Treatment Options for a Congenitally Missing Lateral Incisor

Figure 1. The patient has abundant keratinized tissue and bone, but has limited interdental space. The canine has delayed passive eruption and requires crown lengthening.

Figure 2. The BioHorizons Maximus implant is one piece, 3 mm in diameter, and manufactured in lengths of 12 mm, 15 mm, and 18 mm. The abutment height is 8 mm.

Figure 3. A small-diameter implant is placed to restore a missing lateral incisor.

Figure 4. Two years post implantation, the single-tooth implant crown restored the patient to normal function and aesthetics.

Figure 5. The 3-mm, soft-tissue trephine drill is followed by a coarse diamond bur to contour the soft-tissue drape of the crown. Crown lengthening has begun on the canine.

Figure 6. The alignment drill initially prepares the bone to receive the implant at the angulation. An alignment drill also is used to prepare the crest of the ridge 3 mm below the free gingival margin and prepare a flat area on the bone for the 2-mm drill stop. This region allows the abutment to be positioned ideally within the bone.

Figure 7. A periapical radiograph of the alignment drill is used to evaluate the interdental position.

Figure 8. A trial implant abutment is designed to fit the initial osteotomy.

Figure 9. The trial abutment allows the evaluation of the abutment location for aesthetics and occlusion.

Figure 10. A 2-mm drill prepares the osteotomy to a depth of 12, 15, or 18 mm. Longer implants have an advantage when the implant abutment and body are one piece. A periapical radiograph may confirm the interdental position of the 2-mm drill and is also used to evaluate the length of the implant site.

Figure 11. The 1-piece implant body/abutment may be inserted with a hard wrench or a handpiece at 30 rpm or less.

Figure 12. The final implant position. If required, a No. 702L high-speed bur and copious amounts of water are used to prepare the abutment for the transitional crown.

Figure 13. A transitional crown, completely out of occlusion, is joined to a canine, which is not mobile. The diet is restricted to soft food.

Figure 14. The final crown is cemented into position. The crown-lengthening procedure on the canine improved the aesthetic result.

Seventy percent of the adult population in the United States is missing at least one tooth (not including third molars).¹ The most common missing maxillary anterior teeth are the central incisor (as a result of trauma or endodontic failure) and the lateral incisor (congenital absence).^2,3 This article presents a case report for the treatment of a missing maxillary lateral incisor.

PATIENT PRESENTATION

A 20-year-old male presented to the office with a congenitally missing lateral incisor. All the remaining teeth were healthy. The intertooth space was 5.5 mm. The available bone was greater than 5 mm in width and 16 mm in height. The soft tissues associated with the central incisor and edentulous site were within normal limits. The maxillary canine demonstrated delayed passive eruption and a short clinical crown (Figure 1).

OPTIONS FOR TREATMENT

(1) A traditional, 3-unit fixed partial denture (FPD) with the canine and central incisor as abutments.

(2) A cantilever fixed partial denture with a canine abutment.

(3) A resin-bonded restoration with canine and central incisor as retainers.

(4) Orthodontics to move the canine into the lateral incisor position, a laminate on the canine to make it appear as a lateral incisor, and a crown on the first premolar to make it appear as a canine.

(5) A single-tooth implant and crown to replace the missing tooth.

A traditional, fixed partial denture (FPD) has the advantage of reduced treatment time. However, the aesthetics of a natural central incisor adjacent to a crown abutment is often a challenge. A traditional FPD has a 50% survival rate of 10 to 15 years, and 15% of the time endodontic therapy will be required for the abutment teeth.^4-8 Since prosthesis failure is usually related to caries or endodontic involvement, 8% of abutment teeth are lost within 10 years, and 30% are lost within 15 years.^5-6

A cantilevered FPD from the canine has better aesthetics as compared to a traditional FPD. A disadvantage is an increased risk of loss of the cement seal of the abutment, especially if parafunction exists. Caries as a result of loss of the cement seal and/or failure to perform adequate oral hygiene (use of a floss threader under the pontic region) results in loss of the canine in almost one third of the cases within 10 to 15 years.⁵

A resin-bonded restoration can be considered a transitional prosthesis. The debondingrate can approach 33% within 2 to 5 years.^9,10 The incisal edge of the central incisor must have an adequate thickness of enamel to prevent the metal framework from being seen. The abutment teeth should also have no clinical mobility, or the rate of cement failure dramatically increases. Although the likelihood of the need for endodontic treatment as a result of tooth preparation is reduced, caries associated with a partially debonded restoration and/or inadequate oral hygiene is often an important long-term problem.

