[Morphometric analysis of the infrazygomatic crest zone in 84 mini-implants using cone-beam CT]

Objective: To investigate the soft and hard tissue morphology in the infrazygomatic crest zone by observing the cone-beam computed tomography (CBCT) scans in patients with mini-implants. Methods: CBCT scans of 43 patients performed from January 2014 to December 2016 in the Department of Stomatology, Beijing Friendship Hospital, Capital Medical University, with 84 existing mini-implants in the infrazygomatic crest region were collected. The bone thickness and cortical bone thickness were measured in the palatal and buccal side of the mini-implant. The maxillary sinus membrane thickness, sinus septa, reverse fold, and the sinus opening angle were also determined and analyzed in the maxillary posterior region. Results: The bone thicknesses in the buccal and palatal side of the existing mini-implant were 2.5 (1.5, 3.2) and 5.2 (4.0, 6.4) mm, respectively. However, the corresponding cortical bone thicknesses were only 2.1 (1.3, 2.8) and 1.5 (1.0, 1.9) mm, respectively. The prevalences of the septa and the reverse fold were 33% (28/84) and 45% (38/84), respectively. The sinus opening angle was the largest in the mid-coronal plane of the maxillary first molar (71.6°±15.6°). In the coronal plane located at 10 mm mesially, the angle was the smallest (46.1°±18.0°), and in this area, 20% (16/82) of the angle was less than 30°. Conclusions: The outer cortical plate of the infrazygomatic crest along with the cortical plate of sinus floor could be related to the initial stability of mini-implant anchorage. The anatomic variation such as the reverse fold indicated thorough consideration of insertion depth and angle to avoid unexpected sinus injury.