A Zygomatic Bone Study Using Virtual Dental Implant Planning Software

Zygomatic implants were initially introduced to rehabilitate severely resorbed or atrophic maxilla and patients subjected to oncological resection of the maxilla.¹  Promising results and increased success rates led to the use of zygomatic implants instead of extensive bone grafting procedures, thus reducing morbidity and treatment time and requiring only a single surgical procedure.²  However, postoperative biological complications, such as sinus infections, fistula formation, maxillary sinus infections, among others, were reported.³ 

An earlier study reported a simplified sinus slot technique for improved orientation of zygomatic implants with less invasive and faster surgical procedures that allowed for a better placement of the prosthetic platform on the alveolar ridge crest.⁴  The optimized placement of the implants also contributes to a shorter distal extension of the prosthesis, thus favoring hygiene and biomechanical behavior.⁴ 

Accurate information regarding zygomatic bone anatomy, density, and volume and the adjacent structures is required for a successful implant placement due to anatomical differences between patients.⁵  Cone-beam computerized tomography (CBCT) is indicated for the detailed evaluation of bone structures.⁶  CBCTs also have a lower cost with less radiation dose compared to spiral computerized tomography (CT) scans.⁶  CBCT scans can be used for 3-dimensional digital surgical planning of zygomatic implants with accurate measurements and virtual placement of the implants.⁷ 

To the best of our knowledge, most studies have studied the zygomatic placement of implants by using dry skulls and spiral CT scans.^2,8,9  Few anatomical studies used CBCT scans and digital surgical planning software.¹⁰  There is also the need to evaluate the number of patients with anatomical conditions to receive zygomatic implants, where such implants would be placed (intra- or extrasinus), and the indicated implant sizes. This study evaluated the different anatomical factors that influence the surgical planning of zygomatic implants by using virtual surgery on CBCT scans.

This retrospective cross-sectional study was conducted according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement and was approved by the Ethics Committee of the Curitiba Neurology Institute (protocol number 2.040.111). The study was conducted in accordance with the Helsinki Declaration of 1975, as revised in 2000. Each subject provided signed informed consent before inclusion in the study. Patient records were collected by a single researcher and protected against exposure. All patients were informed of the surgical procedures that were to be performed and consented to treatment.

CBCT scans of 2428 zygomatic bones from 1214 patients from the digital archives of the ILAPEO College Department of Radiology and Diagnostic Imaging (Curitiba, PR) were taken between March 2009 and October 2016 to allow the digital planning of zygomatic implants. Maxillary edentulous patients between 37 and 95 years old were included in the study. Exclusion criteria were as follows: patients with history of trauma or surgical interventions, presence of zygomatic bone pathologies, and CBCT scans with an incorrect field of view (FOV) that would not allow an acceptable visualization of the region of interest (zygomatic bone region). One thousand and ninety-two images were therefore excluded from the study, and the final sample included 268 patients.

Exposure variables were classified as patient age, gender, and right and left sides. Outcome variables were classified as frequency of different maxillary sinuses concavities and their relation with the analyzed anatomical measurements. Maxillary-zygomatic anatomical measurements were performed on the zygomatic implants to assess the implant region area, implant length, and the latero-lateral width. The CBCT scans were taken by using CBCT equipment (Galaxis Galileos Implant, Sirona, Bensheim, Germany) and standardized as follows: patient head and occlusal plane were parallel to the ground, and the midsagittal plane was perpendicular to the ground. Image-acquiring factors were 42 mAs, 85 kV, and slice thickness of 0.3 mm.

Galaxis Galileos Implant was used for the virtual zygomatic implant planning by an experienced examiner. The virtual placement of the implants was based on each patient's maxilla anatomic characteristics, maxillary sinuses, and zygomatic bone, aiming for a favorable prosthetic outcome and adequate bone anchorage. The available implant sizes had a body diameter of 4.4 mm and an apex region diameter of 3.9 with 30-, 35-, 40-, 45-, 47.5-, 50-, and 52.5-mm lengths. The entry perforation point was at the alveolar ridge crest near the zygomatic pillar (region between the second premolar and first molar) (Figure 1).

