Determination of Implant Position for Immediate Implant Placement in Maxillary Central Incisors Using Palatal Soft Tissue Landmarks Shin-Hye Chung, DDS, MSD1/Young-Seok Park, DDS, MSD, PhD2/ Seung-Hwan Chung, DDS, MS3/Won-Jun Shon, DDS, MSD, PhD4 Purpose: The aim of this study was to suggest a clinical guideline to properly position immediate implant placement in the anterior maxillary area. Materials and Methods: The computed tomography (CT) records of 250 patients (125 males, 125 females; mean age 49.5 years) were evaluated, and the relationship of the sagittal root positions of the maxillary central incisor teeth to their respective alveolar bony housing was classified as buccal (B), medial (M), or palatal (P). Then, the clinical guideline was suggested based on the implications derived from the classification. In the second investigation, the validity of the clinical guideline was confirmed by CT analysis. First, a reference line was defined using soft tissue landmarks. Then, a virtual rectangle representing the dental implant was created and merged on the CT image, symbolizing the space where the dental implant would be positioned. The rectangular frame was positioned on the image in accordance with the guideline. The frame on the image was thoroughly assessed to detect the possibility of perforation of the labial plate. The location of the initial drilling access on the respective crown was determined by extending the longitudinal axis of the frame in a coronal direction. Results: Of the 250 samples, 94.0% were classified as type B. Type M and P represented 5.6% and 0.4% of the samples, respectively. In 82.0% of the samples, the frame was safely positioned, not contacting the labial or palatal cortical plate in the alveolar bony housing. Moreover, when a frame representing a tapered implant form with an apical diameter of 3.5 mm was applied, 98.8% of samples were positioned safely in the alveolar bony housing. The longitudinal axis of the frame crossed the respective crown at the incisal tip in 54.4% of the cases. It crossed the labial side of the crown and cingulum of the crown in 42.0% and 3.6% of the cases, respectively. Conclusions: The root of the maxillary central incisor teeth tended to be positioning labially in the alveolar bony housing. The result of the CT analysis confirms the clinical guideline as a dependable clinical reference in immediate implant placement. Int J Oral Maxillofac Implants 2014;29:627–633. doi: 10.11607/jomi.2907 Key words: clinical guideline, computed tomography, immediate implant, maxillary central incisor, sagittal root positions
B
ecause immediate flapless implant placement has added advantages of a short healing period, pres-
1Graduate
Student, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. 2Prosthodontist, Assistant Professor, Department of Oral Anatomy, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. 3Clinical Assistant Professor, Department of Conservative Dentistry, Seoul National University, Seoul, Korea. 4 Associate Professor, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. Correspondence to: Dr Won-Jun Shon, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, 28, Yeongeon-dong, Jongno-gu, Seoul, 110-749, South Korea. Fax: +82-2-2072-3859. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
ervation of surrounding tissues, and reduced need for invasive treatments, it has gained popularity in recent years.1–3 Despite these advantages, however, immediate flapless implant placement in the maxillary anterior area has always been a challenge for clinicians because of its unpredictable outcome associated with implant position and soft tissue stability.4 Continuing resorption of the respective labial cortex has been reported as an unavoidable consequence after extraction.5 Related to these limitations, previous studies reported several anatomical considerations such as extraction socket type, critical gap distance, or the morphology of the root.4,6,7 In sagittal view, however, the thin labial cortex, extensive variation of vestibular depth, and labial positioning of the root in the alveolar bony housing remains to be considered. Maxillary central incisors have characteristic anatomy in relation to alveolar bony structure. Several researchers have approached the issue from the perThe International Journal of Oral & Maxillofacial Implants 627
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Chung et al
B
M
P
a
b
c
d
Figs 1a to 1d (a) Schematic diagram of sagittal relationship. An apical line parallel to the CEJ is divided into three parts: buccal (B), medial (M), and palatal (P). Red dotted line shows the root axis. (b) Example of the root axis toward the buccal aspect (type B). Buccal cortical lining is overlapped with buccal root surface and not clearly distinguishable. Abundant palatal bone is observed. (c) Example of the root axis toward the medial third (type M). (d) Example of the root axis toward the palate (type P) with relatively sufficient buccal bone.
spective of anatomy and physiology, suggesting several presurgical conditions of the anterior maxilla for a successful outcome.7,8 Recently, the relationship between the central incisor and the bony housing in the sagittal aspect was studied using cone beam computed tomography (CBCT).9,10 According to these image analyses, in most of the cases (78.8% to 81.1%) the roots of the maxillary central incisors were located buccally, and the buccal cortical plate was thin. Consistent with the above findings, it is reasonable to conclude that the buccolingual angulation of immediate implants in central incisors is parallel with the palatal bony wall, not the long axis of the extraction socket. To be more applicable in the clinical field, a guiding technique should adopt more stable landmarks with clear definition and fixed position. In the anterior palatal area, the volume or shape of the soft tissue, which is composed of keratinized gingiva, is capable of being maintained along with the contour of the palatal bone. This relatively stable soft tissue structure inside the oral cavity has been used as a landmark for various purposes.11 Therefore, this study aimed to establish an objective clinical guideline that would avoid unwanted complications with immediate implant placement by scrutinizing the sagittal relationship of the root of the maxillary central incisor. The clinical guideline was suggested on the basis of the data, with CT analysis as confirmation of its validity.
