Static frictional force and surface roughness of nickel-titanium arch wires Rubert R. Prososki, DDS, MS,* Michael D. Bagby, DDS, PhD,** and Leslie C. Erickson, DDS, MSD*** La Porte, Ind., Kearney and Lincoln, Neb.
Surface roughness and static frictional force resistance of orthodontic arch wires were measured. Nine nickel-titanium alloy arch wires were studied. One beta-titanium alloy wire, one stainless steel alloy wire, and one cobalt-chromium alloy wire were included for comparison. Arithmetic average roughness in micrometers was measUred with a profilometer. Frictional force resistance was quantified by pushing wire segments through the stainless steel seif-ligating brackets of a four-tooth clinical model. The cobalt-chromium alloy and the nickel-titanium alloy wires, with the exception of Sentalloy and Orthonol, e~
Tooth Movement
Fig. 1. Clinical examples of frictional forces opposing tooth/wire movement. Table I. Manufacturers, wires, and abbreviations
Unitek, Monrovia, Calif. Ormco, Glendora, Calif. Rocky Mountain Orthodontics. Denver, Colo. Glenroe Technologies, Sarasota, Fla. Lancer, Carlsbad, Calif. TP Orthodontics, LaPorte, Ind. Unitek, Monrovia, Calif.
Nitinol Ortho Form 11 (NIOL) NiTi Large Broad Arch (NITI) Orthonol (ORTtt) Marsenol (MARS) Titanal Modem Form (TIAL) Reflex (REFL)
Nitinol SE Ortho Form II (NISE) GAC, Central lslip, N.Y. Sentalloy Medium Accuform (SENT) American Orthodontics, She- Titanium Memory Wire Force boygan, Wis. I (TIUM) Ormco, Glendora, Calif. TMA (TMA) Unitek, Monrovia, Calif. Permachrome Stainless Steel (SS) Rocky Mountain Orthodontics, Blue Elgiloy (ELG) Denver, Colo.
directly with b r a c k e t / w i r e angle but that bracket width had no effect. Peterson et al. 7 also found that nitinol required less force than stainless steel. Garner et al. 8 showed beta-titanium and nitinol wires resisted sliding in stainless steel brackets more than stainless steel wires when tested without bracket angulation. Tidy 9 and Drescher et al. I° measured frictional forces between brackets and arch wires while employ-
ing a variable force simulating biologic resistance. The biologic resistance force changed the angle of the bracket with respect to the wire and was the most significant factor studied. Tidy also found bracket width to be inversely proportional to frictional force, whereas wire and slot size had no effect. Tidy found nitinol wire to require more force than stainless steel wire with stainless steel brackets, but Drescher et al. did not. Kusy et a l . " used laser spectroscopy to study surface roughness o f orthodontic wires. A m o n g the four • wire-alloy types that are commonly used in orthodontics, stainless steel appeared the smoothest, followed by cobalt-chromium, beta-titanium, and nickeltitanium. Kusy et a l . " cautioned that surface roughness and friction in orthodontic appliance systems have yet to be correlated. Kusy and Whitley ~2 were the first to look at the effect o f surface topography on friction coefficients. The results showed that low-surface roughness was not a sufficient condition for low-friction coefficients. The purposes o f this study were to measure surface roughness and static frictional force resistance o f orthodontic (particularly nickel-titanium alloy) arch wires.
MATERIALS AND METHODS Twelve different wire varieties were tested in this research. The manufacturers, wire names, and abbreviations are included as Table I. Nitinol, Ni-Ti, Orthonol, Marsenol, Titanal, Reflex, Nitinol SE, Sentalloy, and Titanium are nickel-titanium alloy arch wires. TMA (beta-titanium alloy),
l,blume I00 Number 4
Friction atzd rouglmess of nickel-titanium arch wires
343
Pcrmachrome (stainless steel alloy), and Elgiloy (cobaltchromium alloy) wires were included for comparison. Ten preformed 0.017 z 0.025-inch maxillary arch wires of each of the 12 wire varieties were cut in half. The posterior segment of one half was used for the roughness measurement. The posterior segment of the other half was used for friction determination. Surface characteristics of each wire variety were examined with scanning electron microscopy (JOEL USA, Inc., Peabody, Mass.). Arch forms of the nickel-titanium wires were not modified. Arch forms of the beta-titanium, stainless steel, and cobalt-chromium alloy wires were modified to conform to the pentamorphic arch form for frictional resistance measurements. With the exception of the noted archform modification, all wires were tested as received from the manufacturers.
Roughness Measurements A surface profilometer (Sloan profilomctcr and Dektak Analysis Program, MTW & Associates, Tustin, Calif.) was used to measure wire surface roughness. Proprietary software was used to calculate the arithmetic average roughness, R ( a a ) . " Scans of 6 mm were obtained from each wire specimen.
