MAXILLARY ANATOMY: ASPECTS RELATED TO IMPLANT SURGERY AND SURGICAL PLANNING

Fig. 2.1 • Example of 3D reconstruction of a CT image of the lower jaw which, compared with traditional transaxial sections (Fig. 2.2), makes it easier to understand the morphology of the bone. In this particular case, it is possible to identify the emergence of the mental nerve (A), the external oblique line (B), the mylohyoid line (C), and the mandibular lingula with the opening into the mandibular canal (mandibular foramen) (D) (software: One Scan 3D, 3D-Med, Brescia).

 

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Fig. 2.2 • Transaxial sections of the previous the left hemi-mandible (software: One-Scan 3D, 3 D-Med, Brescia).

Nowadays, all radiographic systems provide the end user with special “viewer” software that allows the original tomographic image to be viewed in different projections. Unfortunately, simply viewing the radiologist’s reconstruction can sometimes be limiting, given that he/she has no direct experience of surgical implant positioning and often will not be in possession of sufficient data to reconstruct the tomographic sections in accordance with the specific surgical requirements. It is therefore important to provide the radiologist with indications for the correct layering of the tomographic sections, for example, a radiographic template that simulates the occlusal plane and thus the ideal axis of the future implants. If this is not provided, the radiologist will process the image purely on the basis of anatomical landmarks (e.g. the base of the jaw or the infraorbital plane). This could result in even quite marked discrepancies between one plane of reference and the other, and the section displayed by the viewer will contain a certain dimensional error that will have to be taken into account before positioning the implants (Fig. 2.3).

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Fig. 2.3 • Example of discrepancy between the mandibular occlusal plane (indicated to the radiologist through a radiographic template) and the anatomical base of the jaw (when no radiographic template-based indication was provided). The different mandibular section that will result from this discrepancy will lead to different dimensional indications in the individual sections. At the level of the inferior alveolar nerve, the difference may even be in the order of several millimetres.

There also exist programs that are capable of reading “raw” CT output data and that allow the surgeon himself to process them according to the specific requirements of the case. In other words, the implantologist can decide what axis should be used for the correct layering of the tomographic sections and thus the virtual volume reconstructed. Some of these programs allow for the correction of initial scanning errors (Fig.s 2.4, 2.5)

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Fig. 2.4 • Example of correction of a CT scan using specific software (One Scan 3D, 3D-Med, Brescia): in the frontal plane, it is necessary to correct (yellow line) the right tilt of the head so that the layering of the tomographic sections follows the anatomical plane and not the scan plane (blue line).

 

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Fig. 2.5 • In the sagittal plane, a correction has been made (yellow line) to ensure that the sections are reconstructed orthogonally to the future occlusal plane and not according to the scan plane (blue line).

so that the tomographic sections can be formatted (reconstructed) according to the ideal plane (occlusal plane) determined by the prosthetic planning, even in the absence of a radiographic template (prosthetically guided implantology).

Prosthetic planning of the position of the teeth and of the future occlusal plane can easily be transferred to the CT scan using various template systems; initial errors in relation to the scan plane are more easily corrected, which is particularly important for correlating the axis of insertion of the implants to the anatomical structures involved.

Nowadays there also exist methods for guiding the axis of drill insertion into the bone, according to the CT-based prosthetic plan. Use of these methods ensures safe intervention in terms of drilling depth; in addition, it allows such a close correspondence between CT scan and diagnostic wax-up that, exceptionally, it is even possible to carry out preliminary preparation of the prosthetic bar prior to the surgery itself, thereby reducing the time needed to deliver the immediate-load implant (Fig.s 2.6, 2.7a-d).

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Fig. 2.6 • Example of planning, using One Scan 3D software (3D-Med, Brescia), of immediate loading in the lower jaw: scanning was performed using a diagnostic template showing – thanks to the use of radio-opqaue teeth – the future occlusal plane. A perforated Flat-guide is positioned on this plane (Courtesy of Dr M. Jacotti, Brescia).

 

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Fig. 2.7 • 3D images generated by the software used to interpret raw CT data (a): simulation, by the surgeon, of the implant positions, based on the CT (b): occlusal view of the perforated Flat-guide. (c, d) The software transfers the (CT-simulated) implant insertion axis to the Flat-guide (light blue cylinders), determining its spatial coordinates. The data of the coordinates x, y, z of the implant axis are sent to a special centre for the construction of a volume (brown) which will contain the implant drill hole sites. Using the existing holes, the volume, made by sintering, is physically coupled with the initial flat-guide: what is obtained in this way is a guide volume for the placement of the implants according to the axis as predetermined on the CT scan. The use of calibrated drills allows extremely precise preparation of the site and, if desired, it is already possible to construct a prosthesis to mount on the implants during the surgery itself (Courtesy of Dr M. Jacotti, Brescia).

