Tooth #18 (mandibular left second molar) is mesially inclined due to the absence of tooth #19. When using a coil spring to upright #18, what unintended movement may occur?

When a coil spring is placed mesial to a mesially tipped molar to upright it, the principal intended movement is distalization (tipping the crown distally to upright the tooth). However, an uncontrolled coil spring also generates a vertical (extrusive) force component due to the direction of the spring's force vector. This can cause unwanted extrusion of the molar unless intrusive anchorage is provided. Proper orthodontic mechanics for molar uprighting include placing the spring on the arch wire and using tip-back bends or mini-implant anchorage to control the vertical dimension and achieve true uprighting without extrusion.