Fatigue study of cancellous and cortical stainless steel screws used with polymethymethacrylate and the SOP plate system for the immobilisation of vertebral fracture and luxation in canine cadavers

Abstract

Globally, vertebral fracture and luxation (VFL) are one of the most common neurological injuries in dogs and cats. The standard care for vertebral body stabilisation of canine VFL are screws with polymethylmethacrylate (PMMA) and the string of pearls (SOP) plate system fixation. The VFL with implants are usually exposed to force in a repetitive or cyclic loading fashion. Indeed, stress on the implant cyclically at a load significantly less than its ultimate tensile strength can cause fatigue failure, one of the major causes of implant breakage. This study aims to compare the failure mode of four different implants using a specifically designed fatigue testing machine. The four different implants were constructed as follows: 3.5 mm cancellous screw with manually applied PMMA (MP), 3.5 mm cancellous screws with syringe application of PMMA (CanP), 3.5 mm cortical screws with syringe application of PMMA (CorP) and 3.5 mm cortical screws with a SOP plate (SOP). The results revealed that the SOP group tolerated the most cycles before failure, followed by the CorP, CanP and MP groups repectively. The point of weakness, as defined by the failure mode, occurred in the middle of the PMMA bridge, 3.5 mm cancellous screw neck, PMMA at the screw neck position and 3.5 mm cortical screw in MP, CanP, CorP and SOP groups respectively. In conclusion, based on fatigue properties of the four implants tested in this study, the SOP plate is recommended as a standard fixation device for VFL in dogs.