109

CASE REPORT

Implant Placement for Mandibular Overdenturesusing the Neutral Zone ConceptYasunori Suzuki, DMD, PhD, Chikahiro Ohkubo, DMD, PhD, and Toshio Hosoi, DDS, PhDDepartment of Removable Prosthodontics, Tsurumi University School of Dental Medicine, Yokohama, Japan

Clinical significanceImplant placement and position affects the function and esthetics of implant-tissue supported overden-tures. This clinical report describes the position of im-plant placement determined utilizing the neutral zone concept.

AbstractPatients: A 56-year-old woman presented with a se-verely resorbed edentulous mandible, complaining of poor retention and stability of her existing ill-fitting re-movable partial denture. The original maxillary right lateral incisor and canine were present. A conventional overdenture and implant-retained overdenture were selected for maxillary and mandibular rehabilitation, respectively. The existing mandibular denture yielded an unacceptable position of implant placement. After molding of the denture border and cameo surface was conducted, the existing denture modified using the neu-tral zone concept allowed proper implant placement.Discussion: The position of implant placement affects the function and esthetics of an implant-supported overdenture. Preparation of diagnostic and surgical templates using the neutral zone concept facilitates proper implant placement for comfortable dentures. Correctly remolded denture borders and cameo sur-face allow proper implant placement.Conclusion: The cameo surface for implant-retained or -supported overdentures needs to be prepared ap-propriately to obtain satisfactory implant position, in terms of esthetics, function, comfort, or patient satis-faction.

Key words: implant, overdenture, neutral zone, cam-eo surface

IntroductionThe neutral-zone denture concept1-4 has been shown to improve the stabi lity of complete den-tures in patients. Implant placement position af-fects the function and esthetics of implant-tissue supported overdentures. Preparation of diagnos-tic and surgical templates using the neutral zone concept facilitates proper placement of implants for complete dentures. This clinical report de-scribes the position of implant placement consid-ered by the cameo surface of a mandibular com-plete denture. It was shown that correctly remolded denture borders and cameo surfaces fa-cilitate proper implant placement.

Outline of the caseA 56-year-old woman presented with a severely resorbed edentulous mandible, complaing of poor retention and stability of her existing ill-fitting removable partial denture (Fig. 1). The original maxillary right lateral incisor and canine were retained. After informing the patient about the various treatment options available, including new complete denture, implant-retained overden-ture or implant supported prosthesis, a conven-tional overdenture and implant-retained over-denture were selected for maxil lary and mandibular rehabilitation, respectively, for eco-nomic and anatomic reasons. The existing man-dibular denture was duplicated using auto-po-lymerizing poly methyl methacrylate (PMMA) (Palapress, Heraeus Kulzer Gmbh & Co., Weh-rheim, Germany) to fabricate a template for ex-amination by computed tomography (CT). It was proposed to place the two implants between the mental foramina, in the areas of the right and left canines. Temporary sealing material (Tempo-rary Stoping, GC, Tokyo, Japan) was used to cre-ate radiopaque makers. However, CT examina-

Corresponding to: Dr Yasunori SuzukiDepartment of Removable Prosthodontics,Tsurumi University School of Dental Medicine,2-1-3 Tsurumi, Tsurumi-ku,Yokohama 230-8501, JapanTel:+81-45-581-1001, FAX:+81-45-573-9599E-mail: suzuki-ys@tsurumi-u.ac.jp

Received on October 26,2005/Accepted on January 17,2006

Prosthodont Res Pract 5 : 109-112, 2006

Suzuki et al., Prosthodont Res Pract 5 : 109-112, 2006

110

tion indicated that the placement direction was inclined, and the position was shifted to the lin-gual aspect (Fig. 2). In addition, the index made with silicone impression (Coltoflax Putty type, Coltene, Altstatten, Switzerland) afforded inade-quate space for the retainer and the housing (CM rider, CM, Biel-binne, Switzerland) (Fig. 3).

