Effects of exposure length, cortical and trabecular bone contact areas on primary stability of infrazygomatic crest mini-screws at different insertion angles.

ABSTRACT

BACKGROUND: The infrazygomatic crest mini-screw has been widely used, but the biomechanical performance of mini-screws at different insertion angles is still uncertain. The aim of this study was to analyse the primary stability of infrazygomatic crest mini-screws at different angles and to explore the effects of the exposure length (EL), screw-cortical bone contact area (SCA), and screw-trabecular bone contact area (STA) on this primary stability. METHODS: Ninety synthetic bones were assigned to nine groups to insert mini-screws at the cross-combined angles in the occlusogingival and mesiodistal directions. SCA, STA, EL, and lateral pull-out strength (LPS) were measured, and their relationships were analysed. Twelve mini-screws were then inserted at the optimal and poor angulations into the maxillae from six fresh cadaver heads, and the same biomechanical metrics were measured for validation. RESULTS: In the synthetic-bone test, the LPS, SCA, STA, and EL had significant correlations with the angle in the occlusogingival direction (rLPS = 0.886, rSCA = -0.946, rSTA = 0.911, and rEL= -0.731; all P < 0.001). In the cadaver-validation test, significant differences were noted in the LPS (P = 0.011), SCA (P = 0.020), STA (P = 0.004), and EL (P = 0.001) between the poor and optimal angulations in the occlusogingival direction. The STA had positive correlations with LPS (rs = 0.245 [synthetic-bone test] and r = 0.720 [cadaver-validation test]; both P < 0.05). CONCLUSIONS: The primary stability of the infrazygomatic crest mini-screw was correlated with occlusogingival angulations. The STA significantly affected the primary stability of the infrazygomatic crest mini-screw, but the SCA and EL did not.