Even though methods that can combine radiographic images with template prototyping systems are relatively widespread, implant surgery, like all surgical procedures, nevertheless demands explicit knowledge of the anatomy of the region and the structures involved. Another thing always to bear in mind is the possibility of anatomical variants in single cases (relating, for example, to the morphology of the maxillary sinus, to position of the mandibular canal in relation to the bone crest, to the lingual concavity of the mandibular crest, or to the degree of resorption of bone structures).

Preferring to leave purely descriptive accounts of the anatomical structures to specific anatomy texts (2-4), in this context, as indicated, we also assume that readers are familiar with the prosthetic planning procedures carried out during the diagnostic and planning phase.

Instead, in this brief treatise we aim to give the surgeon an idea of the anatomical structures involved in the implant field, setting out the possible consequences of damage to them and the precautions to adopt in order to be able operate with complete peace of mind.

We focus, in particular, on the comparison between real anatomy, studied on cadavers, and its tomographic correlates, in order to make the many aspects of clinical and radiological planning of single cases a little more readily understandable.

 

SURGICAL ANATOMY OF THE UPPER MAXILLA BY REGIONS

ANTERIOR REGION OF THE MAXILLA

Accurate positioning of implants in the anterior part of the upper maxilla is fundamental to an optimal aesthetic and phonetic outcome in prosthetic rehabilitation with immediate loading. In order to obtain an optimal emergence profile of all the teeth involved in the creation of an attractive smile, it is necessary to examine particularly carefully the anatomy of the residual bone crest (Fig. 2.8),

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Fig. 2.8 • Three-dimensional CT reconstruction of an atrophic maxilla in the planning of a full-arch restoration with immediate loading. The presence of severe crestal bone resorption could make it impossible to obtain an aesthetic emergence profile of the restoration and reconstruct the papillary architecture. This consideration may lead, for example, to the decision to design a fixed prosthesis with a flange rather than a bridge with an aesthetic emergence profile.  

remembering that the bone volume will determine the height of the interproximal papilla (5), and also that the interproximal papilla between two or more adjacent implants will not have the same anatomical features as papillas between natural teeth, given the absence of the characteristic termino-terminal microcirculation (6).

Papilla volume and preservation thus depend on anatomical and prosthetic factors that must be known (7-10) and taken into account in the pre-surgical planning stage. Indeed, immediate loading is a difficult undertaking precisely because it depends on the capacity to anticipate, well in advance, possible anterior maxillary soft tissue changes, and thus to ensure timely adoption of all the strategies needed to preserve the anatomical structures (bone, papilla, etc.) (Fig.s 2.9a-e).

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Fig.s 2.9a-b-c • A deciduous central incisor is replaced with an immediate-load implant (Intra-Lock, Boca Raton, FL, US). It is important that to ensure that procedure preserves, and prevents resorption of, the bone peaks between the implants, in order to maintain the trophism of the papillas, as these are elements crucial to a successful aesthetic outcome.

 

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Fig.s 2.9d-e • Outcome at 4 months: good maintenance of the bone structure, the root eminence and the conditions of the papillas. For an optimal aesthetic outcome of an immediate loading procedure, it is essential to take into account, in advance, the need to preserve anatomical structures that contribute to the aesthetics of the smile.

When planning immediate loading in the anterior maxilla (Fig.s 2.10a-d),

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Fig.s 2.10a-b • Positioning of immediate-load implants (Intra-Lock, Boca Raton, FL, US) which support a metal bar that fits passively on the fixtures.

 

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Fig.s 2.10c-d • 72 hours after surgery, the passive-fit bar is delivered for fixing on the implants (c). The emergence profile of the fixed prosthesis will affect the shape and volume of the papillas. In the image taken at 4 months (d), the degree of maturation of the papillas can be observed.

provision should be made, ideally, for a maximum of two implants for the central incisors and two for the canines in order to allow optimal aesthetic management of the spaces for papilla reconstruction (5).

Severe bone resorption of the premaxilla, caused by root loss and centripetal resorption of the crestal bone (11-16), exposes the opening of the nasopalatine canal on the ridge crest (Fig. 2.11).

