Introduction Surgical treatment of supracondylar femoral fractures can be challenging. An additional wire cerclage is a suggested way to facilitate fracture reduction prior to plate osteosynthesis. Denudation to the periosteum remains a problematic disadvantage of this procedure. This study analyzed the effect of an additional wire cerclage on the load to failure in plate osteosynthesis of oblique supracondylar femoral shaft fractures. Materials and methods On eight pairs of non-osteoporotic human femora (mean age 74 years; range 57-95 years), an unstable AO/OTA 32-A2.3 fracture was established. All specimens were treated with a polyaxially locking plate. One femur of each pair was randomly selected to receive an additional fracture fixation with a wire cerclage. A servohydraulic testing machine was used to perform an incremental cyclic axial load with a load to the failure mode. Results Specimens stabilized with solely plate osteosynthesis failed at a mean load of 2450 N (95% CI: 1996-2904 N). In the group with an additional wire cerclage, load to failure was at a mean of 3100 N (95% CI: 2662-3538 N) (p = 0.018). Compression deformation with shearing of the condyle region through cutting of screws out of the condylar bone was the most common reason for failure in both groups of specimens. Whereas axial stiffness was comparable between both groups (p = 0.208), plastic deformation of the osteosynthesis constructs differed significantly (p = 0.035). Conclusions An additional wire cerclage significantly increased the load to failure. Therefore, an additional cerclage represents more than just a repositioning aid. With appropriate fracture morphology, a cerclage can significantly improve the strength of the osteosynthesis.