Narrow-Diameter Implants: A Restorative Option for Limited Interdental Space

Stuart J. Froum, DOS; Sang-Choon Cbo, DDS; Young Sung Cho, DDS;!Nicolas Elian, DDS; Dennis Tarnow, DOS

The present study reports on the results of the use of a screw-retained narrow-diameter implant (NDI) system as an option for implant placement in areas of lim­ned bone volume. This retrospective report followed 48 NDIs in 27 patients for 1 to 5 years postloading. No implant failures were reported, yielding a 100% sur­vival rate. The screw-retained attribute of this system allows retrievability of the restorations, which may require replacement because of porcelain fracture, chip­ping, or a desire to change color The three diameters available—1.8 mm, 2.2 mm, and 2 A mm—allow flexibility for a variety of narrow edentulous spaces. These NDIs present a cost-effective alternative for restoring limited spaces with implant restorations, without the bone augmentation or orthodontic procedures required for conventional fixed restorations  The NDI system is approved by the U.S. Food and Drug Administration for long-term use. (Int J Penodontics Restorative Dent 2007;27:449-455.)

'Clinical Professor, Department of Per todonto logy and Implant Dentistry. College of Dantistry, New York University, New York, NV. 'Clinical Assistant Professor, Department of Penodontdogy and Implant Dentistry, College
of Dentistry, New York University, Hew York, NY. Private Pfscticfi. Jinjoo, South Korea 'Assistant Professor Clinical Director, Department of Penodontology and Implant Dentistry, College of Dentistry, New York University, New York, NY. 'Professor and Chair, Department of Period ontology and Implant Dentistry. College of Dentistry. New York University. New York, NY

Dental implant restorations have been documented to have a high degree of success for completely and partially edentulous patients.1"5 In the latter category, the replacement of a single missing tooth with an implant-sup­ported restoration has widespread use, with high rates of success reported *"' ° The advantages of such a restoration include construction of a freestanding restoration, ease of home care, and the avoidance of a removable or fixed partial denture. Preparation of healthy, noncarious teeth adjacent to the eden­tulous area is thus avoided. An addi­tional advantage to the implant restoration is the maintenance of the alveolar bone, which otherwise would undergo resorption with other restora­tive options.' '~14 Moreover, the long-term benefits of freestanding implant-supported crowns include the fact that the crowns and surrounding tissue are accessible and can be hygiemcally maintained by ordinary home care procedures.

A requirement for implant place­ment is the presence of adequate bone volume and horizontal interden­tal space to allow standard-diameter (4.0 mm or greater) implants to be placed. Procedures to increase bone volume, including guided bone regen­eration and block grafting, have been used to increase available bone. Implants placed in regenerated bone show high degrees of success.1S16 Implant manufacturers have intro­duced smaller-diameter implants (3.0 to 3.5 mm) in an attempt to solve the space problem. However, these implants require a minimum mesiodis-tal space of 6.0 to 6.5 mm to allow ade­quate implant-to-tooth distance.""19 Narrow-diameter implants (NDI) with a diameter of 1.8 mm were originally introduced as transitional implants that would help patients undergoing implant therapy avoid removable pro­visional dentures; these NDIs were ulti­mately intended to be removed.2021 However, these implants became osseointegrated and showed a bone-to-implant contact similar to that of implants with conventional diame­ters. Transitional implants using cementable provisional restorations functioned well but occasionally failed because of repeated removal {tapping off), which caused macromovement of the implants.20 Approximately 5 years ago, narrow-diameter titanium (Ti) alloy implants allowing screw-retained restorations (ANEW) were introduced by Dentatus. Based on the successful use of these implants over extended periods of function, approval of the US Food and Drug Administration was obtained for long-term use. The Ti alloy narrow-body implants are cur­rently available in diameters of 1.8, 2.2, and 2.4 mm and in lengths of 7, 10, and 14 mm.

The present study is a retrospec­tive report of 48 NDIs in 27 patients fol­lowed for 1 to 5 years postloading. All implants and prosthetic components were manufactured by Dentatus and placed either at the Department of Penodontok>gy and Implant Dentistry, New York University Kriser Dental Center, or in the office of one of the authors (SJF). All were placed and pro-visionalized dunng the same visit.

Method and Materials

The following procedures were per­formed prior to implant placement (Figs 1a to 1c).

