Fatigue study of cancellous and cortical stainless steel screws used with polymethymethacrylate and the SOP plate system for immobilisation of vertebral 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 is the use of screws with polymethylmethacrylate
(PMMA) and the string of pearls (SOP) plate system fixation. 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
fatigue property and failure mode of four different implants in canine cadaveric VFL. 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). Results revealed that the SOP group tolerated the most cycles before failure,
followed by the CorP, CanP and MP groups, respectively. The point of weakness, as defined by the failure mode,
occurred in the middle of PMMA bridge (MP group), neck of the cancellous screw (CanP group), PMMA at the screw
neck (CorP group), and 3.5 mm cortical screw (SOP group), respectively. In conclusion, based on the fatigue properties
of the four implants tested in this study, the SOP plate is recommended as a standard fixation device for VFL in dogs.