Orthodontics to close the lateral incisor space is rarely the treatment of choice. The midline often shifts to the side of the missing tooth, since the anterior segment does not have the anchorage to oppose a canine and all posterior teeth. The maxillary canineeminence is also lost with this treatment, so a facial depression may be apparent. The canine rarely appears as a lateral incisor, even when a facial laminate or crown is fabricated for this tooth, since its width and length are different than the contralateral incisor.

A single-tooth implant is often the treatment of choice to replace a congenitally missing lateral incisor. The implant (and crown) have the highest success rate of any treatment option, the adjacent teeth are usually unaffected, and the aesthetic result is often ideal. ^2,11

IMPLANT DESIGN

Most manufacturers fabricate their smallest 2-piece (separate abutment and implant body) implants with a diameter of 3.5 mm at the abutment to implant connection, which may be called the implant crest module, although the implant body may have a smaller diameter. The 2-piece implant should be positioned at least 1.5 mm from the adjacent tooth.^12,13 As a result, the width of an edentulous space that will receive this type of dental implant should be at least 6.5 mm (1.5 mm from each tooth and 3.5 mm for the implant). In addition, many dental implant manufacturers make the abutment larger than the crest module of the implant body to enhance the crown emergence profile. This further increases the minimum space requirement to at least 7 mm.

When a maxillary lateral incisor is congenitally missing, the edentulous space is often 5.5 mm or less. As a result, most current implant designs are too large to replace a single anterior tooth. This case report illustrates the use of a 1-piece implant with a 3-mm diameter that was developed to overcome the challenge of small edentulous spaces in the anterior region of the mouth (Bio-Horizons Maximus Dental Implants, Figure 2). A 1-piece implant does not have a gap between the implant body and abutment connection, and therefore crestal bone loss over time may be reduced.14,15 When the implant is not expected to lose proximal bone when positioned at the height of the crestal bone, the implant may be placed as close as 1 mm to the adjacent tooth root (Figure 3). Therefore, mesio-distal spaces for the implant employed here may be as little as 5 mm (Figure 4).

SURGICAL PROCEDURE

The surgical procedure for the 3-mm diameter implant follows a similar protocol as other implants. A mucoperiosteal flap may be reflected, and direct observation of the bone can be made when the available bone is in question. However, when abundant keratinized tissue and bone are present, a tissue punch and implant osteotomy without tissue reflection is often the surgical protocol of choice.¹¹ This was the surgical method used in this case report.

First, a 3-mm diameter trephine bur was used to penetrate the soft tissue (Figure 5). The soft-tissue emergence was then contoured with a high-speed handpiece and coarse diamond, so the soft-tissue profile was similar to the contralateral tooth.

An alignment drill was then used to prepare the implant site initially andto begin to develop the path of insertion for the implant drills (Figure 6). This drill was also designed to level the crest of the ridge 3 mm below the free gingival margin of the implant crown and allow the abutment head of the implant to be level with the bone. In addition, this stage of the procedure serves as a depth gauge for the subsequent drill. A radiograph was taken with the alignment drill in place to evaluate the path of insertion (Figure 7).

A trial implant was then placed into the initial osteotomy site created by the alignment drill. The top of this device is the same size as that of the final implant abutment (Figure 8). The aesthetic position and interocclusal clearance may be determined with this trial abutment (Figure 9). A periapical radiograph may also be taken of the trial implant to confirm the mesio-distal position and angulation. The position and/or angulation of the initial site may be corrected with the side-cutting feature of the alignment drill.  

