Chris J. Lampert, DMD Printer Friendly Version

Rotary Instrument Separation

Rotary nickel-titanium instrument separation is an event that can easily turn an otherwise ordinary appointment into an undesirable experience. Although many times the long-term prognosis for the tooth is unchanged, the event casts a negative light on the root canal procedure for both the patient and doctor. Understanding the physical and mechanical causes for rotary instrument separation is critical to preventing and limiting its occurrence.

Causes of rotary instrument failure can be divided into two main categories. The first is cyclic fatigue failure. The second is torque failure. Although these two causes are often discussed independently, torque failure is more likely to occur after an instrument has experienced cyclic fatigue. Therefore, cyclic fatigue and torque separation are not necessarily independent.

Understanding Cyclic Fatigue Failure
Cyclic fatigue is defined as the accumulated strain that develops from repeated bending of an object at the same location. The repeated bending action cycles tension and compressive forces at the site of bending. An example of cyclic fatigue is demonstrated when a paper clip is repeatedly bent at the same location. With every bend the paper clip becomes more fragile (fatigued) and eventually breaks. In a canal, cyclic fatigue accumulates at the site of the canal curvature. The main factors affecting cyclic fatigue in rotary endodontic instruments are degree of canal curvature, speed of rotation, and instrument size (diameter).

Instrument size (tip diameter and taper) is the most important variable that influences cyclic fatigue failure because the clinician has control over this variable, where as canal curvature is a fixed variable that is beyond the clinician’s control. Smaller tip size and smaller tapered instruments are more resistant to cyclic fatigue because they are more flexible. The clinical relevance of this principle means that in curved canals, selecting an instrument with a .04 taper would be a safer choice than an .06 taper to prevent cyclic fatigue instrument separation.

Understanding Torque Failure
The second main cause of instrument separation is torsional load or torque failure.  Torque failure occurs when the rotational torque load applied to the instrument exceeds the torque limits of the instrument. An example of torque failure occurs when an instrument is forced apically under too much pressure and the tip binds under rotational force. Multiple studies have pointed out that an instrument’s torsional strength is directly related to its metal mass, and therefore is again influenced by instrument tip size and taper. As the cross-sectional diameter of the instrument increases, its resistance to torque induced separation increases. An important point to remember is that as the instrument diameter increases, the instrument cross-sectional surface area increases exponentially. The difference in diameter between a size 20 instrument and a size 30 instrument is a 50% increase, but the cross-sectional surface area is a 129% increase. This geometric feature of instrument size shows that using a slightly larger instrument size will greatly increase metal mass and therefore torque strength.

Torque related instrument failure has partially been remedied by the introduction of torque controlled auto-reversing motors. Torque controlled electric motors can be pre-set with torque limits just below the instrument torque limit so that the motor automatically reverses when the torque sensor reaches a certain torque. Torque controlled motors, when set just below the limit of elasticity for each instrument, reduces the risk of fracture markedly. It is important to remember that the limit of elasticity is different for different types and sizes of rotary instruments; therefore the proper torque limit must be selected for the individual instrument.

Preventing Instrument Separation
Understanding the mechanical principles behind instrument separation aids the clinician in the prevention or reduction of its occurrence. For example, knowing when the canal possesses a significant curvature, which is more likely to cause cyclic fatigue failure, it would be appropriate to use an .04 tapered instrument. Less tapered, smaller diameter instruments are more resistant to cyclic fatigue failure. Creating straight-line canal access is another method to reduce cyclic fatigue. Removing high cervical bulges in the orifice region or impinging access walls will effectively create a straighter, less curved path for the instrument to follow and reduce cyclic fatigue.

Preventing torque-induced failure is a more difficult task. Utilizing a torque controlled electric motor, larger tip diameter instruments in straighter canals, and creating a glide path for the rotary instrument to follow are all techniques that reduce torque induced instrument failure. Perhaps the most important skill for preventing torque-induced separation is the utilization of mild apical pressure when advancing a rotary instrument down a canal.

Unquestionably, rotary nickel-titanium instruments have revolutionized endodontic treatment. Rotary nickel-titanium instrumentation has produced more efficient, predictable shapes than could be achieved by hand instrumentation. Nickel-titanium endodontic instruments also have a greater ability to negotiate curved canals, and reduce transportation, zipping, ledging, or perforation. They also allow larger apical preparations of curved root canals while maintaining the original canal path. Along with all the benefits of rotary nickel-titanium instrumentation there is an inherent risk of instrument separation. Being mindful of cyclic fatigue and torque failure will help prevent the dreaded instrument separation.

Chris J. Lampert, DMD maintains a fulltime Endodontic practice in Portland, Oregon, and is involved with research, testing, and development of new endodontic products. Dr. Lampert received his DMD from Oregon Health and Sciences University and his Endodontic post-graduate degree from Boston University. 

Dr. Lampert can be reached at drlampert@thedentistsnetwork.net

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