After molding of the border of the existing den-ture with modeling plastic impression compound (Impression Tray Compound, GC), a wash im-pression were made with silicone impression ma-terial (Fit-checker, GC) because of its high fluidi-ty (Fig. 4). While the impression was being made, the patient was instructed to swallow and then purse her lips and move her tongue in order to correctly mold the denture borders and cameo surface. The denture flange and cameo surfaces were extended along the labial and lingual sides within physiological limits.

Since the denture labial flange was extended by remolding of the cameo surface, the tentative position of implant placement moved 3 mm to-wards the labial surface in the existing denture space (Fig. 5). The CT template was modified so that the drilling hole was moved approximately 2 mm towards the labial surface.

The second CT examination showed that the

Fig. 1 Edentulous mandibular arch.

Fig. 2 Computed-tomographic scan of the mandible with template before remolding of the cameo surface (first CT scan).

Fig. 3 Example of trial insertion of the selected implant overdenture attachment to evaluate the fit within the denture space before remolding of the cameo surface.

Fig. 4 The denture flange and cameo surface were extend-ed along the labial, buccal and lingual directions within physiological limits.

Fig. 5 Sectional silicone mold over area of anticipated im-plant position after remolding of the cameo surface. Blue: Expected implant position before remolding of the cameo surface. Red: Expected implant position after remolding of the cameo surface.

Implant Placement for Mandibular Overdentures using the Neutral Zone Concept

111

height of the mandibular bone was 10 mm, and the new implant was positioned vertically in the middle of the alveolar bone (Fig. 6). The surgical template was modified according to the CT tem-plate using a drill guide so that the ideal position and direction determined by the CT examination was accurately transferred. A mouth prop was added to the template to restrict its movement during mouth opening (Fig. 7). The two implants (Brånemark implant, MKIII, Nobel Biocare Ja-pan, Tokyo, Japan) (RP φ3.75, 8.5mm ) were ac-curately placed following the surgical template and the mouth prop.

After allowing 3 months of healing and osseo-integration, the definitive impression was made with a medium-viscosity silicone impression ma-terial (Coltex, Coltene), and the maxillomandibu-lar relationship was registered using the dupli-cate denture instead of the individual tray and occlusion rim. A bar attachment (CM rider, CM) was selected as a retainer for the implant-re-tained prosthesis and fabricated in accordance

with the manufacturer’s instructions. The plastic pattern was cut to length and joined to the gold cylinders with sufficient hard wax to make a strong joint. The pattern, complete with gold cyl-inders, was then invested and casted. A cast met-al (Co-Cr) framework was fabricated to reinforce the denture base, and then auto-polymerizing PMMA (Palapress Vario, Kulzer) was polymer-ized. The retainer and implant-retained overden-ture were inserted after polishing and finishing (Fig. 8). For the maxillaly arch, a conventional overdenture was delivered after removal of the right lateral incisor and a stud attachment was placed on the right canine. The recall of the pa-tient for follow-up and subsequent adjustments as necessary have been performed once every six mouths.

DiscussionThe position and direction of implant placement during oral rehabilitation affect the function and esthetics of implant superstructures5-9 and should be determined using a diagnostic and surgical

Fig. 6 Computed-tomographic scan of the mandible with template after remolding of the cameo surface (second CT scan).

Fig. 7 Surgical template with drill guide and mouth prep.

Fig. 8a,b Defintive prosthesis delivered in the mouth.

a

b

Suzuki et al., Prosthodont Res Pract 5 : 109-112, 2006

112

template of the final prosthesis.10-12 Diagnostic and surgical templates for implant-retained and -supported overdentures are usually prepared by duplicating the existing denture. Under other circumstances, a new denture with accurate oc-clusion and appropriate intaglio and cameo sur-faces may be duplicated to prepare the diagnostic and surgical templates.