MATERIALS AND METHODS A total of 250 patients (125 men and 125 women) who visited the Seoul National University Dental Hospital from June 2006 to June 2011 and had un-
dergone CT scans of their maxillary anterior area were included in the study. The included population had both maxillary central incisors, and CT was taken for diagnostic purposes of another edentulous area. The age of the patients ranged between 20 and 65 years, with a mean age of 49.7 ± 10.2 years (men, 50.4 ± 8.9; women, 49.0 ± 11.1). Criteria for exclusion from the samples were moderate to severe periodontitis, severe crowding, alveolar bone deformities, root canal treatment, or extensive restorations and any pathosis that could lead to changes in the root and alveolar bone structure. The CT images were obtained using SOMATOM Sensation 10 (Siemens), and the measurements were done with PiViewSTAR (Infinitt). The slice interval was 2.00 mm, and the slice thickness was 1.00 mm in reconstructed sagittal images. The CT was set at 120 kVp and 100 mAs for a total of 750 milliseconds. The sagittal section that passes through the center of the mesiodistal dimension of the maxillary central incisor was used. This study was conducted with the approval of the Seoul National University Dental Hospital Ethical Research Committee.
Sagittal Relationship Between Root Axis and Alveolar Process in Maxillary Central Incisors
A line passing through the apex of the maxillary central incisor was created in parallel to the cementoenamel junction (CEJ). Then, the CEJ and the created line were divided into three segments of equal length (Fig 1a). The three parts of the alveolar housing were tagged as buccal (B), medial (M), and palatal (P). The relationship of the sagittal root positions of the maxillary central incisor teeth to their respective alveolar bony housing was categorized as type B, M, and P according to the position of the root (Figs 1b to 1d). The
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Chung et al
Figs 2a to 2d Implant guiding technique with landmarks. (a) Ia is at the posterior margin of the incisive papilla (I), and Ib is 10 mm posterior to Ia on the median palatine raphe. (b) Ia and Ib marked on the sagittal CT image. (c) RL transferred to the center of maxillary central incisor. (d) Virtual implant (5 × 10 mm) located within the alveolar bony housing.
I Ia
Ib
10mm Ib a
Ia b
RL c
implication that was derived from this study was that, in the sagittal view, the long axis of the implant fixture should be as parallel as possible to the contour of the palatal cortex.
Creating and Confirming a Clinical Guideline for Immediate Implant Placement
For precision and reproducibility of the guideline, a reference line (RL) was defined on the stone cast, and the term “contour of the palatal cortex” was revised as RL in the guideline. RL is a line that passes through the two soft tissue landmarks (Ia and Ib) that lie on the median palatal raphe. Point Ia is 1 mm posterior to the posterior margin of the incisive papilla. Point Ib is located 10 mm posterior to point Ia (Fig 2a). The reference line defined on the cast was also recognizable on the CT image in sagittal view (Fig 2b). The RL was transferred and overlapped with the maxillary central incisor tooth in parallel movement using the software (Fig 2c). Then, the 5 × 10–mm rectangular image was created to represent the largest possible dental implant diameter that can be used. Assumed as a real dental implant, the position of the rectangular image was tilted and adjusted within the alveolar bony housing to meet the following criteria (Fig 2d): • The rectangular frame should be parallel to RL as the guideline suggests. • The rectangular frame should be surrounded by sufficient bone to allow maximum bony support for the implant. • The apex of the rectangular frame should be in a safety zone to prevent perforation of the labial plate.
d
Location of Initial Drilling Access Point For the location of drilling access in the surgical guide, assuming that the osteotomy would be drilled straight along the midline of the predetermined position, a line was extended from the center of the rectangular frame (Fig 3). The location of initial drilling access was determined as the point where the extended median line coincided with the surface of the crown. Relative to the extended median line, the location of the initial drilling access points of samples were divided into three groups: labial, incisal, and cingulum (Fig 4).
Tentative Abutment Angle
The angle between the long axis of the implant and the crown was measured for the tentative abutment angle. Assuming the height of the abutment to be 3 mm, the abutment angle was measured to 3 mm from the occlusal plane. The angle from the central axis of the cylinder to the incisal edge was measured (Fig 5). The data were statistically analyzed using SPSS 12.0 (SPSS); statistical significance was determined at α = .05 using the Student t test. To test the reliability, ten CTs were randomly selected and measured again on separate days 2 months after the initial measurement.
RESULTS The intraexaminer reliability coefficients ranged from 0.939 to 0.977. In terms of root mean squares, the random errors of estimation were lower than 2.6 degrees The International Journal of Oral & Maxillofacial Implants 629
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Chung et al
Fig 3 Schematic diagram of the implant guiding technique in the sagittal plane. The initial drilling access was determined as the point at which an extension of the median line of the frame coincided with the surface of the crown.
in angular measurements, and there were no statistically significant differences between the test-retest measurements for any of the variables.
Sagittal Relationship Between Root Axis and Alveolar Bony Housing in Maxillary Central Incisors
In this analysis, a total of 127 right central incisors and 123 left central incisors were used. The relationship between the root and the alveolar bone is shown in Table 1. The result showed no statistical difference between the sexes (P > .05).
