Step by Step
Improving Dental Abutment Retention
Zest Locator Bar Abutments can be used to increase holding-power strength.
By Allen L. Schneider, DDS, FAGD, DICOI
The holding-power strength of a dental abutment is the determining factor in whether or not that abutment will unscrew from an implant or cast bar during mastication. Micro-movements created by mastication will cause an attachment screw that is not torque-tightened according to manufacturer specifications to unthread itself and most likely fracture when it is no longer properly seated. Torque tightness recommended by dental manufacturers varies with the type and size of their abutment screw threads. In the past, manufacturer recommendations for tightening abutment screws with a torque driver to 20 Ncm were common;1 however, the current general rule is that 30 Ncm to 35 Ncm of torque tightness is necessary to eliminate accidental unthreading.
It is also important to know that hand tightening a dental abutment without the use of a torque wrench handle will generate approximately 15 Ncm of torque tightness, only half of what is necessary to prevent unthreading. The fact that it takes less torque to remove an abutment that has been tightened with a torque wrench than the amount of torque used to initially place the part has been attributed to a loss in preload, making it more likely that occlusal forces will cause a separation of the implant and abutment joint.2
Zest Abutment Tests
In an internal study, a series of five Locator Bar Abutments were threaded into the receptacle site of an implantretained cast bar to establish a control. Each abutment was torque-tightened to a value of 30 Ncm, then removed with the removal retention value being recorded for each.
A small amount of FujiCem Cement (GC America, www.gcamerica.com) was applied to a Locator Bar Abutment (Figure 1). This brand of dental cement was chosen because of its thin film thickness of 3 μm, compared to the traditional 8-μm film thickness for other cements. The threaded abutment with cement was then placed into the opposite side of the bar and torquetightened to a value of 30 Ncm (Figure 2). Excess cement was removed and the cement-threaded abutment was allowed to cure completely.
A Tohnichi Torque Gauge (Tohnichi America Corp, www.tohnichi.com) (Figure 3) was used to loosen each of the five threaded-only control abutments and the removal torque value was recorded. The abutments all displayed a removal torque value of 24 Ncm (6 Ncm less than the insertion torque tightness of 30 Ncm used to place the abutment). The Torque Gauge was then used to loosen the cement-threaded abutment. The removal torque value measured 34 Ncm (4 Ncm greater than the insertion increase of 42% in the holding-power strength of the abutment. This simple technique was found to significantly increase dental abutment removal retention beyond the value of its placement on a cast bar.
The torque tightness of 30 Ncm to 35 Ncm necessary to eliminate accidental unthreading and possible fracture of a Locator Bar Abutment was achieved by adding cement to the threaded abutment.
This cement technique is not recommended for a dental abutment being placed directly into an implant. A separate investigation should be made using medical grade, lock-tight, small-screw holding material for that purpose. Allen L. Schneider, DDS, FAGD, DICOI, maintains a private practice in Springfield, Virginia. torque tightness of 30 Ncm used to place the abutment).
The torque value difference between placing and removing the control series of Locator Bar Abutments on an implant- supported cast bar amounted to a 20% reduction in the holding-power strength of the abutment. This reduction in measured value was confirmed with a series of five separate abutments. By adding cement to a Locator Bar Abutment, the removal torque value of the cement-threaded abutment was increased to 34 Ncm from the 24 Ncm value recorded for the threaded-only control abutments. This represents an increase of 42% in the holding-power strength of the abutment.
This simple technique was found to significantly increase dental abutment removal retention beyond the value of its placement on a cast bar. The torque tightness of 30 Ncm to 35 Ncm necessary to eliminate accidental unthreading and possible fracture of a Locator Bar Abutment was achieved by adding cement to the threaded abutment.
This cement technique is not recommended for a dental abutment being placed directly into an implant. A separate investigation should be made using medical grade, lock-tight, small-screw holding material for that purpose.
1. Lavrentiadis G, Yousef H, Luke A, et al. Changes in abutment screw dimensions after off-axis loading of implant-supported crowns: A pilot study. Impl Dent. 2009;18(5):447-453.
2. Cho SC, Small PN, Ellan N, et al. Screw loosening for standard and wide diameter implants in partially edentulous cases: 3- to 7-year longitudinal data. Impl Dent. 2004;13(3):245-250.
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About the Author
Schneider, DDS, FAGD, DICOI