Prevalence of Favorable Anatomy for Palatal Emergence of an Immediate Implant in the Maxillary Central Incisor Post-Extraction Site

Immediate implant placement is becoming a widely used technique to restore natural teeth that need to be extracted.¹  These complex procedures are not without risks. Not only is integration of the implant a requisite for the long-term success of the treatment, but the restoration should be aesthetically pleasing and maintain healthy peri-implant tissues over time.^2,3 

Many factors can have a negative impact on the stability and health of the peri-implant tissues, but few will be as consequential as positioning the implant too labially or buccally. These implants are at a high risk of developing mid-facial tissue recession, compromised aesthetic outcomes, and poor peri-implant health.⁶  Three-dimensional implant positioning can also determine the type of restoration that can be fabricated. Screw-retained crowns have 2 main advantages over cement-retained crowns. First, screw-retained crowns are easier to retrieve,⁷  and, second, they do not have the potential to cause biological issues related to retained cement.⁸  Until recently, only implants emerging palatal to the incisal edge could receive screw-retained restorations because in the aesthetic zone, the screw-retention hole access needs to be hidden.⁹ 

To achieve proper positioning of an implant replacing a maxillary central incisor, the implant should ideally emerge through the cingulum area of the desired prosthesis. This may be challenging when performing a post-extraction implant due to the angulation of the natural root and bone anatomy.¹⁰  Flapless procedures can reduce postoperative swelling and pain. Reflection of a flap would accelerate the resorption of the buccal plate, which is often comprised solely of bundle bone.^11–13  For this reason, flapless procedures are widely used in post-extraction sites.¹⁴  But when performing a flapless procedure, the visualization of the bone during surgery is not possible. Therefore, the implant size, position, and angulation must be preoperatively assessed with 3-dimensional imaging to ensure a proper distance to the cortical plates.¹⁵  Placing an implant with the ideal prosthetic angulation can lead to a fenestration; thus, a bone grafting procedure to cover the exposed threads will be required. Alternatively, the implant angulation can be changed to avoid such fenestration, but then the ideal prosthetic emergence might be compromised.¹⁶ 

The aim of this study is to show the prevalence of adequate bone anatomy for the placement of an immediate implant with proper angulation for a screw-retained restoration in the maxillary central incisor site. Cone beam computerized tomography (CBCT) and implant planning software are the planning devices used to evaluate the existing bone volume, trajectory, and position and thus facilitate a minimally invasive surgery with proper implant placement.

Data from patients who received CBCT of the maxillary anterior region for different types of dental procedures between January 2017 and February 2020 in a private practice in Dublin were used to complete this study. All patients signed a consent form giving permission to utilize their data for research purposes. Inclusion criteria were the following:

1. A maxillary central incisor must be included in the CBCT.

2. A tooth with its clinical crown had to be present in the central incisor site.

3. Adjacent teeth had to be present.

4. There must be clear images of the central incisor and surrounding anatomical structures without scattering.

5. Type 1 sockets¹⁷  with intact bony walls and no pathology had to be present.

Only 1 central incisor per patient was included in the study. The central incisor was randomly selected unless one of the incisors was not fulfilling the above inclusion criteria, in which case the only suitable central incisor was used. The CBCTs were not purposely taken for the study because this was purely a retrospective data collection. Thus, approval by the local ethical committee was not necessary.

CBCTs were taken with a CS 8100 3D (Carestream, Rochester, NY, USA) unit at 90 kVp, 3.20 mA, 15 seconds, and a 150-μm voxel size. CS 3D Imaging (Carestream) was used to interpret the images and virtually place the implants into the central incisor sockets.

The principal investigator (PI) (AJ) performed the virtual implant placements and measurements. Samples were then analyzed and measured by 5 independent, experienced clinicians (DCP/MDP/AEM/MBG/EPP).

When the independent analyses were completed, results were returned to the PI. An interrater reliability test was completed.

The implants were selected from the CS 3D Imaging library, and 4.1-mm-diameter tapered implants (Straumann Bone Level Tapered, Basel, Switzerland) were used.

The implants were placed in a sagittal view that passed through the center of the tooth from incisal edge to apex. Measurements were taken with CS 3D Imaging in millimeters. To position the implant in the socket, 5 rules were followed:

1. The implant platform was placed equicrestal (implant platform is even with the crest of the bone) in relation to the buccal plate. If placing the implant equicrestal was leaving less than 3 mm between the cemento-enamel junction (or desired future gingival margin) and the implant platform (in cases of altered passive eruption), it was placed deeper to achieve 3 mm for the biologic width.