Determination of Implant Position
In merging the rectangular frame (5 × 10 mm) on the CT image, 82.0% of the 250 images showed no sign of contact between the frame and the respective alveolar cortex or incisive canal. The same analysis was performed with an altered shape of frame. A frame with a tapered body form and an apical diameter of 3.5 mm showed an increased success rate (98.8%). The incisive canal was observed in 41.6% of cross-sectional images.
a
Location of Initial Drilling Access Point
The locations of the drilling access points on the surgical guide, as determined on the CT images, are shown in Table 2. The extension of the rectangular axis met the crown on its labial surface, incisal edge, and cingulum in 42.0%, 54.0%, and 3.6% of cases, respectively.
b
Tentative Abutment Angle
The tentative abutment angle showed an average of 24.8 ± 11.1 degrees (men, 23.3 ± 10.4 degrees; women, 26.4 ± 11.5 degrees) with a statistically significant difference between the sexes (P = .02) (Table 3).
Clinical Case c Figs 4a to 4c Schematic diagrams of the location of initial drilling point and CT scans of representative cases. The access hole may be located on the (a) labial aspect, (b) incisal aspect, or (c) cingulum of the crown.
Abutment angle Fig 5 Schematic diagram of the abutment angle. The abutment height was assumed to be 3 mm.
A 36-year-old female patient presented with a fractured maxillary right central incisor root (Fig 6a). Radiographic and clinical findings showed a midroot fracture without signs of active infection and intact adjacent teeth (Fig 6b). Because the fractured tooth could not be salvaged, immediate dental implant placement was recommended. The underlying alveolar crest was 3 mm from the free gingival margin of the fractured tooth at the midbuccal position. According to the suggested clinical guideline, a surgical guide was fabricated, and a drilling hole was made on it using a surveyor (Fig 6c). The implant (NobelReplace Tapered Groovy, Nobel Biocare) was placed with allogenic bone graft material (Bio-Oss, Geistlich) on the buccal side of the extraction socket after tooth extraction. The final impression was made approximately 4 months after the surgery. A titanium computer-aided design/computer-
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Chung et al
Fig 6 (a) Preoperative photograph of the maxillary right central incisor. (b) Radiograph of the failing right central incisor with midroot fracture. (c) The drilling hole was made on the surgical guide following the technique described on the surveyor. (d) Facial view of the definitive restoration. Comparison of the (e) preoperative and (f) postoperative CT scans.
a
b
d
e
assisted manufacture customized abutment (Myplant, Raphabio) was torqued to 35 Ncm, and the definitive restoration was cemented (Fig 6d). Preoperative (Fig 6e) and postoperative (Fig 6f ) CT scans were analyzed to evaluate the implant.
DISCUSSION By experience, rather than by any objective statistics, many clinicians have advocated palatal engagement of implants in the anterior maxilla. The results of this study also partially support this clinical trend. In 94% of the 250 Korean subjects in this study, the root of the maxillary central incisor was located labially in the alveolar bony housing. The term palatal engagement, however, does not provide the clinician with a definitive guideline to follow. Without tangible instruction with visible anatomical references, it is difficult to determine the angulation of the handpiece and the location of the initial drilling access. In addition, palatal engagement does not work in all cases. Although it was rare (6.0% of the participants), the root sometimes was positioned in the middle or even the palatal aspect of the alveolar bony housing. In the present study, the relationship between the maxillary central incisor and the alveolar bone in the sagittal plane was determined. The root axis of the maxillary central incisor was located at the buccal third (type B) in 94.0% of the cases. This result is in line with studies by Kan et al9 and Lau et al,10 which described that the majority of the midroot surface was positioned at the buccal side in relation to the alveolar bone. Several studies reported that roots inclined toward the buccal surface tended to have a thin labial wall.7,12,13 To overcome unfavorable inclinations of the extraction socket wall in immediate implant placement, several
c
f
Table 1 Sagittal Relationship Between the Root and the Alveolar Bony Housing in Maxillary Central Incisors by Sex* Type B
Type M No.
Type P
Sex
No.
%
%
No.
%
Men
120
96.0
4
3.2
1
0.8
Women
115
92.0
10
8.0
0
0
Total
235
94.0
14
5.6
1
0.4
*No statistical difference between the sexes.
Table 2 Distribution of the Location of Drilling Access by Sex* Sex
Labial (%)
Incisal (%)
Cingulum (%)
Men
36.0
60.0
4.0
Women
48.0
48.8
3.2
Total
42.0
54.4
3.6
*No statistical difference between the sexes.
