Restoration of pull-out strength of the failed pedicle screw: biomechanical comparison of calcium sulfate vs polymethylmethacrylate augmentation.

OBJECTIVE The aim of the present study was to compare calcium sulfate (CAS) and polymethylmethacrylate (PMMA) bone cements used for the augmentation of a failed pedicle screw with biomechanical pull-out strength (POS) testing. METHODS Thirty lumbar vertebrae were harvested from 6 calves and bone mineral densities (BMD) were measured. Primary polyaxial pedicle screws were randomly inserted and pulled out and the POSs of the specimen were recorded. For revision, specimens were randomly assigned to the CAS-augmented pedicle screws group (Group 1) or PMMA-augmented pedicle screw group (Group 2). Pull-out tests were repeated to compare both groups. RESULTS Mean BMD of the specimens was 1.006 ± 0.116 g/cm(2). There were no statistically significant differences between BMD results of the two groups (p=0.116). For Group 1, mean POS of primary screws was 2,441.3 ± 936.4 N and was 2,499.5 ± 1,425.1 N after CAS augmentation, demonstrating no statistically significant difference (p=0.865). In Group 2, mean POS of the primary screws was 2,876.6 ± 926.6 N and significantly increased to 3,745.5 ± 1,299.2 N after PMMA augmentation (p=0.047). There was also a significant difference in mean POS between the CAS and PMMA groups (p=0.026). CONCLUSION Although CAS augmentation facilitates a revision screw POS as strong as that of primary screws, it is not as strong as PMMA augmentation.