Implant-Guided Volumetric Analysis of Edentulous Maxillary Bone With Cone-Beam Computerized Tomography Scan. Maxillary Sinus Pneumatization Classification

Since the introduction of dental implant technology, implant reconstruction in partially and completely edentulous cases has shown to be a successful treatment modality. Despite continuous refinement of surgical and restorative techniques over the years, full-arch implant treatment can still be challenging because of alveolar bone atrophy associated with tooth loss. When endosseous root-form implants are considered for an edentulous maxilla, these clinical challenges are often related to a dual resorption of maxillary alveolar bone in the superior-posterior direction (tooth loss-related nonuse atrophy, according to the Wolff's law^1,2 ) and due to the pneumatization of the maxillary sinuses³  in the anterior-inferior direction. Extensive bone resorption and sinus pneumatization (SP) in many maxillary edentulous cases often preclude insertion of implants without additional surgical procedures, such as sinus lifts and bone grafts. Examples of maxillary full-arch implant treatment without the need for sinus lifts and bone grafts include zygomatic implants,^4,5  pterygomaxillary implants with zygomatic and conventional implants,⁶  the Marius bridge for the 6-implant rehabilitation of the resorbed edentulous maxilla,⁷  “All-on-4” technique with 4 immediately loaded implants,⁸  “13-23-30” anatomic maxillary technique of an implant distribution along the alveolar arch,⁹  the V-II-V technique of immediate placement and loading of implants into an edentulous maxilla without bone grafting,¹⁰  treatment of atrophic maxilla with short implants using an osteotome procedure,¹¹  optimal use of anatomic features of the maxillary arch with tilted implants,^12,13  and others.

An attempt to measure the volume of the maxilla or mandible with a high degree of accuracy has been performed previously for different purposes, including the diagnosis and treatment with dental implants.^14–18  The objective of this study was to measure the volume of edentulous maxillary bone in human subjects with cone-beam computerized tomography (CBCT) and attempt to determine if it might be sufficient for placement of at least 4 implants for a full-arch restoration without the need for additional bone grafts or sinus lifts. To make an accurate assessment, only the intersinal region of the anterior maxilla commonly used for maxillary implant reconstruction (implant zone) was carefully selected using designated clinical criteria (see below) and then measured on a CBCT scan. This examination was then followed by the radiographic assessment of the degree of SP and comparison of SP with mean maxillary bone volume (MMBV) in studied subjects.

This was a retrospective analysis of randomly selected maxillary edentulous subjects presented for dental treatment to the University of the Pacific (UOP), Arthur A. Dugoni School of Dentistry, in San Francisco, California, and referred to the Radiology Department for a CBCT radiographic evaluation between 2008 and 2010. To be included in this study, subjects had to be ≥18 years of age, completely maxillary edentulous, and without evidence of recent teeth extractions, pathological jaw lesions, dental combination syndrome, or other jaw conditions. Patients in the study could have been edentulous, or had partial or full mandibular dentition.

The first 30 subjects who met the inclusion criteria were analyzed. There were 9 men and 21 women with a mean age of 67.3 years (range, 41 to 92 years). The subjects were stratified by age in decades for statistical analysis. There were 2 (6.7%) subjects in the fourth decade (ie, 40–49 years), 7 (23.3%) in the fifth decade, 7 (23.3%) in the sixth decade, 9 (30%) in the seventh decade, 4 (13.3%) in the eighth decade, and 1 (3.3%) in the ninth decade (ie, 90–99 years).

This retrospective study of CBCT scans was exempt from institutional review board approval, and we followed the guidelines of the Helsinki Declaration.

Images were obtained at UOP using an i-CAT CBCT scanner (Imaging Sciences International, Hatfield, Penn). Subjects were seated upright with an x-ray tube and image screen rotating around the head. The images were reconstructed and analyzed using volumetric imaging software InVivoDental5.0 (Anatomage, San Jose, Calif) and saved in the Digital Imaging and Communications in Medicine (DICOM) format. The CBCT panoramic and cross-sectional (coronal) views (screens), derived from the original scans with the contrast adjusted for maximum viewing clarity, were analyzed.

The evaluation of maxillary bone volume was based on the measurement of the edentulous maxillary alveolar bone area (S) in each coronal slice using the “area tool” provided by the InVivoDental5.0 software. The dots were positioned along the cortical outline of the selected coronal slice, and when the perimeter markings were completed with the last dot, the software automatically calculated the outlined area (Sx). Multiple areas were measured this way at 2-mm intervals (2-mm interval between coronal slices was selected as a minimum measured distance that would not cause discernible changes of volume within it; Figures 1–3). To determine the bone quantity or volume, all measured areas were added together (S1 + S2 + S3, etc) and multiplied by 2 (interslice interval or distance; Table 1).