The concavity formed by the alveolar ridge crest, the maxillary sinus lateral wall, and the zygomatic bone implant insertion region was classified as follows: small, medium, and large (Figure 2).¹¹  Virtual measurement of the implant insertion angles related to the midsagittal plane and length of the virtual zygomatic implants (A-Ju) were made on the coronal slices of the CBCTs. The length of the apical portion of the implant (B-Ju) in contact with the zygomatic bone was measured on the long axis of the virtual implant in a coronal slice of the CBCT (Figure 3). Width of the zygomatic implant was measured on the latero-lateral axis on an axial slice near the implant apex.

The qualitative variables were described and compared by frequencies and percentages. Normality of the quantitative variables was tested by the Kolmogorov-Smirnov test. Analysis of variance (ANOVA) and least significant difference (LSD) for multiple comparisons were used for the statistical comparison between the 3 concavity sizes that were classified and the A-Ju, B-Ju, and implant angle and width measurements that were performed (α = 0.05). The Levène test was used to analyze homogeneity of the variances. All statistical tests were performed on specialized statistics software (IBM SPSS Statistics v20, Armonk, NY). Data collection and statistical analysis methodology were reviewed by an independent statistician.

Two hundred and sixty-eight patients with an edentulous maxilla were analyzed for this study. The mean age of patients was 61.7 (SD = 11.2) with 184 (68.7%) women and 84 (31.3%) men. Implants were virtually planned on 536 zygomatic bones. Tables 1 and 2 present the maxillary concavity sizes and concavity size combination in patients (ie, small on the left side and medium on the right side in the same patient) that were found.

The insertion angle of the virtual implant averaged 43.2 ± 4.64 degrees (58.3 and 29.6, maximum and minimum degrees, respectively). Significant differences were found on the right side when small concavities were compared with medium concavities (P < .012) (Tables 3 and 4). Distance between the alveolar ridge crest and the zygoma (A-Ju measurement) averaged 39.2 ± 3.63 mm (50 and 30 mm, maximum and minimum distances, respectively). No significant differences were found for the concavity size classification (Table 5) (P > .05).

The distance between the apical region of the implant in contact with the zygomatic bone (B-Ju measurement) averaged 9.1 ± 3.14 mm (19.9 and 0.9 mm, maximum and minimum distances respectively). Significant differences were found on both sides in the comparison between small and medium-sized concavities and between small and large concavities (P < .001) (Tables 6 and 7). Widths of the zygomatic bones were measured on the latero-lateral sides on an axial slice near the implant apex. Widths averaged 8 ± 2.49 mm (18.6 and 1.2 mm, maximum and minimum widths, respectively). Significant differences were found on both sides when small and medium-sized concavities and when small and large concavities were compared (P < .001) (Tables 8 and 9).

Surgical procedures for placing zygomatic implants are currently well established for the treatment of the edentulous maxilla with reported success rates above 95%.^3,4,12  The anatomical characteristics of patients, such as the concavity formed by the alveolar ridge crest, the maxillary sinus lateral wall, and the zygomatic bone implant insertion region, directly influence the choice for the most adequate surgical technique. A small concavity is found with a severely resorbed maxilla, and the original Brånemark technique is recommended.¹¹  Exteriorization of the zygomatic implant (sinus slot technique)⁴  is recommended when a large concavity is generated by maxillary resorption.¹¹ 

This study found 68% patients with concavities of the same size on both maxilla sides. The remaining patients had different concavity sizes between each maxilla side, which could lead to the indication of different surgical techniques on the same patient. Most maxilla concavities found in the patients in this study were small (34.9%) to medium sized (57.1%)—the exteriorized zygomatic implant is recommended in such cases.¹¹  Due to eliminating the extensive dissection recommended for the original Brånemark technique, some advantages of the exteriorized technique include reduced patient discomfort and postoperative edema and ecchymosis, faster surgical procedure with earlier patient recovery, and increased bone-implant contact area.⁴ 

The external concavity on the lateral wall of the maxillary sinus, between the entry point of the zygomatic fixture and the alveolar crest, allows implant placement outside the maxillary sinus. The more the implant is outside the sinus cavity, the higher the zygomatic bone anchorage due to a more lateral implant placement with a smaller chance of sinus infection due to maintenance of sinus cavity integrity. The zygomatic implant is also more vertically placed in the first molar region with a more favorable placement of the prosthetic connection providing a shorter distal extension and enabling traditional prosthetic restoration.⁴  A partially guided protocol is therefore recommended. The use of a surgical guide to direct the entry point near the second premolar minimizes surgical trauma. Drilling the alveolar ridge crest with the aid of a surgical guide reduces the incision extension and leads to more favorable prosthetic outcomes. A totally guided procedure is limited by the implant placement outside the maxillary sinus and tearing of the soft tissues due to not detaching the external mucosa.