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Fig. 2.11 • Crestal resorption in the premaxilla leads to exposure, along the crest, of the nasopalatine canal (cadaver dissection, Laboratoire d’Anatomie, Université Claude Bernard, Lyon; copyright Del Corso).

The nasopalatine, or incisive, canal contains the homonymous neurovascular bundle (the nasopalatine nerve) which sends nerve branches to the septal mucosa and exchanges sensory fibres with the anterior superior alveolar nerves, contributing to the innervation of the central incisor (Fig. 2.12).

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Fig. 2.12 • Transaxial CT sections of the nasopalatine canal: the size of the nasopalatine canal appears normal.  

Although intraoperative damage to this structure is not irreparable, prolonged haemorrhage could impede the surgical procedure.

The presence of a widened nasopalatine canal (Fig.s 2.13a-b)

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Fig.s 2.13a-b • Example of a widened nasopalatine canal (CT X-ray panoramic image and transaxial sections) which can complicate implant placement in the area of the central incisors.

can interfere with (17), for example, implant preparation and the primary stabilisation of fixtures in the central incisor area: in such cases, implants should be used in place of the lateral incisors, even though this complicates the management of the future interproximal papilla spaces. If, instead, it is decided not to modify the treatment plan, transposition of the neurovascular bundle will be indicated.

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The subnasal region (2,3,4) can alter in size as a result of post-extraction bone resorption making it necessary, in rare instances, to intervene in order to elevate the nasal mucosa. In this situation, the treatment must be preceded by a thorough instrumental diagnostic examination, since the nasal mucosa is richly vascularised by the terminal branches of the sphenopalatine artery: indeed, lacerations during interventions at the base of the nose may result in prolonged bleeding but, above all, contamination of the operating field with bacteria from the nose (20).

It is advisable to identify and protect, using an instrument, the margins of the pyriform aperture and, when absolutely necessary, to isolate and protect the nasal mucosa itself.

One anatomical abnormality occasionally encountered consists of contiguity between the nasal cavity and the maxillary sinus.

Planning of the surgical procedure must take into account the two different functions of the mucous membranes, and particular care must be taken to prevent cross-contamination between the nasal and sinus mucosa (Fig.s 2.14a-h).

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Fig.s 2.14a-b • 3D CT reconstruction (software: One-Scan 3D, 3 D-Med, Brescia): rare case of anatomical communication between the left maxillary sinus and the adjacent nasal cavity: in such cases, if wanting to plan immediate load implant surgery, it is important to schedule both conservative antrotomy of the nasal mucosa, and a sinus lift.

 

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Fig.s 2.14c-d • Two separate windows are created for the sinus and for the nose: the bone is cut using a piezosurgical technique [Piezosurgery, Mectron, Carasco (GE)]. The nasal mucosa is isolated extending the elevation distally towards the contiguous maxillary sinus. After the sinus lift, the implant sites are prepared, the biomaterial is placed in the access windows to maintain the lift volume, and the immediate-load implants are inserted; the bone inserts are then replaced.

 

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Fig.s 2.14e-f • Implants (Intra-Lock, Boca Raton, FL, US) positioned together with the transfer copings: the flap is sutured and immediate loading is performed, transferring the implant position to the laboratory.

 

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Fig.s 2.14g-h • On the third day, a fixed prosthesis is delivered to the patient; this is made from aesthetic composite material covering a metal structure which passively fits the implant abutments. Where possible, the prosthesis is designed to guarantee an aesthetic emergence profile from the mucosa; accordingly, it will influence, from the healing stage, the profile and shape of the papillas.

 

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The infraorbital region of the maxilla is the site of emergence of the infraorbital nerve (intermediate terminal branch of the maxillary trigeminal nerve) and of the infraorbital veins and arteries (Fig.s 2.15a-c)(3).

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Fig.s 2.15a-b-c • Three different fresh cadaver dissections (Laboratoire d’Anatomie, Université Claude Bernard, Lyon; copyright Del Corso): the infraorbital neurovascular bundle runs in a groove at the base of the orbit (a), through a canaliculus and emerges from the infraorbital foramen (b), unravelling and branching out, it is distributed to the superficial areas of the face (c).

Whereas direct damage to the main trunk of the nerve is a rare event, periosteal incisions and elevations of the superior fornix causing interruption of the terminal fibres of the infraorbital neurovascular bundle can be more frequent. The terminal branches of these structures are often involved in cases of severe atrophy of the maxillary crest, where the distance between the alveolar crest and the foramen is markedly reduced.