2. At least 3 mm of bone anchorage beyond the apex of the root was necessary.

3. A minimum distance of 2 mm was maintained between the buccal plate and the implant body coronally to apically.

4. The palatal cortical plate had to be left intact and not perforated.

5. The implant was not to invade the nasal floor or the nasopalatine canal.

The clinicians always attempted to place the implants emerging palatal to the incisal edge of the natural teeth. If by doing so the previous rules were violated, then that implant was repositioned until meeting the positioning criteria.

Depending on the prosthetic emergence and immediate implant suitability, the cases were classified into 3 groups:

• Suitable for palatal emergence

• Not suitable for palatal emergence

• Not suitable for immediate implant placement

Statistical analysis was performed with Microsoft Excel. The percentage for each group (A, B, C) was calculated by dividing the cases included in each class by the total number of the samples.

The Percent Agreement for Multiple Readers test was conducted to assess the interrater reliability. A metric was calculated for each observation (patients 1, 2, 3 . . . 3, 2, 1) based on the percentage of pairwise comparisons that coincided. There were 6 examiners, leading to 15 pairwise comparisons per observation. The metric obtained for each observation was averaged throughout the entire sample.

Three hundred and twenty-one CBCTs that met the inclusion criteria were collected between January 2017 and February 2020. The average age of the patients was 53.4, ranging from 19 to 83. There were 148 males and 173 females.

Depending on the root dimension and distance to the nasal floor, the length of the chosen implants ranged between 10 and 16 mm.

From 321 cases that were included in this study, 200 (62.3%) were allocated to Group A, as they showed favorable anatomy to place the implant with a palatal emergence without violation of the 2-mm safe distance between the implant and the buccal plate while respecting the 4 other placement rules (Figures 1 and 2). Ninety-three cases (29%) were assigned to Group B because placing the implant with the desired palatal emergence incurred leaving a <2-mm distance between the implant and buccal plate, so the implant angulation and position were modified to meet the placement criteria. By doing this, all 93 implants from Group B had to be buccally tilted and therefore emerged labially to the incisal edge, making these cases unsuitable to get a screw-retained crown (Figures 3 through 5) (Table 1).

An immediate implant was not indicated in 28 patients (8.7%). In 22 of these cases, the crest was not wide enough bucco-palatally to place a 4.1-mm-diameter implant and still have a 2-mm distance between the implant and the buccal wall without invading the palatal cortical plate; therefore, a narrower implant or grafting procedures were required. In 4 cases, there were less than 3 mm between the apex and the nasal floor; therefore, immediate implant placement was ruled out due to insufficient apical anchorage for primary stability. Finally, in 2 cases, the nasopalatine bundle was too large to accommodate the implant while leaving 2 mm of bone buccally.

One hundred and seventeen of the 200 cases from Group A were placed occupying the palatal bone. In these cases, the palatal bone was wide enough to place the implant into it without cortical perforation. The implant placement rules outlined in this study were adhered to, but these implants, placed more palatally than the socket walls, were also emerging outside of the radiologic crown contours. Clinically, this would result in a more voluminous restoration in the palatal aspect (Figures 6 and 7).

This finding led to a reclassification of the sample (Table 2). The implants that emerged outside of the crown contours in the palatal aspect were thus repositioned to emerge within the prosthetic corridor (Figures 8 and 9). By doing so, Group A cases were reduced from 200 to 83. The 117 implants invading the palatal prosthetic space had to be buccally tilted to maintain 2 mm of bone buccally and therefore passed into Group B.

The addition of this new rule changed the results considerably.

Interrater reliability was 90% for the first round of classification and 88% on the reclassification when considering the palatal prosthetic space alteration.

