Excessive mesial - axial inclination of the mandibular second molar or inadequate posterior arch length cause the mandibular second molar to become impacted against the distal contour of the first permanent molar 摘要下颚牙弓空间不足或第二大臼齿本身过度的近心轴向倾斜，常造成第二大臼齿前倾和阻生。