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LATERAL REGION OF THE MAXILLA

In this region, the most important structure from a surgical point of view is the maxillary sinus.

Pyramidal in shape (2,3,4), with its base corresponding to the medial wall of the nose, this sinus is limited by a superior wall that forms the floor of the orbit (Fig.s 2.16a-b),

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Fig.s 2.16a-b • Diagram of an anatomical preparation (Laboratoire d’Anatomie, Université Claude Bernard, Lyon; copyright Dargaud) of a head section passing through the maxillary sinus molars at the level of the molars.

an inferior wall corresponding to the alveolar process, a posterior wall jutting from the maxillary tuberosity, and a mesiovestibular wall corresponding to the distal depression of the canine eminence (Fig. 2.17).

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Fig. 2.17 • Superior view of a 3D CT reconstruction of the left maxillary sinus: it is possible to observe the medial wall of the nose and the anterior (vestibular) and posterior (tuberous) walls of the sinus. In the middle, an intrasinus bony septum (software: One-Scan 3D, 3 D-Med, Brescia).

The maxillary sinus communicates, by means of the “ostium ad antrum” (sinus ostium), with the middle meatus of the homolateral nasal fossa (Fig. 2.18).

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Fig. 2.18 • Front view of a right maxillary sinus after removal of the mesio-vestibular (anterior) wall. The vascularisation of the medial wall of the sinus is clear to see and, at the top, it is possible to glimpse the ostium communicating with the middle nasal meatus. (Fresh cadaver preparation: Laboratoire d’Anatomie, Université Claude Bernard, Lyon; copyright Del Corso).

 

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Fig. 2.19 • Superior view of a left maxillary sinus and adjacent soft tissue structures: using special software it is also possible to see the intrasinus mucosal formations (software: One-Scan 3D, 3D-Med, Brescia).

Internally, the maxillary sinus is lined with a fine mucous membrane (Fig.s 2.18, 2.19) with a ciliated respiratory epithelium, which is continuous with the mucous membrane of the nasal cavity. The sinus membrane is thinner and less vascularised than the mucous membrane of the nasal cavity (21-27). The function of the ciliated respiratory epithelium is to move fluids, such as mucus and pus, out of the sinus cavity through the ostium (3,4,27).

The sinus is supplied with blood by four groups of arteries originating from the maxillary artery: the infraorbital artery cranially, the greater palatine artery caudally, the sphenopalatine artery and the posterior nasal arteries medially, and the posterior superior alveolar artery (also known as the alveolar antral artery) laterally (2,3,26,27). These arteries normally run in the Schneiderian membrane (Fig.s 2.20, 2.21).

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Fig. 2.20 • Fresh cadaver preparation: with the vestibular window removed, it is possible to see the Schneiderian mucous membrane (lining of the sinus). Arteries are visible in its thickness (Laboratoire d’Anatomie, Université Claude Bernard, Lyon; copyright Del Corso).

 

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Fig. 2.21 • Diagram of a frontal section of a left maxillary sinus. The arteries involved are shown, starting posteriorly, from the maxillary artery, and moving forwards towards the medial-vestibular wall, which has been removed. In this wall, it is sometimes possible to find the alveolar antral artery, and anastomoses between the infraorbital artrery and the posterior superior alveolar artery.

As reported by Solar and Testori (26,27), it is not uncommon to find anastomoses between the infraorbital artery and the posterior superior alveolar artery in the medial wall of the sinus; furthermore, in the vestibular wall of the sinus there runs the alveolar antral artery, which is given off from the internal maxillary. This artery can run outside the vestibular wall of the sinus but it can also pass through the thickness of the sinus mucous membrane, or between the mucous membrane and the bony wall of the sinus; in some cases it can run through a groove in the bony vestibular wall or even pass through the thickness of the bone itself (Fig.s 2.22a-b).

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Fig.s 2.22a-b • The 3D CT reconstruction reveals the presence of a large anastomotic vessel at the level of the vestibular wall of the right sinus. The corresponding transaxial section confirms the diagnosis and demonstrates the involvement of the entire thickness of the bone wall: the risk of haemorrhage during the opening of the window is high.

When a window is made in the vestibular wall, the artery is usually included in the elevation of the membrane or, externally, in the thickness of the flap. If the artery is adjacent to the bony wall, there is a high risk, during osteotomy procedures, of damaging the alveolar antral artery.