Table 3 Measurement Results of Tentative Abutment Angle by Sex* Men (degrees) Mean ± SD
23.3 ± 10.4
Women (degrees) 26.4 ± 11.5
Maximum
55.9
62.1
Minimum
1.4
3.1
*Statistically significant difference between the sexes (P = .02).
techniques for palatal shifting of the drilling path have been suggested.14,15 Still, there are no documented studies about the extent of palatal shifting. According to the result of this study, the percentage of root axes inclined medially was 5.6% and palaThe International Journal of Oral & Maxillofacial Implants 631
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Chung et al
tally was 0.4% of the cases. The widely recommended direction for anterior immediate implant placement is palatal to the extracted root axis to engage more native bone. If this same rule is applied to these cases without considering the alveolar bony relationship with the root axis, it may lead to palatal bony perforation. The possible landmarks for implant placement in this study are the incisive papilla and median palatine raphe. The landmarks of the maxilla can be roughly divided into hard and soft tissue landmarks. Since it is covered with soft tissue, hard tissue structures cannot be identified unless special methods are employed, such as radiographic images. The structural stability of the anterior palatal structure after the growth and developmental stage has been reported.11 Also, the incisive papilla is a frequently used reference in positioning the central incisor tip and analyzing teeth.16,17 For added reliability, the suggested landmarks are located in an area that is easily visible during surgery. The reference line on the palate (RL) was established by connecting the most anterior and posterior points of the median palatine raphe. With a uniform soft tissue thickness in the median palatine suture, it is considered to be a reflection of the cortical bone of the palate. The main idea of the technique is to suggest a stable reference for placing immediate implants in maxillary central incisor sites. The virtual RL is only a substitute for the palatal alveolar bony angle of the palatal cortical lining, which is an actual reference, and therefore care must be taken to avoid potential errors. For example, inappropriate determination of point Ib can tilt the reference line slightly from the true RL. However, such clinical mistakes rarely result in a drastic change of the reference line. In the pilot study performed prior to this research, sharp inflection of the palatal curve was not found along the 10 mm interval from Ia to Ib. The greatest advantage of RL is its reproducibility in the oral cavity. When needed, the operator is able to decide the position of the implant in consideration of RL during treatment planning with CT. Also, the direction of the implant is confirmed and reestablished inside the oral cavity anytime during the surgery using RL. A computer-generated prefabricated surgical guide can also be used to aid in the surgical procedures and reduce technique sensitivity with high accuracy.18 However, a computer-generated surgical guide requires extra time and cost and still does not provide the optimal position of implant placement when the failing tooth remains in the arch. To evaluate the availability of the locating technique, a 5 × 10–mm rectangular frame was chosen for the virtual dental implant. The basis for this figure is the shoulder diameter of less than 5 mm required for maxillary incisors to maintain a safe distance from adja-
cent anatomical structures4 as well as longitudinal factors associated with the initial stability of the implant. When the technique was employed, the virtual dental implant was confined to the alveolar bony housing in 82% of cases, and the perforation of either the incisive canal or buccal/palatal cortical plate occurred in 18% of cases. In the clinical situation, the invasion of both structures is less common since an implant with a tapered body and an apical diameter less than 3.5 mm is commonly used in the esthetic zone.10 When assuming that a tapered implant with an apical diameter of 3.5 mm is used, 98.8% of the virtual implants could be placed in the bony housing without damaging adjacent structures. During treatment planning, the position of the implant is planned three-dimensionally, and the surgical guide is used to transfer the planned implant position to the extraction socket directly. It has an access hole to introduce the drill into the socket. The junction of the central axis of the implant and the labial surface of the crown can be assumed as the initial drilling access point of the surgical guide. The location of the initial drilling access point on the crown surface was divided into three groups in this study. The access hole was positioned on the labial surface in 42% of cases, leading to the use of an angled abutment and cement-retained restoration. The use of an angled abutment has been readily accepted as the need to produce functional and esthetic restorations has become more critical.19 It was reported that removal torque values after dynamic cyclic loading did not vary significantly between straight and angled abutments.20 The tentative abutment angles using the suggested technique showed an average of 24.8 degrees. The angled abutments up to 45 degrees did not compromise the long-term survival of implants. Abutments with an angle of 0 to 60 degrees1 are offered by some manufacturers.19 In the clinical study, there seemed to be no significant difference in function and prognosis when using an angled rather than a straight abutment.20 Still, more research is required on the stress distribution that occurs inside the alveolar bone when an angled abutment is used with a maximum amount of labial bony support in the anterior maxilla.
CONCLUSION The roots of the maxillary central incisor teeth tended to be positioned buccally in the alveolar bony housing. The proposed clinical guideline provides an objective reference to determine the proper location for implant placement in the anterior esthetic zone in order to achieve maximum bony support and favorable esthetic outcome.
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ACKNOWLEDGMENTs The authors reported no conflicts of interest related to this study.
REFERENCES 1. De Rouck T, Collys K, Cosyn J. Immediate single-tooth implants in the anterior maxilla: A 1-year case cohort study on hard and soft tissue response. J Clin Periodontol 2008;35:649–657. 2. Kan JY, Rungcharassaeng K, Lozada J. Immediate placement and provisionalization of maxillary anterior single implants: 1-year prospective study. Int J Oral Maxillofac Implants 2003;18:31–39. 3. Wohrle PS. Single-tooth replacement in the aesthetic zone with immediate provisionalization: Fourteen consecutive case reports. Pract Periodontics Aesthet Dent 1998;10:1107–1114; quiz 1116. 4. Buser D, Martin W, Belser UC. Optimizing esthetics for implant restorations in the anterior maxilla: Anatomic and surgical considerations. Int J Oral Maxillofac Implants 2004;19(Suppl):43–61. 5. Araujo MG, Lindhe J. Ridge alterations following tooth extraction with and without flap elevation: An experimental study in the dog. Clin Oral Implants Res 2009;20:545–549. 6. Garber DA, Salama MA, Salama H. Immediate total tooth replacement. Compend Contin Educ Dent 2001;22:210–216, 218. 7. Januario AL, Duarte WR, Barriviera M, Mesti JC, Araujo MG, Lindhe J. Dimension of the facial bone wall in the anterior maxilla: A cone-beam computed tomography study. Clin Oral Implants Res 2011;22:1168–1171. 8. Lee SL, Kim HJ, Son MK, Chung CH. Anthropometric analysis of maxillary anterior buccal bone of Korean adults using cone-beam CT. J Adv Prosthodont 2010;2:92–96. 9. Kan JY, Roe P, Rungcharassaeng K, et al. Classification of sagittal root position in relation to the anterior maxillary osseous housing for immediate implant placement: A cone beam computed tomography study. Int J Oral Maxillofac Implants 2011;26:873–876.
