The In Vitro Biomechanics of Locking Plates Used in a Supercutaneous Fashion Compared to a Type Ia External Skeletal Fixator

The aim of this study was to determine the biomechanics of a supercutaneously placed String of Pearls plate (SOP) and supercutaneously placed locking compression plate (LCP) to be used in lieu of a type Ia external fixator.Nine surrogate constructs of 3.5 mm SOP, 3.5 mm LCP and type Ia external fixator offset 20 mm from the cortex over a 10-mm fracture gap were tested in non-destructive loading to 200 N and then tested in load to 100% displacement.Maximum displacement was used to calculate median maximum displacement in non-destructive axial compression testing for the SOP, LCP and external fixator, measuring 8.5 mm (interquartile range [IQR] 8.23-8.95), 10.3 mm (IQR 9.99-10.40) and 1.2 mm (IQR 1.17-1.31), respectively. There was a difference between the external fixator to LCP (<i>p</i> = 0.0050), external fixator to SOP (<i>p</i> = 0.0050) and LCP to SOP (<i>p</i> = 0.0050). Maximum load was used in compression to 100% displacement to calculate the median maximum load. The SOP, LCP and external fixator, withstood median loads to 100% displacement of 244 N (IQR 231.83-257.19), 168 N (IQR 165.07-170.86) and 766 N (IQR 688.14-784.29), respectively. There was a difference between external fixator to SOP (<i>p</i> = 0.0047), external fixator to LCP (<i>p</i> ≤ 0.001) and LCP to SOP (<i>p</i> = 0.0024).The type Ia external fixator is significantly stiffer compared with the supercutaneous 3.5 mm SOP and 3.5 mm LCP. Given the low stiffness and high displacement, supercutaneously placed locking plates may be unsuitable for clinical fracture repair in a 20-kg dog with an expected axial load of 200 N.