1. A cancer screening was done.
2. A full-mouth examination for caries and petiodontal disease was carried out.
3. Periapical radiographs were taken, and patients were referred for computerized axial tomographic (CAT) scanning.
4. The edentulous area was measured clinically and on the CAT scans using SimPlant technology (Materialise). Implant placement was simulated on the CAT scan.
5. Informed consent was obtained orally and in writing. The patients
   were informed of the benefits and risks, and all other restorative
   options were  explained.  The patients were also informed that one
   or more NDIs would be placed, with a provisional restoration fabricated and inserted dunng the same visit.
6. Impressions were made, study casts crested, and a surgical guide fabri­cated on an ideal waxup.

Implant Placement

1. In all cases, facial and lingual infiltration utilizing lidocaine with epi-
   nephnne 1:100,000 were used to obtain local anesthesia. In patients
   unable to tolerate this anesthetic, carbocaine 3% without epinephrine was used.
2. A crestal incision was made, and a limited soft tissue flap was reflected to expose the crest of bone.
3. With a 1.3-mm twist drill with 7, 10, and 14-mm laser markings (cor- responding to the size of the implants), osteotomies were drilled at 1,500 revolutions per minute using constant copious irrigation (Fig 1d). In areas of dense cortical bone {mandibular anterior area), the drill was inserted two or three times to the desired depth to allow stress- free placement of the implants.
   4. With the handpiece adapter or man­ual driver included in the surgical kit. the implants were placed to the desired depth at 30 revolutions per minute. A manual tactile driver was used to achieve the final insertion depth. This hand tapping was per­formed to maximize initial stability of the implant.

Fabrication of the provisional

1. The titanium indexing abutment was placed over the implant square head in contact with the 3.2-mm implant platform. This coping may be placed in a buccolingual or mesiodistal direction.)
2. A nonhygroscopic acrylic resin screw cap was inserted over the implant head, locking the index coping firmly in place with the manual square driver {Fig 1 e). The open end of the screw cap was plugged with a brass insert to prevent the acrylic resin from blocking its access.
3. The proper mold and shade of poiy-carbonate crown was selected, or laboratory fabrication of a hollow crown from an ideal waxup was performed, in advance of the clinicalprocedure.
4. The provisional restoration was fabricated using the polycarbonate
   crown, laboratory-prefabricated hollow crown, or a vacuum-formed
   shell filled with quick-curing acrylic resin (Jet, Lang Dental). The provisional was trimmed and polished. The screw cap was disassembled by counterclockwise rotation with the square driver.
5. The provisional restoration was placed over the abutment and the
   occlusion adjusted so as to avoid contact in centric and excursive movements the crown was reassembled with the screw cap. The screw opening was temporarily filled with visible off-color tempo­rary acrylic resin for future access.
6. The soft tissue flap was repositioned and sutured with absorbable inter­rupted 4-0 chromic gut sutures to obtain interproximal closure (Fig 1 f).

All patients were placed on antibi­otic coverage (amoxicillin 2,000 mg 1 hour prior to surgery, followed by 250 mg four times a day for 7 days follow­ing surgery; patients unable to take amoxicillin received clindamycin 600 mg 1 hour prior to surgery, followed by clindamycin 150 mg four times a day for 7 days). Patients were placed on 1.2% chlorhexidine rinse (Peridex, Zila Technical) twice a day for 3 weeks start­ing 2 days prior to surgery. Patients were instructed to avoid hard contact with the implant restoration for 3 to 4 months following surgery. Patients were also advised to lightly brush and gently floss the surgical area starting 3 weeks after surgery. Patients returned every 2 weeks for 2 months after surgery and once a month thereafter for 2 to 4 months for maintenance and monitoring of the area.

Approximately 4 months after implant placement in the mandible and 6 months after placement in the maxilla, the screw-retained provision-als were replaced with a metal ceramic or acrylic resin crown using conven­tional impression analog transfer tech­niques for models and laboratory-con­structed restorations

Fabrication of the definative restoration

1. The indexed impression coping was attached with the gray technical screw cap to the implant.
2. A closed-tray impression was made of the screw-attached index coping using elastomeric impression material {Reprosil Vinyl Polysiloxane Impression Material, Dentsply/Caulk); this was used for making an indexed master cast.
3. The transfer coping was attached to the analog and re-inserted into the impression with an implant analog.
4. The master cast was made with a soft tissue base for the construction of the restoration.
5. The definitive restoration was then fabricated using gold or plastic
   copings.
6. A porcelain-fused-to-metal crown was fabricated using conventional
   clinical and technical laboratory procedures.