10. Lau SL, Chow J, Li W, Chow LK. Classification of maxillary central incisors-implications for immediate implant in the esthetic zone. J Oral Maxillofac Surg 2011;69:142–153. 11. Patil MS, Patil SB, Acharya AB. Palatine rugae and their significance in clinical dentistry: A review of the literature. J Am Dent Assoc 2008;139:1471–1478. 12. Braut V, Bornstein MM, Belser U, Buser D. Thickness of the anterior maxillary facial bone wall—A retrospective radiographic study using cone beam computed tomography. Int J Periodontics Restorative Dent 2011;31:125–131. 13. Ghassemian M, Nowzari H, Lajolo C, Verdugo F, Pirronti T, D’Addona A. The thickness of facial alveolar bone overlying healthy maxillary anterior teeth. J Periodontol 2012;83:187–197. 14. Hwang KG, Park CJ. Ideal implant positioning in an anterior maxillary extraction socket by creating an apico-palatal guiding slot: A technical note. Int J Oral Maxillofac Implants 2008;23:121–122. 15. Koyanagi K. Development and clinical application of a surgical guide for optimal implant placement. J Prosthet Dent 2002;88:548– 552. 16. Lau GC, Clark RF. The relationship of the incisive papilla to the maxillary central incisors and canine teeth in southern Chinese. J Prosthet Dent 1993;70:86–93. 17. Varjao FM, Nogueira SS, Filho JN. The center of the incisive papilla for the selection of complete denture maxillary anterior teeth in 4 racial groups. Quintessence Int 2008;39:841–845. 18. Van Assche N, van Steenberghe D, Quirynen M, Jacobs R. Accuracy assessment of computer-assisted flapless implant placement in partial edentulism. J Clin Periodontol 2010;37:398–403. 19. Sethi A, Kaus T, Sochor P, Axmann-Krcmar D, Chanavaz M. Evolution of the concept of angulated abutments in implant dentistry: 14year clinical data. Implant Dent 2002;11:41–51. 20. Ha CY, Lim YJ, Kim MJ, Choi JH. The influence of abutment angulation on screw loosening of implants in the anterior maxilla. Int J Oral Maxillofac Implants 2011;26:45–55. 21. Eger DE, Gunsolley JC, Feldman S. Comparison of angled and standard abutments and their effect on clinical outcomes: A preliminary report. Int J Oral Maxillofac Implants 2000;15:819–823.
The International Journal of Oral & Maxillofacial Implants 633 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
B
ecause immediate flapless implant placement has added advantages of a short healing period, pres-
1Graduate
Student, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. 2Prosthodontist, Assistant Professor, Department of Oral Anatomy, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. 3Clinical Assistant Professor, Department of Conservative Dentistry, Seoul National University, Seoul, Korea. 4 Associate Professor, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, Seoul, Korea. Correspondence to: Dr Won-Jun Shon, Department of Conservative Dentistry, Dental Research Institute and School of Dentistry, Seoul National University, 28, Yeongeon-dong, Jongno-gu, Seoul, 110-749, South Korea. Fax: +82-2-2072-3859. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
ervation of surrounding tissues, and reduced need for invasive treatments, it has gained popularity in recent years.1–3 Despite these advantages, however, immediate flapless implant placement in the maxillary anterior area has always been a challenge for clinicians because of its unpredictable outcome associated with implant position and soft tissue stability.4 Continuing resorption of the respective labial cortex has been reported as an unavoidable consequence after extraction.5 Related to these limitations, previous studies reported several anatomical considerations such as extraction socket type, critical gap distance, or the morphology of the root.4,6,7 In sagittal view, however, the thin labial cortex, extensive variation of vestibular depth, and labial positioning of the root in the alveolar bony housing remains to be considered. Maxillary central incisors have characteristic anatomy in relation to alveolar bony structure. Several researchers have approached the issue from the perThe International Journal of Oral & Maxillofacial Implants 627
© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
Chung et al
B
M
P
a
b
c
d
Figs 1a to 1d (a) Schematic diagram of sagittal relationship. An apical line parallel to the CEJ is divided into three parts: buccal (B), medial (M), and palatal (P). Red dotted line shows the root axis. (b) Example of the root axis toward the buccal aspect (type B). Buccal cortical lining is overlapped with buccal root surface and not clearly distinguishable. Abundant palatal bone is observed. (c) Example of the root axis toward the medial third (type M). (d) Example of the root axis toward the palate (type P) with relatively sufficient buccal bone.
