CONTROL ID: 2600027
CONTACT: neil meredith
Abstract Details


Torque, tightening and handling charactersitics of a novel abutment screw with an integral PTFE shim.

AUTHORS: meredith, neil

The tightening and preload characteristics of dental implant abutment screws are critical to maintain prosthesis tightening and avoid fracturing during the lieftime of an implant supported restoration. The limitations of titanium and alloyed screws due to its high coeffcient of friction are well understood. This has been treated by the use of alloys or coatings; typically gold or DLC. Such coatings have disadvantages and often result in overtightening leading to an increase in the breaking strain with consequential fracture. A new abutment screw has been designed that has a PTFE shim on the bearing surface that allows a progressive increase in tightening torque whilst controlling friction. This in-vitro study was undertaken to measure and compare the torque and preload characteristics of a range of abutment screws under simulated clinical conditions.

A test rig was designed and fabricated to enable the application of a regulated varying torque (10,20,25 and 32Ncm) torque at speeds of 5,10,15 and 20 rpm to abutment screws inserted into a dental abutment connected to a load cell to measure preload during tightening. Measurements of peak preload were recorded together with a graphical plot of torque against tightening distance as a function of time. Titanium, alloyed, DLC coated, gold and the novel shimmed screw were evaluated. Tests were carried out under dry conditions and then repeated with simulated saliva as a lubricant.

Results indicated that Titanium and uncoated alloyed screws had the lowest preloads at all tightening torques. Plots also indicated that preload was achieved almost instantly. The PTFE shimmed screw generated the maximal preload under the tightening torque and resulted in a gently progressive preload. Moist conditions did not significantly decrease preloads.

Geometry, material and torque characteristics are critical parameters in the successful design of implant abutment screws. The use of a PTFE shim is novel and may overcome some of the issues associated with abutment screw fracture due to inadvertent overtightening attributable to screw coatings. In addition the elastoplastic nature of the shim may contribute to increase abutment-fixture sealing.

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