Volume 8, Issue 7 (July 2011)
Fixture Variations When Evaluating ASTM F1717 Construct Stiffness: Pin Diameter and Material
The ASTM F1717 test standard describes the fixtures to be used when testing spinal implant construct, but leaves some room to interpret. As a result, the same device could be evaluated using different fixturing and it would not be considered a deviation because the standard does not specifically allow or prohibit the method used. Three potential variations in fixturing when conducting F1717 testing were selected for analysis in this study: changes to pin diameter, changes to pin material, and changes to test fixture hole configuration. To evaluate these parameters, a sample F1717 construct was designed and manufactured. Specimens, N = 6, were tested in six different configurations of pin diameter, material, and hole configurations. A review of all F1717 test batteries conducted at Empirical Testing Corporation during CY2009 was conducted to provide context for results from the sample F1717 construct. Differences noted in test results for axial compression bending stiffness occurred when sleeves were used to adapt 3/8 in. pins diam for use in 1/2 in. diam holes. Significant differences were noted in torsional stiffness when using a half-hole configuration, and when comparing 3/8 in. diam pins to using 1/2 in. diam pins. The torsional stiffness and compression yield load of the sample lumbar F1717 construct is within the range of F1717 constructs tested during CY2009, but outside the range for compression stiffness and yield torque. Users of the test standard are advised to not use 3/8 in. diam pins (which are prescribed for cervical construct tests) when testing lumbar constructs, and to avoid the use of half holes in torsion testing.