Due to the arterial pressure, this vessel can initially bleed copiously, however, if the haemorrhage is properly controlled, it is not an irremediable emergency. Adequate preventive diagnostic investigations are of course important to prevent such intra-operative events.

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Before performing any procedure on the maxillary sinus, it is necessary to ascertain its patency, or rather its good aeration, which is synonymous with its good physiological functioning. A patent sinus, with no evidence of chronic (Fig. 2.23) or acute pathological processes, always shows good contrast on traditional radiographic images(28) and particularly on tomographic ones(29,30) (Fig.s 2.23a-b).

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Fig.s 2.23a-b • CT X-ray image: patent maxillary sinuses (a); the right maxillary sinus of the same case as it appears in the transaxial CT sections (b). The scan of the maxilla, however, does not reach the middle concha therefore it is not possible to show the patency of the ostium.

Chronic sinus pathologies result in thickening of the Schneiderian membrane and, when this is present, the sinus can show up opaque even on simple ortho-pan-tomographic (OPT) images (Fig.s 2.24a-b, Fig.s 2.25a-b).

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Fig.s 2.24a-b • X-ray (a) and transaxial (b) sections of a CT scan: the left maxillary sinus, strongly pneumatised, appears patent but it is not possible to form an idea of the mucosal thickness.

 

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Fig.s 2.25a-b • X-ray (a) and transaxial (b) sections of a CT scan: the left maxillary sinus, strongly pneumatised, appears opaque due to the thickening of the mucosa. This is clear evidence of previous chronic sinus pathology. Further anamnestic investigation is called for before intervening.

To be absolutely sure of the patency of the sinus, it is necessary, when using axial CT sections, to request that the scan extend, at the top, to the middle nasal concha, in order to verify the patency of the ostium (Fig. 2.23). Although coronal sections are actually better able to depict the communication of the ostium with the middle nasal meatus (Fig. 2.26), they tend not to be indicated in implantology, and therefore need to be requested explicitly.

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Fig. 2.26 • Coronal section of a CT scan: the red arrows indicate the communication with the middle nasal meatus. The two sinuses appear well aerated without pathological signs.

In the absence of pathological changes, the sinus membrane thickness cannot be ascertained a priori from the radiographic image, which means that the surgical risk of perforating the membrane is highly unpredictable.

The maxillary sinus varies considerably in size and shape during development and from individual to individual (4,27). In the past, perforation of the sinus membrane was considered a major complication; now, thanks to the advance of scientific knowledge and the development of correct surgical practices, it is merely seen as an inconvenient event which commonly occurs when the available bone lacks sufficient height. We are obviously talking about situations in which, thanks to appropriate technology and using blunt-ended implants and instruments, it is possible to perform a mini sinus lift using the osteotome (Summers) technique or using the implant body itself. Follow-up radiographs often show formation of bone apical to the implant.

As regards the lower part of the maxillary sinus, insufficient basal bone may make it impossible to position and, above all, to immediately load implants. In the same way, centripetal crest resorption tends to reduce the extension of the vestibular crest, and if the minimum crest width (5 mm) needed to stabilise the implant is not available, recourse must be had to crest thickening techniques (28,29).

As regards the vertical dimension of the alveolar crest, the literature recommends a minimum height of 4 mm if implants are to be stabilised in a single intervention (28-36). Nowadays there exist implants whose special morphology makes it possible to achieve primary stabilisation in the presence of a minimal amount of bone and thus that allow contemporaneous fixture positioning and intrasinus bone augmentation. When the quantity and quality of the bone are not sufficient to guarantee primary stabilisation of the implant, the solution most widely advocated in the literature is to perform the maxillary sinus lift several months prior to the definitive positioning of the fixtures (28-36) (Fig.s 2.27a-c).

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Fig. 2.27a • The lack of basal bone at the level of the left maxillary sinus and the impossibility of placing implants in a single surgical session indicate the need for prior elevation of the maxillary sinus.

Fig. 2.27b • Radiographic follow up at 6 months: the implants can be inserted.

Fig. 2.27c • Radiographic follow up at 5 years: the residual dental roots of 2.3, 2.4 and 2.5 have been extracted and 2 implants, 4.3×11.5mm, have been inserted (Intra-Lock, Boca Raton, FL, US). The intrasinus bone graft has conserved its dimensions and appears well-calcified.

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May 6, 2017 | Posted by in Prosthodontics | Comments Off on MAXILLARY ANATOMY: ASPECTS RELATED TO IMPLANT SURGERY AND SURGICAL PLANNING
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