Force and Torque Measurements During Rotational Insertion of Endodontic Files

Objectives: to determine the effects of insertion-rate and rotational-velocity on the relative force and torque values occurring during the insertion and cutting of endodontic files with helical and non-helical designs.  Methods:  Acrylic microscope slides with five uniform pilot channels (0.35mm) were mounted in a freely rotating fixture coupled to a LVDT for measurement of induced torque. The fixture was mounted on a load cell to permit measurement of load during insertion of the rotating file.  Files were axially aligned in a handpiece that was affixed to the cross-head of an Instron testing instrument.  The experimental classification variables were: file-type (Pow-R/helical, Liberator/non-helical; Miltex, Inc., York, PA), insertion-rate (7.5/15.0 mm/min) and rotational-velocity (250/500 rpm). For each of the eight experimental groups, 20 files were tested (total=160). Coding was by file-type, insertion-rate, and rotational-velocity, e.g., PR75500 indicates Pow-R, insertion-rate 7.5 mm/min, file rotational-velocity 500 rpm.  Force and torque data were acquired by the Instron's Bluehill2 software. Data were analyzed with SAS software for statistically significant differences among the experimental groups (GLM and post hoc Duncan multiple range tests; α=0.05). Results:  Means and standard deviations were: 

CODE	FORCE (N)	CODE	TORQUE (N-mm)
PR75500	0.0019±0.1864       A	PR150500	0.0004±0.0003         A
PR150500	-0.2170±0.0859       B	PR75500	0.0004±0.0003         A
PR75250	-0.2913±0.0728       B	PR75250	0.0003±0.0003         A
PR150250	-0.4023±0.1208       C	PR150250	0.0002±0.0004         A
LB150250	0.2853±0.0230        A	LB150500	0.0063±0.0112         A
LB150500	0.2609±0.0807        A	LB150250	0.0016±0.0157         AB
LB75250	0.1746±0.1136        B	LB75250	0.0004±0.0004            B
LB75500	0.1130±0.0379        B	LB75500	0.0003±0.0003            B

Same letter=n.s.d.

Conclusions:  (1) The maximum mean force and torque values were measured for the non-helical (LB) files. (2) Force values for the non-helical files were dependent on insertion-rate. (3) Torque values were independent of insertion-rate and rotational-velocity for helical design. (4) Minimal and negative force values were observed for the helical design.    Funding: J. Dean Robertson Society, OUCOD.