Friction Measurement A clinical simulation similar to the posterior segments of Fig. 1, B was constructed. Acrylic dowels were placed in the canine, the first and second premolar, and the first molar positions of a pcntamorphic arch form (Fig. 2, A)." The mesial-distal dimensions (dowel diameters) were obtained from Wheeler's Dental Anatomy, Physiology, and Occlusion." Stainless steel self-ligating 0.022-inch slot brackets (Edgelock, Ormo, Glendora, Calif.) were placed on the plastic dowels, incorporating a 1.5 mm curve of Spee (Fig. 2, B ) The posterior sections of the arch wires were pushed through the bracket assembly by a universal testing machine (lnstron Corp., Canton, Mass.) at a crosshead speed of 2 mm/minute (Fig. 3). Static frictional force was the amount of force required to initiate movement of the arch wire through the bracket assembly. Tests were performed in artificial saliva (Xero-Lube, Scherer Laboratories, Dallas, Texas) at 37 ° C.
Fig. 2. Occlusal view (A) and facial view of bracket assembly (B)
Statistics Data were statistically analyzed with SAS software (SAS Institute, Cary, N.C.). Variances were found to be unequal with the F.~. test'6; thus repeated t tests were used. The t test used a weighted average degree of frccdom. " The probability of committing a type 1 error was maintained at the 5% level by adjusting the level of significance, a. Assuming k number of comparisons, the appropriate critical value, a ' , was obtained with a ' = 1 - (1 - or)'/~. Thus, to obtain an experiment error rate of 0.05 with 66 comparisons, a comparison error rate of 0.0008 was used. ~' Correlation coefficients for roughness and frictional force data were calculated for each wire and for all nickel-titanium wires as a group.
Fig. 3. Segment of arch wire being pushed through the bracket assembly by lnstron testing machine
RESULTS Fig. 4 is a bar graph showing the average surface roughness in micrometers and frictional force values in grams for each o f the wire~ tested. A m o n g the nickeltitanium wires, NITI, M A R S , and O R T H were the roughest, whereas S E N T was the smoothest. A m o n g
344
Prososki, Bagby, and Erickson
Am. J. Orthod. Den:ofac. Orthop. October 1991
2.5
1800 SURFACE
ROUGHNESS
,:~:~:~ F R I C T I O N A i .
FORCE 1500
¢:: 2 . 0
~.~
300
0.0
NITI
ORTH NIOL NISE REFL SENT MARS TMA TIAL ELG TIUM SS
m
0
Fig. 4. Average surface roughness and frictional force. Error bars indicate one standard deviation.
E
2000 r = 0.02
m
zm p"
1600
m
ILl 0 1200 O IJ. .J
,
Surface roughness and static frictional force resistance of orthodontic arch wires were measured. Nine nickel-titanium alloy arch wires were studied. One beta-titanium alloy wire, one stainless steel alloy wire, and one cobalt-chromium alloy wire were included for comparison. Arithmetic average roughness in micrometers was measUred with a profilometer. Frictional force resistance was quantified by pushing wire segments through the stainless steel seif-ligating brackets of a four-tooth clinical model. The cobalt-chromium alloy and the nickel-titanium alloy wires, with the exception of Sentalloy and Orthonol, e~
Tooth Movement
Fig. 1. Clinical examples of frictional forces opposing tooth/wire movement. Table I. Manufacturers, wires, and abbreviations
Unitek, Monrovia, Calif. Ormco, Glendora, Calif. Rocky Mountain Orthodontics. Denver, Colo. Glenroe Technologies, Sarasota, Fla. Lancer, Carlsbad, Calif. TP Orthodontics, LaPorte, Ind. Unitek, Monrovia, Calif.
Nitinol Ortho Form 11 (NIOL) NiTi Large Broad Arch (NITI) Orthonol (ORTtt) Marsenol (MARS) Titanal Modem Form (TIAL) Reflex (REFL)
Nitinol SE Ortho Form II (NISE) GAC, Central lslip, N.Y. Sentalloy Medium Accuform (SENT) American Orthodontics, She- Titanium Memory Wire Force boygan, Wis. I (TIUM) Ormco, Glendora, Calif. TMA (TMA) Unitek, Monrovia, Calif. Permachrome Stainless Steel (SS) Rocky Mountain Orthodontics, Blue Elgiloy (ELG) Denver, Colo.
