Osseus defects around implants. They are classified into four main groups.
Group I defects: These demonstrate moderate horizontal bone loss with a minimal intrabony component. This group of implants is usually covered by a thin buccal/ lingual bone crest at the time of placement and are at an early stage of periimplant breakdown.
Group II defects Present moderate to severe horizontal bone loss with a minimum intrabony component. Its an advanced version of Group I.
Group III defects Demonstrate minimal to moderate horizontal bone loss with an advanced circumferential intra bony lesion. These implants are initially covered by a thin coronal bone crest and a wider apical bone base. Frequently the pattern of bone loss has a symmetric feature with a circular trough of uniform width and depth occurring around the circumference of the implant.
Group IV Presents more complicated implant defects with moderate horizontal bone loss with an advanced circumferential intrabony lesion; additionally the buccal and/or lingual bone plate has been lost.