The ultimate clinical effect of SSI stability and rigidity is not known, but three SSI systems exhibited different displacements during cyclic loading in an in-vitro model. Although no failure of the SSI system occurred at 50,000 cycles, the possibility of fatigue failure cannot be excluded, particularly in view of the minor damage and significant displacements observed at some of the connecting interfaces. For that reason, static testing of SSI should be augmented by cyclic testing whenever possible, as this can elucidate other failure mechanisms, particularly since the expected clinical loading of these systems is greater than 10(6) cycles.