Norman Kwan, DDS, MS*; Shi Bin, DDS, MS**

Abstract:

A comparative study was conducted to investigate the stability of identical crowns retained by cement and screw under dynamic cyclic fatigue conditions. Forty identical crowns were each telescopically inserted onto hexagonal abutments connected to CP titanium threaded implants via an abutment screw torqued to 25 Ncm. Twenty of the crowns were cemented onto the abutment using HY Bond Zinc Phosphate Cement (Shofu Inc., Japan) while the other twenty crowns were secured to the abutment implant assembly with a retaining screw (1.4 x 4mm). Each assembly was embedded in an acrylic block that fits into the cell holder. The cell holder oscillates at 24? from the vertical at 0.02 Hz by a custom designed mechanism that simulates the chewing function. A calibrated Instron Material Testing Machine (Instron, MS, USA) exerted a variable load set at 15 kg at 14Hz on the crown during the test.

Crown mobility was periodically assessed in PTV values using the Periotest (Siemens AG, Bensheim, Germany). The endpoint was defined when the PTV >10, or when the sample reached 20M cycles.

Preliminary results indicate that the initial PTV of the cement and screw retained samples were not significantly different using the t-test with P>0.05. There is also no significant difference in the stability of the two groups. The cement retained group failed on average at 2.60M cycles (SD=2.27M cycles) for n=18, while screw retained samples failed at 2.17M cycles (SD=1.27M cycles) when n=20 (P>0.05). Two cement samples were omitted as a result of machine error. The mode of failure for the two groups is striking. Of the eighteen cemented samples, fifteen fractured at either the fixture or abutment screw level. Cemented samples were more rigid throughout the test and would fracture unexpectedly. In screw retained samples, the retaining screw would gradually become lose and were detected by increasing PTV values. There is also a consistent decrease in the residual torque of the abutment screw. When the screw samples were left to run continuously, it increased the probability of either fixture or abutment screw fracture. These results indicate that the load concentration in the two groups is different under the same test conditions.

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