This study assessed defect depth and volume resulting from root instrumentation using a magnetostrictive ultrasonic scaler with a slim scaling tip (P 12) in vitro. Combinations of the following working parameters were analyzed: lateral forces of 0.5 N, 1 N, and 2 N; tip angulations of 0 degrees, 45 degrees, and 90 degrees; power settings of low, medium, and high; and instrumentation time of 10, 20, 40, and 80 seconds. Defects were quantified using a 3-dimensional optical laser scanner. Lateral force had the greatest influence on defect volume compared to tip angulation and power setting (beta-weights 0.53+/-0.04, 0.17+/-0.04, and 0.19+/-0.04, respectively). Lateral force and tip angulation had similar effects on defect depth, but both effects were greater compared to power setting (beta-weights 0.43+/-0.04, 0.49+/-0.04, and 0.19+/-0.04, respectively). The various combinations of the assessed working parameters showed synergistic effects resulting in a wide range of defect depths (14.1+/-1.7 microm to 410.5+/-51.1 microm) and volumes (0.0084+/-0.0057 mm3 to 1.3+/-0.079 mm3). Severe root damage (defect depth >50 microm) at 40 seconds instrumentation time occurred under most combinations of lateral force, angulation, and power settings. The only exceptions were combinations of: 1) 0 degrees angulation, 0.5 N and 1 N lateral force at any power setting and 2) 45 degrees angulation, 0.5 N lateral force at low and medium power setting. The efficacy of the assessed magnetostrictive ultrasonic scaler may be adapted to the various clinical needs by adjusting the lateral force, tip angulation, and power setting.