The radiological study was conducted in line with the TRIPOD reporting guidelines22 on patients referring to the Department of Dentistry, IRCCS San Raffaele Hospital, Milan, Italy, between 2015 and 2018.
Patients were included according to the following criteria:
- Age > 18 years old
- Edentulous area at least from the first molar to the canine
- Severely atrophic maxilla corresponding to class V of the Cawood and Howell’s Classification23. In particular, a residual flat ridge form, with less than 5 mm in height but without evident basilar loss, was categorized as class V of the Cawood and Howell’s Classification
Patients were not included if they presented one of the following exclusion criteria:
- Contraindications to the CBCT exam
- Systemic diseases that influence bone metabolism
- Radiolucent or radiopaque images in the mid-maxilla area
- Implant or impacted tooth in the mid-maxilla area
Cbct Scan Acquisition
The CBCT scans were taken for diagnostic reasons. Each patient provided a written consent before undergoing the CBCT scan. All CBCT scans were acquired with a Field of View of 12 x 8 cm, at 90kV 10 mA 16 s and 0,2 mm Voxel size with a NewTom VGi evo Cone Beam 3D Imaging Device (Cefla SC, Imola Italy). The default position and orientation of the orthogonal sectional planes relative to the jaws were consistent in all the CBCT datasets of each patient. Thus, standardization of the site and orientation of the reformatted sample sites as achievable through measured shift and angulation of the orthogonal sectional planes.
To guarantee a stable head position, all CBCT scans were checked and re-orientated to place the scan view parallel to the Camper’s plane.
The “mid-maxilla” region is an anatomical district that belongs to the maxillary bone. It could be identified with the Retrocanine Bone Triangle24 extended from the lateral border of the nasal cavity until the sinus cavity, including also the residual alveolar process below the sinus floor (Fig. 1).
Frontally, the antero-lateral side of the maxilla’s body represents the external wall of the mid-maxilla. The residual alveolar crest represents the inferior margin, with a high thickness in the posterior section. The anterior wall of the maxillary sinus represents the posterior limit of the mid-maxilla and it’s extremely concave due to the sinusal cavity. Medially, the mid-maxilla is limited by the lateral nasal cavity’s wall, from the anterior external margin until the naso-lacrimal duct’s emergency, under the inferior turbinate. The inferior medial border is represented by the alveolar process connected with the palatal process.
The bone polygon seems a truncated pyramid with a triangular base, described as follow.
The base: the inferior side of the pyramid is a sort of triangle with a posterior base. It represents the fixture’s entry and allows a good stability to the implant. It belongs to the residual alveolar process. In very severe atrophy it includes also the lateral portion of the palatal process. The antero-lateral wall of the pyramid coincides with the antero-lateral face of the maxilla and includes also the canine pillar, the canine pit and the residual alveolar bone under the sinus floor. The medial alveolar portion, the palatal process and, superiorly, the lateral wall of the nasal cavity constitute the medial wall of the pyramid. Below the inferior nasal turbinate there is the emergency of the naso-lacrimal canal that limits the mid-maxilla.
The posterior wall is generally a large wall with a concavity due to the anterior wall of the maxillary sinus. It narrows apically following the shape of the frontal process of the maxilla25.
Virtual Iuxtameatal Implant Positioning
The residual bone of each patient was analyzed using the panorex imagine construction (Fig. 2). According to the software procedures the panorex image was obtained drawing a panoramic curve on the axial view of the maxillary segment above the residual alveolar crest. The cross sectional views perpendicular to the panoramic curve were automatically elaborated by the software.
In order to standardize the implant positioning, on the panoramic view a horizontal line “H” is drawn from the point “P0” (the lowermost point of the right or left maxillary sinus floor) parallel to the scan view.
Then a vertical line “V” is drawn tangent to the point “PV” (the most medial/anterior point of the right or left sinus cavity) and perpendicular to the line “H”.
A line “1” was drawn with an angle of 30° from the line “V” and tangent to the antero-medial wall of the sinus cavity in the point “T30”.
