Designing Both Mandibular Central Incisors on One 3.0-mm Single-Piece Implant: Five Years' Outcome

Replacement of lost teeth with dental implants is currently the preferred and predictable means of treatment unless indicated otherwise.¹  The definitive outcome varies with patient-related factors such as preoperative hard and soft tissue volume, the need for implant bed improvement before osteotomy, and the patient's consent for the same. Operator-related parameters leading to satisfactory results include optimal implant placement in 3 dimensions plus appropriate prosthesis design and material selection that complement each other. However, for a long-term sustainable outcome, biomechanics of the whole implant-prosthetic system plays a significant role and must be given due consideration.

Implants are available in a myriad of dimensions to suit different clinical presentations. At present, there is no universally accepted classification of dental implants based solely on their diameter. Fixtures ≤2.9 mm in diameter are considered as mini, between 3–3.4 mm as narrow, 3.75–4 mm as regular, and 5–6 mm as wide diameter.²  In general, regular diameter two-piece systems are the first choice for implant-related treatment. Narrow diameter, single-piece implants (SPI) serve their function well when they support individual crowns of relatively small dimensions, particularly in functionally less demanding areas of the oral cavity such as a maxillary lateral incisor, mandibular incisors, and narrow spaces after orthodontic treatment. Narrow diameter implants (NDI) are also considered beneficial for elderly patients who refuse or are medically unfit for bone augmentation procedures.³ 

The documented drawbacks of two-piece implants include weak abutment screw joint and increased bacterial colonization at fixture abutment interphase are resolved by SPI.^4–6  These fixtures incorporate the bone anchoring portion, the intermediate part passing through the soft tissue, and the prosthetic abutment—all as one integral unit.⁷ 

Negligible soft tissue manipulation during and after surgery, control over final prostheses margin, conventional prosthetic procedures of impression making and cementation, reduced prosthetic inventory, and decreased cost offer definite advantages to SPI over two-piece implants.^8,9  Due to lack of abutment versatility, SPIs are required to be placed precisely in three dimensions for desirable prosthetic results.

SPIs are amenable to being manufactured in a small diameter because of their solid design and, therefore, are utilized in narrow esthetic zones to restore a single missing tooth. Their narrow configuration presents a biomechanical disadvantage due to increased stress levels at the implant-bone interface. By virtue of being narrow, SPIs maintain sufficient distance from the adjacent teeth, thereby preserving the interproximal bone and the gingival papillae.^10,11 

This article describes the 5-year outcome in 2 patients for whom both mandibular central incisors were replaced on variably placed single-piece 3.0-mm implants.

Three patients reported for the replacement of missing both mandibular central incisors on dental implants placed 1.5 months prior (Table). Single one-piece 3.0 implants (Maximus 3.0, Biohorizon, Birmingham, Ala) were placed in variable positions in each one (Figure 1a through c). Deficiency in number, suboptimal 3-dimensional positioning, and impending cantilever fixed partial dentures (FPD) posed biomechanical challenges for a sustainable prosthetic restoration. As the prostheses would function in the esthetically critical zones, patients' expectations were very high, and they were eager to achieve the final outcome.

The clinical complexities concerning the implants (number, diameter, position), their impact on designing, prosthetic issues, and biomechanical limitations were discussed with the patients in detail. Placement of another implant was deliberated in 2 patients (patients 2 and 3) but was not agreed upon due to the requirement of second surgical intervention and for financial reasons.

Fully contoured temporary prostheses were delivered to restore the edentulous segment and partly fulfill the patients' unmet need for esthetics. No centric and eccentric contacts were maintained on the transitional restorations, and patients were advised against incising from the artificial teeth. Temporary prostheses were utilized so we could receive patient feedback on the design, tentative esthetic outcome, and the extent of deficiencies to be compensated in the final prostheses. Frequent loss of retention was observed in both patients where the implant was placed in the mandibular left central incisor region, causing their increasing anxiety. Placement of the second implant was again proposed but was denied.

It was then decided to reinforce the dimensions of the supragingival part of the implant in these patients by utilizing computer-aided designing and computer-aided manufacturing (CAD-CAM) designed with primary telescopic coping. We fabricated a Monolith zirconia coping (Procera, Nobel Biocare, Washington, DC) to be cemented on the implant. The abutment of the implant was not prepared to create a margin for the primary coping, as this would have severely jeopardized the mechanical parameters and requirements for a stable and retentive prosthesis. A chamfer margin was created for the superstructure on the primary coping in patient 2, which caused difficulty in identification and removal of the residual cement. Therefore, the margin for patient 3 was designed as knife edge (Figure 2a and b).

