Composite synthetic bones are a commercially available substitute for cadaveric specimens, and they have previously been validated to replicate natural bone under quasi-static, non-destructive testing. Synthetic tibias could be used to analyse injury risk to the lower leg during impact events, but their failure mode must be validated by way of comparative tests to human bone. Synthetic tibias were instrumented with strain gauges and subjected to axial impact loading. Two different projectile masses were used for the tests, and the effects of force, momentum, and energy on failure were compared with previous cadaveric data. The composite tibias failed at forces between 37—45 per cent of those from cadavers, and failed via cortical delamination in combination with fracture. A Weibull analysis generated a survivability curve based on axial force at failure, and was shown to be lower than previous cadaveric curves. Failure was dependent on both the momentum and energy applied. Strain distributions through the synthetic tibias were significantly different from those of cadavers. The convex distal articular surface of the synthetic bones may partially account for the lower fracture tolerance. As a result of the many differences in response, these synthetic tibias are not recommended for use in impact fracture studies.