The meeting point of the line “1” with the line “H” is the point “P30”.
From the point “T30” was drawn a segment “T30-A” with 3mm of length, perpendicular to line 1. This is the minimum distance of the implant axis from the sinus cavity. A line was drawn parallel to the line “1” and passing through point “A”. This line corresponded with the long axis of the virtual iuxtameatal implant and crossed the line “H” in the point “Pe30”.
The distance from “P30” to “Pe30” can be then calculated through the geometrical similarity.
Two triangles are similar if and only if corresponding angles have the same measure. This implies that they are similar if and only if the lengths of corresponding sides are proportional. It can be shown that two triangles having congruent angles (equiangular triangles) are similar, that is, the corresponding sides can be proved to be proportional. This is known as the AAA similarity theorem.
Then, in according to the trigonometry functions related to right triangles:
P30-Pe30 = T30-A / cos 30° = 3mm / cos30° = 3,46 mm
This result is the distance from the center of the implant to the point “P30”. So that the initial drilling point of all the virtual iuxtameatal implants is the projection of the point Pe30 to the residual alveolar crest (Fig. 3).
An analogous procedure can be performed in the case of a 45° degrees angulation implant (Fig. 4).
In this latter case the distance from P45 and Pe45 (the drilling entry point) shall be:
P45-Pe45 = T45-B / cos 45° = 3mm / cos45° = 4,24 mm
In the general case, with the use of an implant of generic diameter “x”, the distance from P45 or P30, shall be measured as follow:
P30-Pegeneric = (1+x/2)/ cos 30°
P45-Pegeneric = (1+x/2)/ cos 45°
In the most examined cases the extreme maxillary atrophy allowed the implants placement only in the mid-maxillary basal bone, between the lateral nasal cavity wall and the anterior wall of the maxillary sinus.
For each patient, two iuxtameatal cone shape implants were virtually placed in tilted position in each side, according to the All-on-four procedure described by Malò et al26 trying to maximally exploit all the bone volume offered (Fig. 5). All the implants were placed in the same way at 30° and at 45° of mesio-distally angulation, because the majority of implant companies produces abutments which can correct an angle of 30° maximum. However, the prosthesis meso-structure can correct additional 15° of angulation with its conic component, so that the maximum of angulation could be 45°.
Implant insertions were simulated with the RealGUIDE 5.0 implant planning software (3DIEMME, Cantù, Italy).
The iuxtameatal implants were positioned in the mid-maxilla with a sufficient amount of bone all around the fixtures. Then the length and the diameter of the implants were measured. The buccal-palatal angulation of each implant were analyzed too in the cross sectional views (Fig. 6). A negative angle indicated that the implant insertion went from the palatal side to the vestibular side of the residual alveolar crest (Fig. 7). With the dedicated software also the bone density around the implants’ apex was measured in gray-scale. Different studies demonstrated how grey levels of CBCT can be used to derive Hounsfield units27. Nevertheless among CBCT scans gray-scales vary widely due to different factors, such as the lack of grey level uniformity, the presence of artifacts, the effects of scatter and beam hardening28-29. So gray-scales are not equivalent among CBCTs, but the outcomes taken from the CBCT scans could suggest the presence or not of cortical bone around the fixtures.
All statistical analyses were performed with a specific software (R, R Core Team, Foundation for Statistical Computing, Vienna, Austria). In particular, the Linear and Nonlinear Mixed Effects models package was used to estimate the Linear Mixed Effects model30.
To evaluate the effect of the tilting degree (30° vs. 45° degrees) on the implant length, a linear mixed-effects model was estimated. The modelling approach here applied allows to properly account for repeated measure data and for unobserved heterogeneity among patients. Actually, along with fixed effects, the model allows to specify in the model random components. An initial complete model was estimated including position and tilting degree as fixed effects along with their interaction. Subject-specific random effect was specified. Hence, a random intercept model was considered. Assumptions for the correct application of the model were checked. A backward stepwise procedure was applied to select a more parsimonious model.
In all the analyses, the significance threshold was set at 0.05.