[BIOMECHANICAL STRENGTH INFLUENCE OF LATERAL WALL VIOLATION ON SPINAL PEDICLE SCREW FIXATION].

OBJECTIVE

To compare the fixation strength of optimum placed pedicle screw (OS) with redirectionally correctly placed pedicle screw (RS) following a violation of lateral pedicle.

METHODS

Thirty fresh lumbar vertebrae (L1-5) were obtained from 6 pigs weighing 95-105 kg, male or female. Each vertebra was instrumented with a monoaxial pedicle screw into each pedicle using two different techniques. On one side, a perfect screw path was created using direct visualization and fluoroscopy. A pedicle screw of 5 mm in diameter and 35 mm in length was placed with a digital torque driver (OS). On the other side, a lateral pedicle wall violation was created at the pedicle-vertebral body junction with a guide wire, a cannulated tap, and a pedicle probe. This path was then redirected into a correct position, developed, and instrumented with a 5-mm-diameter by 35-mm-long pedicle screw (RS). For each pedicle screw, the maximal torque, seating torque, screw loosening force, and post-loosening axial pullout were measured. Screw loosening and axial pullout were assessed using an MTS machine.

RESULTS

Maximal insertion torque was (111.4 ± 8.2) N x cm and (78.9 ± 6.4) N x cm for OS and RS respectively, showing significant difference (Z = 3.038, P = 0.002). The seating torque was (86.3 ± 7.7) N x cm and (59.7 ± 5.3) N x cm for OS and RS respectively, showing significant difference (Z = 2.802, P = 0.005). The screw loosening force was (76.3 ± 6.2) N and (53.0 ± 5.8) N for OS and RS respectively, showing significant difference (Z = 2.861, P = 0.004). The post-loosening axial pullout force was (343.0 ± 12.6) N and (287.0?10.5) N for OS and RS respectively, showing significant difference (Z = 2.964, P = 0.003).

CONCLUSION

Compared with OS, RS placement after a lateral wall violation shows significantly decreased maximal insertion torque, seating torque, screw loosening force, and post-loosening axial pullout. On this occasion, RS augmentation is a probable option for remediation.