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Extension plates for volar locking plates: A mechanical analysis
Daniel J Brown, MB ChB, FRCS
1, Ashley Newton, MB ChB, FRCS
2, Joseph Pia, MSc
3; Graham Cheung, MB ChB, FRCS
1(1)Liverpool University Hospitals, Liverpool, Merseyside, United Kingdom, (2)Liverpool University hospitals, Liverpool, Merseyside, United Kingdom, (3)Acumed LLC, Hilsboro, OR
IntroductionDistal radius extension plates were developed to provide modularity allowing the standard volar locking plates to be extended by up to 110mm. There has been some concern that the linking mechanism between the two plates may be the weak point in the assembly. Our hypothesis is that the linkage is not the weakest point in the system and that the construct is at least as strong as both the standard plate and similar plates currently available on the market.
Materials and MethodsThe testing was carried out in an independent biomechanical testing facility (Element Material Technologies Laboratories, Cincinnati, OH) on calibrated equipment that met industry standards for biomechanical testing (ASTM F382-17). Acu-loc 2 volar locking plate with extension and link screws (Acumed LLC, Hilsboro, OR) (test plates) were compared with ‘2.4mm VA LCP Two-Column Volar Distal Radius Plate' and ‘2.4mm Titanium VA LCP Extra-Articular Volar Distal Radius Plate' (both Depuy Synthes, Raynham, MA) (control plates) in a non-inferiority biomechanical study. All plates were mounted in PMMA filled sleeves so that the link mechanism and adjacent screw holes, and an identical length of the other plates were free. A bending force was applied to a single test plate until it yielded (static load to failure). Cyclical loading was then applied to three other identical test plate constructs, initially at 50% of the yield load, increasing by 10% if the previous load was tolerated for one million cycles. This continued until failure. For the control plates, cyclical testing started at the load that was survived by all three test plates.
ResultsThe initial test plate started to fail (0.2% offset yield) at 551.8N (or 7.174Nm) and ultimately failed at 9.762Nm, by plastic deformation at a point on the VDR plate, just proximal to the link mechanism.
In cyclical loading all three test plates survived one million cycles at 80% yield load, while one failed at the junction with the PMMA at 79,881 cycles at 90% yield load. None failed at the link mechanism.
The control plates were therefore tested cyclically at 80% of the test yield load and failed at between 1,260 and 2,265 cycles, at which point the test was concluded.
ConclusionsA Volar locking plate with an extension plate and link screw is, mechanically, an acceptable way of dealing with distal radial fractures with diaphyseal extension. Further, the link mechanism is not the weakest point of the plate construct.
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