spective of anatomy and physiology, suggesting several presurgical conditions of the anterior maxilla for a successful outcome.7,8 Recently, the relationship between the central incisor and the bony housing in the sagittal aspect was studied using cone beam computed tomography (CBCT).9,10 According to these image analyses, in most of the cases (78.8% to 81.1%) the roots of the maxillary central incisors were located buccally, and the buccal cortical plate was thin. Consistent with the above findings, it is reasonable to conclude that the buccolingual angulation of immediate implants in central incisors is parallel with the palatal bony wall, not the long axis of the extraction socket. To be more applicable in the clinical field, a guiding technique should adopt more stable landmarks with clear definition and fixed position. In the anterior palatal area, the volume or shape of the soft tissue, which is composed of keratinized gingiva, is capable of being maintained along with the contour of the palatal bone. This relatively stable soft tissue structure inside the oral cavity has been used as a landmark for various purposes.11 Therefore, this study aimed to establish an objective clinical guideline that would avoid unwanted complications with immediate implant placement by scrutinizing the sagittal relationship of the root of the maxillary central incisor. The clinical guideline was suggested on the basis of the data, with CT analysis as confirmation of its validity.
MATERIALS AND METHODS A total of 250 patients (125 men and 125 women) who visited the Seoul National University Dental Hospital from June 2006 to June 2011 and had un-
dergone CT scans of their maxillary anterior area were included in the study. The included population had both maxillary central incisors, and CT was taken for diagnostic purposes of another edentulous area. The age of the patients ranged between 20 and 65 years, with a mean age of 49.7 ± 10.2 years (men, 50.4 ± 8.9; women, 49.0 ± 11.1). Criteria for exclusion from the samples were moderate to severe periodontitis, severe crowding, alveolar bone deformities, root canal treatment, or extensive restorations and any pathosis that could lead to changes in the root and alveolar bone structure. The CT images were obtained using SOMATOM Sensation 10 (Siemens), and the measurements were done with PiViewSTAR (Infinitt). The slice interval was 2.00 mm, and the slice thickness was 1.00 mm in reconstructed sagittal images. The CT was set at 120 kVp and 100 mAs for a total of 750 milliseconds. The sagittal section that passes through the center of the mesiodistal dimension of the maxillary central incisor was used. This study was conducted with the approval of the Seoul National University Dental Hospital Ethical Research Committee.
Sagittal Relationship Between Root Axis and Alveolar Process in Maxillary Central Incisors
A line passing through the apex of the maxillary central incisor was created in parallel to the cementoenamel junction (CEJ). Then, the CEJ and the created line were divided into three segments of equal length (Fig 1a). The three parts of the alveolar housing were tagged as buccal (B), medial (M), and palatal (P). The relationship of the sagittal root positions of the maxillary central incisor teeth to their respective alveolar bony housing was categorized as type B, M, and P according to the position of the root (Figs 1b to 1d). The
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Figs 2a to 2d Implant guiding technique with landmarks. (a) Ia is at the posterior margin of the incisive papilla (I), and Ib is 10 mm posterior to Ia on the median palatine raphe. (b) Ia and Ib marked on the sagittal CT image. (c) RL transferred to the center of maxillary central incisor. (d) Virtual implant (5 × 10 mm) located within the alveolar bony housing.
I Ia
Ib
10mm Ib a
Ia b
RL c
implication that was derived from this study was that, in the sagittal view, the long axis of the implant fixture should be as parallel as possible to the contour of the palatal cortex.
Creating and Confirming a Clinical Guideline for Immediate Implant Placement
For precision and reproducibility of the guideline, a reference line (RL) was defined on the stone cast, and the term “contour of the palatal cortex” was revised as RL in the guideline. RL is a line that passes through the two soft tissue landmarks (Ia and Ib) that lie on the median palatal raphe. Point Ia is 1 mm posterior to the posterior margin of the incisive papilla. Point Ib is located 10 mm posterior to point Ia (Fig 2a). The reference line defined on the cast was also recognizable on the CT image in sagittal view (Fig 2b). The RL was transferred and overlapped with the maxillary central incisor tooth in parallel movement using the software (Fig 2c). Then, the 5 × 10–mm rectangular image was created to represent the largest possible dental implant diameter that can be used. Assumed as a real dental implant, the position of the rectangular image was tilted and adjusted within the alveolar bony housing to meet the following criteria (Fig 2d): • The rectangular frame should be parallel to RL as the guideline suggests. • The rectangular frame should be surrounded by sufficient bone to allow maximum bony support for the implant. • The apex of the rectangular frame should be in a safety zone to prevent perforation of the labial plate.
d
Location of Initial Drilling Access Point For the location of drilling access in the surgical guide, assuming that the osteotomy would be drilled straight along the midline of the predetermined position, a line was extended from the center of the rectangular frame (Fig 3). The location of initial drilling access was determined as the point where the extended median line coincided with the surface of the crown. Relative to the extended median line, the location of the initial drilling access points of samples were divided into three groups: labial, incisal, and cingulum (Fig 4).