The depth of the osteotomy was established using a 2-mm diameter depth drill of 12, 15, or 18 mm. A 15-mm was used in this patient. The longer the implant, the greater the initial stability. In addition, if the opposing dense cortical plate can be engaged, a further benefit of rigid fixation occurs. A radiograph may be used to confirm the proper drill length and position (Figure 10). The osteotomy can be widened to 2.5 mm using the finishing drill when the bone is of a dense quality. Use of the final drill is not necessary in softer bone types, since the implant will condense the bone during insertion and provide greater fixation. A bone tap may be used when the bone is very dense (ie, as is occasionally found in the anterior mandible).

The 1-piece implant body/abutment was then inserted with a handpiece mount at 30 rpm (a hand wrench insertion with a ratchet adapter may also be used) (Figure 11). The implant was positioned so the threads were 1 to 2 mm below the crest of the bone. A periapical radiograph confirmed the position.

A No. 702L preparation bur was used to modify the abutment as required, with consideration of the opposing teeth in occlusion (Figure 12). A transitional crown without any occlusal contact was then fabricated for the implant.

TRANSITIONAL RESTORATION

There are 2 options for the transitional restoration for a 1-piece, 3-mm diameter implant. The first option is an acrylic crown (Figure 13). However, this crown cannot have occlusal load for 3 to 4 months. In addition, the transitional crown may be splinted to a natural tooth that has no clinical mobility (ie, a canine). The diet should be restricted during the initial 3-month healing period to only soft food such as pasta and fish. Hard bread and raw fruits or vegetables should be avoided, and the patient is told to avoid placing pressure on this restoration.

After this initial period of healing, the final restoration can be fabricated, and the diet is not restricted.

The second option also uses a premade crown and was used in this case report. The crown was modified to fit over the abutment and places the gingival margin in close approximation to the tissue. The premade crown is not relined with acrylic. A hole was then made in the mesial and distal interproximal surfaces of the crown. The adjacent teeth were acid-etched, and composite resin was placed in the interproximal regions of the crown to lute it to the adjacent teeth. The occlusion was modified to eliminate occlusal contact. This approach provided an aesthetic fixed replacement for the missing tooth without the excess force on the implant. This approach will offer slightly less risk to the implant during initial healing, since the crown is not actually attached to the implant and tooth contact will not overload the implant. The diet should be restricted to soft foods for the initial bone-implant healing period. After 3 to 4 months, the transitional crown was removed and a final restoration was fabricated. Occlusal equilibration was performed to reduce the occlusal load (Figure 14). A periapical radiograph was taken to confirm the position of the crown and implant (Figure 15).

DISCUSSION

Implants with smaller diameters have several limitations. Smaller diameter implants have a smaller surface area for bone-implant contact, and this could reduce the long-term survival of the fixture.

The surface area of an implant is related to the amount of force the implant is able to resist when serving as a prosthetic abutment. The roots of posterior natural teeth have greater surface area than anterior teeth, and forces are greater on posterior teeth. Likewise, an implant with greater surface area is less likely to be overloaded during function.¹⁵ A 1-mm decrease in width of an implant may decrease the surface area of an implant by more than 40%16 (Figure 16). Hence, a 3-mm diameter implant may have almost one-third less surface area of contact with bone as compared to a 4-mm diameter implant.

The fatigue strength of an implant is affected by the diameter.¹⁷ The formula for the fracture strength of a circular implant is Pi/4(R4). This means that a unit decrease in width decreases the strength of the implant by a factor of 4. For example, a 4-mm diameter implant is 16 times stronger than a 2-mm diameter implant. Hence, clinicians may use a 2-mm diameter transitional implant, but regular occlusal loads over an extended period of time would result in an unstable situation.

The prosthetic platform of a 2-stage, small-diameter implant is likely to have screw loosening.¹⁸ The narrower the abutment to implant attachment diameter, the more force applied to the abutment screw during occlusal loading. When the abutment screw becomes loose under a cemented crown, the crown may need to be cut off to gain access to the abutment screw. Abutment screw loosening is the most common prosthetic complication of single-tooth implants and has been reported to occur in 7% to 40% of cases (depending on patient factors and the implant system used).¹⁹ A 1-piece implant body and abutment has a distinct advantage, since abutment screw loosening does not occur.

