C

Because the open-coil spring is placed only on the buccal side, the force line of action passes buccal to the first premolar’s center of resistance, generating a moment that tends to spin the tooth. Clinical studies of NiTi buccal coil springs report significantly greater rotation of premolars/canines compared with other mechanics, confirming rotation as the most frequent side-effect [Hashemzadeh 2022, PMID 34866026].