Not all available maxillary bone observed on CBCT scans was measured. Because of the clinical relevance (root-form endosseous implant placement) of this study, the sensible measurement envelope (envelope of bone) had to be determined to accommodate at least 4 dental implants inserted in a typical fashion between both maxillary sinuses. The following measurement borders were established on CBCT images:

• 1. Distal border:

• a.

  If maxillary sinuses were significantly enlarged, the distal extension of maxillary measurements was set at a 7-mm vertical distance (the practical smallest length of a root-form implant without need for a sinus lift procedure) on the panoramic image of the CBCT scan. When this vertical height was met, the line was drawn with a line tool and became the far posterior extension (distal border) of measurement. This was calculated as the shortest distance from the alveolar crest (the most convex area) to the maxillary sinus (the most inferior extension at that particular coronal plane). The measurements were started from the very right maxillary distal border and progressed to the midline (ML) and then to the very left border in 2-mm increments using the area tool (Figures 1–3; Table 1).

• b.

  If maxillary sinuses were small and positioned high above the alveolar crest, the anatomically determined first molar region was considered as the most distal extension of measurement (first molar implant occlusion is commonly used in full-arch implant reconstructions). The position of the first molar (its most medial border contacting the second premolar) in these cases was determined based on the classic anatomic studies of teeth by H. Sicher.¹⁹  For 5 maxillary teeth on each side (2 incisors, canine, 2 premolars), the overall mean average width by H. Sicher was equal to 35.8 mm from the ML (sum of 8.4-mm average width for the central incisor, 6.5 mm for the lateral incisor, 7.6 mm for the canine, 6.8 mm for the first premolar, and 6.5 mm for the second premolar). Considering a typical pattern of maxillary alveolar bone atrophy that parallels the tooth loss that inevitably contracts the maxillary arch (in the superior and posterior direction), 35.8 mm was reduced to 30.0 mm for our study.

• 2. Inferior and superior border:

The inferior border was the alveolar crest (the most convex portion of it). The superior border was set at the nasal floor (pyriform aperture) in the anterior region or the inferior border of the maxillary sinus in the posterior region. In cases of large alveolar process with little evidence of atrophy and/or small and high-positioned maxillary sinuses, a 15-mm height from the alveolar crest was set as the superior plane that limited the vertical extension of measurements based on the longest clinically relevant endosseous implant that could be used for a full-arch implant treatment (7 mm was chosen as the commonly used smallest implant, 15 mm was selected as the longest implant; Figures 1–3).

• 3. Anterior and posterior border:

Analyzing the coronal slices, the anterior border was determined by following the cortical outline of the alveolar ridge clearly visible on the CBCT scan. The posterior border or the depth of measurement was more challenging to determine. Anatomically, the alveolar and palatal processes of an adult maxilla are interconnected and do not have a discernable border. Dental implants are typically placed in the middle of the alveolar ridge (close to the natural tooth position in the same area) or slightly towards the palate (lingualized occlusion. The commonly used standard 4-mm diameter root-form implant when inserted into the bone should ideally be surrounded by 2 to 3 mm of bone for osseointegration. Therefore, the posterior extension was set at 10-mm depth (4 mm + [3 mm × 2] = 10 mm) from the anterior border (at the 7-mm vertical distance). This determinant defined a 10-mm volumetric corridor that was measured from one side of the edentulous maxilla to the other side on the coronal slices (Figures 1–3). A slight adjustment of this measurement was made at the maxillary ML for 2 to 3 cross-sectional slices where the incisive (nasopalatine) canal served as the posterior border.

• 4.

  Maxillary SP:

Maxillary bone volume measurements helped to define the second goal of the study: to classify maxillary sinuses based on the degree of their pneumatization. In this study of edentulous maxillary subjects, we encountered maxillary sinuses of small size and/or positioned high that would have not limited any full-arch implant procedure, as well as very large (pneumatized) sinuses that would have considerably impeded any implant treatment. Authors attempted to stratify the size of sinuses (SP) in the study subjects based on already discussed anatomic studies by H. Sicher¹⁹  into 5 categories: SP1, mild degree of enlargement: greater than 25 mm of alveolar length from the ML to the anterior border of the sinus that can accommodate an upright implant of 7-mm length (about the level of the second premolar); SP2, moderate degree of enlargement: 21- to 25-mm distance from the ML (about the first premolar position); SP3, severe SP: 16–20 mm from the ML (canine area); SP4, extreme SP: less than 15 mm from the ML to the anterior border of the sinus that can accommodate an upright implant of 7-mm length. The SP0 or “clear” (likely, more than 30 mm from the ML) category signified sinuses that were small and/or high positioned and not interfering with any implant treatment (clear/no interference to place an implant in any region of maxilla; Table 1).

