Bone is shrinking after tooth is extracted (Fig.6 vs. Fig.5), particularly the width: from 7.21 mm to 1.93 mm.  To place an implant, we need to increase the width of the bone.  One way to do it is to expand the bone, back to original article

Xin Wei, DDS, PhD, MS 1st edition 03/16/2011, last revision 09/27/2012