**2. Anatomy of the skeleton**

The fundamental bony skeleton of the jaws consist of a mandible and two maxillary bones. Because of the functional aspect of these structures and their atrophic changes during aging, anatomical features have specific importance to distinguish defects and determine the proper treatment plan. The quantity and quality of bone in the alveolar process and adjacent structures are the key elements of this issue. The anatomical knowledge of these structures is also a determinant factor when using them as donor sites for reconstruction.

**The alveolar bone of mandible and maxilla** is a functional bony process which harbors teeth in a dentate human. After tooth loss, this bony structure loses its dimensions both vertically and horizontally [3]. After atrophic sequences, the maxillary alveolar arch diameter decreases, despite the fact that the mandibular alveolar arch enlarges in diameter and a pseudo-class III relation may appear in severe atrophic alveolar ridges (Figure 1).

**Figure 1.** A, The atrophic changes of mandible. B, The atrophic changes of maxilla.

The quality of edentulous alveolar bone is classified to D1, D2, D3 and D4 based on cortical bone thickness and density of trabecular bone respectively.

**D1** demonstrates the thickest cortical bone and the most dense trabecular part and is usually located in anterior mandible;

**D4** demonstrates a large volume of low density trabecular bone and thin cortices and is located mainly in posterior maxilla.

**D2 and D3** with intermediate characteristics are located in posterior mandible and anterior maxilla respectively [4].

**The maxillary tuberosity** is located in the posterior maxillary bone on each side and contains low density D4 bone and attached to the pterygoid plates at the pterygomaxillary junction. It is located next to important anatomical structures- the pterygomaxillary fissure and pterygo‐ palatine fossa.

**The maxillary sinus** is a pyramidal cavity in each maxilla with a broad base medially and an apex laterally. Its size varies depending on the patient's age and presence of teeth. During the lifetime the sinus enlarges continuously and at the age about 12, the floor of the sinus is almost at the level of the nasal floor. Maxillary posterior teeth loss and sinus pneumatization are responsible for decreasing bone volume in this area.

**The mandible** is the largest bone of the face and generally consists of thicker cortical bone compared to the maxilla. The anterior border of ramus as runs toward the mandibular body creates external oblique ridges bilaterally. The mandibular canal begins from the mandibular foramen at the middle medial surface of ramus horizontally and vertically and ends at the mental foramen on the buccal surface of the mandibular body near the apices of the premolar teeth on both sides. The least distance from the mandibular canal to the buccal cortex is in the distal part of the mandibular first molars. The canal course through the mandible usually makes a loop near the mental foramen with about a 3 mm diameter. The neurovascular bundle travels through this canal to supply sensation and blood to the mandibular teeth and some part of the chin.

**The buccal fat pads** or Bichat's fat are located lateral to the buccinator muscles bilaterally and consist of four parts; body, temporal, buccal, and pterygoid extensions. Buccal fat pads are supplied by the temporal and transverse facial arteries. The buccal fat pads are very useful structures in reconstruction of oral defects [5, 6].
