a bar overdenture ().

The patient did not want the implants placed on the same day of the extractions, so a new maxillary denture and mandibular immediate denture were fabricated for her. All of her remaining lower teeth were removed, an alveoloplasty was performed, and the dentures were delivered. The patient was allowed to heal for 3 months. Next, the lower immediate denture was duplicated, and the duplicate was used first to hold radiographic markers and later as a surgical guide by placing holes in the selected osteotomy sites.

Surgical Techniques

The letters A through E were assigned by Misch to the recommended osteotomy sites in the intraforaminal region,6 with A and E being the most distally placed osteotomies, and B and D the anterior ones. Undoubtedly, the distal osteotomy sites must be selected carefully to avoid the vital structures associated with the mental foramen and related anterior loops. Several techniques are useful here. The most reliable is gently exposing the mental foramen and placing the implants 6 mm or 7 mm mesial to it. Another technique designed to minimize the surgical flap extension is to use radiographic markers (which are radiopaque objects of known dimensions—eg, a 10-mm long orthodontic wire) attached to the surgical template and expose some films ().

The anterior-posterior spread of the implants should be as large as possible to be able to extend a small cantilevered bar distal from the most posterior implants for better support of the overdenture in the posterior region. The literature suggests that the surgeon can cantilever as much as 2.5 times the anterior–posterior spread,8 but the author does not extend more than 10 mm when cantilevering (). The sites are selected, and in preparing the osteotomies, the surgeon must take into account the anatomy of the area, which allows a fairly straight up and down angulation in the first premolar region; however, in the anterior region, the osteotomy may need to be angled slightly anteriorly to avoid perforating the lingual plate while proceeding apically, because the bone is more anterior in that area and perforating the lingual plate apically can result in severe bleeding complications.

The surgeon should always try to align the implants parallel to each other, but should not be concerned if they are not perfectly parallel, because the bar makes it possible to correct moderate amounts of divergence in the implants ().

One technique that optimizes the surgical placement of the implants uses cone beam computed tomography-based surgical guides.9,10 The surgeon can use the guide to locate the predetermined implant sites and even control angulation and depth to prevent damage to important anatomic structures.

The implants are placed at crestal bone level and will heal submerged for 3 to 4 months to prevent pressure from mastication that may cause their failure to osseointegrate. Next, the implants are exposed, but not with a tissue punch; instead, the surgeon should use a small incision to be able to keep all of the scarce keratinized attached gingiva, which is desirable around the implants ().

Seating the Overdenture

After the gums heal, impression posts are placed and an X-ray is exposed to verify complete seating before taking an impression (). The impression posts used are either transfer or pick-up impression copings. Transfer copings require a conventional impression technique and pick-up copings use an open tray and are unscrewed from the implant before removing the impression from the mouth. The open-tray impression posts are necessary when the implants have different angulations; a closed tray could lock the impression in the mouth. The open tray may arguably be a more accurate technique because the impression posts remain inside the impression.

The laboratory pours the impression and the resulting model has the implant analogs replicating the position of the implants. One of the most popular bar designs uses three attachments—one anteriorly and two posteriorly (one on each side)— and these attachments are usually clips. The most common bar patterns are the Dolder Bar (Preat Corporation, www.preat.com), and the Hader Bar® (Sterngold™, www.sterngold.com). The Hader bar, which was used in this case, is fabricated using a plastic pattern and DuraLay™ resin (DuraLay, Reliance Dental, www.reliancedental.net) by casting it in Type 3 or Type 4 noble alloy (). The Dolder bar is premade and soldered or welded. Other more expensive options are a milled cast bar and a CAD/CAM overdenture bar, which is computer-designed and milled from a titanium blank.11 This milled bar is stronger and often more accurate than the conventional cast bar. The bar needs to fit passively on the implants, and is verified by placing only one screw and taking X-rays to determine the fit (Figure 8). Once the fit is verified, the bar is placed back on the model and the laboratory can construct the overdenture ().

When the overdenture is returned, the bar is placed and torqued to the manufacturer’s specifications, delivered () to the clinician, and checked for fit, extension, balanced contacts, and group function ().

Conclusion

In the author’s opinion, the mandibular overdenture is one of the best and most comfortable modalities of treatment for the edentulous patient. The above discussion is meant to provide some useful information to help dentists adequately select, plan, and deliver a bar overdenture to their patients.

References

1. Carlsson GE, www.ncbi.nlm.nih.gov/pubmed?term=%22Omar R%22%5BAuthor%5D'>Omar R. The future of complete dentures in oral rehabilitation. A critical review. J Oral Rehabil. 2010;37(2):143-156.

2. Pietrokovski J, Starinsky R, Arensburg B, Kaffe I. Morphologic characteristics of bony edentulous jaws. J Prosthodont. 2007;16(2):141-147.

3. Misch CE. The endentulous mandible: an organized approach to implant-supported overdentures.Contemporary Implant Dentistry. 3rd ed. St. Louis, Mo: Mosby; 2008.

4. Şahin S, Kaya Y, Şençimen M, et al. Retrospective radiographic evaluation of the interforaminal region with spiral computerized tomography: adequacy for dental implant placement related to age and dental status. Gulhane Med J. 2010;52(2):69-75.

5. Worthington P, Rubenstein J, Hatcher DC. The role of cone-beam computed tomography in the planning and placement of implants. J Am Dent Assoc. 2010;141(Suppl 3):19S-24S.

6. Dreiseidler T, Neugebauer J, Ritter L, et al. Accuracy of a newly developed integrated system for dental implant planning. Clin Oral Implants Res. 2009;20(11):1191-1199.

7. Nokar S, Moslehifard E, Bahman T, et al. Accuracy of implant placement using a CAD/CAM surgical guide: an in vitro study. Int J Oral Maxillofac Implants. 2011;26(3):520-526.

8. Misch CE. Preimplant prosthodontics. Dental Implant Prosthetics. St Louis, Mo: Mosby; 2005:178.

Mandibular Denture Anatomy

9. Al-Harbi SA, Sun AY. Implant placement accuracy when using stereolithographic template as a surgical guide: preliminary results. Implant Dent. 2009;18(1):46-56.

10. Valente F, Schiroli G, Sbrenna A. Accuracy of computer-aided oral implant surgery: a clinical and radiographic study. Int J Oral Maxillofac Implants. 2009;24(2):234-242.

11. Carpentieri JR. Clinical protocol for an overdenture bar prosthesis fabricated with CAD/CAM technology. Pract Proced Aesthet Dent. 2004;16(10):755-757.

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About the Author

Image

Carlos Boudet, DDS, DICOI
Private Practice
West Palm Beach, Florida

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.

Prosthetic Principles

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:

Maxillary

• Hamular notches
• Tuberosities
• Fovea palatina
• Labial and buccal frenums

Mandibular

• Retromolar pads
• Mylohyoid
• 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.

Conclusion

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.

Buccal Flange Denture

Acknowledgements:

Anatomy Of A Denture

Edentulous photos and scans courtesy of Valerie Cooper, DDS (“The Denture Queen”). Figure 5 and Figure 6 regenerated with permission from the ehuman project (ehuman.com).

About the Author

Robert Kreyer, CDT
Owner,
Kreyer Prosthetic Consulting
San Ramon, California

Disclosure: The author has no disclosures to report.

Mandibular Dental Anatomy

References

Lingual Tuberosity

1. Goodacre B. Comparison of denture base adaptation between CAD-CAM and conventional fabrication techniques. J Prosthet Dent. 2016;116:249-256.

Mandibular Denture Anatomy

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.

Edentulous Maxilla Landmarks

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.