Root canal therapy may include clinical instrumentation that is driven by an endodontic motor. Endodontic motors have traditionally been designed to maintain constant rotation of between 300 RPM and 1000 RPM. Typically, a clinician sets the rotation speed. Despite the application of variable torque and friction to the motor during use, constant motor rotation is often maintained by a feedback control loop that dynamically monitors and then acts to maintain the speed of the motor. In this manner, the power output to the endodontic motor varies in accordance with the variable torque and friction observed by the motor to maintain the preset motor speed.
More recently, developments within the endodontic field suggest a performance benefit from a reciprocating endodontic motor. Reciprocating endodontic motors drive the rotor and attached endodontic drill or file through a clockwise rotation and then a counterclockwise rotation. This clockwise-counterclockwise rotation cycle may be repeated very rapidly during treatment. By way of example, the reciprocating endodontic motor may rotate a file clockwise 160 degrees followed by a counterclockwise rotation of 40 degrees. This reciprocating motion may be repeated with any combination of clockwise and counterclockwise rotations of differing degrees to create novel and complex filing cycles. Moreover, by introducing two directions of movement, the attached drill or file has two potentially useful directions, each rotation direction of which is capable of specialization. For instance, a file may be designed for dual use such that it may cut while rotating clockwise, but grind or buff while rotating counterclockwise.
A further modification to the reciprocating endodontic motor is a coast feature. In this regard, at the end of a clockwise or counterclockwise rotation, the motor and attached file will coast under only the influence of its own momentum. For example, an endodontic motor programmed with the coast feature may be programmed to drive a file through a 120 degree clockwise rotation followed by an 80 degree counterclockwise rotation. Following the 120 degree clockwise rotation, the endodontic motor and attached file will continue to rotate, but such rotation will be the result of coasting beyond the specified clockwise rotation. Thus, coasting occurs prior to beginning a counterclockwise rotation of 80 degrees. Regardless of the prescribed angular values, the coast feature permits additional, though unpowered, rotation of the endodontic motor. The amount of such additional rotational coast is dynamically determined and may depend on the time permitted to coast, the angular momentum of the system, and/or the amount of friction encountered by the system. It will be appreciated that generally angular momentum of the motor increases coast time while increased friction reduces coast time.
As applied to reciprocating endodontic motors, the coast feature is a desirable feature because it allows the motor and file to dynamically and automatically respond to various and complex environmental factors, including the specific anatomy encountered during root canal therapy. Rather than the traditional preset and forced control drive which forces a highly prescribed motion, the coast feature allows for greater flexibility of rotational movement. For instance, where the file is spinning freely within the root canal, coast will allow the file to continue spinning while also cutting efficiently to reduce the time to complete the procedure. On the other hand, where the file encounters increased friction due to resistance, coast will decrease the rotation of the endodontic motor to reduce torsional and bending loads applied to the file. In either case, the coast feature provides significant procedural benefits.
Presently, a clinician must choose an amount of coast that is symmetrically applied to the tool. That is, coasting following a clockwise rotation is the same as the coasting following a counterclockwise rotation. Unfortunately, different tools create different frictional forces, which, in turn, affect the amount of optimal coast for a counterclockwise rotation and a clockwise rotation. For instance, a file cutting in the clockwise direction will encounter greater friction and resistance than the same file which produces a buffing action in the counterclockwise direction. Such divergent uses may force a clinician to choose a coast for either cutting or buffing, but not both simultaneously. Thus, a clinician desiring a particular coast in the clockwise direction must also accept this coast in the counterclockwise direction due to the nature of such symmetrical coast control. Identical coast settings are not necessarily advantageous, because significant rotation in the counterclockwise direction may lead to undesirable procedural phenomena such as extrusion of canal debris out of the canal apex, creating post operative pain for the patient.
There is a need for an apparatus and method for use in endodontic procedures, such as root canal therapy, that addresses present challenges and characteristics such as those discussed above.