The calculated volume of maxillary bone and degree of SP in each case were added to the database. All measurements were entered and stored in the Excel program (Microsoft, Redmond, Wash). The statistical data were then analyzed.

Many clinical determinants (bone quality, morphology of the alveolar ridge, etc) are important in treatment planning of the edentulous maxilla. One of them is bone volume (bone quantity). It appears that four implants are the very minimal amount of fixtures that are necessary for a maxillary full-arch implant-supported or implant-retained prosthesis.^8,20  Considering bone volume only, authors attempted to estimate the necessary (minimal) amount of bone (volumetrically) to accommodate 4 maxillary implants of average diameter (4 mm) and average length (10 mm) and optimally occlusally spaced and not tilted (placed upright in the lateral incisor and first premolar positions). According to anatomic studies (mentioned above) of normal teeth measurements by H. Sicher,¹⁹  the level of lateral incisor (with correction to edentulism) would be approximately 10 mm and first premolar 25 mm from the ML (50 mm of total alveolar length from first premolar implant on one side to the first premolar implant on the other side). If each implant requires at least 1.5 mm of surrounding bone for proper osseointegration (4 mm + 1.5 mm × 2 = 7 mm), the following bone volume can be easily calculated: 50 (alveolar length) × 10 (bone height = implant length) × 7 (bone width to accommodate 4-mm diameter implants) = 3500 mm³ (Figure 4). We compared this hypothetical (and minimal) bone volume necessary to place and restore 4 implants in the edentulous maxilla (Vh) with the measured total CBCT-generated volume in 30 edentulous maxillary cases in this study (Vt).

The mean total volume of the anterior maxillary alveolar bone (Vt) suitable for implantation was 4408.1 mm³ and ranged from 1489.7 to 7 263.1 mm³. The mean maxillary bone volume for men (Vm) was 4365.9 mm³ with a range from 1549.9 to 7263.1 mm³ and for women (Vf) was 4426.2 mm³ with a range from 1489.7 to 6773.4 mm³. Mean male age was 65.8 years and mean female age was 68.0 years in the study. The subjects were stratified by age in decades: 2 (6.7%) subjects in the fourth decade (age 40 to 49) had a mean maxillary bone volume of 3834.3 mm³, with an escalating bone volume as age increased from 3244.0 mm³ (age 41) to 4424.6 mm³ (age 48); 7 (23.3%) in the fifth decade had a mean bone volume of 4901.9 mm³; 7 (23.3%) in the sixth decade had a mean bone volume of 4659.6 mm³; 9 (30%) subjects in the seventh decade (70–79 years) had a mean bone volume of 4418.7 mm³; 4 (13.3%) subjects in the eighth decade had a mean bone volume of 3086.0 mm³; and a single (3.3%) subject in the ninth decade had a maxillary bone volume of 5531.5 mm³. The study reaffirmed an inverse proportion that often exists between the age of edentulous patients and their remaining bone volume. Among the study subjects, it was observed in the fifth to eighth decades that as the subjects' ages increased, their MMBVs decreased from 4901.9 mm³ (fifth decade) to 3086.0 mm³ (eighth decade; Figure 5; Table 2).

The degree of SP in 60 sinuses (as analyzed on panoramic images of 30 CBCT scans) had the following representation in the study: SP0 (clear: not interfering with implant treatment in cases of high/small sinus), 2 sinuses or 3.3% (1 patient for right and 1 patient for the left sinus); SP1 (mild sinus enlargement), 29 sinuses or 48.3% (14 on the right side and 15 on for the left); SP2 (moderate SP), 16 sinuses or 26.7% (9 right and 7 left); SP3 (severe SP), 9 or 15.0% (4 right and 5 left); and SP4 (extreme), 4 sinuses in the study or 6.7% (2 on each side). Most analyzed maxillary sinuses—47 of 60 (24 right and 23 left) or 78.3%—were in clear, mild, or moderate category of SP (SP0, SP1, and SP2) with a sufficient amount of maxillary bone beneath maxillary sinuses to allow a full-arch implant treatment. With other clinical criteria in mind (alveolar ridge morphology, maxillary bone quality, occlusal considerations, etc), SP0 should not interfere with any implant treatment, SP1 could be suitable for placement of up to ≥6 implants, SP2 might be appropriate for planning of 4 to 6 implants, SP3 might be appropriate for 4 implants only and may require an additional bone graft/sinus lift, and SP4 nearly occupies the whole maxilla and implant treatment may not be a practical choice (options: conventional full denture, large bone grafting, zygoma implants, etc; Table 2).