Regarding the materials from which implants are fabricated, the most common implant body is fabricated from titanium, since a direct bone to implant interface has been shown to develop.²⁰ Five grades of titanium are used for implants, and the bone to implant contact is similar for each.²¹ Grade 1 to 4 is 99% titanium, and Grade 5 is titanium alloy (90% titanium, 6% vanadium, and 4% aluminum). The strength of each of these materials is different.²¹

Grade 1 titanium is 4 times weaker than Grade 5 titanium, and although a few manufacturers have used this grade for 4-mm diameter implants, it is inappropriate to use for a permanent, small-diameter implant (Figure 17). Some manufacturers of transitional implants select a lower grade titanium so the clinician can bend the abutment post for parallelism. Several implant companies use softer Grade 3 titanium for their implants, yet Grade 3 is 2 times weaker than Grade 5. Further, Grade 4 titanium is 1.6 times weaker than Grade 5. Therefore, when small-diameter implants are used to replace a natural tooth, a Grade 5 implant material should be used. The bone to implant contact is similar for all grades of titanium, because a similar oxide layer is formed regardless of the titanium grade.²¹

The 1-piece, small-diameter implant has several advantages when used to replace maxillary lateral incisors and mandibular incisors. The 1-piece design eliminates the risk of abutment screw loosening. Since there is no microgap between the abutment and implant, the amount of crestal bone loss may also be reduced.15 The abutment-implant connection of 2-piece implants is often at or below the crestal bone. With traditional implant designs, bone loss of up to 3 mm from the microgap has been reported.^13,14

Traditional implant sizes of 3.5 mm and greater at the crest module are often too large to replace a missing tooth in the anterior regions of the jaws. On the other hand, temporary implants of less than 3-mm diameter risk fatigue fracture. A fractureof an implant also will place the adjacent teeth at risk during implant removal. An integrated implant that is fractured must be removed from the bone using a bur, with damage likely occurring to the bone and adjacent teeth.

The fracture of an implant is directly related to the amount of force placed on the implant component or body. Greater force is more likely to fracture an implant than lesser force. The maximum bite forces in the mouth are less in the anterior regions (25 to 50 lb/in2) compared to the molar regions (200 to 250 lb/in2).²² Therefore, smaller-diameter implants should be limited to the anterior regions of the mouth to reduce the occurrence of fracture.

The primary disadvantage for a 1-piece, small-diameter implant is the requirement of immediate restoration. Since the implant abutment is intraoral at the time of surgical placement (the implant body and abutment are a single component), an increased risk of overload is present during initial bone healing. Oral habits or activities such as gum chewing, tongue thrust, and playing some musical instruments (ie, woodwinds) may overload the developing interface. The “open” transitional crown concept, whereby the premade crown is not relined with acrylic, reduces this risk.

CONCLUSION

Single-tooth replacement with an implant and restoration is becoming common, especially in the anterior regions of the jaws. Maxillary lateral incisors are one of the most common congenitally missing teeth, especially in females.3 Traditional, small-diameter, 2-piece implants are often too large to insert into the mesio-distal space of a missing maxillary lateral incisor. In addition, 2-piece implant designs have increased risk of screw loosening, fatigue fracture, and crestal bone loss.23

The treatment presented in this article used a 1-piece, small-diameter implant/abutment design. The 1-piece design reduces the risk of screw loosening and crestal bone loss since there are no abutment screws and no microgap. The implant is made of Grade 5 titanium to reduce the risk of fatigue fracture. Future studies are needed to evaluate the long-term success of this implant therapy.

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Dr. Misch is a clinical professor at the University of Michigan, department of periodontics/geriatrics. He has authored more than 175 clinical articles. He can be reached at (248) 642-3199.

Disclosure: Dr. Misch is a paid consultant of BioHorizons dental implants and is also on the board of directors.