For complete dentures, an appropriate cameo surface is necessary to obtain proper denture re-tention and stability.1-4,13-15 The neutral zone is defined as the potential space surrounding the mandibular denture between the lips, cheeks and tongue.2 The neutral zone concept has been used to obtain an appropriate cameo surface of den-tures. Implant-retained or -supported overden-tures also require an appropriate cameo surface to prevent harmful forces from acting on the im-plant.16 In these situations, a diagnostic and sur-gical template should be prepared utilizing the neutral zone concept. In the case of the mandible, if there is a high degree of absorption of the re-sidual ridge bone, it becomes difficult to predict the implant position. In such cases, the neutral zone concept is useful for predicting the correct implant position. Therefore, recording of the neu-tral zone as part of the diagnostic work-up before implant placement is important. Diagnostic and surgical templates prepared using the neutral zone concept improve the position of implant placement for comfortable dentures. The advan-tage of recording of the neutral zone is that it en-ables the most optimal positions for fixtures to be assessed based on the cameo surface. However, the disadvantage is that the process is complicat-ed.

ConclusionThe existing denture did not allow correct im-

plant placement and afforded inadequate space for the retainer. After molding of the border and making impressions on the cameo surface, the existing denture modified using to the neutral zone concept allowed proper implant placement.

Acknowledgments: The authors appreciate the clini-cal guidance of Dr. Susumu Nisizaki, University of Uruguay, Faculty of Dentistry. The editorial assistance of Mrs. Jeanne Santa Cruz is also much appreciated.

References1. Lott F, Levin B. Flange technique: an anatomic

and physiologic approach to increased retention, function, comfort and appearance of denture. J Prosthet Dent 16:394-413,1966.

2. Beresin VE, Schiesser FJ. The neutral zone in complete and partial dentures, 2nd ed. St Louis: CV Mosby, 1978.

3. Sciesser FJ. The neutral zone and polished sur-faces in complete dentures. J Prosthet Dent 14: 854-865, 1964.

4. Fahmi FM. The position of the neutral zone in re-lation to the alveolar ridge. J Prosthet Dent 67: 805-809, 1992.

5. Murrell GA, Davis WH. Presurgical prosthodon-tics. J Prosthet Dent 59: 447-452, 1988.

6. Engelman MJ, Sorensen JA, Moy P. Optimum placement of osseointegrated implants. J Prosthet Dent 59: 467-473, 1988.

7. Neidlinger J, Lilien BA, and Kalant DC. Surgical implant stent: A design modification and simpli-fied fabrication technique. J Prosthet Dent 69: 70-72, 1993.

8. Fumihiko W, Yoshiaki H, Izumi M. Retrieval and replacement of a malpositioned dental implant. J Prosthet Dent 88: 255-258, 2002.

9. Yoav G, David M. Prosthetic treatment for severe-ly implants. J Prosthet Dent 88: 259-262, 2002.

10. Garber DA. The esthetic dental implant: letting restoration be the guide. J Am Dent Assoc 126: 319-325, 1995.

11. Balshi TJ, Garver DG. Surgical guidestents for placement of implants. J Oral Maxillofac Surg 45: 463-465, 1987.

12. Harel S. Use of transitional implants to support a surgical guide: Enhancing the accuracy of implant placement. J Prosthet Dent 87: 229-232, 2002.

13. Kapur KK, Soman S. The effect of denture factors on masticatory performance. Part 2: influence of the polished surface contour of denture base. J Prosthet Dent 15: 231-240, 1965.

14. Cantor R, Curtis TA. Prosthetic management of edentulous mandibulectomy patients. Part 2. Clinical procedures. J Prosthet Dent 25: 546-555, 1971.

15. Ohkubo C, Shigeru H, Hosoi T. Neutral zone ap-proach for denture fabrication for a partial glos-sectomy patient. J Prosthet Dent 84: 390-393, 2000.

16. Graham E.White. Osseointegrated Dental Technol-ogy. 153-168, London: Quintessence; 1993.