2. Part-time retention is recommended up to a year and often longer. • intramembranous bone formation; without In the cranial vault, the growth process is entirely the result of periosteal activity at the surfaces of the bones. Remodeling and growth occur primarily at the periosteum-lined contact areas between skull bones, theskeletalsutures,but periosteal activity also changes both the inner and outer surfaces of these plate-like hones. Although themajority of growthin the cranial vault occurs at thesutures,there is a tendency for bone to beremoved from the inner surfaceof the cranial vault, while at the same time new bone is addedohthe exterior surface. In contrastto the cranial vault, the bones(i.e., ethmoid, sphenoid, and occipital bones)of thecranial baseare formed initially incartilageand are later transformed byendochondral ossificationto bone. As ossification proceeds, bands of cartilage calledsynchondrosesremain between the centers of ossi-fication. These important growth sites are thesynchondrosisbetween the sphenoid and occipital bones, or spheno-occipital synchondrosis, theintersphenoidsynchondrosis, between the two parts of the sphe-noid bone, and thesphenoethmoidalsynchondrosis, between the sphenoid and ethmoid bones. Even-tually, these synchondroses become inactive.Note:The bones of the cranial base are not affected to a great degree by growth of the brain(since they are endochondral bones). 1.After age 6, the greatest increase in size of the mandible occursdistal to the first molars. Notes2.Thecondyle of the mandible grows byproliferation of cartilage. 3. The chief factor in the formation of the alveolar process is theeruption of teeth. 4. Arch length space for the eruption of permanent mandibular second and third molars is cre-ated byresorption at the anterior border of the ramus. 5. At birth the greatest dimension of the face iswidth. 6. After a tooth has been moved from one position to another, the resulting bone istransi-tional bone. 7. The dependence of tooth development and tooth eruption upon growth of bone and bones isconsiderable. 8. Growth of the cranial base generallyprecedesgrowth of the jaws. Important: Mandibular growthis in the downward and forward directions. Growth in thecondylein-creases the anterior-posterior(downward and forward pattern of growth)dimension of the mandible.