Tentative Abutment Angle
The angle between the long axis of the implant and the crown was measured for the tentative abutment angle. Assuming the height of the abutment to be 3 mm, the abutment angle was measured to 3 mm from the occlusal plane. The angle from the central axis of the cylinder to the incisal edge was measured (Fig 5). The data were statistically analyzed using SPSS 12.0 (SPSS); statistical significance was determined at α = .05 using the Student t test. To test the reliability, ten CTs were randomly selected and measured again on separate days 2 months after the initial measurement.
RESULTS The intraexaminer reliability coefficients ranged from 0.939 to 0.977. In terms of root mean squares, the random errors of estimation were lower than 2.6 degrees The International Journal of Oral & Maxillofacial Implants 629
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Fig 3 Schematic diagram of the implant guiding technique in the sagittal plane. The initial drilling access was determined as the point at which an extension of the median line of the frame coincided with the surface of the crown.
in angular measurements, and there were no statistically significant differences between the test-retest measurements for any of the variables.
Sagittal Relationship Between Root Axis and Alveolar Bony Housing in Maxillary Central Incisors
In this analysis, a total of 127 right central incisors and 123 left central incisors were used. The relationship between the root and the alveolar bone is shown in Table 1. The result showed no statistical difference between the sexes (P > .05).
Determination of Implant Position
In merging the rectangular frame (5 × 10 mm) on the CT image, 82.0% of the 250 images showed no sign of contact between the frame and the respective alveolar cortex or incisive canal. The same analysis was performed with an altered shape of frame. A frame with a tapered body form and an apical diameter of 3.5 mm showed an increased success rate (98.8%). The incisive canal was observed in 41.6% of cross-sectional images.
a
Location of Initial Drilling Access Point
The locations of the drilling access points on the surgical guide, as determined on the CT images, are shown in Table 2. The extension of the rectangular axis met the crown on its labial surface, incisal edge, and cingulum in 42.0%, 54.0%, and 3.6% of cases, respectively.
b
Tentative Abutment Angle
The tentative abutment angle showed an average of 24.8 ± 11.1 degrees (men, 23.3 ± 10.4 degrees; women, 26.4 ± 11.5 degrees) with a statistically significant difference between the sexes (P = .02) (Table 3).
Clinical Case c Figs 4a to 4c Schematic diagrams of the location of initial drilling point and CT scans of representative cases. The access hole may be located on the (a) labial aspect, (b) incisal aspect, or (c) cingulum of the crown.
Abutment angle Fig 5 Schematic diagram of the abutment angle. The abutment height was assumed to be 3 mm.
A 36-year-old female patient presented with a fractured maxillary right central incisor root (Fig 6a). Radiographic and clinical findings showed a midroot fracture without signs of active infection and intact adjacent teeth (Fig 6b). Because the fractured tooth could not be salvaged, immediate dental implant placement was recommended. The underlying alveolar crest was 3 mm from the free gingival margin of the fractured tooth at the midbuccal position. According to the suggested clinical guideline, a surgical guide was fabricated, and a drilling hole was made on it using a surveyor (Fig 6c). The implant (NobelReplace Tapered Groovy, Nobel Biocare) was placed with allogenic bone graft material (Bio-Oss, Geistlich) on the buccal side of the extraction socket after tooth extraction. The final impression was made approximately 4 months after the surgery. A titanium computer-aided design/computer-
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Fig 6 (a) Preoperative photograph of the maxillary right central incisor. (b) Radiograph of the failing right central incisor with midroot fracture. (c) The drilling hole was made on the surgical guide following the technique described on the surveyor. (d) Facial view of the definitive restoration. Comparison of the (e) preoperative and (f) postoperative CT scans.
a
b
d
e
assisted manufacture customized abutment (Myplant, Raphabio) was torqued to 35 Ncm, and the definitive restoration was cemented (Fig 6d). Preoperative (Fig 6e) and postoperative (Fig 6f ) CT scans were analyzed to evaluate the implant.
DISCUSSION By experience, rather than by any objective statistics, many clinicians have advocated palatal engagement of implants in the anterior maxilla. The results of this study also partially support this clinical trend. In 94% of the 250 Korean subjects in this study, the root of the maxillary central incisor was located labially in the alveolar bony housing. The term palatal engagement, however, does not provide the clinician with a definitive guideline to follow. Without tangible instruction with visible anatomical references, it is difficult to determine the angulation of the handpiece and the location of the initial drilling access. In addition, palatal engagement does not work in all cases. Although it was rare (6.0% of the participants), the root sometimes was positioned in the middle or even the palatal aspect of the alveolar bony housing. In the present study, the relationship between the maxillary central incisor and the alveolar bone in the sagittal plane was determined. The root axis of the maxillary central incisor was located at the buccal third (type B) in 94.0% of the cases. This result is in line with studies by Kan et al9 and Lau et al,10 which described that the majority of the midroot surface was positioned at the buccal side in relation to the alveolar bone. Several studies reported that roots inclined toward the buccal surface tended to have a thin labial wall.7,12,13 To overcome unfavorable inclinations of the extraction socket wall in immediate implant placement, several
c
f
Table 1 Sagittal Relationship Between the Root and the Alveolar Bony Housing in Maxillary Central Incisors by Sex* Type B
Type M No.
Type P
Sex
No.
%
%
No.
%
Men
120
96.0
4
3.2
1
0.8
Women
115
92.0
10
8.0
0
0
Total
235
94.0
14
5.6
1
0.4
*No statistical difference between the sexes.