Results

In this study, 27 patients received 48 NDIs, which were loaded for periods of 12 to 64 months postinsertion. To date, no implant or prosthesis has had to be removed or replaced. Two screw-retained crowns loosened; the com­posite was removed and the screw cap was tightened without patient dis­comfort or interruption of the function of the implant restoration. Each patient has been recalled at 2- to 3-month intervals for maintenance and rexamination.

Discussion

Re-formation of the horizontal width around an implant requires a minimum distance of 1.5 mm between the implant surface and the neighboring tooth to allow maintenance of ade­quate interproximal bone. Standard-diameter implants of 4.0 mm or greater, therefore, require a mesial-to-distal edentulous distance of at least 7 mm between two teeth to place an implant and maintain the proper restorative distances. Moreover, stan­dard-diameter implants require at least 2 mm of bone buccal to the implant to avoid bone resorption and gingival shrinkage, which then requires a greater apicocoronal dimension of the crown restoration. Restoration of the implant in such cases requires an arti­ficially long implant crown, resulting in compromised esthetics

Ridge augmentation procedures are often required to create adequate buccolingual bone to maintain a 2-mm bone thickness following implant placement NDIs {1.8, 2.2, and 2.4 mm) present a solution for the aforemen­tioned requirements ot adequate buc-colingual bone dimensions and proper implant spacing without the need for ridge augmentation. Because of their smaller diameters, NDIs also result in a greater thickness of remaining buc-cal bone following the osteotomies for implant placement. They also offer the advantage of immediate temponza-tion; the patient thus avoids the use of removable provisional appliances or the need to 90 edentulous until the implant fully osseointegrates and is able to be restored. In the current ret­rospective study, implants were nonoc-clusally loaded immediately after placement of a provisional restoration, for a period of 4 to 6 months before definitive impression and restoration. Each of the 48 NDIs has been in func­tion for 1 to 5 years,

The 100% success rate of NDIs as used and documented in the present study is similar to that observed by Mazorand colleagues,23 who reported on 32 single-tooth replacements in 32 patients. One implant failed as a result of "mechanical overload." Several dif­ferences, however, should be noted between the two studies. The implants used in the Mazor et al study were ail 2.4 mm in diameter. All were immedi­ately loaded at the time of placement. The implants used in the present study were 1.8, 2.2, or 2.4 mm, depending on the volume of remaining bone. The additional option of using a smaller-diameter implant in the present study allowed preservation of more bone in the implant site, both buccally and mterdentally. The implants used in the current study (ANEW, Dentatus) allow immediate loading and have been used with this protocol for multiple implants. However, in the present study, for the replacement of one or more teeth in patients who had enough remaining teeth to provide a stable, trauma-free occlusion, the authors felt that there was lirttle need to load these implants early and risk macromovement with loss of osseom-tegration. The immediate, nonocclud-ing provisional provided for esthetics and performed as a space maintainer, thus avoiding adjacent tooth migra­tion. The NDI system employed in the present study also has the advantage of delivenng a screw-retained definitive restoration. This provides an option for retrivability, which is extremely useful should the crown require replacement because of porcelain frac­ture, chipping, or the desire to change porcelain color when the color of the adjacent teeth changes. The latter may occur as a result of aging, professional or home whitening procedures, or future additional restorations. These NDIs present a useful adjunct to the implant restorative armamentarium by providing an implant option in patients with congenitally missing incisors, reduced interdental space following orthodontic movement, retained pri­mary incisors that am lost, one or two missing mandibular incisors (Figs 2a to 2c), or space collapse in the maxillary anterior area following a lack of needed orthodontic therapy.

Although there are several NDI systems available today, the data in the present study with the acrylic resin screw cap-retained system demon­strate similar high survival rates for the 1.8- to 2.4-mm-diameter implants

The advantage of the NDI system used in the present study is that it offers the same ease of insertion with a greater prosthetic flexibility and retriev-ability than purely cementable sys­tems. Evaluation of these NDIs for long-term use will require additional multicenter prospective longitudinal studies.

References

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