Simple SummaryIdentifying risk factors for implant failure in small animal orthopedics could improve outcomes in clinical patients. Abnormal fracture healing has been hypothesized to correlate with implant failure. This study retrospectively reviewed a consecutive series of implant failure cases in dogs and cats. Area moment of inertia (AMI), plate working length, and bone screw density were determined as appropriate for each case. Implant failure occurred in 23% of the fracture cases in the study period and was associated with an increased risk of delayed union, malunion, or non-union of the fracture. Major complications were found in 56% of the fracture repairs and were associated with delayed union. Surgical revision was performed in 49% of implant failure cases. Common problems were implant loosening and failure of low AMI plates. Implant AMI should routinely be considered during preoperative planning.AbstractImplant failure is common in small animal orthopedics, but risk factors are rarely reported. Our objective was to determine whether abnormal fracture healing was associated with implant failure after fracture fixation in dogs and cats in a consecutive series of cases. Thirty-seven client-owned animals (thirty-two dogs, five cats) diagnosed with implant failure after fracture treatment from January 2013–September 2018 were studied. Medical and radiographic records were retrospectively reviewed to identify patients that underwent fracture fixation using open reduction and internal fixation with subsequent radiographic evidence of implant failure. Area moment of inertia (AMI), plate working length, and bone screw density were determined. Implant failure was found in 39 fractures in 37 animals, representing 23% of fracture cases during the study period. Cases of implant failure were at increased risk of delayed union, malunion, or non-union (p< 0.0001). The most common cause of implant failure was loosening (54%); the second most common was plate failure that included low AMI locking plates (28%). Major complications found in 22/39 fractures (56%) were associated with delayed union (p< 0.01). Surgical revision was performed in 49% of implant failure cases. Complications were most frequently identified after treatment of humeral fractures (26%). We conclude mechanical failure of implants increases the risk for delayed or abnormal fracture healing and often requires revision surgery. Implant AMI should be considered during preoperative planning. Locking plates are associated with implant failure if plate bending stiffness is not sufficient, based on findings from this case series.