Mechanical Fatigue Analysis Comparing Two Locking Plates in a Metaphyseal Fracture Model of the Distal Ulna

Abstract

The purpose of the study was to test the mechanical fatigue properties (fatigue limit and fatigue strength) of two locking plates designed for the distal ulna. Distal Ulna Plate 2.5 (Medartis, Basel, Switzerland) and 2.0 mm LCP Distal Ulna Plate (DePuySynthes, West Chester (PA), USA) were used. A metaphyseal fracture situation was chosen (AO 23-A1.3). Six plate-screw constructs of each design were mounted on custom made fixtures using a Sawbone's 3D ulna model. Testing was performed on monoaxial material testing machines. Endpoints were fatigue strength (load) and fatigue limit (cycles) when the sample fails. Initial load was set at Fmax 10 N. Sinusoidal loading was carried out at 4 Hz. After 50'000 cycles load was increased by 15% and again by 15% after each additional 10'000 cycles. 2.5 mm plates showed median fatigue strength of 40.5 N (range 5.3) and median fatigue limit of 141'307 cycles (range 5'436). 2.0 mm plates 13.2 N (range 5.2) and 67'287 cycles (range 28'718) respectively. Distal Ulna Plate 2.5 showed better mechanical properties.