Anatomic Considerations

The implant surgeon should be fully conversant with all anatomical structures that they are likely to encounter or that will affect the implant placement, which includes the maxilla, mandible, teeth and know in detail about the available bone at the implant site.

Anatomic Considerations of Maxilla
Firstly, lets discuss the important anatomical structures in maxilla which the implant surgeon must know and which will influence the implant placement.

a)Maxillary sinus : is a pyramidal shaped large cavity in the body of the maxilla containing many structures of concern during surgery. The sinus is lined by a membrane known as the Schneiderian membrane and it opens into the middle meatus of nose.

Schematic diagram depicting the maxillary sinus and its relations in a frontal section

In normal conditions, the maxillary sinus maintains a close relationship with the alveolar ridge associated with second premolar and first molar teeth.With age the sinus floor expands and is often in close relationship with the apices of the posterior teeth separated from the sinus mucosa by a thin layer of bone but occasionally may be in direct contact with mucosa. Loss of teeth causes the antrum to further expand, so that the sinus floor ultimately joins the crest of the ridge. This position of the sinus floor affects the implant placement, in that sinus lift procedure is necessary so that the implants can be placed without perforating the sinus floor.

Fig. : Extension of maxillary sinus in an extremely atrophic maxilla. Often only a paper-thin lamella of compact bone (1) Anterior nasal spine (2) Alveolar process of maxilla (3) separates the sinus from the oral mucosa. (4) Maxillary sinus 

The bony walls of the Maxillary sinus:
The Anterior wall of the sinus has thin compact bone above the apex of the canine teeth which may extend to the lateral piriform of the nose. The infraorbital neurovascular bundle lies directly in the bone and within the sinus mucosa which should be not be injured during surgery. The Superior wall represents the thin orbital floor. The infraorbital neurovascular bundle run in the infraorbital canal traversing this wall. Manipulation of this wall is not indicated during the sinus graft procedure. Posterior wall separates the antrum from the infra-temporal fossa and contains two important structures.

This wall should not be perforated during surgery. This limits bleeding from the pterygoid plexus or branches of the maxillary artery (Posterior superior alveolar artery) as complications. Medial wall separates the sinus from the nasal fossa. The maxillary ostium which drains the sinus into the middle meatus of the nasal cavity is located in the highest, most medial aspect of the sinus wall. Although its high location makes dependant drainage (causing increased susceptibility to infection) difficult, its location is favourable for the implant surgeon in that, it is unlikely to become obstructed by the routine maxillary augmentation in the inferior sinus region.Complications to be avoided during implant Maxillary Sinus Lift and Graft Surgery

Membrane Perforation: It is the most common complication during the sinus graft surgery resulting from the scoring of lateral access window for surgery and when elevating the membrane from the bony walls. Small size tear can be sealed by initially elevating membrane from all sides (circumelevation) and then in the tear region excess membrane seals the hole. Larger size tear requires a resorbable collagen membrane of longer resorption cycle.

Septa: Septa are thin bony plates, which divide the inferior portion of the antrum into sections and may even create separate compartments. A buttress or web formation may be present in the lateral wall of the maxilla. These septa are of benefit to bone augmentation creating additional bone growth regions within the graft. Inaddition these dense septa enhance initial stability for the implants & can be used as ideal future implant placement sites.

However, they may create added difficulty at the time of surgery in several ways. Elevation of the membrane may be difficult over the sharp webs often resulting in tear in the membrane. Also the separation of the lateral access window may not be possible, requiring the division of the access window into two with an additional vertical scoring line.

