Hardware failure in an unconstrained lumbar pedicle screw system - A 2-year follow-up study

Study Design. A consecutive study of patients who underwent lumbar spinal arthrodesis with an unconstrained pedicle screw system. Objectives. To determine the rate of arthrodesis and of clinical success and to examine and characterize the cases of hardware failure with the AO/Dynamic Compression Plate system (Synthes, Paoli, PA). Summary of Background Data. Although the advantages and disadvantages of nonconstrained versus constrained systems have been studied extensively, instrumentation failure has not. Additionally, the association between pseudarthrosis and hardware failure per se is unclear. Methods. Seventy-four consecutive cases of lumbar spinal fusion are reviewed. Standard outcome scores based on pain relief and medication usage were tabulated, along with pertinent demographic data. The patients were observed at five intervals after surgery for at least 2 years (range, 24 to 35 months; mean, 27 months). Standard statistical analyses were used to analyze data. Status of the arthrodesis was determined by standard radiographic criteria. Results. The overall fusion rate was 61%. At final follow-up, 60% of patients believed that their back pain had improved, whereas 70% believed that their limb pain had improved. The presence of a solid fusion (r = 3.3, P = 0.010) was correlated positively with a successful clinical outcome; the presence of pseudarthrosis and preoperative narcotic use were negatively correlated with a successful clinical outcome. Twenty-two percent of patients (16) experienced hardware failure. Twelve of the 16 had pseudarthrosis; in the majority of these patients, hardware failure occurred at the level of the pseudarthrosis. Conclusions. The results of this study demonstrate an extremely high rate of hardware failure and pseudarthrosis using an unconstrained pedicle screw system. Interestingly, the initial rate of pain relief was higher and declined over time and was quite possibly associated with loosening of the hardware. Based on these data, it is difficult to recommend the use of an unconstrained fixation system in the lumbar spine.