Comparison of short-segment monoaxial and polyaxial pedicle screw fixation combined with intermediate screws in traumatic thoracolumbar fractures: a finite element study and clinical radiographic review

Abstract

OBJECTIVES:No studies have compared monoaxial and polyaxial pedicle screws with regard to the von Mises stress of the instrumentation, intradiscal pressures of the adjacent segment and adjacent segment degeneration.METHODS:Short-segment monoaxial/polyaxial pedicle screw fixation techniques were compared using finite element methods, and the redistributed T11-L1 segment range of motion, largest maximal von Mises stress of the instrumentation, and intradiscal pressures of the adjacent segment under displacement loading were evaluated. Radiographic results of 230 patients with traumatic thoracolumbar fractures treated with these fixations were reviewed, and the sagittal Cobb’s angle, vertebral body angle, anterior vertebral body height of the fractured vertebrae and adjacent segment degeneration were calculated and evaluated.RESULTS:The largest maximal values of the von Mises stress were 376.8 MPa for the pedicle screws in the short-segment monoaxial pedicle screw fixation model and 439.9 MPa for the rods in the intermediate monoaxial pedicle screw fixation model. The maximal intradiscal pressures of the upper adjacent segments were all greater than those of the lower adjacent segments. The maximal intradiscal pressures of the monoaxial pedicle screw fixation model were larger than those in the corresponding segments of the normal model. The radiographic results at the final follow-up evaluation showed that the mean loss of correction of the sagittal Cobb’s angle, vertebral body angle and anterior vertebral body height were smallest in the intermediate monoaxial pedicle screw fixation group. Adjacent segment degeneration was less likely to be observed in the intermediate polyaxial pedicle screw fixation group but more likely to be observed in the intermediate monoaxial pedicle screw fixation group.CONCLUSION:Smaller von Mises stress in the pedicle screws and lower intradiscal pressure in the adjacent segment were observed in the polyaxial screw model than in the monoaxial pedicle screw fixation spine models. Fracture-level fixation could significantly correct kyphosis and reduce correction loss, and adjacent segment degeneration was less likely to be observed in the intermediate polyaxial pedicle screw fixation group.

Incomplete insertion of pedicle screws in a standard construct reduces the fatigue life: A biomechanical analysis

“…In this current study, leaving two threads of the screw exposed (unsupported screw length of 7.6 mm) increased the von Mises stress on the caudal side by 58%. This may explain why screws are often reported to break on the proximal side [7,25]. The experimental setup in this study is similar to La Barbera et al[17], but not completely consistent.…”

Extent and location of fixation affects the biomechanical stability of short- or long-segment pedicle screw technique with screwing of fractured vertebra for the treatment of thoracolumbar burst fractures

“…In current studies, posterior intermediate screw fixation at the fracture level can help improve and maintain the kyphosis correction, and the biomechanical stability can also be increased compared with SSPF. [ 1 – 15 ]…”

A comparative study between the Universal Spinal System® (USS) and the CD Horizon® Legacy™ (CDH) in the management of thoracolumbar fractures

“…No intended fusion was performed in the underlying study and Elkhateeb and Mahmoud found that there was no difference if the dorsolumbar fracture had a load sharing score < 7 [22]. Short-segment fixation with intermediate pedicular screw application into the fractured vertebrae unilaterally or bilaterally was shown to have favorable biomechanical characteristics as reduced von Mises stress and lower intradiscal pressure in the adjacent segments and lower occurrence of adjacent segment degeneration, especially when using a polyaxial screw system [23]. Hence many surgeons treating thoracolumbar fractures clearly shifted their practice to short-segment fixation with intermediate fixation [3].…”