directly with b r a c k e t / w i r e angle but that bracket width had no effect. Peterson et al. 7 also found that nitinol required less force than stainless steel. Garner et al. 8 showed beta-titanium and nitinol wires resisted sliding in stainless steel brackets more than stainless steel wires when tested without bracket angulation. Tidy 9 and Drescher et al. I° measured frictional forces between brackets and arch wires while employ-
ing a variable force simulating biologic resistance. The biologic resistance force changed the angle of the bracket with respect to the wire and was the most significant factor studied. Tidy also found bracket width to be inversely proportional to frictional force, whereas wire and slot size had no effect. Tidy found nitinol wire to require more force than stainless steel wire with stainless steel brackets, but Drescher et al. did not. Kusy et a l . " used laser spectroscopy to study surface roughness o f orthodontic wires. A m o n g the four • wire-alloy types that are commonly used in orthodontics, stainless steel appeared the smoothest, followed by cobalt-chromium, beta-titanium, and nickeltitanium. Kusy et a l . " cautioned that surface roughness and friction in orthodontic appliance systems have yet to be correlated. Kusy and Whitley ~2 were the first to look at the effect o f surface topography on friction coefficients. The results showed that low-surface roughness was not a sufficient condition for low-friction coefficients. The purposes o f this study were to measure surface roughness and static frictional force resistance o f orthodontic (particularly nickel-titanium alloy) arch wires.
MATERIALS AND METHODS Twelve different wire varieties were tested in this research. The manufacturers, wire names, and abbreviations are included as Table I. Nitinol, Ni-Ti, Orthonol, Marsenol, Titanal, Reflex, Nitinol SE, Sentalloy, and Titanium are nickel-titanium alloy arch wires. TMA (beta-titanium alloy),
l,blume I00 Number 4
Friction atzd rouglmess of nickel-titanium arch wires
343
Pcrmachrome (stainless steel alloy), and Elgiloy (cobaltchromium alloy) wires were included for comparison. Ten preformed 0.017 z 0.025-inch maxillary arch wires of each of the 12 wire varieties were cut in half. The posterior segment of one half was used for the roughness measurement. The posterior segment of the other half was used for friction determination. Surface characteristics of each wire variety were examined with scanning electron microscopy (JOEL USA, Inc., Peabody, Mass.). Arch forms of the nickel-titanium wires were not modified. Arch forms of the beta-titanium, stainless steel, and cobalt-chromium alloy wires were modified to conform to the pentamorphic arch form for frictional resistance measurements. With the exception of the noted archform modification, all wires were tested as received from the manufacturers.
Roughness Measurements A surface profilometer (Sloan profilomctcr and Dektak Analysis Program, MTW & Associates, Tustin, Calif.) was used to measure wire surface roughness. Proprietary software was used to calculate the arithmetic average roughness, R ( a a ) . " Scans of 6 mm were obtained from each wire specimen.
Friction Measurement A clinical simulation similar to the posterior segments of Fig. 1, B was constructed. Acrylic dowels were placed in the canine, the first and second premolar, and the first molar positions of a pcntamorphic arch form (Fig. 2, A)." The mesial-distal dimensions (dowel diameters) were obtained from Wheeler's Dental Anatomy, Physiology, and Occlusion." Stainless steel self-ligating 0.022-inch slot brackets (Edgelock, Ormo, Glendora, Calif.) were placed on the plastic dowels, incorporating a 1.5 mm curve of Spee (Fig. 2, B ) The posterior sections of the arch wires were pushed through the bracket assembly by a universal testing machine (lnstron Corp., Canton, Mass.) at a crosshead speed of 2 mm/minute (Fig. 3). Static frictional force was the amount of force required to initiate movement of the arch wire through the bracket assembly. Tests were performed in artificial saliva (Xero-Lube, Scherer Laboratories, Dallas, Texas) at 37 ° C.
Fig. 2. Occlusal view (A) and facial view of bracket assembly (B)
Statistics Data were statistically analyzed with SAS software (SAS Institute, Cary, N.C.). Variances were found to be unequal with the F.~. test'6; thus repeated t tests were used. The t test used a weighted average degree of frccdom. " The probability of committing a type 1 error was maintained at the 5% level by adjusting the level of significance, a. Assuming k number of comparisons, the appropriate critical value, a ' , was obtained with a ' = 1 - (1 - or)'/~. Thus, to obtain an experiment error rate of 0.05 with 66 comparisons, a comparison error rate of 0.0008 was used. ~' Correlation coefficients for roughness and frictional force data were calculated for each wire and for all nickel-titanium wires as a group.
Fig. 3. Segment of arch wire being pushed through the bracket assembly by lnstron testing machine
RESULTS Fig. 4 is a bar graph showing the average surface roughness in micrometers and frictional force values in grams for each o f the wire~ tested. A m o n g the nickeltitanium wires, NITI, M A R S , and O R T H were the roughest, whereas S E N T was the smoothest. A m o n g
344
Prososki, Bagby, and Erickson
Am. J. Orthod. Den:ofac. Orthop. October 1991
2.5
1800 SURFACE
ROUGHNESS
,:~:~:~ F R I C T I O N A i .
FORCE 1500
¢:: 2 . 0
~.~
300
0.0
NITI
ORTH NIOL NISE REFL SENT MARS TMA TIAL ELG TIUM SS
m
0
Fig. 4. Average surface roughness and frictional force. Error bars indicate one standard deviation.
E
2000 r = 0.02
m
zm p"
1600
m
ILl 0 1200 O IJ. .J
,
