The Procedures, Limitations and Indications for Small Diameter Implants and a Case Report

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The Procedures, Limitations and Indications for Small Diameter Implants and a Case Report
August 2004
By Carl E. Misch, DDS, MDS and Hom-Lay Wang, DDS, MSD
Oral Health Journal
The most common congenital missing maxillary anterior tooth is a lateral incisor.1 The restoration of one maxillary anterior crown is one of the more difficult prosthetic treatments in a general practice. 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
The following article presents the indications for a small diameter implant and presents a case report of a patient missing one maxillary lateral incisor.
The average lateral incisor is 6.5mm in width, 1mm less than the canine and 2mm less than the central incisor.3 When a permanent lateral incisor is missing, the deciduous tooth is retained longer then ideal, and the collapse of the permanent incisor mesio-distal space occurs as the canine positions itself in a more mesial position. In addition, when a lateral incisor is congenitally missing, the contralateral incisor is often peg or deficient in mesio-distal size.1
This condition further reduces the space dimension. As a consequence, the missing lateral incisor space is often less than average and approaches 5mm. Mandibular central and lateral incisors are missing less often, but also have a smaller mesial-distal dimension, and average 5.4mm.3 Therefore, single tooth replacement may on occasion require implants of small diameter to restore the missing tooth situation. For example, a 20-year-old male presented to the office with a congenitally missing lateral incisor. All the remaining teeth were healthy. The inter-tooth space was only 5.5mm. The available bone was greater than 5mm in width and 16mm in height. The soft tissues associated with the central incisor and edentulous site were within normal limits. The maxillary canines demonstrated delayed passive eruption and short clinical crowns (Fig. 1).
Most manufacturers fabricate their smallest two-piece implants in a dimension of 3.5mm or larger at the crest module, although the implant body may be smaller (i.e. 3.2mm). A two-piece implant should have the microgap of the abutment at least 1.5mm from the adjacent tooth.4 As a result, the ideal minimum inter-tooth space for a dental implant with this dimension should be 6.5mm (1.5mm from each tooth and 3.5mm for the top of the implant). This dimension is sufficient for the average dimension of a maxillary lateral incisor.
However, mandibular incisors are usually 5.2 to 5.4mm in width, and congenitally missing maxillary lateral incisors are often less than 6.0mm. As a result, most current implant designs are too large for smaller intra-tooth spaces.
A one-piece implant with a 3.0mm diameter was developed to overcome the challenge of small edentulous spaces in the anterior region of the mouth (Biohorizons¨? MaximusÇ—˘ Dental Implants) (Fig. 2). A one-piece implant does not have a microgap between the implant body and abutment connection, and therefore initial crestal bone loss over time may be reduced.5,6
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 1mm to the adjacent tooth root (Fig. 3). Therefore, mesio-distal spaces for a 3.0mm diameter implant employed here may be as little as 5.0mm (Fig. 4). The primary indications for an implant of this dimension is for single maxillary lateral incisors and mandibular anterior lateral and/or central incisors.
SURGICAL PROCEDURE
The surgical procedure for the 3mm 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.2 This was the surgical method used in this case report. An esthetic crown lengthening procedure was also performed on the maxillary canines.
A 3mm diameter trephine bur was used to penetrate the soft tissue in the missing lateral incisor region (Fig.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 initially prepared the implant site and begin to develop the path of insertion for the implant drills (Fig. 6). This drill was also designed to level the crest of the ridge 3mm below the free gingival margin of the implant crown and allow the abutment head of the implant to be level with the bone. A radiograph was taken with the alignment drill in place to evaluate the path of insertion (Fig. 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 the final implant abutment (Fig. 8). The aesthetic position and interocclusal clearance may be determined with this trial abutment (Fig. 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 2.0mm diameter depth drill of 12, 15 or 18mm. A 15mm was used in this patient. The longer the implant, the greater is 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 (Fig. 10). The osteotomy can be widened to 2.5mm 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 (i.e., as is occasionally found in the anterior mandible).
The one piece implant body/ abutment was then inserted with a handpiece mount at 30rpm (a hand wrench insertion with a ratchet adapter may also be used) (Fig. 11). The implant was positioned so the threads were 1 to 2mm below the crest of the bone. A periapical radiograph confirmed the position.
A #702L preparation bur was used to modify the abutment as required, with consideration of the opposing teeth in occlusion (Fig. 12). A transitional crown, without any occlusal contact, was then fabricated for the implant.
TRANSITIONAL RESTORATION
There are two options for the transitional restoration for a one-piece, 3mm diameter implant. The first option is an acrylic crown (Fig. 13). However, this crown has no occlusal load for three to four months. In addition, the transitional crown may be splinted to a natural tooth, which has no clinical mobility (i.e. a canine). The diet should be restricted to only soft food and the patient is told to avoid this restoration as much as possible during the initial healing period. After the initial healing, the final restoration may be fabricated and the diet returned to normal.
The second option also uses a pre-made crown. The crown is modified to fit over the abutment, and places the gingival margin in close approximation to the tissue. The pre-made crown is not relined with acrylic. A hole is then made in the mesial and distal interproximal surfaces of the crown. The adjacent teeth are acid etched and composite resin is 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 provides 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 restoration.
The diet is also restricted with this technique to soft foods for the initial bone-implant healing period. After three to four months the transitional crown is removed and a final restoration was fabricated. Occlusal equilibration is performed to reduce the occlusal load (Fig. 14). A periapical radiograph was taken to confirm the position of the crown and implant (Fig. 15).
DISCUSSION
Limitations
Implants with smaller diameters have several limitations including less surface area, lower fatigue strength, and higher risk of screw loosening. 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.6 A 1mm decrease in width of an implant may decrease the surface area of an implant by more than 40 percent7 (Fig. 16). Hence, a 3mm diameter implant may have almost one-third less surface area of contact with bone as compared to a 4mm diameter implant.
The fatigue strength of an implant is affected by the diameter, the implant material and amount of force applied to the system.8 The formula for the fracture strength of a circular implant is */4(R4). This means that a unit decrease in width decreases the strength of the implant by a factor of 4. For example, a 2mm diameter implant is 16 times weaker than a 4mm diameter implant. Hence, clinicians may use a 2mm diameter transitional implant, but regular occlusal loads over an extended period of time would r