Table 2 Distribution of the Location of Drilling Access by Sex* Sex
Labial (%)
Incisal (%)
Cingulum (%)
Men
36.0
60.0
4.0
Women
48.0
48.8
3.2
Total
42.0
54.4
3.6
*No statistical difference between the sexes.
Table 3 Measurement Results of Tentative Abutment Angle by Sex* Men (degrees) Mean ± SD
23.3 ± 10.4
Women (degrees) 26.4 ± 11.5
Maximum
55.9
62.1
Minimum
1.4
3.1
*Statistically significant difference between the sexes (P = .02).
techniques for palatal shifting of the drilling path have been suggested.14,15 Still, there are no documented studies about the extent of palatal shifting. According to the result of this study, the percentage of root axes inclined medially was 5.6% and palaThe International Journal of Oral & Maxillofacial Implants 631
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Chung et al
tally was 0.4% of the cases. The widely recommended direction for anterior immediate implant placement is palatal to the extracted root axis to engage more native bone. If this same rule is applied to these cases without considering the alveolar bony relationship with the root axis, it may lead to palatal bony perforation. The possible landmarks for implant placement in this study are the incisive papilla and median palatine raphe. The landmarks of the maxilla can be roughly divided into hard and soft tissue landmarks. Since it is covered with soft tissue, hard tissue structures cannot be identified unless special methods are employed, such as radiographic images. The structural stability of the anterior palatal structure after the growth and developmental stage has been reported.11 Also, the incisive papilla is a frequently used reference in positioning the central incisor tip and analyzing teeth.16,17 For added reliability, the suggested landmarks are located in an area that is easily visible during surgery. The reference line on the palate (RL) was established by connecting the most anterior and posterior points of the median palatine raphe. With a uniform soft tissue thickness in the median palatine suture, it is considered to be a reflection of the cortical bone of the palate. The main idea of the technique is to suggest a stable reference for placing immediate implants in maxillary central incisor sites. The virtual RL is only a substitute for the palatal alveolar bony angle of the palatal cortical lining, which is an actual reference, and therefore care must be taken to avoid potential errors. For example, inappropriate determination of point Ib can tilt the reference line slightly from the true RL. However, such clinical mistakes rarely result in a drastic change of the reference line. In the pilot study performed prior to this research, sharp inflection of the palatal curve was not found along the 10 mm interval from Ia to Ib. The greatest advantage of RL is its reproducibility in the oral cavity. When needed, the operator is able to decide the position of the implant in consideration of RL during treatment planning with CT. Also, the direction of the implant is confirmed and reestablished inside the oral cavity anytime during the surgery using RL. A computer-generated prefabricated surgical guide can also be used to aid in the surgical procedures and reduce technique sensitivity with high accuracy.18 However, a computer-generated surgical guide requires extra time and cost and still does not provide the optimal position of implant placement when the failing tooth remains in the arch. To evaluate the availability of the locating technique, a 5 × 10–mm rectangular frame was chosen for the virtual dental implant. The basis for this figure is the shoulder diameter of less than 5 mm required for maxillary incisors to maintain a safe distance from adja-
cent anatomical structures4 as well as longitudinal factors associated with the initial stability of the implant. When the technique was employed, the virtual dental implant was confined to the alveolar bony housing in 82% of cases, and the perforation of either the incisive canal or buccal/palatal cortical plate occurred in 18% of cases. In the clinical situation, the invasion of both structures is less common since an implant with a tapered body and an apical diameter less than 3.5 mm is commonly used in the esthetic zone.10 When assuming that a tapered implant with an apical diameter of 3.5 mm is used, 98.8% of the virtual implants could be placed in the bony housing without damaging adjacent structures. During treatment planning, the position of the implant is planned three-dimensionally, and the surgical guide is used to transfer the planned implant position to the extraction socket directly. It has an access hole to introduce the drill into the socket. The junction of the central axis of the implant and the labial surface of the crown can be assumed as the initial drilling access point of the surgical guide. The location of the initial drilling access point on the crown surface was divided into three groups in this study. The access hole was positioned on the labial surface in 42% of cases, leading to the use of an angled abutment and cement-retained restoration. The use of an angled abutment has been readily accepted as the need to produce functional and esthetic restorations has become more critical.19 It was reported that removal torque values after dynamic cyclic loading did not vary significantly between straight and angled abutments.20 The tentative abutment angles using the suggested technique showed an average of 24.8 degrees. The angled abutments up to 45 degrees did not compromise the long-term survival of implants. Abutments with an angle of 0 to 60 degrees1 are offered by some manufacturers.19 In the clinical study, there seemed to be no significant difference in function and prognosis when using an angled rather than a straight abutment.20 Still, more research is required on the stress distribution that occurs inside the alveolar bone when an angled abutment is used with a maximum amount of labial bony support in the anterior maxilla.
CONCLUSION The roots of the maxillary central incisor teeth tended to be positioned buccally in the alveolar bony housing. The proposed clinical guideline provides an objective reference to determine the proper location for implant placement in the anterior esthetic zone in order to achieve maximum bony support and favorable esthetic outcome.
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ACKNOWLEDGMENTs The authors reported no conflicts of interest related to this study.
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