To the authors' knowledge, in the current literature there is no study that shows the prevalence of favorable anatomy for palatal emergence of the implant in the maxillary central incisor site when considering the previously stated virtual planning rules. According to the results of this study, if implants are systematically placed to get a palatal emergence without considering the anatomy, 29% can be placed too close to the buccal plate apically or even perforate it. This is in line with other CBCT studies that have shown 22.6% and 26.07% apical perforation rates.^18,19  This can be addressed by raising a flap to graft over the exposed implant threads but will add cost, morbidity, and higher complication rates. This issue could also be minimized by preparing the osteotomy into the palatal bone and placing the implant more palatally; however, this could lead to a bulky restoration on the palatal aspect, which can affect phonetics, occlusion, and comfort.²⁰  In this study, when this restorative aspect was not taken into consideration, 200 (62.3%) cases were suitable for palatal emergence and therefore for receiving a screw-retained crown. But if palatal prosthetic contour alteration was avoided, the numbers were reduced to 83 (25.9%) cases.

In this study, the 5 rules for virtual implant placement were set according to current knowledge on immediate implants.

First, in the aesthetic area, the depth of the implant will be guided by the desired future buccal gingival margin. It is well established that there should be at least 3 mm between the implant platform and the gingival margin to allow adequate space for biologic width organization.^21,22  On average, in type I sockets, the buccal plate will be 3 mm below the buccal gingival margin; therefore, the implant should be placed at the level of this buccal bone.²³ 

Second, obtaining primary stability is a key factor necessary to achieve osseointegration. In post-extraction sites, bone availability can often be limited due to the presence of the sinus or nasal floor. A minimum of 3 mm of bone beyond the tooth apex should be present to predictably achieve implant primary stability.^21,24 

Third, the buccal plate in the maxillary anterior region is usually very thin (1 mm or less) and can present a mid-alveolar undercut.^25,26  It has been published that 2 mm of buccal bone over the implant would be ideal to maintain hard and soft tissues long term.^27,28  For this reason, it is recommended to place immediate implants at least 2 mm lingual to the buccal plate.²⁹  It can be debated whether these 2 mm of buccal bone over the implant are necessary for the entire implant length or only in the crestal region. The crestal bone presence is most critical to maintain support for the soft tissues and thus the emergence profile. The authors decided to keep a 2-mm distance to the buccal plate for the entire implant length, considering that during clinical implant placement, deviations might occur and that this safety distance could help reduce apical fenestrations of the buccal bone. When performing flapless procedures, deviation and buccal shifting of the implants are a common occurrence even when surgical guides are used. This could result in implants being positioned too close to the buccal plate in the apical area or even lead to perforations.³⁰ 

Fourth, the palatal cortical plate should also be assessed and measured to avoid perforations.³¹ 

Fifth, the invasion of the nasal floor and nasopalatine canal will leave the implant in direct contact with connective tissue and can lead to complications, such as surgical hemorrhage, sensory disturbance of the nasopalatine area, nonintegration of the implant, and nasopalatine duct cyst formation.^18,32 

Wide-diameter implants require a larger osteotomy and have a higher potential of violating the safe distance to the buccal plate and other important structures, such as adjacent roots, nasopalatine canal, and palatal bone, as they take up more volume in the socket. To maintain enough bone surrounding the implant and to prevent hard and soft tissue recession, it is safer to use a standard- or narrow-diameter implant in the maxillary central incisor site.³³  A 4.1-mm-diameter fixture was chosen for this study, as found in other, similar CBCT studies.^18,19  The use of a narrower implant could have resulted in a higher number of cases included in Group A (suitable for palatal emergence).

Two-dimensional radiologic images such as periapical and panoramic radiographs offer insufficient information for planning implant therapy in a safe, predictable, and aesthetic manner. With 2-dimensional radiographic images, (1) vertical height distances will often be distorted, (2) it is not possible to take measurements in a bucco-lingual direction, (3) the angulation of the ridge and roots cannot be addressed, (4) it is not possible to accurately assess the nasopalatine canal, and (5) superimposition of anatomical structures will occur.^34–37  The American Academy of Oral and Maxillofacial Radiology recommends the use of 3-dimensional imaging for dental implant planning.³⁸  The accuracy, easy accessibility and handling, and low radiation dose of modern CBCT equipment makes cross-section images and 3-dimensional implant planning software the diagnostic tool of choice in implantology.³⁹  CBCT is also paramount to preoperatively detect those cases in which an immediate implant could be at risk or contraindicated due to bone and anatomic limitations.

A simple, minimally invasive and effective technique was used to determine the implant angulation for a post-extraction maxillary central incisor site in this study. The 3-dimensional planning software is an accurate way of virtually placing the implant into the socket using a 1:1 scale. CBCT has minimum distortion; therefore, the virtual implant will give the surgeon a highly precise reproduction of what to expect during the surgery.^40,41  The 5 rules proposed in this study to obtain a predictable and safe post-extraction implant placement in the maxillary central incisor were easily followed and integrated into the planning of each case thanks to the implant planning software.