Maxillary sinus is multicompartmental with radiopaque bony septa/ buttress or web formation

Bleeding: Bleeding from injury to small blood vessels pushed along the lateral wall of expanding maxillary sinus should be contained by bone wax or electrocautery rather than crushing which can cause fracture of the thin bone, or sinus membrane tearing.

b)Associated nerves and vessels
The Anterior superior alveolar, Infraorbital, and Posterior superior alveolar nerves and arteries provide both the innervation and blood supply to the sinus. Sensory innervations to maxilla is from the Maxillary division of the Trigeminal nerve Posterior superior alveolar nerve and vessels: found in the posterior aspect of the maxilla running between the bone and the lining of the maxillary sinus may be injured during a sinus augmentation procedure with a lateral approach.Infraorbital nerve and vessels: The infraorbital nerve exits through the infraorbital foramen. This foramen and the neurovascular contents are within 5 to10mm of extremely resorbed maxilla. When applying onlay grafts which expose the entire maxilla, the implant dentist must be aware of the situation. Fixation screws or implants may cause paraesthesia when inserted through the graft and into this structure.

Subperiosteal implants designed for atrophied maxilla should not extend into the site of the infraorbital nerve and vessels.Anterior superior alveolar nerve: is a branch of infraorbital nerve which arises within the infraorbital canal. It gives a nasal branch which passes into nasal cavity to supply the mucosal lining of a portion of the nasal cavity. Before elevation of nasal mucosa and placement of grafts this nerve must be anaesthetized. Infraorbital nerve block is suggested.

The anterior, middle and posterior superior alveolar nerves run in the facial wall of the maxillary sinus between its lining membrane and the bone. During antrostomy procedures to augment the floor of the sinus the operator should be aware of these structures which are present even in the absence of teeth.Greater Palatine nerve and vessel : The greater palatine nerve runs forward in a groove on the inferior surface of the hard palate to communicate with   nerve in incisor region. The greater palatine artery and vein accompany the nerve.As the maxillary alveolar process atrophies it shifts to the palate bringing the crest of the ridge closer to the greater palatine neurovascular bundle. The implant dentist should be aware that an incision too palatal to the crest of the ridge in the atrophied maxilla may injure these vital structures.

Naso palatine nerve and vessels :- found in the nasopalatine canal, must be taken care off when placing implants in the maxillary incisor region. Subperiosteal implant placement often results in the severing of these vessels causing a minor degree of paraesthesia on the palate.

c) Posterior Maxilla

Posterior maxilla has very poor quality of bone demanding special consideration in implant placement:

•  Increasing the number of implants to decrease the crestal stresses.
•  Hydroxyapatite coatings(HA) on implant to improve surface contact between the implant and bone
• Use of threaded implant design(deep threads)which increase the surface area of support
•  Use of relatively longer and wider implants for initial stability and early loading.

The maintenance of the 2mm zone of safety from the opposing anatomic landmark for implant placement is not indicated in posterior maxilla. As long as the rotary instruments do not perforate the thin cortical plate and/or membrane lining of sinus, no contraindications exist to preparation or implant placement within this boundary.

d)Floor of nose and Nasal spine

Although variable, the nasal floor is typically situated 5-9mm below the level of inferior turbinates. The inferior piriform rim is used as a guideline for the height of the opposing cortical plate to determine the implant length, and care should be taken to prevent perforation of the nasal floor and mucosa during implant placement.Another consideration is the gradual loss of canine eminence distal to the lateral piriform rim in edentulous patients, forming a recess behind it. Implants placed in this position in extremely resorbed maxilla which is palatally placed and closer to nasal cavity will penetrate the nasal cavity.

Implant perforating the nasal floor

e) Maxillary Tuberosity

Incision and reflection of the mucosa overlying the areas of the maxillary tuberosity and hamular notch when taking impressions for maxillary subperiosteal implant or harvesting bone graft must avoid injury to the tendon of tensor veli palatini muscle which passes around the pterygoid hamulus.Maxillary tuberosity offers a variable amount of cancellous bone for grafting procedure. This area is convenient for use in maxillary sinus grafting and may also be considered for smaller areas of ridge augmentation. The anatomic limitations of this area include the maxillary sinus, pterygoid plates, adjacent teeth when present, and the greater palatine canal. Although the sinus may inadvertently be entered during removal of the graft, coverage of the perforation with the thick mucosa should preclude the development of an oroantral communication. Care should be taken to avoid fracturing the posterior maxilla.