Are Synonymous Cortical Screws Equivalent? Surface and Fatigue Properties of Screws from Two Manufacturers

Abstract

Small-fragment bone screws are everyday products in orthopedic surgery practice. There are several manufacturers of apparently synonymous products, their basic properties being described in international standards. The aim of the study was to compare surface topography and mechanical properties of two synonymous cortical screws from two manufacturers. Screws (3.5 × 24 mm) from two manufacturers (SM: Stratec Medical; SN: Smith & Nephew) were topographically examined, including scanning electron microscopy and testing according to ISO requirements; an accelerated fatigue test, using a staircase loading mode, was applied. The screws were subjected to 12 000 torsional cycles starting at ± 0.6 Nm, followed by another 12 000 cycles at a torque level which was incremented by ± 0.2 Nm. The total number of cycles as well as the torque level at failure was recorded. Both brands of apparently similar screws fulfilled the requirements of current product standards. Screws from SN displayed more surface features and sharper threads than did the screws from SM. The new accelerated fatigue test revealed, however, that the screws from SN sustained more loading cycles on a higher load level that did the screws from SM (p = 0.005). The clinical significance is difficult to interpret without data from systematic problem reporting systems for medical devices.

Key words

bone screws - mechanical testing - materials testing - standards - surface properties

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