When performing procedures in sockets that present apical undercuts, a correction of the implant angulation is necessary. This apical correction leads to shifting the implant apex more palatal to avoid apical bone perforations, resulting in a buccal implant angulation. In these situations, cement-retained restorations will still be necessary in a considerable percentage of cases in maxillary central incisor sites unless a dynamic abutment and screw are utilized where the screw channel can be angulated up to 30°.⁴²  These dynamic abutments can overcome buccal implant angulation, enabling the fabrication of a palatal screw channel to screw retain implant crowns in most cases where otherwise a cemented restoration would be necessary.⁴³ 

Immediate implants are advanced surgical techniques. Clinicians must understand that proper diagnosis and planning go further than just the 3D assessment shown in this study. Beyond the ideal sagittal implant positioning and prosthetic emergence, each site must be assessed for the risk of post-extraction bone loss, which may affect peri-implant tissue health and aesthetics over time. For these situations, techniques such as bone and/or soft tissue grafting and immediate temporization must be considered during case analysis, preparation, and execution.^44,45 

A sagittal root position classification published in 2011 showed Class I sockets (root positioned against the buccal plate) to be the most common position for the maxillary central incisor site (86.5%). The authors suggested this was the most favorable scenario for an immediate implant.⁴⁶  The authors further found that when the root of a maxillary central incisor is in contact with both the buccal and the palatal cortical plates (Class IV socket) (8% of their sample), an immediate implant would be contraindicated. The results of the present study are in accordance with this; 22 cases (6.9%) of the 321 cases examined were Class IV sockets. The present authors found all 22 cases unsuitable for immediate implants.

This study has some limitations; therefore, the results must be interpreted with caution. A single implant type and diameter was used in all cases, and even though a tapered implant with a slim apex was used, other implant geometries or narrower diameters could offer different results. The 2-mm distance from the implant to the buccal plate was respected over the entire length of the implant position. The clinical need for this might not be as significant in the apical area, and virtual planning of the implant apexes closer to the buccal plate could change the results. The implants were placed equicrestal (implant platform is even with the crest of the bone) in relation to the buccal plate. Currently, some implant manufacturers advocate placing implants subcrestally; this could also impact the results but probably not significantly, as the implant angulation is not affected by placing an implant deeper. The virtual implants were placed by clinicians, introducing some subjectivity into the process, although the placement rules were clear and precise. Measurements were taken by the clinicians while placing the implants, and corrections were made accordingly, but a human error factor could be influencing the accuracy. A single sagittal view running through the center of the tooth was used for the virtual placement and measurements. Mesio-distal root angulation or distance between adjacent roots was not taken into consideration, which could contraindicate implants in narrow mesio-distal spaces or require smaller-diameter fixations. Patients with past orthodontic treatment were not excluded from the sample. The authors understand that orthodontics can change root angulation(s) and affect the results of this study. They also understand that past orthodontic treatment is a common factor in the patient population and therefore had to be incorporated into the sample. This article does not intend to prioritize the palatal emergence of an immediate implant over other important surgical and dental aspects. Clinically multiple factors must be considered individually and collectively before attempting immediate implant placement(s). These factors include bone density, periodontal health, adjacent teeth, medical condition, restorative space, occlusion, aesthetics, and so on.

Immediate implants in the maxillary central incisor should be approached with meticulous planning. The results of this study showed that 62.3% of the sockets were candidates for palatal emergence in preparation for screw-retained restorations. Furthermore, when aiming to not have an end prosthetic result with excessive palatal prosthetic volume due to placing the implant too palatally, the percentage of cases suitable for palatal emergence was reduced to 25.9%. Three-dimensional images and implant planning software are the methods of choice to assess implant sites. The restorative technique, whether screw or cement retained, should not be preestablished without a careful examination of cross-section images. The surgeon should determine where and how to place the implant for each patient without standardizing and adapt the angulation to the anatomy of each individual rather than to the operator's restorative preferences. The prosthetic emergence of an immediate implant is only 1 of the many surgical aspects that need to be considered to achieve predictable results and avoid pitfalls and complications.

CBCT: cone beam computed tomography

Special acknowledgments to Daniel Jones BMath for the statistical analysis.