An adequate implant insertion angle is recommended to avoid injuring anatomical structures, such as the orbit and the infratemporal fossa. The midsagittal plane was used as a reference to measure the insertion angle according to an earlier study.¹⁰  This study found an average 43.2-degree insertion angle, whereas an earlier study¹⁰  found an average 48.09-degree insertion angle. Zygomatic implant length is then determined from the insertion point on the alveolar ridge crest to the zygoma external cortical surface (A-Ju measurement). The most commonly used zygomatic implant lengths range from 40 to 47.5 mm.¹  This study virtually planned mostly 40-mm-long zygomatic implants. Other studies found an average 53.21-,¹⁰  50.2-,¹³  and 42.4²-mm-long zygomatic implants. Ethnic differences between the population analyzed in the mentioned studies and different measurement methodologies could have influenced such differences. An earlier study¹¹  found no significant differences between surgical technique and zygomatic implant length. This is in agreement with the results found in this study when the zygoma external cortical surface was correlated with the concavity size.

Bone contact with the implant surface provides primary stability for conventional implants. However, zygomatic implants present bone contact mostly in the apical region regardless of surgical technique. The B-Ju measurement performed for this study found an average of 7.4 mm of bone contact when small concavities were present and an average of 9.7 mm when a medium concavity was found. The higher bone contact was found when a large concavity was present (12.5 mm). Earlier studies found significant differences in the B-Ju measurement with different surgical techniques with 8.39 mm of bone contact for the original Brånemark technique and 14.11 mm for the sinus slot technique.¹¹  Less apical bone contact does not seem to influence osseointegration since the success of zygomatic fixtures is achieved by the employment of at least 4 cortical portions.⁹ 

Measuring width of the zygomatic bone is recommended to avoid exposure of implant threads. A 2-mm bone width around the implant is recommended for osseointegration;¹⁴  therefore, the recommended zygomatic bone width in the apical region is at least 5.9 mm. This study used a latero-lateral measurement in an axial slice and found an average 8-mm zygomatic bone width. Another study used an antero-posterior measurement in an axial slice and found a 5.84-mm average width.¹⁰  One thousand and ninety-two images had to be excluded from the study for not meeting the inclusion criteria and thus limiting the amount of analyzed images. Among the exclusion criteria that were followed, a more careful FOV CBCT scan that allows an acceptable visualization of the region of interest (zygomatic bone region) is recommended in future studies. Different implant planning software are available to evaluate zygomatic bone volume and therefore bone availability for zygomatic implants. Future studies could use such novel software for further bone availability evaluation.

Measurements performed for this study used the virtual implant itself for reference. The use of specialized implant planning software for the measurements allows better visualization of the relationship between implants and anatomical structures and achieving an optimized implant placement. The measurements were therefore performed on more specific sites compared to other studies that used anatomical measurements in the zygomatic bone implant region. Implant planning software is a valid tool to aid the achievement of predictable outcomes for zygomatic implant fixtures. Virtual planning additionally aids in avoidance of accidental contact with vital anatomical structures and limits potential prosthetic complications that may arise due to misangulation of the implant, thus placing the platform too lingual to the crestal midline.

Abbreviations

ANOVA:

analysis of variance

CBCT:

cone-beam computerized tomography

CT:

computerized tomography

FOV:

field of view

LSD:

least significant difference

The authors would like to acknowledge the consulting statistician Márcia Olandoski, PhD, from the Pontifícia Universidade Católica do Paraná (PUC-PR) at Curitiba, PR, Brazil.

The authors have no conflicts of interest to declare.