The study confirmed a common tendency often observed in clinical settings demonstrating an inverse correlation between the maxillary bone volume and pneumatization of maxillary sinuses in maxillary edentulous cases: with age, the maxillary bone volume decreased (increased resorption) and SP increased. Two patients (4 sinuses) in the study (SD and RN; Table 1) who were diagnosed with SP4 (extreme SP on both sides) had also the most advanced maxillary bone atrophy of all subjects (1487.7 and 1549.9 mm³). One patient (2 sinuses; VM; Table 1) who was diagnosed with SP0 (clear) had also the largest presence of maxillary bone (very little bone loss) of all subjects (7263.1 mm³). The inverse correlation (proportion) between SP and MMBV can also be observed in Table 2 and Figure 6.

Accuracy of CBCT images for assessing 3D linear measurements and volume of bone in the maxilla or mandible has been demonstrated in a few studies.^{14–18,21,22}  Veyre-Goulet et al¹⁴  showed the reliability of CBCT in the assessment of bone quantity in the posterior maxilla for implant planning. King et al¹⁷  determined that CBCT allows an accurate assessment of the paramedian palatal bone volume of adolescents for orthodontic implant anchorage. Pinsky et al¹⁸  also proved an accuracy of 3D CBCT measurements for mandibular osseous lesions.

Many edentulous patients demonstrate a significant degree of maxillary bone resorption as well as pneumatization of maxillary sinuses that prohibit or complicate full-arch implant treatment. This research seems to be one of just a few that are focused on the volumetric measurement of maxillary bone using CBCT technology for the purpose of implant reconstruction.

The data from this implant-centered radiographic CBCT study of maxillary edentulous bone volume has demonstrated the following: (1) the measured total mean maxillary bone volume (Vt = 4408.1 mm³), mean maxillary bone volume in men (Vm = 4365.9 mm³), and mean maxillary bone volume in women (Vf = 4426.2 mm³) in the study were all above the hypothesized (assumed) bone volume (Vh = 3500 mm³) minimally suitable for placement of 4 implants into edentulous maxilla. This indicates that in some clinical situations, careful analysis of bone availability (quantity or volume) may suggest the possibility of reconstruction with implant-supported or combined implant-mucosa–supported restorations without the need for additional surgical procedures, such as sinus lifts and bone grafts. All other clinical parameters, such as the anatomy of the alveolar crest, bone quality, occlusal relations, and so forth, also need to be considered. (2) The study confirmed the commonly encountered trend of inverse proportion between the age of edentulous patients and their remaining bone volume (Figure 5). Among the study subjects, it was observed in the fifth to eighth decades that as the subjects' age increased, their MMBV decreased from 4901.9 mm³ (fifth decade) to 3086.0 mm³ (eighth decade). (3) Most maxillary sinuses in the study—47 (of 60 in 30 subjects) or 78.3%—were in clear (SP0), mild (SP1), or moderate (SP2) categories of SP with an adequate amount of maxillary bone beneath the maxillary sinuses to allow a full-arch implant treatment with at least 4 implants. (4) The inverse correlation between maxillary bone volume and pneumatization of maxillary sinuses in maxillary edentulous cases was observed (Figure 6). The study confirmed that with age and edentulism, there is a tendency for patients to show progressive maxillary bone atrophy with a decrease of MMBV and enlargement of maxillary sinuses (SP), making them less suitable for implant treatment (Table 2). (5) Although many other clinical surgical and prosthetic criteria are important (bone quality, bone morphology, occlusal scheme, maxillomandibular relationship, etc), bone availability appears to be one of the most important factors in implant treatment.

The primary goal of this anatomic study was to measure the average bone volume of the edentulous maxilla with a CBCT scan for the placement of implants. Other surgical factors, such as bone quality and alveolar ridge morphology and restorative factors, such as occlusal scheme, skeletal jaw classification, and so forth, were not analyzed in this study. This CBCT study indicates that if other clinical factors are present, in many maxillary edentulous cases the existing bone quantity (volume) can be often sufficient for a full-arch maxillary implant treatment with at least 4 implants without an additional trauma or expense associated with bone grafting or sinus lift.

This study also demonstrates the enormous benefits of current 3D CBCT technology that allows measuring bone volume for clinical purposes with high precision. If dental implants are considered, the treatment plan can rely on these radiographic (in addition to clinical) data as a guiding tool to determine the most successful implant rehabilitation approach for each particular case. CAD/CAM virtual surgery is one important tool in guiding the implant insertion using a CBCT scan. Further improvements to volumetric software programs are needed to make measurements more easily determined. The CBCT volumetric data can also help expand our understanding of the bone quantity, pattern of bone resorption, SP, and other factors that are associated with loss of teeth and is important for diagnosis and implant treatment.

CBCT

cone-beam computerized tomography

MMBV

mean maxillary bone volume

SP

sinus pneumatization