An in vitro mechanical study was performed to compare the fatigue properties and the loads to failure of four plate designs that are used to stabilise the tibial plateau levelling osteotomy (TPLO). Seventy-two gapped osteotomy models were created using 3.5 mm versions of the following plates: a standard TPLO plate, a low profile TPLO plate, a locking TPLO plate (LocP), and a broad locking TPLO plate (bLocP). The 18 constructs for each plate design were sub-divided into three identical groups of six. Six constructs were mounted in a materials testing device and subjected to cyclic compressive loading until failure was observed or one-million cycles were achieved. Additionally, six constructs of each plate design were tested in compressive axial loading and six others in four-point bending in a load-to-failure manner. To provide residual strength comparisons, the six constructs undergoing cyclic fatigue compression testing were also tested in axial compressive loading. There were not any significant differences in stiffness between plates tested in axial compressive loading. However, mean loads at failure were higher for the LocP and bLocP constructs. The bLocP had a significantly higher mean stiffness and mean load at failure compared to the other constructs tested in four-point bending.