In patient 1, a geminated tooth-like morphology was designed in the provisional prosthesis. There were no mechanical or esthetic issues during the entire evaluation period; therefore, the design of the provisional prosthesis was replicated in the final prosthesis (Figure 3a and b). In patients 2 and 3, subsequent to checking the primary coping fit, a pickup impression was made in polyvinylsiloxane (Imprint II Grant, 3M, ESPE, St Paul, Minn) for fabrication of the all-ceramic secondary superstructure. All FPDs were CAD-CAM designed and fabricated in the zirconia-based material (Procera, Nobel Biocare). Glass ionomer cement (GC, Fuji, Japan) was used as the luting agent.

The laboratory technician was given specific instructions to design the prostheses with light contact in maximum intercuspation; anterior guidance was placed only on the implant-supported part of the prostheses and the cantilever portion of the prostheses without contact during excursive movements (Figure 4a through c). The axial walls of the primary coping were designed to be as parallel as technically feasible to aid in the superstructure retention.

During the entire 5-year period, none of the prostheses showed loss of retention, and there was no reported functional or esthetic complication of such severity that necessitated removal of the prostheses or implant. Patient 1 had a transient complaint (up to 3 months) of mild saliva ooze while speaking. The reason for this self-limiting issue was excessive space due to the narrow configuration at the cervical end of the prosthesis and the root of right lateral incisor (marked with an arrow in Figure 3a). Peri-implantitis in patient 3 was reported at around 4 years post cementation due to poor oral hygiene maintenance. It was resolved with oral prophylaxis and oral hygiene reinforcement (Figure 5a through e). No antimicrobials were prescribed. The supporting bone around the implants remained stable through the evaluation period in all 3 patients (Figure 6a through c).

Treatment with NDI is a reliable option for a single-tooth replacement. Evidence of equally predictable survival rates with NDI (94–100%) to those obtained in restorations using larger diameter implants is available.^12,13  Although the results with NDI are encouraging, it has been emphasized that the indications, treatment protocol, success, survival, frequency, and type of complications related to implant treatment vary with the decreasing diameter of dental implants.^14–18 

A meta-analysis showed that narrower implants (<3.3 mm) had significantly lower survival rates compared with wider implants (>3.3 mm). Many confounding variables impact the outcome: type of prosthesis, implant type, occlusal loads, parafunctional habits, and timing of prosthetic loading.²  The failure rate of narrow implants (<3.3 mm) was reported to be almost four times greater than regular implants and even higher for narrow and short implants.^13,19 

One-piece NDI are placed in an already deficient edentulous site and, therefore, there is an increased possibility of surgical, prosthetic, and mechanical complications. A study by Ding et al¹⁰  revealed significantly higher stress values at the implant-bone interface when implant diameter was reduced to 3.3 from 4.1 mm in comparison to reducing the diameter to 4.1 from 4.8 mm. Further, the chances of implant fatigue fracture increase with reducing implant diameter.²⁰  Even though it has been advised to use NDI only in areas of less functional loads, indications have been extended to support prostheses in posterior areas of the mouth²¹  and to replace multiple missing teeth with FPDs.²² 

To best of author's knowledge, to date, there is no documented literature reporting the outcome of cantilever prosthesis on one single-piece 3.0-mm diameter implants. Except for the transient issue of peri-implantitis in one patient, all 3 patients experienced 5 years of complication-free treatment. It is reasonable to presume that the biomechanically unfavorable cantilever prostheses were made sustainable through (1) precise framework designing, (2) increasing the abutment dimensions with primary telescopic coping to aid in retention and stability of the superstructure, (3) controlling occlusal forces by selective contact only on the implant part of the prosthesis, and (4) preventing all excursive contacts accrued on the cantilever part of the FPDs.

The promising results achieved should be interpreted with caution, as the outcomes are based on a clinical report in only 3 patients. In the author's opinion, randomized control trials should be conducted to gather more data and to further investigate and compare the treatment outcome of different variables associated with cantilever prostheses on single-piece 3.0-mm implants.

In the absence of other available treatment options, single-piece 3.0-mm implants may serve as a less morbid, low-cost solution for replacement of two missing teeth in non–load-bearing areas in the oral cavity. Excellent control of occlusal parameters, along with mandatory systematic follow-up evaluations to diagnose and treat impending complications are prerequisites for such prostheses.

Abbreviations

CAD-CAM

computer-aided designing and computer-aided manufacturing

FPD

fixed partial dentures

NDI

narrow diameter implants

SPI

single-piece implants

The manuscript was presented as a scientific poster at the 17th Biennial Meeting of the International College of Prosthodontists, September 7–9, 2017, in Santiago, Chile. The manuscript bears no conflict of interest.